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Abstract Furans are a major class of volatile organic compounds emitted from biomass burning. Their high reactivity with atmospheric oxidants leads to the formation of secondary organic aerosol (SOA), including secondary brown carbon (BrC) that can affect global climate via interactions with solar radiation. Here, we investigate the optical properties and chemical composition of SOA generated via photooxidation of furfural, 2‐methylfuran, and 3‐methylfuran under dry (RH

Gas- and particle-phase molecular markers provide highly specific information about the sources and atmospheric processes that contribute to air pollution. In urban areas, major sources of pollution are changing as regulation selectively mitigates some pollution sources and climate change impacts the surrounding environment. In this study, a comprehensive thermal desorption aerosol gas chromatograph (cTAG) was used to measure volatile, intermediate-volatility and semivolatile molecular markers every other hour over a 10 d period from 11 to 21 April 2018 in suburban Livermore, California. Source apportionment via positive matrix factorization (PMF) was performed to identify major sources of pollution. The PMF analysis identified 13 components, including emissions from gasoline, consumer products, biomass burning, secondary oxidation, aged regional transport and several factors associated with single compounds or specific events with unique compositions. The gasoline factor had a distinct morning peak in concentration but lacked a corresponding evening peak, suggesting commute-related traffic emissions are dominated by cold starts in residential areas. More monoterpene and monoterpenoid mass was assigned to consumer product emissions than biogenic sources, underscoring the increasing importance of volatile chemical products to urban emissions. Daytime isoprene concentrations were controlled by biogenic sunlight- and temperature-dependent processes, mediated by strong midday mixing, but gasoline was found to be the dominant and likely only source of isoprene at night. Biomass burning markers indicated residential wood burning activity remained an important pollution source even in the springtime. This study demonstrates that specific high-time-resolution molecular marker measurements across a wide range of volatility enable more comprehensive pollution source profiles than a narrower volatility range would allow.

The chemical composition of ambient organic aerosols plays a critical role in driving their climate and health-relevant properties and holds important clues to the sources and formation mechanisms of secondary aerosol material. In most ambient atmospheric environments, this composition remains incompletely characterized, with the number of identifiable species consistently outnumbered by those that have no mass spectral matches in the literature or the National Institute of Standards and Technology/National Institutes of Health/Environmental Protection Agency (NIST/NIH/EPA) mass spectral databases, making them nearly impossible to definitively identify. This creates significant challenges in utilizing the full analytical capabilities of techniques which separate and generate spectra for complex environmental samples. In this work, we develop the use of machine learning techniques to quantify and characterize novel, or unidentifiable, organic material. This work introduces Ch3MS-RF (Chemical Characterization by Chromatography–Mass Spectrometry Random Forest Modeling), an open-source, R-based software tool, for efficient machine-learning-enabled characterization of compounds separated in chromatography–mass spectrometry applications but not identifiable by comparison to mass spectral databases. A random forest model is trained and tested on a known 130 component representative external standard to predict the response factors of novel environmental organics based on position in volatility–polarity space and mass spectrum, enabling the reproducible, efficient, and optimized quantification of novel environmental species. Quantification accuracy on a reserved 20 % test set randomly split from the external standard compound list indicates that random forest modeling significantly outperforms the commonly used methods in both precision and accuracy, with a median response factor percent error of −2 %, for modeled response factors, compared to > 15 %, for typically used proxy assignment-based methods. Chemical properties modeling, evaluated on the same reserved 20 % test set and an extrapolation set of species identified in ambient organic aerosol samples collected in the Amazon rainforest, also demonstrate robust performance. Extrapolation set property prediction mean absolute errors for carbon number, oxygen to carbon ratio (O : C), average carbon oxidation state (OSc‾), and vapor pressure are 1.8, 0.15, 0.25, and 1.0 (log(atm)), respectively. Extrapolation set out-of-sample R2 for all properties modeled are above 0.75, with the exception of vapor pressure. While predictive performance for vapor pressure is less robust compared to the other chemical properties modeled, random-forest-based modeling was significantly more accurate than other commonly used methods of vapor pressure prediction, decreasing the mean vapor pressure prediction error to 0.24 (log(atm)) from 0.55 (log(atm)) (chromatography-based vapor pressure prediction) and 1.2 (log(atm)) (chemical formula-based vapor pressure prediction). The random forest model significantly advances an untargeted analysis of the full scope of chemical speciation yielded by two-dimensional gas chromatography (GCxGC-MS) techniques and can be applied to gas chromatography coupled with electron ionization mass spectrometry (GC-MS) as well. It enables the accurate estimation of key chemical properties commonly utilized in the atmospheric chemistry community, which may be used to more efficiently identify important tracers for further individual analysis and to characterize compound populations uniquely formed under specific ambient conditions.

Brown carbon (BrC) associated with aerosol particles in western United States wildfires was measured between July and August 2019 aboard the NASA DC-8 research aircraft during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) study. Two BrC measurement methods are investigated, highly spectrally resolved light absorption in solvent (water and methanol) extracts of particles collected on filters and in situ bulk aerosol particle light absorption measured at three wavelengths (405, 532 and 664 nm) with a photoacoustic spectrometer (PAS). A light-absorption closure analysis for wavelengths between 300 and 700 nm was performed. The combined light absorption of particle pure black carbon material, including enhancements due to internally mixed materials, plus soluble BrC and a Mie-predicted factor for conversion of soluble BrC to aerosol particle BrC, was compared to absorption spectra from a power law fit to the three PAS wavelengths. For the various parameters used, at a wavelength of roughly 400 nm they agreed, at lower wavelengths the individual component-predicted particle light absorption significantly exceeded the PAS and at higher wavelengths the PAS absorption was consistently higher but more variable. Limitations with extrapolation of PAS data to wavelengths below 405 nm and missing BrC species of low solubility that more strongly absorb at higher wavelengths may account for the differences. Based on measurements closest to fires, the emission ratio of PAS-measured BrC at 405 nm relative to carbon monoxide (CO) was on average 0.13 Mm−1 ppbv−1; emission ratios for soluble BrC are also provided. As the smoke moved away from the burning regions, the evolution over time of BrC was observed to be highly complex; BrC enhancement, depletion or constant levels with age were all observed in the first 8 h after emission in different plumes. Within 8 h following emissions, 4-nitrocatechol, a well-characterized BrC chromophore commonly found in smoke particles, was largely depleted relative to the bulk BrC. In a descending plume where temperature increased by 15 K, 4-nitrocatechol dropped, possibly due to temperature-driven evaporation, but bulk BrC remained largely unchanged. Evidence was found for reactions with ozone, or related species, as a pathway for secondary formation of BrC under both low and high oxides of nitrogen (NOx) conditions, while BrC was also observed to be bleached in regions of higher ozone and low NOx, consistent with complex behaviors of BrC observed in laboratory studies. Although the evolution of smoke in the first hours following emission is highly variable, a limited number of measurements of more aged smoke (15 to 30 h) indicate a net loss of BrC. It is yet to be determined how the near-field BrC evolution in smoke affects the characteristics of smoke over longer timescales and spatial scales, where its environmental impacts are likely to be greater.

