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Madrid–Barajas Airport (MAD) is the fourth-busiest airport in Europe. The aerosol chemical composition and the concentrations of other key pollutants were measured at the airport perimeter during October 2021 to assess the impact of airport emissions on local air quality. A high-fidelity ambient instrumentation system was deployed at Madrid–Barajas Airport to measure the following: concentrations of organic aerosols (with their composition), black carbon (eBC), carbon dioxide (CO2), carbon monoxide (CO), nitrogen dioxide (NOx), sulfur dioxide (SO2), particulate matter (PM2.5, PM10), total hydrocarbon (THC), and total particle number. The average concentrations of eBC, NOx, SO2, PM2.5, PM10, CO, and THC at the airport for the entire campaign were 1.07 µg m−3, 22.7 µg m−3, 4.10 µg m−3, 9.35 µg m−3, 16.43 µg m−3, 0.23 mg m−3, and 2.30 mg m−3, respectively. The source apportionment analysis of the non-refractory organic aerosol (OA) using positive matrix factorisation (PMF) allowed us to discriminate between different sources of pollution, namely less oxidised oxygenated organic aerosol (LO-OOA), alkane organic aerosol (AlkOA), and more oxidised oxygenated organic aerosol (MO-OOA). The results showed that LO-OOA and MO-OOA account for more than 80 % of the total organic particle mass measured near the runway. Trace gases correlate better with the AlkOA factor than LO-OOA and MO-OOA, indicating that AlkOA is mainly related to primary combustion emissions. Bivariate polar plots were used for pollutant source identification. Significantly higher concentrations of the obtained factors were observed at low wind speeds ( m s−1) from the southwest, where two of the runways and all terminals are located. Higher SO2/NOx and CO/eBC ratios were observed when the winds originated from the northeast, where the two northern runways are located. These elevated ratios are attributed to the aircraft activity being the major pollutant source in the northeast area.

Cloud feedbacks associated with deep convective anvils remain highly uncertain. In part, this uncertainty arises from a lack of understanding of how microphysical processes influence the cloud radiative effect. In particular, climate models have a poor representation of microphysics processes, thereby encouraging the collection and study of observation data to enable better representation of these processes in models. As such, the Deep Convective Microphysics Experiment (DCMEX) undertook an in situ aircraft and ground-based measurement campaign of New Mexico deep convective clouds during July–August 2022. The campaign coordinated a broad range of instrumentation measuring aerosol, cloud physics, radar, thermodynamics, dynamics, electric fields, and weather. This paper introduces the potential data user to DCMEX observational campaign characteristics, relevant instrument details, and references to more detailed instrument descriptions. Also included is information on the structure and important files in the dataset in order to aid the accessibility of the dataset to new users. Our overview of the campaign cases illustrates the complementary operational observations available and demonstrates the breadth of the campaign cases observed. During the campaign, a wide selection of environmental conditions occurred, ranging from dry, northerly air masses with low wind shear to moist, southerly air masses with high wind shear. This provided a wide range of different convective growth situations. Of 19 flight days, only 2 d lacked the formation of convective cloud. The dataset presented (https://doi.org/10.5285/B1211AD185E24B488D41DD98F957506C; Facility for Airborne Atmospheric Measurements et al., 2024) will help establish a new understanding of processes on the smallest cloud- and aerosol-particle scales and, once combined with operational satellite observations and modelling, can support efforts to reduce the uncertainty of anvil cloud radiative impacts on climate scales.

Atmospheric aerosol particles are known to have detrimental effects on human health and climate. Black carbon is an important constituent of atmospheric aerosol particulate matter (PM), emitted from incomplete combustion. Source apportionment of BC is very important, to evaluate the influence of different sources. The high-resolution soot particle aerosol mass spectrometer (HR-SP-AMS) instrument uses a laser vaporiser, which allows the real-time detection and characterisation of refractory black carbon (rBC) and its internally mixed particles such as metals, coating species, and rBC subcomponents in the form of HOA + fullerene. In this case study, the soot data were collected by using HR-SP-AMS during Guy Fawkes Night on 5 November 2014. Positive matrix factorisation was applied to positively discriminate between different wood-burning and bonfire sources for the first time, which no existing black carbon source apportionment technique is currently able to do. Along with this, the use of the fullerene signals in differentiating between soot sources and the use of metals as a tracer for fireworks has also been investigated, which did not significantly contribute to the rBC concentrations. The addition of fullerene signals and successful positive matrix factorisation (PMF) application to HR-SP-AMS data apportioned rBC into more than two sources. These bonfire sources are HOA + fullerene, biomass burning organic aerosol, more oxidised oxygenated organic aerosol (MO-OOA), and non-bonfire sources such as hydrocarbon-like OA and domestic burning. The result of correlation analysis between HR-SP-AMS data and previously published Aethalometer, MAAP, and CIMS data provides an effective way of gaining insights into the relationships between the variables and provide a quantitative estimate of the source contributions to the BC budget during this period. This research study is an important demonstration of using HR-SP-AMS for the purpose of BC source apportionment.

Seasonal biomass burning (BB) over West Africa is a globally significant source of carbonaceous particles in the atmosphere, which have important climate impacts but are poorly constrained. Here, the evolution of smoke aerosols emitted from flaming-controlled burning of agricultural waste and wooded savannah in the Senegal region was characterized over a timescale of half-day advection from the source during the MOYA-2017 (Methane Observation Yearly Assessment-2017) aircraft campaign. Plumes from such fire types are rich in black carbon (BC) emissions. Concurrent measurements of chemical composition, organic aerosol (OA) oxidation state, bulk aerosol size and BC mixing state reveal that emitted BB submicron aerosols changed dramatically with time. Various aerosol optical properties (e.g. absorption Ångström exponent – AAE – and mass absorption coefficients – MACs) also evolved with ageing. In this study, brown carbon (BrC) was a minor fractional component of the freshly emitted BB aerosols (< 0.5 h), but the increasing AAE with particle age indicates that BrC formation dominated over any loss process over the first ∼ 12 h of plume transport. Using different methods, the fractional contribution of BrC to total aerosol absorption showed an increasing trend with time and was ∼ 18 %–31 % at the optical wavelength of 405 nm after half-day transport. The generated BrC was found to be positively correlated with oxygenated and low-volatility OA, likely from the oxidation of evaporated primary OA and secondary OA formation. We found that the evolution of BrC with particle age was different in this region compared with previous BB field studies that mainly focused on emissions from smouldering fires, which have shown a high contribution from BrC at the source and BrC net loss upon ageing. This study suggests an initial stage of BrC absorption enhancement during the evolution of BB smoke. Secondary processing is the dominant contributor to BrC production in this BB region, in contrast to the primary emission of BrC previously reported in other BB studies. The total aerosol absorption normalized to BC mass (MACmeas-BC) was also enhanced with ageing due to the lensing effect of increasingly thick coatings on BC and the absorption by BrC. The effect of ageing on aerosol absorption, represented by the absorption enhancement (EAbs-MAC), was estimated over timescales of hours. MOYA-2017 provides novel field results. The comparisons between MOYA-2017 and previous field studies imply that the evolution of absorbing aerosols (BC and BrC) after emission varies with source combustion conditions. Different treatments of absorbing aerosol properties from different types of fires and their downwind evolution should be considered when modelling regional radiative forcing. These observational results will be very important for predicting climate effects of BB aerosol in regions controlled by flaming burning of agricultural waste and savannah-like biomass fuels.

Seasonal biomass burning (BB) from June to October in central and southern Africa leads to absorbing aerosols being transported over the South Atlantic Ocean every year and contributes significantly to the regional climate forcing. The vertical distribution of submicron aerosols and their properties were characterized over the remote southeast Atlantic, using airborne in situ measurements made during the CLoud-Aerosol-Radiation Interactions and Forcing for Year 2017 (CLARIFY-2017) campaign. BB aerosols emitted from flaming-controlled fires were intensively observed in the region surrounding Ascension Island, in the marine boundary layer (MBL) and free troposphere (FT) up to 5 km. We show that the aerosols had undergone a significant ageing process during > 7 d transit from source, as indicated by the highly oxidized organic aerosol. The highly aged BB aerosols in the far-field CLARIFY region were also especially rich in black carbon (BC), with relatively low single-scattering albedos (SSAs), compared with those from other BB transported regions. The column-weighted dry SSAs during CLARIFY were observed to be 0.85, 0.84 and 0.83 at 405, 550 and 658 nm respectively. We also found significant vertical variation in the dry SSA, as a function of relative chemical composition and size. The lowest SSA in the column was generally in the low FT layer around 2000 m altitude (averages: 0.82, 0.81 and 0.79 at 405, 550 and 658 nm). This finding is important since it means that BB aerosols across the southeast Atlantic region are more absorbing than currently represented in climate models, implying that the radiative forcing from BB may be more strongly positive than previously thought. Furthermore, in the FT, average SSAs at 405, 550 and 658 nm increased to 0.87, 0.86 and 0.85 with altitude up to 5 km. This was associated with an enhanced inorganic nitrate mass fraction and aerosol size, likely resulting from increased partitioning of ammonium nitrate to the existing particles at higher altitude with lower temperature and higher relative humidity. After entrainment into the boundary layer (BL), aerosols were generally smaller in dry size than in the FT and had a larger fraction of scattering material with resultant higher average dry SSA, mostly due to marine emissions and aerosol removal by drizzle. In the BL, the SSA decreased from the surface to the BL top, with the highest SSA in the column observed near the surface. Our results provide unique observational constraints on aerosol parameterizations used in modelling regional radiation interactions over this important region. We recommend that future work should consider the impact of this vertical variability on climate models.

The optical properties of black carbon (BC) are a major source of uncertainty in regional and global climate studies. In the past, detailed investigation of BC absorption has been hampered by systematic biases in the measurement instrumentation. We present airborne measurements of aerosol absorption and black carbon microphysical properties in highly aged biomass burning plumes measured 4–8 d from their source over the southeast Atlantic Ocean during CLARIFY-2017, using a suite of novel photoacoustic spectrometers to measure aerosol absorption at 405, 514, and 655 nm and a single-particle soot photometer to measure the BC mass concentration, size, and mixing state. These measurements are of sufficient quality and detail to provide constraint on optical schemes used in climate models for the first time in biomass burning plumes far from their source – an aerosol environment that is one of the most important climatically. The average absorption Ångström exponents (AAE) were 1.38 over the wavelength range from 405 to 514 nm and 0.88 over the range from 514 to 655 nm, suggesting that brown carbon (BrC) contributed to 11±2 % of absorption at 405 nm. The effective organic aerosol (OA) mass absorption coefficient (MAC) was 0.31±0.09 m2 g−1 at 405 nm. The BC particles were universally thickly coated, and almost no externally mixed BC particles were detected. The average MAC of BC was 20±4, 15±3, and 12±2 m2g−1 at wavelengths of 405, 514, and 655 nm respectively, with equivalent absorption enhancements of around 1.85±0.45 at all three wavelengths, suggesting that the thick coatings acted as a lens that enhanced light absorption by the BC. We compared the measured MAC and AAE values with those calculated using several optical models and absorption parameterisations that took the measured BC mass and mixing state as inputs. Homogeneous grey-sphere Mie models were only able to replicate MAC for some low (real and imaginary) values of the complex BC refractive index (mBC) at the shortest wavelength, but they would have to use unrealistically low values of mBC to accurately replicate the AAE. A core–shell Mie model was able to generate good agreement for MAC in the green–red end of the visible spectrum for most values of mBC. However, there are no possible values of mBC that produce MAC values that agree with our observations at all three wavelengths, due to a wavelength-dependent underestimation of the MAC of the underlying BC core. Four semiempirical parameterisations from the literature were also tested, linking the BC mixing state to either the MAC or absorption enhancement. Two of these schemes produced results that agreed within a few percent with the measured MAC at all three wavelengths, and the AAE agreed well when discounting the effects of BrC. Our results uniquely demonstrate the validity of absorption parameterisations, as well as the failings of Mie calculations, in this highly aged environment. We recommend that future work should conduct similar analyses in environments where BC has different properties; future studies should also investigate the impact of implementing these types of schemes within climate models as well as the impact of developing equivalent schemes for light scattering by soot particles at visible wavelengths.

