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When making measurements of air quality, having a reliable estimate of the measurement uncertainty is key to assessing the information content that an instrument is capable of providing, and thus its usefulness in a particular application. This is especially important given the widespread emergence of low cost sensors (LCS) to measure air quality. To do this, end users need to clearly identify the data requirements a priori and design quantifiable success criteria by which to judge the data. All measurements suffer from errors, with the degree to which these errors impact the accuracy of the final data often determined by our ability to identify and correct for them. The advent of LCS has provided a challenge in that many error sources show high spatial and temporal variability, making laboratory derived corrections difficult. Characterising LCS performance thus currently depends primarily on colocation studies with reference instruments, which are very expensive and do not offer a definitive solution but rather a glimpse of LCS performance in specific conditions over a limited period of time. Despite the limitations, colocation studies do provide useful information on measurement device error structure, but the results are non-trivial to interpret and often difficult to extrapolate to future device performance. A problem that obscures much of the information content of these colocation performance assessments is the exacerbated use of global performance metrics (R2, RMSE, MAE, etc.). Colocation studies are complex and time-consuming, and it is easy to fall into the temptation to only use these metrics when trying to define the most appropriate sensor technology to subsequently use. But the use of these metrics can be limited, and even misleading, restricting our understanding of the error structure and therefore the measurements' information content. In this work, the nature of common air pollution measurement errors is investigated, and the implications they have on traditional metrics and other empirical, potentially more insightful approaches to assess measurement performance. With this insight we demonstrate the impact these errors can have on measurements, using a selection of LCS deployed alongside reference measurements as part of the QUANT project, and discuss the implications this has on device end use.

Aerosol-cloud interaction contributes to one of the highest uncertainties in radiative forcing estimations. Aerosols from ship emissions alter the cloud properties and have become an important source of anthropogenic air pollution in recent decades. We have measured the size distributions and number concentrations of aerosols in the cloud and outside clouds using various onboard instruments such as cloud droplet probe (CDP; DMT), passive cavity aerosol spectrometer (PCASP, DMT), Scanning Mobility Particle Sizer (SMPS) and Single Particle Soot Photometer (SP2; DMT). The measurements were performed in the ship emission-dominated environments and relatively cleaner regions of the Celtic Sea. We discuss the difference in the characteristics of in-cloud and out-cloud measurements in these relatively contrasting environments. The measurements were made between 29th September and 12th October 2021 using the Facility for Airborne Atmospheric Measurements (FAAM) research aircraft as a part of the Atmospheric Composition and Radiative forcing changes due to UN International Ship Emissions regulations (ACRUISE) Project.

Sulfur dioxide (SO2) plays a pivotal role in the chemistry of the troposphere, ultimately affecting the Earth’s radiation balance and climate. Within the atmosphere, SO2 is oxidised by gas- and aqueous-phase chemistry to sulfate and is therefore a major precursor to atmospheric aerosols, particularly in the remote marine atmosphere. Both the direct radiative forcing from aerosols and the indirect forcing from aerosol-cloud interactions are poorly understood and produce large uncertainties in climate models. Therefore, it is of interest to precisely quantify the concentration of atmospheric SO2 if we are to predict the effects of changing emission rates on both climate and air quality.Anthropogenic SO2 emissions have fallen dramatically in recent decades, resulting in significant reductions in atmospheric concentrations. Current commercial SO2 detection techniques, for example pulsed fluorescence, are no longer sensitive enough to detect trace levels of SO2 such as those found in remote marine environments. We report the development of a laser-induced fluorescence instrument for in situ SO2 measurements using a custom-built, tunable fiber-amplified laser system. Based on the system initially developed by Rollins et al. (2016), the University of York LIF-SO2 system has a detection limit of 50 ppt for 30 seconds and its relatively small size, weight and power requirements makes this instrument suitable for a variety of field campaigns.Here we present aircraft measurements of SO2 made by the York LIF-SO2 instrument on board the UK FAAM research aircraft in both the remote and polluted marine boundary layer. These are then compared to simultaneous SO2 measurements made by the University of Manchester I- chemical ionisation mass spectrometer (I- CIMS) instrument and the FAAM pulsed fluorescence commercial SO2 detector.