Brown carbon (BrC) consists of particulate organic species that preferentially absorb light at visible and ultraviolet wavelengths. Ambient studies show that as a component of aerosol particles, BrC affects photochemical reaction rates and regional to global climate. Some organic chromophores are especially toxic, linking BrC to adverse health effects. The lack of direct measurements of BrC has limited our understanding of its prevalence, sources, evolution, and impacts. We describe the first direct, online measurements of water-soluble BrC on research aircraft by three separate instruments. Each instrument measured light absorption over a broad wavelength range using a liquid waveguide capillary cell (LWCC) and grating spectrometer, with particles collected into water by a particle-into-liquid sampler (CSU PILS-LWCC and NOAA PILS-LWCC) or a mist chamber (MC-LWCC). The instruments were deployed on the NSF C-130 aircraft during WE-CAN 2018 as well as the NASA DC-8 and the NOAA Twin Otter aircraft during FIREX-AQ 2019, where they sampled fresh and moderately aged wildfire plumes. Here, we describe the instruments, calibrations, data analysis and corrections for baseline drift and hysteresis. Detection limits (3σ) at 365 nm were 1.53 Mm−1 (MC-LWCC; 2.5 min sampling time), 0.89 Mm−1 (CSU PILS-LWCC; 30 s sampling time), and 0.03 Mm−1 (NOAA PILS-LWCC; 30 s sampling time). Measurement uncertainties were 28 % (MC-LWCC), 12 % (CSU PILS-LWCC), and 11 % (NOAA PILS-LWCC). The MC-LWCC system agreed well with offline measurements from filter samples, with a slope of 0.91 and R2=0.89. Overall, these instruments provide soluble BrC measurements with specificity and geographical coverage that is unavailable by other methods, but their sensitivity and time resolution can be challenging for aircraft studies where large and rapid changes in BrC concentrations may be encountered.

Aerosols are a source of great uncertainty in radiative forcing predictions and have poorly understood health impacts. Most aerosol mass is formed in the atmosphere from reactive gas-phase organic precursors, forming secondary organic aerosol (SOA). Semivolatile organic compounds (SVOCs) (effective saturation concentration, C*, of 10−1–103 µg m−3) comprise a large fraction of organic aerosol, while intermediate-volatility organic compounds (IVOCs) (C* of 103–106 µg m−3) and volatile organic compounds (VOCs) (C* ≥ 106 µg m−3) are gas-phase precursors to SOA and ozone. The Comprehensive Thermal Desorption Aerosol Gas Chromatograph (cTAG) is the first single instrument simultaneously quantitative for a broad range of compound-specific VOCs, IVOCs and SVOCs. cTAG is a two-channel instrument which measures concentrations of C5–C16 alkane-equivalent-volatility VOCs and IVOCs on one channel and C14–C32 SVOCs on the other coupled to a single high-resolution time-of-flight mass spectrometer, achieving consistent quantification across 15 orders of magnitude of vapor pressure. cTAG obtains concentrations hourly and gas–particle partitioning for SVOCs every other hour, enabling observation of the evolution of these species through oxidation and partitioning into the particle phase. Online derivatization for the SVOC channel enables detection of more polar and oxidized species. In this work we present design details and data evaluating key parameters of instrument performance such as I/VOC collector design optimization, linearity and reproducibility of calibration curves obtained using a custom liquid evaporation system for I/VOCs and the effect of an ozone removal filter on instrument performance. Example timelines of precursors with secondary products are shown, and analysis of a subset of compounds detectable by cTAG demonstrates some of the analytical possibilities with this instrument.

Determination of transition metals in ambient aerosols is important due to their toxicity to human health. However, the traditional measurement techniques for metal analysis are often costly and require sophisticated instruments. In this study, we developed and verified relatively low-cost liquid spectrophotometric methods for the measurements of iron (Fe) and copper (Cu), the two most abundant transition metals in ambient fine particulate matter (PM2.5). For Fe analysis, we utilized a ferrozine-based colorimetric method, which has been frequently used for water-soluble (WS) Fe determination, and further extended this approach for the measurement of total Fe (water-soluble + water-insoluble). In this method, Fe is quantified through the formation of a light-absorbing ferrozine–Fe(II) complex (absorbance at 562 nm). A similar colorimetric method, which forms a bathocuproine–Cu(I) complex absorbing light at 484 nm, was developed and examined for measurement of WS and total Cu. These methods were applied to 24 h integrated filter samples collected in urban Atlanta. Based on PM2.5 ambient aerosols, total and water-soluble Fe and Cu concentrations were in good agreement with inductively coupled plasma mass spectrometry (ICP-MS) measurements (slopes 1.0±0.1, r2>0.89). The water-soluble components, operationally defined as those species in the aqueous filter extract that pass through a 0.45 µm filter, were further characterized by ultrafiltration, which showed that roughly 85 % of both the Fe and Cu in the water-soluble fraction was composed of species smaller than nominally 4 nm.

Wildfires have become more common and intense in the western US over recent decades due to a combination of historical land management practices and warming climate. Emissions from large-scale fires now frequently affect populated regions such as the San Francisco Bay Area during the fall wildfire season, with documented impacts of the resulting particulate matter on human health. Health impacts of exposure to wildfire emissions depend on the chemical composition of particulate matter, but the molecular composition of the real biomass burning organic aerosol (BBOA) that reaches large population centers remains insufficiently characterized. We took PM2.5 (particles having aerodynamic diameters less than or equal to 2.5 µm) samples at the University of California, Berkeley campus (∼ 60 km downwind of the fires) during the October 2017 Northern California wildfires period and analyzed molecular composition of OA using a two-dimensional gas chromatography coupled with high-resolution time-of-flight mass spectrometry (GC×GC HR-ToF-MS). Sugar-like compounds were the most abundant component of BBOA, followed by mono-carboxylic acids, aromatic compounds, other oxygenated compounds, and terpenoids. The vast majority of compounds detected in smoke have unknown health impacts. Regression models were trained to predict the saturation vapor pressure and averaged carbon oxidation state (OSc‾) of detected compounds. The compounds speciated have a wide volatility distribution and most of them are highly oxygenated. In addition, time series of primary BBOA tracers observed in Berkeley were found to be indicative of the types of plants in the ecosystems burned in Napa and Sonoma, and could be used to differentiate the regions from which the smoke must have originated. Commonly used secondary BBOA markers like 4-nitrocatechol were enhanced when plumes aged, but their very fast formation caused them to have similar temporal variation as primary BBOA tracers. Using hierarchical clustering analysis, we classified compounds into seven factors indicative of their sources and transformation processes, identifying a unique daytime secondary BBOA factor. Chemicals associated with this factor include multifunctional acids and oxygenated aromatic compounds. These compounds have high OSc‾, and they are also semi-volatile. We observed no net particle-phase organic carbon formation, which indicates an approximate balance between the mass of evaporated organic carbonaceous compounds and the addition of secondary organic carbonaceous compounds.

Ecosystem productivity is strongly modulated by the atmospheric deposition of inorganic reactive nitrogen (the sum of ammonium and nitrate). The individual contributions of ammonium and nitrate vary considerably over space and time, giving rise to complex patterns of nitrogen deposition. In the absence of rain, much of this complexity is driven by the large difference between the dry deposition velocity of nitrogen-containing molecules in the gas or condensed phase. Here we quantify how aerosol liquid water and acidity, through their impact on gas–particle partitioning, modulate the deposition velocity of total NH3 and total HNO3 individually while simultaneously affecting the dry deposition of inorganic reactive nitrogen. Four regimes of deposition velocity emerge: (i) HNO3 – fast, NH3 – slow, (ii) HNO3 – slow, NH3 – fast, (iii) HNO3 – fast, NH3 – fast, and (iv) HNO3 – slow, NH3 – slow. Conditions that favor partitioning of species to the aerosol phase strongly reduce the local deposition of reactive nitrogen species and promote their accumulation in the boundary layer and potential for long-range transport. Application of this framework to select locations around the world reveals fundamentally important insights: the dry deposition of total ammonia displays little sensitivity to pH and liquid water variations, except under conditions of extreme acidity and/or low aerosol liquid water content. The dry deposition of total nitric acid, on the other hand, is quite variable, with maximum deposition velocities (close to gas deposition rates) found in the eastern United States and minimum velocities in northern Europe and China. In the latter case, the low deposition velocity leads to up to 10-fold increases in PM2.5 nitrate aerosol, thus contributing to the high PM2.5 levels observed during haze episodes. In this light, aerosol pH and associated liquid water content can be considered to be control parameters that drive dry deposition flux and can accelerate the accumulation of aerosol contributing to intense haze events throughout the globe.