Methane emission fluxes from many facility-scale sources may be poorly quantified, potentially leading to uncertainties in the global methane budget. Accurate atmospheric measurement-based flux quantification is urgently required to address this. This paper describes the first test (using unbiased sampling) of a near-field Gaussian plume inversion (NGI) technique, suitable for facility-scale flux quantification, using a controlled release of methane gas. Two unmanned-aerial-vehicle (UAV) platforms were used to perform 22 flight surveys downwind of a point-source methane gas release from a regulated cylinder with a flowmeter. One UAV was tethered to an instrument on the ground, while the other UAV carried an on-board prototype instrument (both of which used the same near-infrared laser technology). Both instruments were calibrated using certified standards to account for variability in the instrumental gain factor, assuming fixed temperature and pressure. Furthermore, a water vapour correction factor, specifically calculated for the instrument, was applied and is described here in detail. We also provide guidance on potential systematic uncertainties associated with temperature and pressure, which may require further characterisation for improved measurement accuracy. The NGI technique was then used to derive emission fluxes for each UAV flight survey. We found good agreement of most NGI fluxes with the known controlled emission flux, within uncertainty, verifying the flux quantification methodology. The lower and upper NGI flux uncertainty bounds were, on average, 17 %±10(1σ) % and 227 %±98(1σ) % of the controlled emission flux, respectively. This range of conservative uncertainty bounds incorporate factors including the variability in the position of the time-invariant plume and potential for under-sampling. While these average uncertainties are large compared to methods such as tracer dispersion, we suggest that UAV sampling can be highly complementary to a toolkit of flux quantification approaches and may be a valuable alternative in situations where site access for tracer release is problematic. We see tracer release combined with UAV sampling as an effective approach in future flux quantification studies. Successful flux quantification using the UAV sampling methodology described here demonstrates its future utility in identifying and quantifying emissions from methane sources such as oil and gas extraction infrastructure facilities, livestock agriculture, and landfill sites, where site access may be difficult.

Black carbon (BC) is known to have major impacts on both climate and human health and is therefore of global importance, particularly in regions close to large populations that have strong sources. The size-resolved mixing state of BC-containing particles was characterised using a single-particle soot photometer (SP2). The study focusses on the Indo-Gangetic Plain (IGP) during the pre-monsoon and monsoon seasons. Data presented are from the UK Facility for Airborne Atmospheric Measurements BAe-146 research aircraft that performed flights during the pre-monsoon (11 and 12 June) and monsoon (30 June to 11 July) seasons of 2016. Over the IGP, BC mass concentrations were greater (1.95 µg m−3) compared to north-west India (1.50 µg m−3) and north-east India (0.70 µg m−3) during the pre-monsoon season. Across northern India, two distinct BC modes were recorded; a mode of small BC particles (core diameter  µm and coating thickness  nm) and a mode of moderately coated BC (core diameter  µm and coating thickness of 50–200 nm). The IGP and north-east India locations exhibited moderately coated black carbon particles with enhanced coating thicknesses, core sizes, mass absorption cross sections, and scattering enhancement values compared to much lower values present in the north-west. The coating thickness and mass absorption cross section increased with altitude (13 %) compared to those in the boundary layer. As the monsoon arrived across the region, mass concentration of BC decreased over the central IGP and north-east locations (38 % and 28 % respectively), whereas for the north-west location BC properties remained relatively consistent. Post-monsoon onset, the coating thickness, core size, mass absorption cross section, and scattering enhancement values were all greatest over the central IGP much like the pre-monsoon season but were considerably reduced over both north-east and north-west India. Increases in mass absorption cross section through the atmospheric column were still present during the monsoon for the north-west and central IGP locations, but less so over the north-east due to lack of long-range transport aerosol aloft. Across the Indo-Gangetic Plain and north-east India during the pre-monsoon and monsoon seasons, solid-fuel (wood burning) emissions form the greatest proportion of BC with moderately coated particles. However, as the monsoon develops in the north-east there was a switch to small uncoated BC particles indicative of traffic emissions, but the solid-fuel emissions remained in the IGP into the monsoon. For both seasons in the north-west, traffic emissions form the greatest proportion of BC particles. Our findings will prove important for greater understanding of the BC physical and optical properties, with important consequences for the atmospheric radiative forcing of BC-containing particles. The findings will also help constrain the regional aerosol models for a variety of applications such as space-based remote sensing, chemistry transport modelling, air quality, and BC source and emission inventories.

Biases in absorption coefficients measured using a filter-based absorption photometer (Tricolor Absorption Photometer, or TAP) at wavelengths of 467, 528 and 652 nm are evaluated by comparing to measurements made using photoacoustic spectroscopy (PAS). We report comparisons for ambient sampling covering a range of aerosol types including urban, fresh biomass burning and aged biomass burning. Data are also used to evaluate the performance of three different TAP correction schemes. We found that photoacoustic and filter-based measurements were well correlated, but filter-based measurements generally overestimated absorption by up to 45 %. Biases varied with wavelength and depended on the correction scheme applied. Optimal agreement to PAS data was achieved by processing the filter-based measurements using the recently developed correction scheme of Müller et al. (2014), which consistently reduced biases to 0 %–18 % at all wavelengths. The biases were found to be a function of the ratio of organic aerosol mass to light-absorbing carbon mass, although applying the Müller et al. (2014) correction scheme to filter-based absorption measurements reduced the biases and the strength of this correlation significantly. Filter-based absorption measurement biases led to aerosol single-scattering albedos that were biased low by values in the range 0.00–0.07 and absorption Ångström exponents (AAEs) that were in error by ± (0.03–0.54). The discrepancy between the filter-based and PAS absorption measurements is lower than reported in some earlier studies and points to a strong dependence of filter-based measurement accuracy on aerosol source type.

The vertical distribution in the physical and chemical properties of submicron aerosol has been characterised across northern India for the first time using airborne in situ measurements. This study focusses primarily on the Indo-Gangetic Plain, a low-lying area in the north of India which commonly experiences high aerosol mass concentrations prior to the monsoon season. Data presented are from the UK Facility for Airborne Atmospheric Measurements BAe-146 research aircraft that performed flights in the region during the 2016 pre-monsoon (11 and 12 June) and monsoon (30 June to 11 July) seasons. Inside the Indo-Gangetic Plain boundary layer, organic matter dominated the submicron aerosol mass (43 %) followed by sulfate (29 %), ammonium (14 %), nitrate (7 %) and black carbon (7 %). However, outside the Indo-Gangetic Plain, sulfate was the dominant species, contributing 44 % to the total submicron aerosol mass in the boundary layer, followed by organic matter (30 %), ammonium (14 %), nitrate (6 %) and black carbon (6 %). Chlorine mass concentrations were negligible throughout the campaign. Black carbon mass concentrations were higher inside the Indo-Gangetic Plain (2 µg m−3) compared to outside (1 µg m−3). Nitrate appeared to be controlled by thermodynamic processes, with increased mass concentration in conditions of lower temperature and higher relative humidity. Increased mass and number concentrations were observed inside the Indo-Gangetic Plain and the aerosol was more absorbing in this region, whereas outside the Indo-Gangetic Plain the aerosol was larger in size and more scattered in nature, suggesting greater dust presence, especially in north-western India. The aerosol composition remained largely similar as the monsoon season progressed, but the total aerosol mass concentrations decreased by ∼50 % as the rainfall arrived; the pre-monsoon average total mass concentration was 30 µg m−3 compared to a monsoon average total mass concentration of 10–20 µg m−3. However, this mass concentration decrease was less noteworthy (∼20 %–30 %) over the Indo-Gangetic Plain, likely due to the strength of emission sources in this region. Decreases occurred in coarse mode aerosol, with the fine mode fraction increasing with monsoon arrival. In the aerosol vertical profile, inside the Indo-Gangetic Plain during the pre-monsoon, organic aerosol and absorbing aerosol species dominated in the lower atmosphere ( km), with sulfate, dust and other scattering aerosol species enhanced in an elevated aerosol layer above 1.5 km with maximum aerosol height ∼6 km. The elevated concentration of dust at altitudes >1.5 km is a clear indication of dust transport from the Great Indian Desert, also called the Thar Desert, in north-western India. As the monsoon progressed into this region, the elevated aerosol layer diminished, the aerosol maximum height reduced to ∼2 km. The dust and sulfate-dominated aerosol layer aloft was removed upon monsoon arrival, highlighted by an increase in fine mode fraction throughout the profile.

The mineralogy and mixing state of dust particles originating from the African continent influences climate and marine ecosystems in the North Atlantic due to its effect on radiation, cloud properties and biogeochemical cycling. However, these processes are difficult to constrain because of large temporal and spatial variability, and the lack of in situ measurements of dust properties at all stages of the dust cycle. This lack of measurements is in part due to the remoteness of potential source areas (PSAs) and transport pathways but also because of the lack of an efficient method to report the mineralogy and mixing state of single particles with a time resolution comparable to atmospheric processes, which may last a few hours or less. Measurements are equally challenging in laboratory simulations where dust particles need to be isolated and characterised in low numbers whilst conditions are dynamically controlled and monitored in real time. This is particularly important in controlled expansion cloud chambers (CECCs) where ice-nucleating properties of suspended dust samples are studied in cold and mixed phase cloud conditions. In this work, the mineralogy and mixing state of the fine fraction ( µm) in laboratory-suspended dust from PSAs in north Africa were made using novel techniques with online single-particle mass spectrometry (SPMS) and traditional offline scanning electron microscopy (SEM). A regional difference in mineralogy was detected, with material sourced from Morocco containing a high number fraction of illite-like particles in contrast to Sahelian material which contains potassium- and sodium-depleted clay minerals like kaolinite. Single-particle mixing state had a much greater local variation than mineralogy, particularly with respect to organic–biological content. Applying the same methods to ambient measurement of transported dust in the marine boundary layer at Cabo Verde in the remote North Atlantic enabled the number fractions of illite/smectite clay mineral (ISCM), non-ISCM and calcium-containing particles to be reported at a 1 h time resolution over a 20-day period. Internal mixing of silicate particles with nitrate, chlorine and organic–biological material was also measured and compared to that in the suspended soils. The results show SPMS and SEM techniques are complementary and demonstrate that SPMS can provide a meaningful high-resolution measurement of single-particle mineralogy and mixing state in laboratory and ambient conditions. In most cases, the differences in the mineralogical composition between particles within a soil sample were small. Thus, particles were not composed of discrete mineral phases. In ambient measurements, the ISCM and nitrate content was found to change significantly between distinct dust events, indicating a shift in source and transport pathways which may not be captured in offline composition analysis or remote sensing techniques.