Since 1st January 2020 the legal sulphur content of shipping fuel was decreased – from 3.5% to 0.5% by mass outside of the Sulphur Emission Control Areas (SECAs) to improve coastal air quality. A possible downside of this change was acceleration of climate change since sulphur is believed to be a negative climate forcer and sipping is one of its main sources. Further question was the level of compliance to the new rules, especially in the open waters. Another climate related aspect of shipping is recent growth in the liquified natural gas (LNG) tanker fleets. LNG is considered the greenest of the fossil fuels, however there are few empirical studies of methane emissions from marine LNG transport.The Atmospheric Composition and Radiative forcing changes due to UN International Ship Emissions regulations (ACRUISE) project aims to address the above considerations. During three field campaigns the FAAM Airborne Laboratories’ large research aircraft was deployed to target ships in coastal shipping lanes and open waters. First measurements were performed in July 2019 (before regulation change) in shipping lanes along the Portuguese coast, the English Channel SECA and the Celtic Sea. Further two campaigns were delayed by the COVID-19 pandemic until September 2021 and April 2022, targeting ships in the Bay of Biscay, the English Channel SECA and the Celtic Sea. Throughout the project, nearly 300 ships were measured during 30 research flights, varying from plume aging and cloud interaction studies, through collecting bulk statistics in busy shipping lanes to comparing emissions in and out of SECA. This work focuses on the gaseous species measurements (SO2, CO2, CH4 and VOCs from whole air samples). They are used to study changes in apparent sulphur fuel content of the ships observed throughout ACRUISE, plume composition and methane emissions from LNG tankers.

Despite the reduced nitrogen (N) cycle being central to global biogeochemistry, there are large uncertainties surrounding its sources and rate of cycling. Here, we present the first observations of gas-phase urea (CO(NH₂)₂) in the atmosphere from airborne high-resolution mass spectrometer measurements over the North Atlantic Ocean. We show that urea is ubiquitous in the marine lower troposphere during the Summer, Autumn and Winter flights but was found to be below the limit of detection during the Spring flights. The observations suggest the ocean is the primary emission source but further studies are required to understand the processes responsible for the air-sea exchange of urea. Urea is also frequently observed aloft due to long-range transport of biomass-burning plumes. These observations alongside global model simulations point to urea being an important, and as yet unaccounted for, component of reduced-N to the remote marine environment. Since we show it readily partitions between gas and particle phases, airborne transfer of urea between nutrient rich and poor parts of the ocean can occur readily and could impact ecosystems and oceanic uptake of CO2, with potentially important atmospheric implications.

1 January 2020 marked a major change in the legal sulphur content of shipping fuel – from 3.5% to 0.5% by mass outside of the Sulphur Emission Control Areas (SECAs). The anticipated effect of the new regulation is improvement of coastal air quality, supporting both environmental and human health. In addition, since sulphur is believed to be a negative climate forcer, removal of its substantial source might have positive influence on the global climate.The Atmospheric Composition and Radiative forcing changes due to UN International Ship Emissions regulations (ACRUISE) project demonstrates the use of a large aircraft to measure emissions from ships and their impact on local air quality and cloud formation. The Facility for Airborne Atmospheric Measurements (FAAM) research aircraft was deployed first in July 2019 (before regulation change) in shipping lanes along the Portuguese coast, the English Channel SECA and the Celtic Sea. Over 100 ships were sampled, 15 specifically targeted for plume aging and cloud interaction. A large container ship showed significant reduction in apparent fuel sulphur content upon entering SECA. Bulk statistics in and out of extremely busy shipping lanes were collected. The second, post regulation change, part of the fieldwork was postponed by the COVID-19 pandemic until September 2021. Over 150 ships were measured in the shipping lanes of the Bay of Biscay, the English Channel SECA and Celtic Sea. This part of the work focussed more on targeting specific ships, than on bulk measurements due to lower density of ships in the region and improved sampling strategy.This study presents a range of aspects of measurements. Onboard measurements of SO2, CO2, CH4 and speciated PM provide emission factors and apparent fuel sulphur content for a variety of ships. Moreover, about 100 whole air samples were taken during each fieldwork and analysed for VOCs. The encountered vessels included container ships, bulk carriers, cruise ships, ferries, crude oil tankers and even elusive LNG tankers. Some ships were measured both in and out of SECA and a few ships were measured both in 2019 and 2021.