The formation and evolution of secondary organic aerosol (SOA) were investigated at Yorkville, GA, in late summer (mid-August to mid-October 2016). The organic aerosol (OA) composition was measured using two online mass spectrometry instruments, the high-resolution time-of-flight aerosol mass spectrometer (AMS) and the Filter Inlet for Gases and AEROsols coupled to a high-resolution time-of-flight iodide-adduct chemical ionization mass spectrometer (FIGAERO-CIMS). Through analysis of speciated organics data from FIGAERO-CIMS and factorization analysis of data obtained from both instruments, we observed notable SOA formation from isoprene and monoterpenes during both day and night. Specifically, in addition to isoprene epoxydiol (IEPOX) uptake, we identified isoprene SOA formation from non-IEPOX pathways and isoprene organic nitrate formation via photooxidation in the presence of NOx and nitrate radical oxidation. Monoterpenes were found to be the most important SOA precursors at night. We observed significant contributions from highly oxidized acid-like compounds to the aged OA factor from FIGAERO-CIMS. Taken together, our results showed that FIGAERO-CIMS measurements are highly complementary to the extensively used AMS factorization analysis, and together they provide more comprehensive insights into OA sources and composition.

The capability of ambient particles to generate in vivo reactive oxygen species (ROS), called oxidative potential (OP), is a potential metric for evaluating the health effects of particulate matter (PM) and is supported by several recent epidemiological investigations. Studies using various types of OP assays differ in their sensitivities to varying PM chemical components. In this study, we systematically compared two health-relevant acellular OP assays that track the depletion of antioxidants or reductant surrogates: (i) the synthetic respiratory-tract lining fluid (RTLF) assay that tracks the depletion of ascorbic acid (AA) and glutathione (GSH) and (ii) the dithiothreitol (DTT) assay that tracks the depletion of DTT. Yearlong daily samples were collected at an urban site in Atlanta, GA (Jefferson Street), during 2017, and both DTT and RTLF assays were performed to measure the OP of water-soluble PM2.5 components. PM2.5 mass and major chemical components, including metals, ions, and organic and elemental carbon were also analyzed. Correlation analysis found that OP as measured by the DTT and AA depletion (OPDTT and OPAA, respectively) were correlated with both organics and some water-soluble metal species, whereas that from the GSH depletion (OPGSH) was exclusively sensitive to water-soluble Cu. These OP assays were moderately correlated with each other due to the common contribution from metal ions. OPDTT and OPAA were moderately correlated with PM2.5 mass with Pearson's r=0.55 and 0.56, respectively, whereas OPGSH exhibited a lower correlation (r=0.24). There was little seasonal variation in the OP levels for all assays due to the weak seasonality of OP-associated species. Multivariate linear regression models were developed to predict OP measures from the particle composition data. Variability in OPDTT and OPAA were not only attributed to the concentrations of metal ions (mainly Fe and Cu) and organic compounds but also to antagonistic metal–organic and metal–metal interactions. OPGSH was sensitive to the change in water-soluble Cu and brown carbon (BrC), a proxy for ambient humic-like substances.

Nitrogen oxides (NOx) and ammonia (NH3) from anthropogenic and biogenic emissions are central contributors to particulate matter (PM) concentrations worldwide. The response of PM to changes in the emissions of both compounds is typically studied on a case-by-case basis, owing in part to the complex thermodynamic interactions of these aerosol precursors with other PM constituents. Here we present a simple but thermodynamically consistent approach that expresses the chemical domains of sensitivity of aerosol particulate matter to NH3 and HNO3 availability in terms of aerosol pH and liquid water content. From our analysis, four policy-relevant regimes emerge in terms of sensitivity: (i) NH3 sensitive, (ii) HNO3 sensitive, (iii) NH3 and HNO3 sensitive, and (iv) insensitive to NH3 or HNO3. For all regimes, the PM remains sensitive to nonvolatile precursors, such as nonvolatile cations and sulfate. When this framework is applied to ambient measurements or predictions of PM and gaseous precursors, the “chemical regime” of PM sensitivity to NH3 and HNO3 availability is directly determined. The use of these regimes allows for novel insights, and this framework is an important tool to evaluate chemical transport models. With this extended understanding, aerosol pH and associated liquid water content naturally emerge as previously ignored state parameters that drive PM formation.

Carbonaceous aerosols significantly affect global radiative forcing and climate through absorption and the scattering of sunlight. Black carbon (BC) and brown carbon (BrC) are light-absorbing carbonaceous aerosols. The direct radiative effect (DRE) of BrC is uncertain. A recent study suggests that BrC absorption is comparable to BC in the upper troposphere over biomass burning regions and that the resulting radiative heating tends to stabilize the atmosphere. Yet current climate models do not include proper physical and chemical treatments of BrC. In this study, we derived a BrC global biomass burning emission inventory on the basis of the Global Fire Emissions Database version 4 (GFED4), developed a module to simulate the light absorption of BrC in the Community Atmosphere Model version 5 (CAM5) of the Community Earth System Model (CESM), and investigated the photobleaching effect and convective transport of BrC on the basis of Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) and Deep Convective Clouds and Chemistry Project (DC3) measurements. The model simulations of BC were also evaluated using HIAPER (High-Performance Instrumented Airborne Platform for Environmental Research) Pole-to-Pole Observations (HIPPO) measurements. We found that globally BrC is a significant absorber, the DRE of which is 0.10 W m−2, more than 25 % of BC DRE (+0.39 W m−2). Most significantly, model results indicated that BrC atmospheric heating in the tropical mid and upper troposphere is larger than that of BC. The source of tropical BrC is mainly from wildfires, which are more prevalent in the tropical regions than higher latitudes and release much more BrC relative to BC than industrial sources. While BC atmospheric heating is skewed towards the northern mid-latitude lower atmosphere, BrC heating is more centered in the tropical free troposphere. A possible mechanism for the enhanced convective transport of BrC is that hydrophobic high molecular weight BrC becomes a larger fraction of the BrC and less easily activated in a cloud as the aerosol ages. The contribution of BrC heating to the Hadley circulation and latitudinal expansion of the tropics is likely comparable to BC heating.