Over the past decade, there has been an increasing interest in short-term events that negatively affect air quality such as bonfires and fireworks. High aerosol and gas concentrations generated from public bonfires or fireworks were measured in order to understand the night-time chemical processes and their atmospheric implications. Nitrogen chemistry was observed during Bonfire Night with nitrogen containing compounds in both gas and aerosol phases and further N2O5 and ClNO2 concentrations, which depleted early next morning due to photolysis of NO3 radicals and ceasing production. Particulate organic oxides of nitrogen (PONs) concentrations of 2.8 µg m−3 were estimated using the m ∕ z 46 : 30 ratios from aerosol mass spectrometer (AMS) measurements, according to previously published methods. Multilinear engine 2 (ME-2) source apportionment was performed to determine organic aerosol (OA) concentrations from different sources after modifying the fragmentation table and it was possible to identify two PON factors representing primary (pPON_ME2) and secondary (sPON_ME2) contributions. A slight improvement in the agreement between the source apportionment of the AMS and a collocated AE-31 Aethalometer was observed after modifying the prescribed fragmentation in the AMS organic spectrum (the fragmentation table) to determine PON sources, which resulted in an r2 = 0.894 between biomass burning organic aerosol (BBOA) and babs_470wb compared to an r2 = 0.861 obtained without the modification. Correlations between OA sources and measurements made using time-of-flight chemical ionisation mass spectrometry with an iodide adduct ion were performed in order to determine possible gas tracers to be used in future ME-2 analyses to constrain solutions. During Bonfire Night, strong correlations (r2) were observed between BBOA and methacrylic acid (0.92), acrylic acid (0.90), nitrous acid (0.86), propionic acid, (0.85) and hydrogen cyanide (0.76). A series of oxygenated species and chlorine compounds showed good correlations with sPON_ME2 and the low volatility oxygenated organic aerosol (LVOOA) factor during Bonfire Night and an event with low pollutant concentrations. Further analysis of pPON_ME2 and sPON_ME2 was performed in order to determine whether these PON sources absorb light near the UV region using an Aethalometer. This hypothesis was tested by doing multilinear regressions between babs_470wb and BBOA, sPON_ME2 and pPON_ME2. Our results suggest that sPON_ME2 does not absorb light at 470 nm, while pPON_ME2 and LVOOA do absorb light at 470 nm. This may inform black carbon (BC) source apportionment studies from Aethalometer measurements, through investigation of the brown carbon contribution to babs_470wb.

Particulate emissions from biomass burning can both alter the atmosphere's radiative balance and cause significant harm to human health. However, due to the large effect on emissions caused by even small alterations to the way in which a fuel burns, it is difficult to study particulate production of biomass combustion mechanistically and in a repeatable manner. In order to address this gap, in this study, small wood samples sourced from Côte D'Ivoire in West Africa were burned in a highly controlled laboratory environment. The shape and mass of samples, available airflow and surrounding thermal environment were carefully regulated. Organic aerosol and refractory black carbon emissions were measured in real time using an Aerosol Mass Spectrometer and a Single Particle Soot Photometer, respectively. This methodology produced remarkably repeatable results, allowing aerosol emissions to be mapped directly onto different phases of combustion. Emissions from pyrolysis were visible as a distinct phase before flaming was established. After flaming combustion was initiated, a black-carbon-dominant flame was observed during which very little organic aerosol was produced, followed by a period that was dominated by organic-carbon-producing smouldering combustion, despite the presence of residual flaming. During pyrolysis and smouldering, the two phases producing organic aerosol, distinct mass spectral signatures that correspond to previously reported variations in biofuel emissions measured in the atmosphere are found. Organic aerosol emission factors averaged over an entire combustion event were found to be representative of the time spent in the pyrolysis and smouldering phases, rather than reflecting a coupling between emissions and the mass loss of the sample. Further exploration of aerosol yields from similarly carefully controlled fires and a careful comparison with data from macroscopic fires and real-world emissions will help to deliver greater constraints on the variability of particulate emissions in atmospheric systems.

In situ airborne observations of cloud microphysics, aerosol properties, and thermodynamic structure over the transition from sea ice to ocean are presented from the Aerosol-Cloud Coupling And Climate Interactions in the Arctic (ACCACIA) campaign. A case study from 23 March 2013 provides a unique view of the cloud microphysical changes over this transition under cold-air outbreak conditions. Cloud base lifted and cloud depth increased over the transition from sea ice to ocean. Mean droplet number concentrations, Ndrop, also increased from 110 ± 36 cm−3 over the sea ice to 145 ± 54 cm−3 over the marginal ice zone (MIZ). Downstream over the ocean, Ndrop decreased to 63 ± 30 cm−3. This reduction was attributed to enhanced collision-coalescence of droplets within the deep ocean cloud layer. The liquid water content increased almost four fold over the transition and this, in conjunction with the deeper cloud layer, allowed rimed snowflakes to develop and precipitate out of cloud base downstream over the ocean. The ice properties of the cloud remained approximately constant over the transition. Observed ice crystal number concentrations averaged approximately 0.5–1.5 L−1, suggesting only primary ice nucleation was active; however, there was evidence of crystal fragmentation at cloud base over the ocean. Little variation in aerosol particle number concentrations was observed between the different surface conditions; however, some variability with altitude was observed, with notably greater concentrations measured at higher altitudes ( > 800 m) over the sea ice. Near-surface boundary layer temperatures increased by 13 °C from sea ice to ocean, with corresponding increases in surface heat fluxes and turbulent kinetic energy. These significant thermodynamic changes were concluded to be the primary driver of the microphysical evolution of the cloud. This study represents the first investigation, using in situ airborne observations, of cloud microphysical changes with changing sea ice cover and addresses the question of how the microphysics of Arctic stratiform clouds may change as the region warms and sea ice extent reduces.

Heavy rainfall from convective clouds can lead to devastating flash flooding, and observations of aerosols and clouds are required to improve cloud parameterisations used in precipitation forecasts. We present measurements of boundary layer aerosol concentration, size, and composition from a series of research flights performed over the southwest peninsula of the UK during the COnvective Precipitation Experiment (COPE) of summer 2013. We place emphasis on periods of southwesterly winds, which locally are most conducive to convective cloud formation, when marine air from the Atlantic reached the peninsula. Accumulation-mode aerosol mass loadings were typically 2–3 µg m−3 (corrected to standard cubic metres at 1013.25 hPa and 273.15 K), the majority of which was sulfuric acid over the sea, or ammonium sulfate inland, as terrestrial ammonia sources neutralised the aerosol. The cloud condensation nuclei (CCN) concentrations in these conditions were ∼ 150–280 cm−3 at 0.1 % and 400–500 cm−3 at 0.9 % supersaturation (SST), which are in good agreement with previous Atlantic measurements, and the cloud drop concentrations at cloud base ranged from 100 to 500 cm−3. The concentration of CCN at 0.1 % SST was well correlated with non-sea-salt sulfate, meaning marine sulfate formation was likely the main source of CCN. Marine organic aerosol (OA) had a similar mass spectrum to previous measurements of sea spray OA and was poorly correlated with CCN. In one case study that was significantly different to the rest, polluted anthropogenic emissions from the southern and central UK advected to the peninsula, with significant enhancements of OA, ammonium nitrate and sulfate, and black carbon. The CCN concentrations here were around 6 times higher than in the clean cases, and the cloud drop number concentrations were 3–4 times higher. Sources of ice-nucleating particles (INPs) were assessed by comparing different parameterisations used to predict INP concentrations, using measured aerosol concentrations as input. The parameterisations based on total aerosol produced INP concentrations that agreed within an order of magnitude with measured first ice concentrations at cloud temperatures as low as −12 °C. Composition-specific parameterisations for mineral dust, fluorescent particles, and sea spray OA were 3–4 orders of magnitude lower than the measured first ice concentrations, meaning a source of INPs was present that was not characterised by our measurements and/or one or more of the composition-specific parameterisations greatly underestimated INPs in this environment.

We present microphysical observations of cumulus clouds measured over the southwest peninsula of the UK during the COnvective Precipitation Experiment (COPE) in August 2013, which are framed into a wider context using ground-based and airborne radar measurements. Two lines of cumulus clouds formed in the early afternoon along convergence lines aligned with the peninsula. The lines became longer and broader during the afternoon due to new cell formation and stratiform regions forming downwind of the convective cells. Ice concentrations up to 350 L−1, well in excess of the expected ice nuclei (IN) concentrations, were measured in the mature stratiform regions, suggesting that secondary ice production was active. Detailed sampling focused on an isolated liquid cloud that glaciated as it matured to merge with a band of cloud downwind. In the initial cell, drizzle concentrations increased from ∼ 0.5 to ∼ 20 L−1 in around 20 min. Ice concentrations developed up to a few per litre, which is around the level expected of primary IN. The ice images were most consistent with freezing drizzle, rather than smaller cloud drops or interstitial IN forming the first ice. As new cells emerged in and around the cloud, ice concentrations up to 2 orders of magnitude higher than the predicted IN concentrations developed, and the cloud glaciated over a period of 12–15 min. Almost all of the first ice particles to be observed were frozen drops, while vapour-grown ice crystals were dominant in the latter stages. Our observations are consistent with the production of large numbers of small secondary ice crystals/fragments, by a mechanism such as Hallett–Mossop or droplets shattering upon freezing. Some of the small ice froze drizzle drops on contact, while others grew more slowly by vapour deposition. Graupel and columns were seen in cloud penetrations up to the −12 °C level, though many ice particles were mixed habit due to riming and growth by vapour deposition at multiple temperatures. Our observations demonstrate that the freezing of drizzle/raindrops is an important process that dominates the formation of large ice in the intermediate stages of cloud development. As these frozen drops were the first precipitation observed, interactions between the warm-rain and secondary ice production processes appear to be key to determining the timing and location of precipitation.

Trace element measurements in PM10–2.5, PM2.5–1.0 and PM1.0–0.3 aerosol were performed with 2 h time resolution at kerbside, urban background and rural sites during the ClearfLo winter 2012 campaign in London. The environment-dependent variability of emissions was characterized using the Multilinear Engine implementation of the positive matrix factorization model, conducted on data sets comprising all three sites but segregated by size. Combining the sites enabled separation of sources with high temporal covariance but significant spatial variability. Separation of sizes improved source resolution by preventing sources occurring in only a single size fraction from having too small a contribution for the model to resolve. Anchor profiles were retrieved internally by analysing data subsets, and these profiles were used in the analyses of the complete data sets of all sites for enhanced source apportionment. A total of nine different factors were resolved (notable elements in brackets): in PM10–2.5, brake wear (Cu, Zr, Sb, Ba), other traffic-related (Fe), resuspended dust (Si, Ca), sea/road salt (Cl), aged sea salt (Na, Mg) and industrial (Cr, Ni); in PM2.5–1.0, brake wear, other traffic-related, resuspended dust, sea/road salt, aged sea salt and S-rich (S); and in PM1.0–0.3, traffic-related (Fe, Cu, Zr, Sb, Ba), resuspended dust, sea/road salt, aged sea salt, reacted Cl (Cl), S-rich and solid fuel (K, Pb). Human activities enhance the kerb-to-rural concentration gradients of coarse aged sea salt, typically considered to have a natural source, by 1.7–2.2. These site-dependent concentration differences reflect the effect of local resuspension processes in London. The anthropogenically influenced factors traffic (brake wear and other traffic-related processes), dust and sea/road salt provide further kerb-to-rural concentration enhancements by direct source emissions by a factor of 3.5–12.7. The traffic and dust factors are mainly emitted in PM10–2.5 and show strong diurnal variations with concentrations up to 4 times higher during rush hour than during night-time. Regionally influenced S-rich and solid fuel factors, occurring primarily in PM1.0–0.3, have negligible resuspension influences, and concentrations are similar throughout the day and across the regions.