Airborne sampling of methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), and nitrous oxide (N2O) mole fractions was conducted during field campaigns targeting fires over Senegal in February and March 2017, and Uganda in January 2019. The majority of fire plumes sampled were close to, or directly over burning vegetation, with the exception of two longer-range flights over the West African Atlantic seaboard, (100–300 km from source) where the continental outflow of biomass burning emissions from a wider area of West Africa was sampled. Fire Emission Factors (EFs) and modified combustion efficiencies (MCEs) were estimated from the enhancements in measured mole fractions. For the Senegalese fires, mean EFs and corresponding one-standard deviation variabilities, in units of g per kg of dry fuel were 1.8 (± 0.06) for CH4, 1633 (± 56.4) for CO2 and 679 (± 1.6) for CO, with a mean MCE of 0.94 (± 0.005). For the Ugandan fires, mean EFs (in units of g kg−1) were 3.1 (± 0.1) for CH4, 1610 (± 54.9) for CO2 and 78 (± 1.9) for CO, with a mean modified combustion efficiency of 0.93 (± 0.004). A mean N2O EF of 0.08 (± 0.002) g kg−1 is also reported for one flight over Uganda; issues with temperature control of the instrument optical bench prevented N2O EFs from being obtained for other flights over Uganda. This study has provided new datasets of African biomass burning EFs and MCEs for two distinct study regions, in which both have been studied little by aircraft measurement previously. These results highlight the important intracontinental variability of biomass burning trace gas emissions, and can be used to better constrain future biomass burning emission budgets. More generally, these results highlight the importance of regional and fuel-type variability when attempting to spatially scale biomass burning emissions. Further work to constrain EFs at more local scales and for more specific (and quantifiable) fuel types will serve to improve global estimates of biomass burning emissions of climate-relevant gases.

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 Combining double low line 0.894 between biomass burning organic aerosol (BBOA) and babs-470wb compared to an r2 Combining double low line 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.

To predict atmospheric partitioning of organic compounds between gas and aerosol particle phase based on explicit models for gas phase chemistry, saturation vapor pressures of the compounds need to be estimated. Estimation methods based on functional group contributions require training sets of compounds with well established saturation vapor pressures. However, vapor pressures of semi- and low volatile organic molecules at atmospheric temperatures reported in the literature often differ by several orders of magnitude between measurement techniques. These discrepancies exceed the stated uncertainty of each technique which is generally reported to be smaller than a factor of two. At present, there is no general reference technique for measuring saturation vapor pressures of atmospherically relevant compounds with low vapor pressures at atmospheric temperatures. To address this problem we measured vapor pressures with different techniques over a wide temperature range for intercomparison and to establish a reliable training set. We determined saturation vapor pressures for the homologous series of polyethylene glycols (H–(O–CH2–CH2)n–OH) for n = 3 to n = 8 ranging in vapor pressure at 298 K from 10−7 Pa to 5 · 10−2 Pa and compare them with quantum chemistry calculations. Such a homologous series provides a reference set that covers several orders of magnitude in saturation vapor pressure, allowing a critical assessment of the lower limits of detection of vapor pressures for the different techniques as well as permitting the identification of potential sources of systematic error. Also, internal consistency within the series allows to reject outlying data more easily. Most of the measured vapor pressures agreed within the stated uncertainty range. Deviations mostly occurred for vapor pressures values approaching the lower detection limit of a technique. The good agreement between the measurement techniques (some of which are sensitive to the mass accommodation coefficient and some not) suggest that the mass accommodation coefficients of the studied compounds are close to unity. The quantum chemistry calculations were about one order of magnitude higher than the measurements. We find that extrapolation of vapor pressures from elevated to atmospheric temperatures is permissible over a range of about 100 K for these compounds, suggesting that measurements should be performed best at temperatures yielding the highest accuracy data allowing subsequent extrapolation to atmospheric temperatures.

This study presents the first gaseous formic acid (HC(O)OH) concentration measurements collected over the Fenno-Scandinavian wetlands (67.9–68.0° N, 22.1–27.8° E) as part of the MAMM (Methane and other greenhouse gases in the Arctic-Measurements, process studies and Modelling) aircraft campaigns conducted in August and September 2013. A boundary layer box model approach has been used to calculate a regionally representative (~240 km2) surface flux for HC(O)OH of 0.0098 (±0.0057) mg[HCOOH]·m−2·h−1. A surface-type classification map was used to estimate proportional source contributions to the observed HC(O)OH flux over the measurement region. The removal of expected source contributions (using available literature parameterisations) from the calculated surface flux identified that 75% remained unaccounted for. This may suggest that HC(O)OH emission from wetland within the Fenno-Scandinavian region could contribute up to 29 times higher per unit area than previous theoretical HC(O)OH globally-averaged wetland estimates, highlighting a need for further constrained wetland studies of HC(O)OH emission to better understand its potentially significant impact on the Arctic HC(O)OH budget and consequent impacts on oxidative capacity.