Water-soluble organic carbon (WSOC) is a ubiquitous and significant fraction of fine particulate matter. Despite advances in aerosol thermodynamic equilibrium models, there is limited understanding on the comprehensive impacts of WSOC on aerosol acidity (pH). We address this limitation by studying submicron aerosols that represent the two extremes in acidity levels found in the atmosphere: strongly acidic aerosol from Baltimore, MD, and weakly acidic conditions characteristic of Beijing, China. These cases are then used to construct mixed inorganic–organic single-phase aqueous particles and thermodynamically analyzed by the Extended Aerosol Inorganics Model (E-AIM) and ISORROPIA models in combination with activity coefficient model AIOMFAC (Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficient) to evaluate the effects of WSOC on the H+ ion activity coefficients (γH+) and activity (pH). We find that addition of organic acids and nonacid organic species concurrently increases γH+ and aerosol liquid water. Under the highly acidic conditions typical of the eastern US (inorganic-only pH ∼1), these effects mostly offset each other, giving pH changes of ∼4.5), the nonacidic WSOC compounds had similarly minor effects on aerosol pH, but organic acids imparted the largest changes in pH compared to the inorganic-only simulations. Organic acids affect pH in the order of their pKa values (oxalic acid > malonic acid > glutaric acid). Although the inorganic-only pH was above the pKa value of all three organic acids investigated, pH changes in excess of 1 pH unit were only observed at unrealistic organic acid levels (aerosol organic acid concentrations > 35 µg m−3) in Beijing. The model simulations were run at 70 %, 80 %, and 90 % relative humidity (RH) levels and the effect of WSOC was inversely related to RH. At 90 % RH, WSOC altered aerosol pH by up to ∼0.2 pH units, though the effect was up to ∼0.6 pH units at 70 % RH. The somewhat offsetting nature of these effects suggests that aerosol pH is sufficiently constrained by the inorganic constituents alone under conditions where liquid–liquid phase separation is not anticipated to occur.

Biomass burning is a major source of atmospheric brown carbon (BrC), and through its absorption of UV/VIS radiation, it can play an important role in the planetary radiative balance and atmospheric photochemistry. The considerable uncertainty of BrC impacts is associated with its poorly constrained sources, transformations, and atmospheric lifetime. Here we report laboratory experiments that examined changes in the optical properties of the water-soluble (WS) BrC fraction of laboratory-generated biomass burning particles from hardwood pyrolysis. Effects of direct UVB photolysis and OH oxidation in the aqueous phase on molecular-weight-separated BrC were studied. Results indicated that the majority of low-molecular-weight (MW) BrC ( Da) was rapidly photobleached by both direct photolysis and OH oxidation on an atmospheric timescale of approximately 1 h. High MW BrC (≥400 Da) underwent initial photoenhancement up to ∼15 h, followed by slow photobleaching over ∼10 h. The laboratory experiments were supported by observations from ambient BrC samples that were collected during the fire seasons in Greece. These samples, containing freshly emitted to aged biomass burning aerosol, were analyzed for both water- and methanol-soluble BrC. Consistent with the laboratory experiments, high-MW BrC dominated the total light absorption at 365 nm for both methanol and water-soluble fractions of ambient samples with atmospheric transport times of 1 to 68 h. These ambient observations indicate that overall, biomass burning BrC across all molecular weights has an atmospheric lifetime of 15 to 28 h, consistent with estimates from previous field studies – although the BrC associated with the high-MW fraction remains relatively stable and is responsible for light absorption properties of BrC throughout most of its atmospheric lifetime. For ambient samples of aged (>10 h) biomass burning emissions, poor linear correlations were found between light absorptivity and levoglucosan, consistent with other studies suggesting a short atmospheric lifetime for levoglucosan. However, a much stronger correlation between light absorptivity and total hydrous sugars was observed, suggesting that they may serve as more robust tracers for aged biomass burning emissions. Overall, the results from this study suggest that robust model estimates of BrC radiative impacts require consideration of the atmospheric aging of BrC and the stability of high-MW BrC.

Overprediction of fine-particle ammonium-sulfate molar ratios (R) by thermodynamic models is suggested as evidence for interactions with organic constituents that inhibit the equilibration of gas-phase ammonia with aerosol sulfate and questions the equilibrium assumption long thought to apply for submicron aerosol. This hypothesis is tested through thermodynamic analysis of ambient observations. We find that the deviation between R from a molar ratio of 2 is strongly correlated with the concentration of sodium (Na+), a nonvolatile cation (NVC), but exhibits no correlation to organic aerosol (OA) mass concentration or mass fraction. Thermodynamic predictions of both R and ammonia gas–particle partitioning can accurately reproduce observations when small amounts of NVCs are included in the calculations, whereas exclusion of NVCs results in a predicted R consistently near 2. The sensitivity of R to small amounts of NVCs arises because, when the latter are present but not included in the thermodynamic calculations, the missing cations are replaced with ammonium in the model (NH3–NH4+ equilibrium shifts to the particle), resulting in an R that is biased high. Results and conclusions based on bulk aerosol considerations that assume all species are internally mixed are not changed even if NVCs and sulfate are largely externally mixed; fine-particle pH is found to be much less sensitive to mixing state assumptions than molar ratios. We also show that the data used to support the organic inhibition of NH3 from equilibrium, when compared against other network and field campaign datasets, display a systematically and significantly lower NH4+ (thought to be from an evaporation bias), that is of the order of the effect postulated to be caused by organics. Altogether, these results question the postulated ability of organic compounds to considerably perturb aerosol acidity and prevent ammonia from achieving gas–particle equilibrium, at least for the locations considered. Furthermore, the results demonstrate the limitations of using molar ratios to infer aerosol properties or processes that depend on particle pH.

In this work, the design of a pulsed magnetic field generator, with user-selective pulsed modulation frequency is described. The ability to operate at various frequencies (single-frequency below 10 MHz) makes the system valuable to several areas such as medical treatments and pulsed switching systems. In this work, the pulsed magnetic field generator is designed to create localized field effects in portable magnetic resonance systems. Users may operate at a Larmor precession frequency between 100 kHz - 10 MHz and can achieve high currents through the load. Certain tunability can also be obtained by varying the load inductance or switching device conditions. In summary, this paper will describe the design considerations and challenges for portable monophasic pulsed magnetic field systems.

The sources and atmospheric chemistry of gas-phase organic acids are currently poorly understood, due in part to the limited range of measurement techniques available. In this work, we evaluated the use of SF6− as a sensitive and selective chemical ionization reagent ion for real-time measurements of gas-phase organic acids. Field measurements are made using chemical ionization mass spectrometry (CIMS) at a rural site in Yorkville, Georgia, from September to October 2016 to investigate the capability of this measurement technique. Our measurements demonstrate that SF6− can be used to measure a range of organic acids in the atmosphere. One-hour averaged ambient concentrations of organic acids ranged from a few parts per trillion by volume (ppt) to several parts per billion by volume (ppb). All the organic acids displayed similar strong diurnal behaviors, reaching maximum concentrations between 17:00 and 19:00 EDT. The organic acid concentrations are dependent on ambient temperature, with higher organic acid concentrations being measured during warmer periods.

In some regions, reducing aerosol ammonium nitrate (NH4NO3) concentrations may substantially improve air quality. This can be accomplished by reductions in precursor emissions, such as nitrogen oxides (NOx) to lower nitric acid (HNO3) that partitions to the aerosol, or reductions in ammonia (NH3) to lower particle pH and keep HNO3 in the gas phase. Using the ISORROPIA-II thermodynamic aerosol model and detailed observational data sets, we explore the sensitivity of aerosol NH4NO3 to gas-phase NH3 and NOx controls for a number of contrasting locations, including Europe, the United States, and China. NOx control is always effective, whereas the aerosol response to NH3 control is highly nonlinear and only becomes effective at a thermodynamic sweet spot. The analysis provides a conceptual framework and fundamental evaluation on the relative value of NOx versus NH3 control and demonstrates the relevance of pH as an air quality parameter. We find that, regardless of the locations examined, it is only when ambient particle pH drops below an approximate critical value of 3 (slightly higher in warm and slightly lower in cold seasons) that NH3 reduction leads to an effective response in PM2.5 mass. The required amount of NH3 reduction to reach the critical pH and efficiently decrease NH4NO3 at different sites is assessed. Owing to the linkage between NH3 emissions and agricultural productivity, the substantial NH3 reduction required in some locations may not be feasible. Finally, controlling NH3 emissions to increase aerosol acidity and evaporate NH4NO3 will have other effects, beyond reduction of PM2.5 NH4NO3, such as increasing aerosol toxicity and potentially altering the deposition patterns of nitrogen and trace nutrients.