Black carbon aerosol (BC) deposited to the Arctic sea ice or present in the free troposphere can significantly affect the Earth's radiation budget at high latitudes yet the BC burden in these regions and the regional source contributions are poorly constrained. Aircraft measurements of aerosol composition in the European Arctic were conducted during the Aerosol–Cloud Coupling And Climate Interactions in the Arctic (ACCACIA) campaign in March 2013. Pollutant plumes were encountered throughout the lower to upper Arctic troposphere featuring enhancements in CO and aerosol mass loadings, which were chemically speciated into BC and non-refractory sulphate and organic matter. FLEXPART-WRF simulations have been performed to evaluate the likely contribution to the pollutants from regional ground sources. By combining up-to-date anthropogenic and open fire biomass burning (OBB) inventories, we have been able to compare the contributions made to the observed pollution layers from the sources of eastern/northern Asia (AS), Europe (EU) and North America (NA). Over 90 % of the contribution to the BC was shown to arise from non-OBB anthropogenic sources. AS sources were found to be the major contributor to the BC burden, increasing background BC loadings by a factor of 3–5 to 100.8 ± 48.4 ng sm−3 (in standard air m3 at 273.15 K and 1013.25 mbar) and 55.8 ± 22.4 ng sm−3 in the middle and upper troposphere respectively. AS plumes close to the tropopause (about 7.5–8 km) were also observed, with BC concentrations ranging from 55 to 73 ng sm−3, which will potentially have a significant radiative impact. EU sources influenced the middle troposphere with a BC mean concentration of 70.8 ± 39.1 ng sm−3 but made a minor contribution to the upper troposphere due to the relatively high latitude of the source region. The contribution of NA was shown to be much lower at all altitudes with BC mean concentration of 20 ng sm−3. The BC transported to the Arctic is mixed with a non-BC volume fraction representing between 90–95 % of the mass, and has a relatively uniform core size distribution with mass median diameter 190–210 nm and geometric standard deviation σg = 1.55–1.65 and this varied little across all source regions. It is estimated that 60–95 % of BC is scavenged between emission and receptor based on BC / ΔCO comparisons between source inventories and measurement. We show that during the springtime of 2013, the anthropogenic pollution particularly from sources in Asia, contributed significantly to BC across the European Arctic free troposphere. In contrast to previous studies, the contribution from open wildfires was minimal. Given that Asian pollution is likely to continue to rise over the coming years, it is likely that the radiative forcing in the Arctic will also continue to increase.

For the first time, the behaviour of non-refractory inorganic and organic submicron particulate through an entire annual cycle is investigated using measurements from an Aerodyne compact time-of-flight aerosol mass spectrometer (cToF-AMS) located at a UK urban background site in North Kensington, London. We show that secondary aerosols account for a significant fraction of the submicron aerosol burden and that high concentration events are governed by different factors depending on season. Furthermore, we demonstrate that on an annual basis there is no variability in the extent of secondary organic aerosol (SOA) oxidation, as defined by the oxygen content, irrespective of amount. This result is surprising given the changes in precursor emissions and contributions as well as photochemical activity throughout the year; however it may make the characterisation of SOA in urban environments more straightforward than previously supposed. Organic species, nitrate, sulphate, ammonium, and chloride were measured during 2012 with average concentrations (±1 standard deviation) of 4.32 (±4.42), 2.74 (±5.00), 1.39 (±1.34), 1.30 (±1.52), and 0.15 (±0.24) μg m−3, contributing 44, 28, 14, 13, and 2 % to the total non-refractory submicron mass (NR-PM1) respectively. Components of the organic aerosol fraction are determined using positive matrix factorisation (PMF), in which five factors are identified and attributed as hydrocarbon-like OA (HOA), cooking OA (COA), solid fuel OA (SFOA), type 1 oxygenated OA (OOA1), and type 2 oxygenated OA (OOA2). OOA1 and OOA2 represent more and less oxygenated OA with average concentrations of 1.27 (±1.49) and 0.14 (±0.29) μg m−3 respectively, where OOA1 dominates the SOA fraction (90%). Diurnal, monthly, and seasonal trends are observed in all organic and inorganic species due to meteorological conditions, specific nature of the aerosols, and availability of precursors. Regional and transboundary pollution as well as other individual pollution events influence London's total submicron aerosol burden. High concentrations of non-refractory submicron aerosols in London are governed by particulate emissions in winter, especially nitrate and SFOA, whereas SOA formation drives the high concentrations during the summer. The findings from this work could have significant implications for modelling of urban air pollution as well as for the effects of atmospheric aerosols on health and climate.

Accurately accounting for new particle formation (NPF) is crucial to our ability to predict aerosol number concentrations in many environments and thus cloud properties, which is in turn vital in simulating radiative transfer and climate. Here we present an analysis of NPF events observed in the Greenland Sea during the summertime as part of the Aerosol-Cloud Coupling And Climate Interactions in the Arctic (ACCACIA) project. While NPF events have been reported in the Arctic before, we were able, for the first time, to detect iodine in the growing particles using an Aerosol Mass Spectrometer (AMS) during a persistent event in the region of the coastal sea-ice near Greenland. Given the potency of iodine as a nucleation precursor, the results imply that iodine was responsible for the initial NPF, a phenomenon that has been reported at lower latitudes and associated with molecular iodine emissions from coastal macroalgae. The initial source of iodine in this instance is not clear, but it was associated with air originating approximately 1 day previously over melting coastal sea-ice. These results show that atmospheric models must consider iodine as a source of new particles in addition to established precursors such as sulfur compounds.

Ambient concentrations of trace elements with 2 h time resolution were measured in PM10–2.5, PM2.5–1.0 and PM1.0–0.3 size ranges at kerbside, urban background and rural sites in London during winter 2012. Samples were collected using rotating drum impactors (RDIs) and subsequently analysed with synchrotron radiation-induced X-ray fluorescence spectrometry (SR-XRF). Quantification of kerb and urban increments (defined as kerb-to-urban and urban-to-rural concentration ratios, respectively), and assessment of diurnal and weekly variability provided insight into sources governing urban air quality and the effects of urban micro-environments on human exposure. Traffic-related elements yielded the highest kerb increments, with values in the range of 10.4 to 16.6 for SW winds (3.3–6.9 for NE) observed for elements influenced by brake wear (e.g. Cu, Sb, Ba) and 5.7 to 8.2 for SW (2.6–3.0 for NE) for other traffic-related processes (e.g. Cr, Fe, Zn). Kerb increments for these elements were highest in the PM10–2.5 mass fraction, roughly twice that of the PM1.0–0.3 fraction. These elements also showed the highest urban increments (~ 3.0), although no difference was observed between brake wear and other traffic-related elements. All elements influenced by traffic exhibited higher concentrations during morning and evening rush hours, and on weekdays compared to weekends, with the strongest trends observed at the kerbside site, and additionally enhanced by winds coming directly from the road, consistent with street canyon effects. Elements related to mineral dust (e.g. Al, Si, Ca, Sr) showed significant influences from traffic-induced resuspension, as evidenced by moderate kerb (3.4–5.4 for SW, 1.7–2.3 for NE) and urban (~ 2) increments and increased concentrations during peak traffic flow. Elements related to regional transport showed no significant enhancement at kerb or urban sites, with the exception of PM10–2.5 sea salt (factor of up to 2), which may be influenced by traffic-induced resuspension of sea and/or road salt. Heavy-duty vehicles appeared to have a larger effect than passenger vehicles on the concentrations of all elements influenced by resuspension (including sea salt) and wearing processes. Trace element concentrations in London were influenced by both local and regional sources, with coarse and intermediate fractions dominated by traffic-induced resuspension and wearing processes and fine particles influenced by regional transport.

Solid fuel emissions, including those from biomass burning, are increasing in urban areas across the European Union due to rising energy costs and government incentives to use renewable energy sources for heating. In order to help protect human health as well as to improve air quality and pollution abatement strategies, the sources of combustion aerosols, their contributions, and the processes they undergo need to be better understood. A high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was therefore deployed at an urban background site between January and February 2012 to investigate solid fuel organic aerosols (SFOA) in London. The variability of SFOA was examined and the factors governing the split between the two SFOA factors derived from Positive Matrix Factorisation (PMF) were assessed. The concentrations of both factors were found to increase during the night and during cold periods, consistent with domestic space heating activities. The split between the two factors is likely governed predominantly by differences in burn conditions where SFOA1 best represents more efficient burns and SFOA2 best represents less efficient burns. The differences in efficiency may be due to burner types or burn phase, for example. Different fuel types and levels of atmospheric processing also likely contribute to the two factors. As the mass spectral profile of SFOA is highly variable, the findings from this study may have implications for improving future source apportionment and factorisation analyses. During the winter, SFOA was found to contribute 38% to the total non-refractory submicron organic aerosol (OA) mass, with similar contributions from both SFOA factors (20% from SFOA1 and 18% from SFOA2). A similar contribution of SFOA was derived for the same period from a compact time-of-flight AMS (cToF-AMS), which measured for a full calendar year at the same site. The seasonality of SFOA was investigated using the year-long data set where concentrations were greatest in the autumn and winter. During the summer, SFOA contributed 11% to the organic fraction, where emissions resulted from different anthropogenic activities such as barbecues and domestic garden wood burning. The significant contribution of SFOA to total organic mass throughout the year suggests that the negative effects on health and air quality, as well as climate, are not just confined to winter as exposure to these aerosols and the associated black carbon can also occur during the summer, which may have significant implications for air-quality policies and mitigation strategies.

PM2.5 was collected during a winter campaign at two southern England sites, urban background North Kensington (NK) and rural Harwell (HAR), in January–February 2012. Multiple organic and inorganic source tracers were analysed and used in a Chemical Mass Balance (CMB) model, which apportioned seven separate primary sources, that explained on average 53% (NK) and 56% (HAR) of the organic carbon (OC), including traffic, woodsmoke, food cooking, coal combustion, vegetative detritus, natural gas and dust/soil. With the addition of source tracers for secondary biogenic aerosol at the NK site, 79% of organic carbon was accounted for. Secondary biogenic sources were represented by oxidation products of α-pinene and isoprene, but only the former made a substantial contribution to OC. Particle source contribution estimates for PM2.5 mass were obtained by the conversion of the OC estimates and combining with inorganic components ammonium nitrate, ammonium sulfate and sea salt. Good mass closure was achieved with 81% (92% with the addition of the secondary biogenic source) and 83% of the PM2.5 mass explained at NK and HAR respectively, with the remainder being secondary organic matter. While the most important sources of OC are vehicle exhaust (21 and 16%) and woodsmoke (15 and 28%) at NK and HAR respectively, food cooking emissions are also significant, particularly at the urban NK site (11% of OC), in addition to the secondary biogenic source, only measured at NK, which represented about 26%. In comparison, the major source components for PM2.5 at NK and HAR are inorganic ammonium salts (51 and 56%), vehicle exhaust emissions (8 and 6%), secondary biogenic (10% measured at NK only), woodsmoke (4 and 7%) and sea salt (7 and 8%), whereas food cooking (4 and 1%) showed relatively smaller contributions to PM2.5. Results from the CMB model were compared with source contribution estimates derived from the AMS-PMF method. The overall mass of organic matter accounted for is rather similar for the two methods. However, appreciably different concentrations were calculated for the individual primary organic matter contributions, although for most source categories the CMB and AMS-PMF results were highly correlated (r2 = 0.69–0.91). In comparison with the CMB model, the AMS appears to overestimate the biomass burning/coal and food cooking sources by a factor of around 1.5 to 2 while estimates of the traffic source are rather similar for each model. The largest divergence is in the primary/secondary organic matter split, with the AMS estimating an appreciably smaller secondary component. Possible reasons for these discrepancies are discussed, but despite these substantial divergences, the strong correlation of the two methods gives some confidence in their application.

Aerosol chemical composition was found to influence nighttime atmospheric chemistry during a series of airborne measurements in northwestern Europe in summer conditions, which has implications for regional air quality and climate. The uptake of dinitrogen pentoxide, γ (N2O5), to particle surfaces was found to be modulated by the amount of water content and ammonium nitrate present in the aerosol. The conditions prevalent in this study suggest that the net uptake rate of N2O5 to atmospheric aerosols was relatively efficient compared to previous studies, with γ (N2O5) values in the range 0.01–0.03. This is likely a consequence of the elevated relative humidity in the region, which promotes greater aerosol water content. Increased nitrate concentrations relative to particulate water were found to suppress N2O5 uptake. The results presented here contrast with previous ambient studies of N2O5 uptake, which have generally taken place in low-nitrate environments in the USA. Comparison of the N2O5 uptake derived from the measurements with a parameterised scheme that is based on the ratio of particulate water to nitrate yielded reasonably good agreement in terms of the magnitude and variation in uptake, provided the effect of chloride was neglected. An additional suppression of the parameterised uptake is likely required to fully capture the variation in N2O5 uptake, which could be achieved via the known suppression by organic aerosol. However, existing parameterisations representing the suppression by organic aerosol were unable to fully represent the variation in N2O5 uptake. These results provide important ambient measurement constraint on our ability to predict N2O5 uptake in regional and global aerosol models. N2O5 uptake is a potentially important source of nitrate aerosol and a sink of the nitrate radical, which is the main nocturnal oxidant in the atmosphere. The results further highlight the importance of ammonium nitrate in northwestern Europe as a key component of atmospheric composition in the region.

Wet deposition is the dominant mechanism for removing black carbon (BC) from the atmosphere and is key in determining its atmospheric lifetime, vertical gradient and global transport. Despite the importance of BC in the climate system, especially in terms of its ability to modulate the radiative energy budget, there are few quantitative case studies of wet removal in ambient environments. We present a case study of BC wet removal by examining aerosol size distributions and BC coating properties sampled in three Canadian boreal biomass burning plumes, one of which passed through a precipitating cloud. This depleted the majority of the plume's BC mass, and the largest and most coated BC-containing particles were found to be preferentially removed, suggesting that nucleation scavenging was likely the dominant mechanism. Calculated single-scattering albedo (SSA) showed little variation, as a large number of non-BC particles were also present in the precipitation-affected plume. The remaining BC cores were smaller than those observed in previous studies of BC in post-precipitation outflow over Asia, possibly due to the thick coating by hydrophilic compounds associated with the Canadian biomass burning particles. This study provides measurements of BC size, mixing state and removal efficiency to constrain model parameterisations of BC wet removal in biomass burning regions, which will help to reduce uncertainty in radiative forcing calculations.

Isoprene is a potentially highly significant but currently poorly quantified source of secondary organic aerosols (SOA). This is especially important in the tropics, where large rainforests act as significant sources of isoprene. Methylfuran, produced through thermal decomposition during analysis, has recently been suggested as a marker for isoprene SOA formation through the isoprene epoxydiol (IEPOX) route, which mostly occurs under low NOx conditions. This is manifested as a peak at m/z=82 in Aerodyne Aerosol Mass Spectrometer (AMS) data. Here we present a study of this marker measured during five flights over the Amazon rainforest on board the UK Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft during the South American Biomass Burning Analysis (SAMBBA) campaign. Cases where this marker is and is not present are contrasted and linked to the presence of acidic seed particles, lower NOx concentrations and higher humidities. There are also data to suggest a role of organic nitrogen in the particulate composition. Furthermore, an inspection of the vertical trends of the marker indicates that concentrations are highest at the top of the boundary layer (possibly due to semivolatile repartitioning) and that upwards through the free troposphere, the mass spectral profile evolves towards that of low volatility oxygenated aerosol. These observations offer insights into the behaviour of IEPOX-derived SOA formation above the Amazon rainforest and the suitability of methylfuran as a marker for this process.

Black carbon aerosols (BC) at a London urban site were characterised in both winter- and summertime 2012 during the Clean Air for London (ClearfLo) project. Positive matrix factorisation (PMF) factors of organic aerosol mass spectra measured by a high-resolution aerosol mass spectrometer (HR-AMS) showed traffic-dominant sources in summer but in winter the influence of additional non-traffic sources became more important, mainly from solid fuel sources (SF). Measurements using a single particle soot photometer (SP2, DMT), showed the traffic-dominant BC exhibited an almost uniform BC core size (Dc) distribution with very thin coating thickness throughout the detectable range of Dc. However, the size distribution of sf (project average mass median Dc = 149 ± 22 nm in winter, and 120 ± 6 nm in summer) and BC coating thickness varied significantly in winter. A novel methodology was developed to attribute the BC number concentrations and mass abundances from traffic (BCtr) and from SF (BCsf), by using a 2-D histogram of the particle optical properties as a function of BC core size, as measured by the SP2. The BCtr and BCsf showed distinctly different sf distributions and coating thicknesses, with BCsf displaying larger Dc and larger coating thickness compared to BCtr. BC particles from different sources were also apportioned by applying a multiple linear regression between the total BC mass and each AMS-PMF factor (BC–AMS–PMF method), and also attributed by applying the absorption spectral dependence of carbonaceous aerosols to 7-wavelength Aethalometer measurements (Aethalometer method). Air masses that originated from westerly (W), southeasterly (SE), and easterly (E) sectors showed BCsf fractions that ranged from low to high, and whose mass median Dc values were 137 ± 10 nm, 143 ± 11 nm and 169 ± 29 nm, respectively. The corresponding bulk relative coating thickness of BC (coated particle size/BC core – Dp/Dc) for these same sectors was 1.28 ± 0.07, 1.45 ± 0.16 and 1.65 ± 0.19. For W, SE and E air masses, the number fraction of BCsf ranged from 6 ± 2% to 11 ± 5% to 18 ± 10%, respectively, but importantly the larger BC core sizes lead to an increased fraction of BCsf in terms of mass than number (for W, SE and E air masses, the BCsf mass fractions ranged from 16 ± 6%, 24 ± 10% and 39 ± 14%, respectively). An increased fraction of non-BC particles (particles that did not contain a BC core) was also observed when SF sources were more significant. The BC mass attribution by the SP2 method agreed well with the BC–AMS–PMF multiple linear regression method (BC–AMS–PMF : SP2 ratio = 1.05, r2 = 0.80) over the entire experimental period. Good agreement was found between BCsf attributed with the Aethalometer model and the SP2. However, the assumed absorption Ångström exponent (αwb) had to be changed according to the different air mass sectors to yield the best comparison with the SP2. This could be due to influences of fuel type or burn phase.

We present a case study of Aitken and accumulation mode aerosol observed downwind of the anvil of a deep tropical thunderstorm. The measurements were made by condensation nuclei counters flown on the Egrett high-altitude aircraft from Darwin during the ACTIVE campaign, in monsoon conditions producing widespread convection over land and ocean. Maximum measured concentrations of aerosol with diameter greater than 10 nm were 25 000 cm−3 (STP). By calculating back-trajectories from the observations, and projecting onto infrared satellite images, the time since the air exited cloud was estimated. In this way a time scale of about 3 hours was derived for the Aitken aerosol concentration to reach its peak. We examine the hypothesis that the growth in aerosol concentrations can be explained by production of sulphuric acid from SO2 followed by particle nucleation and coagulation. Estimates of the sulphuric acid production rate show that the observations are only consistent with this hypothesis if the particles coagulate to sizes >10 nm much more quickly than is suggested by current theory. Alternatively, other condensible gases (possibly organic) drive the growth of aerosol particles in the TTL.

Optical particle counters (OPCs) are used regularly for atmospheric research, measuring particle scattering cross sections to generate particle size distribution histograms. This manuscript presents two methods for calibrating OPCs with case studies based on a Passive Cavity Aerosol Spectrometer Probe (PCASP) and a Cloud Droplet Probe (CDP), both of which are operated on the Facility for Airborne Atmospheric Measurements BAe-146 research aircraft. A probability density function based method is provided for modification of the OPC bin boundaries when the scattering properties of measured particles are different to those of the calibration particles due to differences in refractive index or shape. This method provides mean diameters and widths for OPC bins based upon Mie-Lorenz theory or any other particle scattering theory, without the need for smoothing, despite the highly nonlinear and non-monotonic relationship between particle size and scattering cross section. By calibrating an OPC in terms of its scattering cross section the optical properties correction can be applied with minimal information loss, and performing correction in this manner provides traceable and transparent uncertainty propagation throughout the whole process. Analysis of multiple calibrations has shown that for the PCASP the bin centres differ by up to 30% from the manufacturer's nominal values and can change by up to approximately 20% when routine maintenance is performed. The CDP has been found to be less sensitive than the manufacturer's specification with differences in sizing of between 1.6 ± 0.8 μm and 4.7 ± 1.8 μm for one flight. Over the course of the Fennec project in the Sahara the variability of calibration was less than the calibration uncertainty in 6 out of 7 calibrations performed. As would be expected from Mie-Lorenz theory, the impact of the refractive index corrections has been found to be largest for absorbing materials and the impact on Saharan dust measurements made as part of the Fennec project has been found to be up to a factor of 3 for the largest particles measured by CDP with diameters of approximately 120 μm. In an example case, using the calibration and refractive index corrections presented in this work allowed Saharan dust measurement from the PCASP, CDP and a Cloud Imaging Probe to agree within the uncertainty of the calibration. The agreement when using only the manufacturer's specification was poor. Software tools have been developed to perform these calibrations and corrections and are now available as open source resources for the community via the SourceForge repository.