The implementation of stringent emission regulations has resulted in the decline of anthropogenic pollutants including sulfur dioxide (SO2), nitrogen oxides (NOx), and carbon monoxide (CO). In contrast, ammonia (NH3) emissions are largely unregulated, with emissions projected to increase in the future. We present real-time aerosol and gas measurements from a field study conducted in an agriculturally intensive region in the southeastern US during the fall of 2016 to investigate how NH3 affects particle acidity and secondary organic aerosol (SOA) formation via the gas–particle partitioning of semi-volatile organic acids. Particle water and pH were determined using the ISORROPIA II thermodynamic model and validated by comparing predicted inorganic HNO3-NO3− and NH3-NH4+ gas–particle partitioning ratios with measured values. Our results showed that despite the high NH3 concentrations (average 8.1±5.2 ppb), PM1 was highly acidic with pH values ranging from 0.9 to 3.8, and an average pH of 2.2±0.6. PM1 pH varied by approximately 1.4 units diurnally. Formic and acetic acids were the most abundant gas-phase organic acids, and oxalate was the most abundant particle-phase water-soluble organic acid anion. Measured particle-phase water-soluble organic acids were on average 6 % of the total non-refractory PM1 organic aerosol mass. The measured molar fraction of oxalic acid in the particle phase (i.e., particle-phase oxalic acid molar concentration divided by the total oxalic acid molar concentration) ranged between 47 % and 90 % for a PM1 pH of 1.2 to 3.4. The measured oxalic acid gas–particle partitioning ratios were in good agreement with their corresponding thermodynamic predictions, calculated based on oxalic acid's physicochemical properties, ambient temperature, particle water, and pH. In contrast, gas–particle partitioning ratios of formic and acetic acids were not well predicted for reasons currently unknown. For this study, higher NH3 concentrations relative to what has been measured in the region in previous studies had minor effects on PM1 organic acids and their influence on the overall organic aerosol and PM1 mass concentrations.

Organic aerosols (OA) that strongly absorb solar radiation in the near-UV are referred to as brown carbon (BrC). The sources, evolution, and optical properties of BrC remain highly uncertain and contribute significantly to uncertainty in the estimate of the global direct radiative effect (DRE) of aerosols. Previous modeling studies of BrC optical properties and DRE have been unable to fully evaluate model performance due to the lack of direct measurements of BrC absorption. In this study, we develop a global model simulation (GEOS-Chem) of BrC and test it against BrC absorption measurements from two aircraft campaigns in the continental US (SEAC4RS and DC3). To the best of our knowledge, this is the first study to compare simulated BrC absorption with direct aircraft measurements. We show that BrC absorption properties estimated based on previous laboratory measurements agree with the aircraft measurements of freshly emitted BrC absorption but overestimate aged BrC absorption. In addition, applying a photochemical scheme to simulate bleaching/degradation of BrC improves model skill. The airborne observations are therefore consistent with a mass absorption coefficient (MAC) of freshly emitted biomass burning OA of 1.33 m2 g−1 at 365 nm coupled with a 1-day whitening e-folding time. Using the GEOS-Chem chemical transport model integrated with the RRTMG radiative transfer model, we estimate that the top-of-the-atmosphere all-sky direct radiative effect (DRE) of OA is −0.344 Wm−2, 10 % higher than that without consideration of BrC absorption. Therefore, our best estimate of the absorption DRE of BrC is +0.048 Wm−2. We suggest that the DRE of BrC has been overestimated previously due to the lack of observational constraints from direct measurements and omission of the effects of photochemical whitening.

An automated analytical system was developed for measuring the oxidative potential (OP) with the dithiothreitol (DTT) assay of filter extracts that include both water-soluble and water-insoluble (solid) aerosol species. Three approaches for measuring total oxidative potential were compared. These include using methanol as the solvent with (1) and without (2) filtering the extract, followed by removing the solvent and reconstituting with water, and (3) extraction in pure water and performing the OP analysis in the extraction vial with the filter. The water extraction method (the third approach, with filter remaining in the vial) generally yielded the highest DTT responses with better precision (coefficient of variation of 1–5 %) and was correlated with a greater number of other aerosol components. Because no organic solvents were used, which must be mostly eliminated prior to DTT analysis, it was easiest to automate by modifying an automated analytical system for measuring water-soluble OP developed by Fang et al. (2015). Therefore, the third method was applied to the field study for the determination of total OP. Daily 23 h filter samples were collected simultaneously at a roadside (RS) and a representative urban (Georgia Tech, GT) site for two 1-month study periods, and both water-soluble (OPWS-DTT) and total (OPTotal-DTT) OP were measured. Using PM2. 5 (aerodynamic diameter WS-DTT-to-OPTotal-DTT ratio at the urban site was 65 % with a correlation coefficient (r) of 0.71 (N = 35; p value r = 0. 56 (N = 31; p value −1) with Teflon filters. Comparison of measurements between sites showed only slightly higher levels of both OPWS-DTT and OPTotal-DTT at the RS site, indicating both OPWS-DTT and OPTotal-DTT were largely spatially homogeneous. These results are consistent with roadway emissions as sources of DTT-quantified PM2. 5 OP and indicate that both soluble and insoluble aerosol components contributing to OP are largely secondary.

pH is a fundamental aerosol property that affects ambient particle concentration and composition, linking pH to all aerosol environmental impacts. Here, PM1 and PM2. 5 pH are calculated based on data from measurements during the California Research at the Nexus of Air Quality and Climate Change (CalNex) study from 15 May to 15 June 2010 in Pasadena, CA. Particle pH and water were predicted with the ISORROPIA-II thermodynamic model and validated by comparing predicted to measured gas–particle partitioning of inorganic nitrate, ammonium, and chloride. The study mean ± standard deviation PM1 pH was 1.9 ± 0.5 for the SO42−–NO3−–NH4+–HNO3–NH3 system. For PM2. 5, internal mixing of sea salt components (SO42−–NO3−–NH4+–Na+–Cl−–K+–HNO3–NH3–HCl system) raised the bulk pH to 2.7 ± 0.3 and improved predicted nitric acid partitioning with PM2. 5 components. The results show little effect of sea salt on PM1 pH, but significant effects on PM2. 5 pH. A mean PM1 pH of 1.9 at Pasadena was approximately one unit higher than what we have reported in the southeastern US, despite similar temperature, relative humidity, and sulfate ranges, and is due to higher total nitrate concentrations (nitric acid plus nitrate) relative to sulfate, a situation where particle water is affected by semi-volatile nitrate concentrations. Under these conditions nitric acid partitioning can further promote nitrate formation by increasing aerosol water, which raises pH by dilution, further increasing nitric acid partitioning and resulting in a significant increase in fine particle nitrate and pH. This study provides insights into the complex interactions between particle pH and nitrate in a summertime coastal environment and a contrast to recently reported pH in the eastern US in summer and winter and the eastern Mediterranean. All studies have consistently found highly acidic PM1 with pH generally below 3.