The REgents PARk and Tower Environmental Experiment (REPARTEE) comprised two campaigns in London in October 2006 and October/November 2007. The experiment design involved measurements at a heavily trafficked roadside site, two urban background sites and an elevated site at 160–190 m above ground on the BT Tower, supplemented in the second campaign by Doppler lidar measurements of atmospheric vertical structure. A wide range of measurements of airborne particle physical metrics and chemical composition were made as well as measurements of a considerable range of gas phase species and the fluxes of both particulate and gas phase substances. Significant findings include (a) demonstration of the evaporation of traffic-generated nanoparticles during both horizontal and vertical atmospheric transport; (b) generation of a large base of information on the fluxes of nanoparticles, accumulation mode particles and specific chemical components of the aerosol and a range of gas phase species, as well as the elucidation of key processes and comparison with emissions inventories; (c) quantification of vertical gradients in selected aerosol and trace gas species which has demonstrated the important role of regional transport in influencing concentrations of sulphate, nitrate and secondary organic compounds within the atmosphere of London; (d) generation of new data on the atmospheric structure and turbulence above London, including the estimation of mixed layer depths; (e) provision of new data on trace gas dispersion in the urban atmosphere through the release of purposeful tracers; (f) the determination of spatial differences in aerosol particle size distributions and their interpretation in terms of sources and physico-chemical transformations; (g) studies of the nocturnal oxidation of nitrogen oxides and of the diurnal behaviour of nitrate aerosol in the urban atmosphere, and (h) new information on the chemical composition and source apportionment of particulate matter size fractions in the atmosphere of London derived both from bulk chemical analysis and aerosol mass spectrometry with two instrument types.

The global organic aerosol (OA) budget is highly uncertain and past studies suggest that models substantially underestimate observed concentrations. Few of these studies have examined the vertical distribution of OA. Furthermore, many model-measurement comparisons have been performed with different models for single field campaigns. We synthesize organic aerosol measurements from 17 aircraft campaigns from 2001–2009 and use these observations to consistently evaluate a GEOS-Chem model simulation. Remote, polluted and fire-influenced conditions are all represented in this extensive dataset. Mean observed OA concentrations range from 0.2–8.2 μg sm−3 and make up 15 to 70% of non-refractory aerosol. The standard GEOS-Chem simulation reproduces the observed vertical profile, although observations are underestimated in 13 of the 17 field campaigns (the median observed to simulated ratio ranges from 0.4 to 4.2), with the largest model bias in anthropogenic regions. However, the model is best able to capture the observed variability in these anthropogenically-influenced regions (R2=0.18−0.57), but has little skill in remote or fire-influenced regions. The model bias increases as a function of relative humidity for 11 of the campaigns, possibly indicative of missing aqueous phase SOA production. However, model simulations of aqueous phase SOA suggest a pronounced signature in the mid-troposphere (2–6 km) which is not supported in the observations examined here. Spracklen et al. (2011) suggest adding ~100 Tg yr−1 source of anthropogenically-controlled SOA to close the measurement-model gap, which we add as anthropogenic SOA. This eliminates the model underestimate near source, but leads to overestimates aloft in a few regions and in remote regions, suggesting either additional sinks of OA or higher volatility aerosol at colder temperatures. Sensitivity simulations indicate that fragmentation of organics upon either heterogeneous or gas-phase oxidation could be an important (missing) sink of OA in models, reducing the global SOA burden by 15% and 47% respectively. The best agreement with observations is obtained when the simulated anthropogenically-controlled SOA is increased to ~100 Tg yr−1 accompanied by either a gas-phase fragmentation process or a reduction in the temperature dependence of the organic aerosol partitioning (by decreasing the enthalpy of vaporization from 42 kJ mol−1 to 25 kJ mol−1). These results illustrate that models may require both additional sources and additional sinks to capture the observed concentrations of organic aerosol.

Atmospheric composition affects the radiative balance of the Earth through the creation of greenhouse gases and the formation of aerosols. The latter interact with incoming solar radiation, both directly and indirectly through their effects on cloud formation and lifetime. The tropics have a major influence on incoming sunlight however the tropical atmosphere is poorly characterised, especially outside Amazonia. The origins of air masses influencing a measurement site in a protected rainforest in Borneo, South East Asia, were assessed and the likely sources of a range of trace gases and particles were determined. This was conducted by interpreting in situ measurements made at the site in the context of ECMWF backwards air mass trajectories. Two different but complementary methods were employed to interpret the data: comparison of periods classified by cluster analysis of trajectories, and inspection of the dependence of mean measured values on geographical history of trajectories. Sources of aerosol particles, carbon monoxide and halocarbons were assessed. The likely source influences include: terrestrial organic biogenic emissions; long range transport of anthropogenic emissions; biomass burning; sulphurous emissions from marine phytoplankton, with a possible contribution from volcanoes; marine production of inorganic mineral aerosol; and marine production of halocarbons. Aerosol sub- and super-saturated water affinity was found to be dependent on source (and therefore composition), with more hygroscopic aerosol and higher numbers of cloud condensation nuclei measured in air masses of marine origin. The prevailing sector during the majority of measurements was south-easterly, which is from the direction of the coast closest to the site, with a significant influence inland from the south-west. This analysis shows that marine and terrestrial air masses have different dominant chemical sources. Comparison with the AMAZE-08 project in the Amazon basin shows Bornean composition to arise from a different, more complex mixture of sources. In particular sulphate loadings are much greater than in Amazonia which is likely to mainly be the result of the marine influence on the site. This suggests that the significant region of the tropics made up of island networks is not well represented by extrapolation from measurements made in the Amazon. In addition, it is likely that there were no periods where the site was influenced only by the rainforest, with even the most pristine inland periods showing some evidence of non-rainforest aerosol. This is in contrast to Amazonia which experienced periods dominated by rainforest emissions.

Nanoparticles emitted from road traffic are the largest source of respiratory exposure for the general public living in urban areas. It has been suggested that adverse health effects of airborne particles may scale with airborne particle number, which if correct, focuses attention on the nanoparticle (less than 100 nm) size range which dominates the number count in urban areas. Urban measurements of particle size distributions have tended to show a broadly similar pattern dominated by a mode centred on 20–30 nm diameter emitted by diesel engine exhaust. In this paper we report the results of measurements of particle number concentration and size distribution made in a major London park as well as on the BT Tower, 160 m aloft. These measurements taken during the REPARTEE project (Regents Park and BT Tower experiment) show a remarkable shift in particle size distributions with major losses of the smallest particle class as particles are advected away from the traffic source. In the Park, the traffic related mode at 20–30 nm diameter is much reduced with a new mode at

The VAMOS Ocean-Cloud-Atmosphere-Land Regional Experiment (VOCALS-REx) was conducted from 15 October to 15 November 2008 in the South East Pacific (SEP) region to investigate interactions between land, sea and atmosphere in this unique tropical eastern ocean environment and to improve the skill of global and regional models in representing the region. This study synthesises selected aircraft, ship and surface site observations from VOCALS-REx to statistically summarise and characterise the atmospheric composition and variability of the Marine Boundary Layer (MBL) and Free Troposphere (FT) along the 20° S parallel between 70° W and 85° W. Significant zonal gradients in mean MBL sub-micron aerosol particle size and composition, carbon monoxide, sulphur dioxide and ozone were seen over the campaign, with a generally more variable and polluted coastal environment and a less variable, more pristine remote maritime regime. Gradients in aerosol and trace gas concentrations were observed to be associated with strong gradients in cloud droplet number. The FT was often more polluted in terms of trace gases than the MBL in the mean; however increased variability in the FT composition suggests an episodic nature to elevated concentrations. This is consistent with a complex vertical interleaving of airmasses with diverse sources and hence pollutant concentrations as seen by generalised back trajectory analysis, which suggests contributions from both local and long-range sources. Furthermore, back trajectory analysis demonstrates that the observed zonal gradients both in the boundary layer and the free troposphere are characteristic of marked changes in airmass history with distance offshore – coastal boundary layer airmasses having been in recent contact with the local land surface and remote maritime airmasses having resided over ocean for in excess of ten days. Boundary layer composition to the east of 75° W was observed to be dominated by coastal emissions from sources to the west of the Andes, with evidence for diurnal pumping of the Andean boundary layer above the height of the marine capping inversion. Analysis of intra-campaign variability in atmospheric composition was not found to be significantly correlated with observed low-frequency variability in the large scale flow pattern; campaign-average interquartile ranges of CO, SO2 and O3 concentrations at all longitudes were observed to dominate over much smaller differences in median concentrations calculated between periods of different flow regimes. The campaign climatology presented here aims to provide a valuable dataset to inform model simulation and future process studies, particularly in the context of aerosol-cloud interaction and further evaluation of dynamical processes in the SEP region for conditions analogous to those during VOCALS-REx. To this end, our results are discussed in terms of coastal, transitional and remote spatial regimes in the MBL and FT and a gridded dataset are provided as a resource.

The experiment presented in this paper was conducted at the Holme Moss site, which is located in the southern Pennines region in Northwestern England during November–December 2006. The strong southwesterly wind during the experimental period, which enhanced the transport of urban pollutants from the conurbations of Greater Manchester and Liverpool, in addition to the seasonally increased nearby residential heating activities, made this site a receptor for pollutants from a range of sources. A factor analysis is applied to the mass spectra of organic matter (OM) measured by the Aerodyne Aerosol Mass Spectrometer (AMS) to attribute the pollutant sources. Besides the oxygenated organic aerosol (OOA), this site was found to contain a considerable fraction of primary organic aerosols (POA, mass fraction 50–70% within total mass of OM). The POA sources are attributed to be traffic emission and solid fuel burning, which are identified as hydrocarbon-like organic aerosol (HOA) and solid fuel organic aerosol (SFOA) respectively. There were strongly combined emissions of black carbon (BC) particles from both sources. The refractory BC component (rBC) was characterized by a single particle soot photometer. This site began to be influenced during the late morning by fresh traffic emissions, whereas solid fuel burning became dominant from late afternoon until night. A covariance analysis of rBC and POA was used to derive source specific emission factors of 1.61 μgHOA/μgrBC and 1.96 μgHOA/μgrBC. The absorbing properties of aerosols were characterized at multiple wavelengths (λ), and a stronger spectral dependence of absorption was observed when this site was significantly influenced by solid fuel burning. The rBC was estimated to contribute 3–16% of submicron aerosol mass. The single scattering albedo at λ = 700 nm (SSA700 nm) was significantly anti-correlated with the rBC mass fraction, but also associated with the BC mixing state. The BC incorporation/removal process therefore may play a role in modulating the radiative properties of aerosols at the site under the influence of fresh sources. Given that traffic and residential combustion of solid fuels are significant contributors of carbonaceous aerosols over Europe, these results provide important source-specific information on modeling the anthropogenic carbonaceous aerosols.