Particulate matter (PM), of which a significant fraction is comprised of secondary organic aerosols (SOA), has received considerable attention due to its health implications. In this study, the water-soluble oxidative potential (OPWS) of SOA generated from the photooxidation of biogenic and anthropogenic hydrocarbon precursors (isoprene, α-pinene, β-caryophyllene, pentadecane, m-xylene, and naphthalene) under different reaction conditions (RO2+ HO2 vs. RO2+ NO dominant, dry vs. humid) was characterized using dithiothreitol (DTT) consumption. The measured intrinsic OPWS-DTT values ranged from 9 to 205 pmol min−1 µg−1 and were highly dependent on the specific hydrocarbon precursor, with naphthalene and isoprene SOA generating the highest and lowest OPWS-DTT values, respectively. Humidity and RO2 fate affected OPWS-DTT in a hydrocarbon-specific manner, with naphthalene SOA exhibiting the most pronounced effects, likely due to the formation of nitroaromatics. Together, these results suggest that precursor identity may be more influential than reaction condition in determining SOA oxidative potential, demonstrating the importance of sources, such as incomplete combustion, to aerosol toxicity. In the context of other PM sources, all SOA systems, with the exception of naphthalene SOA, were less DTT active than ambient sources related to incomplete combustion, including diesel and gasoline combustion as well as biomass burning. Finally, naphthalene SOA was as DTT active as biomass burning aerosol, which was found to be the most DTT-active OA source in a previous ambient study. These results highlight a need to consider SOA contributions (particularly from anthropogenic hydrocarbons) to health effects in the context of hydrocarbon emissions, SOA yields, and other PM sources.

The chemical and physical speciation of atmospheric sulfur was investigated in ambient aerosol samples using a combination of sulfur near-edge x-ray fluorescence spectroscopy (S-NEXFS) and X-ray fluorescence (XRF) microscopy. These techniques were used to determine the composition and oxidation state of sulfur in common primary emission sources and ambient particulate matter collected from the greater Atlanta area. Ambient particulate matter samples contained two oxidation states: S0 and S+VI. Ninety-five percent of the individual aerosol particles (> 1 µm) analyzed contain S0. Linear combination fitting revealed that S+VI in ambient aerosol was dominated by ammonium sulfate as well as metal sulfates. The finding of metal sulfates provides further evidence for acidic reactions that solubilize metals, such as iron, during atmospheric transport. Emission sources, including biomass burning, coal fly ash, gasoline, diesel, volcanic ash, and aerosolized Atlanta soil, and the commercially available bacterium Bacillus subtilis, contained only S+VI. A commercially available Azotobacter vinelandii sample contained approximately equal proportions of S0 and S+VI. S0 in individual aerosol particles most likely originates from primary emission sources, such as aerosolized bacteria or incomplete combustion.

In the southeastern US, substantial emissions of isoprene from deciduous trees undergo atmospheric oxidation to form secondary organic aerosol (SOA) that contributes to fine particulate matter (PM2.5). Laboratory studies have revealed that anthropogenic pollutants, such as sulfur dioxide (SO2), oxides of nitrogen (NOx), and aerosol acidity, can enhance SOA formation from the hydroxyl radical (OH)-initiated oxidation of isoprene; however, the mechanisms by which specific pollutants enhance isoprene SOA in ambient PM2.5 remain unclear. As one aspect of an investigation to examine how anthropogenic pollutants influence isoprene-derived SOA formation, high-volume PM2.5 filter samples were collected at the Birmingham, Alabama (BHM), ground site during the 2013 Southern Oxidant and Aerosol Study (SOAS). Sample extracts were analyzed by gas chromatography–electron ionization-mass spectrometry (GC/EI-MS) with prior trimethylsilylation and ultra performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-QTOFMS) to identify known isoprene SOA tracers. Tracers quantified using both surrogate and authentic standards were compared with collocated gas- and particle-phase data as well as meteorological data provided by the Southeastern Aerosol Research and Characterization (SEARCH) network to assess the impact of anthropogenic pollution on isoprene-derived SOA formation. Results of this study reveal that isoprene-derived SOA tracers contribute a substantial mass fraction of organic matter (OM) ( ∼ 7 to ∼ 20 %). Isoprene-derived SOA tracers correlated with sulfate (SO42−) (r2 = 0.34, n = 117) but not with NOx. Moderate correlations between methacrylic acid epoxide and hydroxymethyl-methyl-α-lactone (together abbreviated MAE/HMML)-derived SOA tracers with nitrate radical production (P[NO3]) (r2 = 0.57, n = 40) were observed during nighttime, suggesting a potential role of the NO3 radical in forming this SOA type. However, the nighttime correlation of these tracers with nitrogen dioxide (NO2) (r2 = 0.26, n = 40) was weaker. Ozone (O3) correlated strongly with MAE/HMML-derived tracers (r2 = 0.72, n = 30) and moderately with 2-methyltetrols (r2 = 0.34, n = 15) during daytime only, suggesting that a fraction of SOA formation could occur from isoprene ozonolysis in urban areas. No correlation was observed between aerosol pH and isoprene-derived SOA. Lack of correlation between aerosol acidity and isoprene-derived SOA is consistent with the observation that acidity is not a limiting factor for isoprene SOA formation at the BHM site as aerosols were acidic enough to promote multiphase chemistry of isoprene-derived epoxides throughout the duration of the study. All in all, these results confirm previous studies suggesting that anthropogenic pollutants enhance isoprene-derived SOA formation.

The ability of certain components of particulate matter to induce oxidative stress through the generation of reactive oxygen species (ROS) in vivo may be one mechanism accounting for observed linkages between ambient aerosols and adverse health outcomes. A variety of assays have been used to measure this so-called aerosol oxidative potential. We developed a semi-automated system to quantify oxidative potential of filter aqueous extracts utilizing the dithiothreitol (DTT) assay and report here the development of a similar semi-automated system for the ascorbic acid (AA) assay. Approximately 500 PM2.5 filter samples collected in contrasting locations in the southeastern US were analyzed for a host of aerosol species, along with AA and DTT activities. We present a detailed contrast in findings from these two assays. Water-soluble AA activity was higher in summer and fall than in winter, with highest levels near heavily trafficked highways, whereas DTT activity was higher in winter compared to summer and fall and more spatially homogeneous. AA activity was nearly exclusively correlated with water-soluble Cu (r = 0.70–0.94 at most sites), whereas DTT activity was correlated with organic and metal species. Source apportionment models, positive matrix factorization (PMF) and a chemical mass balance method with ensemble-averaged source impact profiles (CMB-E), suggest a strong contribution from traffic emissions and secondary processes (e.g., organic aerosol oxidation or metals mobilization by secondary acids) to both AA and DTT activities in urban Atlanta. In contrast, biomass burning was a large source for DTT activity, but insignificant for AA. AA activity was not correlated with PM2.5 mass, while DTT activity co-varied strongly with mass (r = 0.49–0.86 across sites and seasons). Various linear models were developed to estimate AA and DTT activities for the central Atlanta Jefferson Street site, based on the CMB-E sources. The models were then used to estimate daily oxidative potential at this site over the 1998–2009 period. Time series epidemiological analyses were conducted to assess daily emergency department (ED) visits data for the five-county Atlanta metropolitan area based on the estimated 10-year backcast oxidative potential. Estimated AA activity was not statistically associated with any tested health outcome, while DTT activity was associated with ED visits for both asthma or wheeze and congestive heart failure. The findings point to the importance of both organic components and transition metals from biomass burning and mobile sources to adverse health outcomes in this region.