Simultaneous observations of cloud microphysical properties were obtained by in-situ aircraft measurements and ground based Radar/Lidar. Widespread mid-level stratus cloud was present below a temperature inversion (~5 °C magnitude) at 3.6 km altitude. Localised convection (peak updraft 1.5 m s−1) was observed 20 km west of the Radar station. This was associated with convergence at 2.5 km altitude. The convection was unable to penetrate the inversion capping the mid-level stratus. The mid-level stratus cloud was vertically thin (~400 m), horizontally extensive (covering 100 s of km) and persisted for more than 24 h. The cloud consisted of supercooled water droplets and small concentrations of large (~1 mm) stellar/plate like ice which slowly precipitated out. This ice was nucleated at temperatures greater than −12.2 °C and less than −10.0 °C, (cloud top and cloud base temperatures, respectively). No ice seeding from above the cloud layer was observed. This ice was formed by primary nucleation, either through the entrainment of efficient ice nuclei from above/below cloud, or by the slow stochastic activation of immersion freezing ice nuclei contained within the supercooled drops. Above cloud top significant concentrations of sub-micron aerosol were observed and consisted of a mixture of sulphate and carbonaceous material, a potential source of ice nuclei. Particle number concentrations (in the size range 0.1

A case study of atmospheric aerosol measurements exploring the impact of the vertical distribution of aerosol chemical composition upon the radiative budget in North-Western Europe is presented. Sub-micron aerosol chemical composition was measured by an Aerodyne Aerosol Mass Spectrometer (AMS) on both an airborne platform and a ground-based site at Cabauw in the Netherlands. The examined period in May 2008 was characterised by enhanced pollution loadings in North-Western Europe and was dominated by ammonium nitrate and Organic Matter (OM). Both ammonium nitrate and OM were observed to increase with altitude in the atmospheric boundary layer. This is primarily attributed to partitioning of semi-volatile gas phase species to the particle phase at reduced temperature and enhanced relative humidity. Increased ammonium nitrate concentrations in particular were found to strongly increase the ambient scattering potential of the aerosol burden, which was a consequence of the large amount of associated water as well as the enhanced mass. During particularly polluted conditions, increases in aerosol optical depth of 50–100% were estimated to occur due to the observed increase in secondary aerosol mass and associated water uptake. Furthermore, the single scattering albedo was also shown to increase with height in the boundary layer. These enhancements combined to increase the negative direct aerosol radiative forcing by close to a factor of two at the median percentile level. Such increases have major ramifications for regional climate predictions as semi-volatile components are often not included in aerosol models. The results presented here provide an ideal opportunity to test regional and global representations of both the aerosol vertical distribution and subsequent impacts in North-Western Europe. North-Western Europe can be viewed as an analogue for the possible future air quality over other polluted regions of the Northern Hemisphere, where substantial reductions in sulphur dioxide emissions have yet to occur. Anticipated reductions in sulphur dioxide in polluted regions will result in an increase in the availability of ammonia to form ammonium nitrate as opposed to ammonium sulphate. This will be most important where intensive agricultural practises occur. Our observations over North-Western Europe, a region where sulphur dioxide emissions have already been reduced, indicate that failure to include the semi-volatile behaviour of ammonium nitrate will result in significant errors in predicted aerosol direct radiative forcing. Such errors will be particularly significant on regional scales.

The spatial distribution of aerosol chemical composition and the evolution of the Organic Aerosol (OA) fraction is investigated based upon airborne measurements of aerosol chemical composition in the planetary boundary layer across Europe. Sub-micron aerosol chemical composition was measured using a compact Time-of-Flight Aerosol Mass Spectrometer (cToF-AMS). A range of sampling conditions were evaluated, including relatively clean background conditions, polluted conditions in North-Western Europe and the near-field to far-field outflow from such conditions. Ammonium nitrate and OA were found to be the dominant chemical components of the sub-micron aerosol burden, with mass fractions ranging from 20–50% each. Ammonium nitrate was found to dominate in North-Western Europe during episodes of high pollution, reflecting the enhanced NOx and ammonia sources in this region. OA was ubiquitous across Europe and concentrations generally exceeded sulphate by 30–160%. A factor analysis of the OA burden was performed in order to probe the evolution across this large range of spatial and temporal scales. Two separate Oxygenated Organic Aerosol (OOA) components were identified; one representing an aged-OOA, termed Low Volatility-OOA and another representing fresher-OOA, termed Semi Volatile-OOA on the basis of their mass spectral similarity to previous studies. The factors derived from different flights were not chemically the same but rather reflect the range of OA composition sampled during a particular flight. Significant chemical processing of the OA was observed downwind of major sources in North-Western Europe, with the LV-OOA component becoming increasingly dominant as the distance from source and photochemical processing increased. The measurements suggest that the aging of OA can be viewed as a continuum, with a progression from a less oxidised, semi-volatile component to a highly oxidised, less-volatile component. Substantial amounts of pollution were observed far downwind of continental Europe, with OA and ammonium nitrate being the major constituents of the sub-micron aerosol burden. Such anthropogenically perturbed air masses can significantly perturb regional climate far downwind of major source regions.

Results from a measurement study performed in the Tropical Atlantic on board the RHaMBLe Discovery Cruise D319 are presented. Measurements of aerosol composition, hygroscopicity and CCN activity were used to test the ability of a single parameter model to describe water uptake in sub- and supersaturated conditions. It was found that the magnitude and temporal variability of the sub-saturated water uptake could be well represented using the non-refractory composition to derive the model input for 2 periods when the large majority of the aerosol mass was non-refractory. As may be expected, when a significant fraction of the aerosol volume is refractory the sub-saturated water uptake is not well predicted by the non-refractory composition. When predicting the cloud activation potential from the composition and the hygroscopicity there is a consistent under-prediction of the CCN activity. The prediction of CCN activity from the sub-saturated water uptake gives a better prediction of the CCN activity than the composition when the non-refractory components are not fully representative of the aerosol composition. Based on these observations it appears that a single parameter cannot always capture the behavior fully across the sub- and supersaturated regimes. Measurements made at relative humidities (RHs) up to 94% showed that the water activity appears satisfactorily represented by a single parameter derived at 90% RH. It therefore appears that the change in the observed hygroscopicity take place between 94% RH and the point of activation. This change may be due in part to a change solution non-ideality, surface tension effects or the presence of sparingly soluble compounds for example, but cannot be reconciled without measurements at higher RHs.

Organic matter frequently represents the single largest fraction of fine particulates in urban environments and yet the exact contributions from different sources and processes remain uncertain, owing in part to its substantial chemical complexity. Positive Matrix Factorisation (PMF) has recently proved to be a powerful tool for the purposes of source attribution and profiling when applied to ambient organic aerosol data from the Aerodyne Aerosol Mass Spectrometer (AMS). Here we present PMF analysis applied to AMS data from UK cities for the first time. Three datasets are analysed, with the focus on objectivity and consistency. The data were collected in London during the Regent's Park and Tower Environmental Experiment (REPARTEE) intensives and Manchester. These occurred during the autumn and wintertime, such that the primary fraction would be prominent. Ambiguities associated with rotationality within sets of potential solutions are explored and the most appropriate solution sets selected based on comparisons with external data. In addition to secondary organic aerosols, three candidate sources of primary organic aerosol (POA) were identified according to mass spectral and diurnal profiles; traffic emissions, cooking and solid fuel burning (for space heating). Traffic represented, on average, 40% of POA during colder conditions and exhibited a hydrocarbon-like mass spectrum similar to those previously reported. Cooking aerosols represented 34% of POA and through laboratory work, their profile was matched with that sampled from the heating of seed oils, rather than previously-published spectra derived from charbroiling. This suggests that in these locations, oil from frying may have contributed more to the particulate than the meat itself. Solid fuel aerosols represented 26% of POA during cold weather conditions but were not discernable during the first REPARTEE campaign, when conditions were warmer than the other campaigns. This factor showed features associated with biomass burning and occurred mainly at night. Grid-scale emission factors of the combustion aerosols suitable for use in chemical transport models were derived relative to CO and NOx. The traffic aerosols were found to be 20.5 μg m−3 ppm−1 relative to CO for Manchester and 31.6 μg m−3 ppm−1 relative to NOx for London. Solid fuel emissions were derived as 24.7 μg m−3 ppm−1 relative to CO for Manchester. These correspond to mass emission ratios of 0.018, 0.026 (as NO) and 0.021 respectively and are of a similar order to previously published estimates, derived from other regions or using other approaches.

Marine aerosol composition continues to represent a large source of uncertainty in the study of climate and atmospheric chemistry. In addition to their physical size and chemical composition, hygroscopicity plays a significant role, increasing the particles' surface areas and scattering potential. Simultaneous aerosol measurements were performed on board the RRS Discovery and at the Cape Verde atmospheric observatory during the Aerosol Composition and Modelling in the Marine Environment (ACMME) and Reactive Halogens in the Marine Boundary Layer (RHAMBLE) experiments. These included online measurements of number and dry size and bulk collection for offline analysis of aqueous ions. In addition, the measurements on board the Discovery included online measurements of composition using an Aerodyne Aerosol Mass Spectrometer, optical absorption using a Multi Angle Absorption Photometer, ambient humidity size distribution measurements using a humidified differential mobility particle sizer (DMPS) and optical particle counter (OPC) and hygroscopicity measurements with a hygroscopicity tandem differential mobility analyser (HTDMA). Good agreement between platforms in terms of the sea salt (ss) and non sea salt (nss) modes was found during the period when the Discovery was in close proximity to Cape Verde and showed a composition consistent with remote marine air. As the Discovery approached the African coast, the aerosol showed signs of continental influence such as an increase in particle number, optical absorption, enhancement of the nss mode and dust particles. The Cape Verde site was free of this influence during this period. Chloride and bromide showed concentrations with significant deviations from seawater relative to sodium, indicating that atmospheric halogen processing (and/or acid displacement for chloride) had taken place. The time dependent ambient size distribution was synthesised using humidified DMPS and OPC data, corrected to ambient humidity using HTDMA data. Heterogeneous uptake rates of hypoiodous acid (HOI) were also predicted and the nss accumulation mode was found to be the most significant part of the size distribution, which could act as an inert sink for this species. The predicted uptake rates were enhanced by around a factor of 2 during the African influence period due to the addition of both coarse and fine particles. The hygroscopicity of the nss fraction was modelled using the Aerosol Diameter Dependent Equilibrium Model (ADDEM) using the measured composition and results compared with the HTDMA data. This was the first time such a reconciliation study with this model has been performed with marine data and good agreement was reached within the resolution of the instruments. The effect of hygroscopic growth on HOI uptake was also modelled and ambient uptake rates were found to be approximately doubled compared to equivalent dry particles.

A synthesis of UK based airborne in-situ measurements of aerosol properties representing air masses from North-West Europe and the North-East Atlantic is presented. The major focus of the study is the vertical distribution of sub-micron aerosol chemical composition. Vertical profiles are derived from a Quadrupole Aerosol Mass Spectrometer (Q-AMS). Background sub-micron aerosol vertical profiles are identified and are primarily composed of organic matter and sulphate aerosol. Such background conditions occurred predominantly during periods associated with long-range air mass transport across the Atlantic. These instances may serve as useful model input of aerosol to Western Europe. Increased mass concentration episodes are coincident with European outflow and periods of stagnant/recirculating air masses. Such periods are characterised by significantly enhanced concentrations of nitrate aerosol relative to those of organic matter and sulphate. Periods of enhanced ground level PM2.5 loadings are coincident with instances of high nitrate mass fractions measured on-board the aircraft, indicating that nitrate is a significant contributor to regional pollution episodes. The vertical structure of the sulphate and organic aerosol profiles were shown to be primarily driven by large-scale dynamical processes. The vertical distribution of nitrate is likely determined by both dynamic and thermodynamic processes, with chemical partitioning of gas phase precursors to the particle phase occurring at lower temperatures at the top of the boundary layer. Such effects have profound implications for the aerosol's lifetime and subsequent impacts, highlighting the requirement for accurate representation of the aerosol vertical distribution.