Sources of methane (CH4) and nitrous oxide (N2O) were investigated using measurements from a site in southeast Bakersfield as part of the CalNex (California at the Nexus of Air Quality and Climate Change) experiment from mid-May to the end of June 2010. Typical daily minimum mixing ratios of CH4 and N2O were higher than daily minima that were simultaneously observed at a mid-oceanic background station (NOAA, Mauna Loa) by approximately 70 ppb and 0.5 ppb, respectively. Substantial enhancements of CH4 and N2O (hourly averages > 500 and > 7 ppb, respectively) were routinely observed, suggesting the presence of large regional sources. Collocated measurements of carbon monoxide (CO) and a range of volatile organic compounds (VOCs) (e.g., straight-chain and branched alkanes, cycloalkanes, chlorinated alkanes, aromatics, alcohols, isoprene, terpenes and ketones) were used with a positive matrix factorization (PMF) source apportionment method to estimate the contribution of regional sources to observed enhancements of CH4 and N2O. The PMF technique provided a "top-down" deconstruction of ambient gas-phase observations into broad source categories, yielding a seven-factor solution. We identified these emission source factors as follows: evaporative and fugitive; motor vehicles; livestock and dairy; agricultural and soil management; daytime light and temperature driven; non-vehicular urban; and nighttime terpene biogenics and anthropogenics. The dairy and livestock factor accounted for the majority of the CH4 (70–90 %) enhancements during the duration of experiments. The dairy and livestock factor was also a principal contributor to the daily enhancements of N2O (60–70 %). Agriculture and soil management accounted for ~ 20–25 % of N2O enhancements over a 24 h cycle, which is not surprising given that organic and synthetic fertilizers are known to be a major source of N2O. The N2O attribution to the agriculture and soil management factor had a high uncertainty in the conducted bootstrapping analysis. This is most likely due to an asynchronous pattern of soil-mediated N2O emissions from fertilizer usage and collocated biogenic emissions from crops from the surrounding agricultural operations that is difficult to apportion statistically when using PMF. The evaporative/fugitive source profile, which resembled a mix of petroleum operation and non-tailpipe evaporative gasoline sources, did not include a PMF resolved-CH4 contribution that was significant (< 2 %) compared to the uncertainty in the livestock-associated CH4 emissions. The uncertainty of the CH4 estimates in this source factor, derived from the bootstrapping analysis, is consistent with the ~ 3 % contribution of fugitive oil and gas emissions to the statewide CH4 inventory. The vehicle emission source factor broadly matched VOC profiles of on-road exhaust sources. This source factor had no statistically significant detected contribution to the N2O signals (confidence interval of 3 % of livestock N2O enhancements) and negligible CH4 (confidence interval of 4 % of livestock CH4 enhancements) in the presence of a dominant dairy and livestock factor. The CalNex PMF study provides a measurement-based assessment of the state CH4 and N2O inventories for the southern San Joaquin Valley (SJV). The state inventory attributes ~ 18 % of total N2O emissions to the transportation sector. Our PMF analysis directly contradicts the state inventory and demonstrates there were no discernible N2O emissions from the transportation sector in the southern SJV region.

Water-soluble redox-active metals are potentially toxic due to its ability to catalytically generate reactive oxygen species (ROS) in vivo, leading to oxidative stress. As part of the Southeastern Center for Air Pollution and Epidemiology (SCAPE), we developed a method to quantify water-soluble elements, including redox-active metals, from a large number of filter samples (N = 530) in support of the center's health studies. PM2.5 samples were collected during 2012–2013 at various sites (three urban, two rural, a near-road site, and a road-side site) in the southeastern United States, using high-volume samplers. Water-soluble elements (S, K, Ca, Ti, Mn, Fe, Cu, Zn, As, Se, Br, Sr, Ba, and Pb) were determined by extracting filters in deionized water and re-aerosolized for analyses by X-ray fluorescence (XRF) using an online aerosol element analyzer (Xact, Cooper Environmental). Concentrations ranged from detection limits (nominally 0.1 to 30 ng m−3) to 1.2 μg m−3, with S as the most abundant element, followed by Ca, K, Fe, Cu, Zn, and Ba. Positive matrix factorization (PMF) identified four factors that were associated with specific sources based on relative loadings of various tracers. These include brake/tire wear (with tracers Ba and Cu), biomass burning (K), secondary formation (S, Se, and WSOC), and mineral dust (Ca). Of the four potentially toxic and relatively abundant metals (redox-active Cu, Mn, Fe, and redox-inactive Zn), 51 % of Cu, 32 % of Fe, 17 % of Mn, and 45 % of Zn were associated with the brake/tire factor. Mn was mostly associated with the mineral dust factor (45 %). Zn was found in a mixture of factors, with 26 % associated with mineral dust, 14 % biomass burning, and 13 % secondary formation. Roughly 50 % of Fe and 40 % of Cu were apportioned to the secondary formation factor, likely through increases in the soluble fraction of these elements by sulfur-driven aerosol water and acidity. Linkages between sulfate and water-soluble Fe and Cu may account for some of the past observed associations between sulfate/sulfur oxide and health outcomes. For Cu, Mn, Fe, and Zn, only Fe was correlated with PM2.5 mass (r = 0.73–0.80). Overall, mobile source emissions generated through mechanical processes (re-entrained road dust, tire and break wear) and processing by secondary sulfate were major contributors to water-soluble metals known to be capable of generating ROS.

Chemical components of organic aerosol (OA) selectively absorb light at short wavelengths. In this study, the prevalence, sources, and optical importance of this so-called brown carbon (BrC) aerosol component are investigated throughout the North American continental tropospheric column during a summer of extensive biomass burning. Spectrophotometric absorption measurements on extracts of bulk aerosol samples collected from an aircraft over the central USA were analyzed to directly quantify BrC abundance. BrC was found to be prevalent throughout the 1 to 12 km altitude measurement range, with dramatic enhancements in biomass-burning plumes. BrC to black carbon (BC) ratios, under background tropospheric conditions, increased with altitude, consistent with a corresponding increase in the absorption Ångström exponent (AAE) determined from a three-wavelength particle soot absorption photometer (PSAP). The sum of inferred BC absorption and measured BrC absorption at 365 nm was within 3 % of the measured PSAP absorption for background conditions and 22 % for biomass burning. A radiative transfer model showed that BrC absorption reduced top-of-atmosphere (TOA) aerosol forcing by ~ 20 % in the background troposphere. Extensive radiative model simulations applying this study background tropospheric conditions provided a look-up chart for determining radiative forcing efficiencies of BrC as a function of a surface-measured BrC : BC ratio and single scattering albedo (SSA). The chart is a first attempt to provide a tool for better assessment of brown carbon's forcing effect when one is limited to only surface data. These results indicate that BrC is an important contributor to direct aerosol radiative forcing.