Nitrate aerosols make a very major contribution to PM2.5 and PM10 in western Europe, but their sources and pathways have not been fully elucidated. An Aerosol Time-of-Flight Mass Spectrometer (ATOFMS) and a Compact Time of Flight Aerosol Mass Spectrometer (C-ToF-AMS) were deployed in an urban background location in London, UK, collecting data as part of the REPARTEE-I experiment. During REPARTEE-I, daily PM10 concentrations ranged up to 43.6 μg m−3, with hourly nitrate concentrations (measured by AMS) of up to 5.3 μg m−3. The application of the ART-2a neural network algorithm to the ATOFMS data characterised the nitrate particles as occurring in two distinct clusters (i.e. particle types). The first (33.6% of particles by number) appeared to be locally produced in urban locations during nighttime, whilst the second (22.8% of particles by number) was regionally transported from continental Europe. Nitrate in locally produced aerosol was present mainly in particles smaller than 300 nm, whilst the regional nitrate presented a coarser mode, peaking at 600 nm. In both aerosol types, nitrate was found to be internally mixed with sulphate, ammonium, elemental and organic carbon. Nitrate in regional aerosol appeared to be more volatile than that locally formed. During daytime, a core of the regionally transported nitrate aerosol particle type composed of organic carbon and sulphate was detected.

This paper presents measurements of organic aerosols above subtropical West Africa during the wet season using data from the UK Facility for Airborne Atmospheric Measurements (FAAM) aircraft. Measurements of biogenic volatile organic compounds (BVOC) at low altitudes over these subtropical forests were made during the African Monsoon Multidisciplinary Analysis (AMMA) field experiment during July and August 2006 mainly above Benin, Nigeria and Niger. Data from an Aerodyne Quadrupole Aerosol Mass Spectrometer show a median organic aerosol loading of 1.07 μg m−3 over tropical West Africa, which represents the first regionally averaged assessment of organic aerosol mass (OM) in this region during the wet season. This is broadly in agreement with global model predictions based on partitioning schemes, although there are large uncertainties associated with such estimates. In contrast our own calculations based on aerosol yields from isoprene and monoterpenes during chamber studies under represent the OM measured in this region on a comparable scale to the under representations of OM by predictive models in the mid latitudes. As global models rely on similar yield calculations in their global estimates, as our calculations this points to further systematic differences between global model estimates and measurements of SOA, most likely caused by use of incorrect BVOC emission rates. The under predictions of OM by our calculations and those in the mid latitudes employ yields extrapolated from chamber data obtained at higher mass concentrations – more recent yield data for α-pinene obtained at ambient concentrations in a flow through chamber (Shilling et al., 2008) show considerably better agreement with our data.

Measurements of aerosol properties were made in aged polluted and clean background air masses encountered at the North Norfolk (UK) coastline as part of the TORCH2 field campaign in May 2004. Hygroscopic growth factors (GF) at 90% relative humidity (RH) for D0=27–217 nm particles and size-resolved chemical composition were simultaneously measured using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA) and an Aerodyne aerosol mass spectrometer (Q-AMS), respectively. Both hygroscopic properties and chemical composition showed pronounced variability in time and with particles size. With this data set we could demonstrate that the Zdanovskii-Stokes-Robinson (ZSR) mixing rule combined with chemical composition data from the AMS makes accurate quantitative predictions of the mean GF of mixed atmospheric aerosol particles possible. In doing so it is crucial that chemical composition data are acquired with high resolution in both particle size and time, at least matching the actual variability of particle properties. The closure results indicate an ensemble GF of the organic fraction of ~1.20±0.10 at 90% water activity. Thus the organics contribute somewhat to hygroscopic growth, particularly at small sizes, however the inorganic salts still dominate. Furthermore it has been found that most likely substantial evaporation losses of NH4NO3 occurred within the HTDMA instrument, exacerbated by a long residence time of ~1 min. Such an artefact is in agreement with our laboratory experiments and literature data for pure NH4NO3, both showing similar evaporation losses within HTDMAs with residence times of ~1 min. Short residence times and low temperatures are hence recommended for HTDMAs in order to minimise such evaporation artefacts.

Aerosol size distribution measurements from 0.03 µm to 25 µm diameter were taken at ambient humidity aboard the German research vessel, FS-Polarstern, during a transect from Bremerhaven in northern Germany, to Cape Town in South Africa across latitudes 53°32' N to 33°55' S, denoted cruise number ANT XXI/1. The data were segregated according to air mass history, wind speed and latitude. Under clean marine conditions, the averaged size distributions were generally in good agreement with those reported previously for diameters less than 0.5 µm and can be approximated by two log-normal modes, with significant variation in the mean modal diameters. Two short periods of tri-modal behaviour were observed. Above 0.5 µm, there is indication of a limit to the mechanical generation of marine aerosol over the range of wind speeds observed (~1.7–14.7 m s−1). A new technique to determine the errors associated with aerosol size distribution measurements using Poisson statistics has been applied to the dataset, providing a tool to determine the necessary sample or averaging times for correct interpretation of such data. Finally, the data were also used to investigate the loss rate of condensing gases with potentially important consequences for heterogeneous marine photochemical cycles.

Several zero-dimensional box-models with different levels of chemical complexity, based on the Master Chemical Mechanism (MCM), have been used to study the chemistry of OH and HO2 in a coastal environment in the Northern Hemisphere. The models were constrained to and compared with measurements made during the NAMBLEX campaign (Mace Head, Ireland) in summer 2002. The base models, which were constrained to measured CO, CH4 and NMHCs, were able to reproduce [OH] within 25%, but overestimated [HO2] by about a factor of 2. Agreement was improved when the models were constrained to oxygenated compounds (acetaldehyde, methanol and acetone), highlighting their importance for the radical budget. When the models were constrained to measured halogen monoxides (IO, BrO) and used a more detailed, measurements-based, treatment to describe the heterogeneous uptake, modelled [OH] increased by up to 15% and [HO2] decreased by up to 30%. The actual impact of halogen monoxides on the modelled concentrations of HOx was dependant on the uptake coefficients used for HOI, HOBr and HO2. Better agreement, within the combined uncertainties of the measurements and of the model, was achieved when using high uptake coefficients for HO2 and HOI (γHO2=1, γHOI=0.6). A rate of production and destruction analysis of the models allowed a detailed study of OH and HO2 chemistry under the conditions encountered during NAMBLEX, showing the importance of oxygenates and of XO (where X=I, Br) as co-reactants for OH and HO2 and of HOX photolysis as a source for OH.

A model of iodine chemistry in the marine boundary layer (MBL) has been used to investigate the impact of daytime coastal emissions of molecular iodine (I2). The model contains a full treatment of gas-phase iodine chemistry, combined with a description of the nucleation and growth, by condensation and coagulation, of iodine oxide nano-particles. In-situ measurements of coastal emissions of I2 made by the broadband cavity ring-down spectroscopy (BBCRDS) and inductively coupled plasma-mass spectrometry (ICP/MS) techniques are presented and compared to long path differential optical absorption spectroscopy (DOAS) observations of I2 at Mace Head, Ireland. Simultaneous measurements of enhanced I2 emissions and particle bursts show that I2 is almost certainly the main precursor of new particles at this coastal location. The ratio of IO to I2 predicted by the model indicates that the iodine species observed by the DOAS are concentrated over a short distance (about 8% of the 4.2 km light path) consistent with the intertidal zone, bringing them into good agreement with the I2 measurements made by the two in-situ techniques. The model is then used to investigate the effect of iodine emission on ozone depletion, and the production of new particles and their evolution to form stable cloud condensation nuclei (CCN).

Measurements are presented from a sampling location 50 km downwind of Greater London, UK, to investigate the timescales required for the atmospheric transformations of aerosol in urban emissions plumes in the context of photochemical age based on the benzene to toluene ratio. It is shown that particles at or around 100 nm in diameter exhibit the greatest systematic variability in chemical properties, and thus hygroscopic properties, on a timescale of 1–2 days. The smaller Aitken mode and larger accumulation mode particles exhibit less variability on these timescales, which we propose is as a result of their different residence times in the atmosphere. The larger accumulation particles have been in the atmosphere longer than the 100 nm particles and their chemistry and hygroscopic properties have been integrated over several days and potentially over several source regions. In contrast, the smaller Aitken mode particles show little systematic variability with photochemical age because their atmospheric lifetimes are short, thus chemical changes and hence changes in water affinity have not had time to occur. Increases in the particle diameter of up to 40% are observed at 90% relative humidity in the accumulation mode from the uptake of water as the particles become increasingly soluble in nature.

A suite of aerosol physical and chemical measurements were made at the Mace Head Atmospheric Research Station, Co. Galway, Ireland, a coastal site on the eastern seaboard of the north Atlantic Ocean during NAMBLEX. The data have been used in this paper to show that over a wide range of aerosol sizes there is no impact of the inter-tidal zone or the surf zone on measurements made at 7 m above ground level or higher. During the measurement period a range of air mass types were observed. During anticyclonic periods and conditions of continental outflow Aitken and accumulation mode were enhanced by a factor of 5 compared to the marine sector, whilst coarse mode particles were enhanced during westerly conditions. Baseline marine conditions were rarely met at Mace Head during NAMBLEX and high wind speeds were observed for brief periods only. The NAMBLEX experiment focussed on a detailed assessment of photochemistry in the marine environment, investigating the linkage between the HOx and the halogen radical cycles. Heterogeneous losses are important in both these cycles. In this paper loss rates of gaseous species to aerosol surfaces were calculated for a range of uptake coefficients. Even when the accommodation coefficient is unity, lifetimes due to heterogeneous loss of less than 10 s were never observed and rarely were they less than 500 s. Diffusional limitation to mass transfer is important in most conditions as the coarse mode is always significant. We calculate a minimum overestimate of 50% in the loss rate if this is neglected and so it should always be considered when calculating loss rates of gaseous species to particle surfaces. HO2 and HOI have accommodation coefficients of around 0.03 and hence we calculate lifetimes due to loss to particle surfaces of 2000 s or greater under the conditions experienced during NAMBLEX. Aerosol composition data collected during this experiment provide representative information on the input aerosol characteristics to western Europe. During NAMBLEX the submicron aerosol was predominately acidified sulphate and organic material, which was most likely internally mixed. The remaining accumulation mode aerosol was sea salt. The organic and sulphate fractions were approximately equally important, though the mass ratio varies considerably between air masses. Mass spectral fingerprints of the organic fraction in polluted conditions are similar to those observed at other locations that are characterised by aged continental aerosol. In marine conditions, the background input of both sulphate and organic aerosol into Europe was observed to be between 0.5 and 1 µg m−3. Key differences in the mass spectra were observed during the few clean periods but were insufficient to ascertain whether these changes reflect differences in the source fingerprint of the organic aerosol. The coarse mode was composed of sea salt and showed significant displacement of chloride by nitrate and to a lesser extent sulphate in polluted conditions.