We deployed a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and an Aerosol Chemical Speciation Monitor (ACSM) to characterize the chemical composition of submicron non-refractory particulate matter (NR-PM$_{1}$) in the southeastern USA. Measurements were performed in both rural and urban sites in the greater Atlanta area, Georgia (GA), and Centreville, Alabama (AL), for approximately 1 year as part of Southeastern Center for Air Pollution and Epidemiology study (SCAPE) and Southern Oxidant and Aerosol Study (SOAS). Organic aerosol (OA) accounts for more than half of NR-PM1 mass concentration regardless of sampling sites and seasons. Positive matrix factorization (PMF) analysis of HR-ToF-AMS measurements identified various OA sources, depending on location and season. Hydrocarbon-like OA (HOA) and cooking OA (COA) have important, but not dominant, contributions to total OA in urban sites (i.e., 21–38 % of total OA depending on site and season). Biomass burning OA (BBOA) concentration shows a distinct seasonal variation with a larger enhancement in winter than summer. We find a good correlation between BBOA and brown carbon, indicating biomass burning is an important source for brown carbon, although an additional, unidentified brown carbon source is likely present at the rural Yorkville site. Isoprene-derived OA factor (isoprene-OA) is only deconvolved in warmer months and contributes 18–36 % of total OA. The presence of isoprene-OA factor in urban sites is more likely from local production in the presence of NOx than transport from rural sites. More-oxidized and less-oxidized oxygenated organic aerosol (MO-OOA and LO-OOA, respectively) are dominant fractions (47–79 %) of OA in all sites. MO-OOA correlates well with ozone in summer but not in winter, indicating MO-OOA sources may vary with seasons. LO-OOA, which reaches a daily maximum at night, correlates better with estimated nitrate functionality from organic nitrates than total nitrates. Based on the HR-ToF-AMS measurements, we estimate that the nitrate functionality from organic nitrates contributes 63–100 % to the total measured nitrates in summer. Furthermore, the contribution of organic nitrates to total OA is estimated to be 5–12 % in summer, suggesting that organic nitrates are important components in the ambient aerosol in the southeastern USA. The spatial distribution of OA is investigated by comparing simultaneous HR-ToF-AMS measurements with ACSM measurements at two different sampling sites. OA is found to be spatially homogeneous in summer due possibly to stagnant air mass and a dominant amount of regional secondary organic aerosol (SOA) in the southeastern USA. The homogeneity is less in winter, which is likely due to spatial variation of primary emissions. We observe that the seasonality of OA concentration shows a clear urban/rural contrast. While OA exhibits weak seasonal variation in the urban sites, its concentration is higher in summer than winter for rural sites. This observation from our year-long measurements is consistent with 14 years of organic carbon (OC) data from the SouthEastern Aerosol Research and Characterization (SEARCH) network. The comparison between short-term measurements with advanced instruments and long-term measurements of basic air quality indicators not only tests the robustness of the short-term measurements but also provides insights in interpreting long-term measurements. We find that OA factors resolved from PMF analysis on HR-ToF-AMS measurements have distinctly different diurnal variations. The compensation of OA factors with different diurnal trends is one possible reason for the repeatedly observed, relatively flat OA diurnal profile in the southeastern USA. In addition, analysis of long-term measurements shows that the correlation between OC and sulfate is substantially stronger in summer than winter. This seasonality could be partly due to the effects of sulfate on isoprene SOA formation as revealed by the short-term intensive measurements.

Particle water and pH are predicted using meteorological observations (relative humidity (RH), temperature (T)), gas/particle composition, and thermodynamic modeling (ISORROPIA-II). A comprehensive uncertainty analysis is included, and the model is validated. We investigate mass concentrations of particle water and related particle pH for ambient fine-mode aerosols sampled in a relatively remote Alabama forest during the Southern Oxidant and Aerosol Study (SOAS) in summer and at various sites in the southeastern US during different seasons, as part of the Southeastern Center for Air Pollution and Epidemiology (SCAPE) study. Particle water and pH are closely linked; pH is a measure of the particle H+ aqueous concentration and depends on both the presence of ions and amount of particle liquid water. Levels of particle water, in turn, are determined through water uptake by both the ionic species and organic compounds. Thermodynamic calculations based on measured ion concentrations can predict both pH and liquid water but may be biased since contributions of organic species to liquid water are not considered. In this study, contributions of both the inorganic and organic fractions to aerosol liquid water were considered, and predictions were in good agreement with measured liquid water based on differences in ambient and dry light scattering coefficients (prediction vs. measurement: slope = 0.91, intercept = 0.5 μg m−3, R2 = 0.75). ISORROPIA-II predictions were confirmed by good agreement between predicted and measured ammonia concentrations (slope = 1.07, intercept = −0.12 μg m−3, R2 = 0.76). Based on this study, organic species on average contributed 35% to the total water, with a substantially higher contribution (50%) at night. However, not including contributions of organic water had a minor effect on pH (changes pH by 0.15 to 0.23 units), suggesting that predicted pH without consideration of organic water could be sufficient for the purposes of aqueous secondary organic aerosol (SOA) chemistry. The mean pH predicted in the Alabama forest (SOAS) was 0.94 ± 0.59 (median 0.93). pH diurnal trends followed liquid water and were driven mainly by variability in RH; during SOAS nighttime pH was near 1.5, while daytime pH was near 0.5. pH ranged from 0.5 to 2 in summer and 1 to 3 in the winter at other sites. The systematically low pH levels in the southeast may have important ramifications, such as significantly influencing acid-catalyzed reactions, gas–aerosol partitioning, and mobilization of redox metals and minerals. Particle ion balances or molar ratios, often used to infer pH, do not consider the dissociation state of individual ions or particle liquid water levels and do not correlate with particle pH.

Given significant challenges with available measurements of aerosol acidity, proxy methods are frequently used to estimate the acidity of atmospheric particles. In this study, four of the most common aerosol acidity proxies are evaluated and compared: (1) the ion balance method, (2) the molar ratio method, (3) thermodynamic equilibrium models, and (4) the phase partitioning of ammonia. All methods are evaluated against predictions of thermodynamic models and against direct observations of aerosol–gas equilibrium partitioning acquired in Mexico City during the Megacity Initiative: Local and Global Research Objectives (MILAGRO) study. The ion balance and molar ratio methods assume that any deficit in inorganic cations relative to anions is due to the presence of H+ and that a higher H+ loading and lower cation / anion ratio both correspond to increasingly acidic particles (i.e., lower pH). Based on the MILAGRO measurements, no correlation is observed between H+ levels inferred with the ion balance and aerosol pH predicted by the thermodynamic models and NH3–NH4+ partitioning. Similarly, no relationship is observed between the cation / anion molar ratio and predicted aerosol pH. Using only measured aerosol chemical composition as inputs without any constraint for the gas phase, the E-AIM (Extended Aerosol Inorganics Model) and ISORROPIA-II thermodynamic equilibrium models tend to predict aerosol pH levels that are inconsistent with the observed NH3–NH4+ partitioning. The modeled pH values from both E-AIM and ISORROPIA-II run with gas &plus; aerosol inputs agreed well with the aerosol pH predicted by the phase partitioning of ammonia. It appears that (1) thermodynamic models constrained by gas &plus; aerosol measurements and (2) the phase partitioning of ammonia provide the best available predictions of aerosol pH. Furthermore, neither the ion balance nor the molar ratio can be used as surrogates for aerosol pH, and previously published studies with conclusions based on such acidity proxies may need to be reevaluated. Given the significance of acidity for chemical processes in the atmosphere, the implications of this study are important and far reaching.

Formic acid (HCOOH) is one of the most abundant carboxylic acids in the atmosphere. However, current photochemical models cannot fully explain observed concentrations and in particular secondary formation of formic acid across various environments. In this work, formic acid measurements made at an urban receptor site (Pasadena) in June–July 2010 during CalNex (California Research at the Nexus of Air Quality and Climate Change) and a site in an oil and gas producing region (Uintah Basin) in January–February 2013 during UBWOS 2013 (Uintah Basin Winter Ozone Studies) will be discussed. Although the VOC (volatile organic compounds) compositions differed dramatically at the two sites, measured formic acid concentrations were comparable: 2.3 ± 1.3 in UBWOS 2013 and 2.0 ± 1.0 ppb in CalNex. We determine that concentrations of formic acid at both sites were dominated by secondary formation (> 99%). A constrained box model using the Master Chemical Mechanism (MCM v3.2) underestimates the measured formic acid concentrations drastically at both sites (by a factor of > 10). Compared to the original MCM model that includes only ozonolysis of unsaturated organic compounds and OH oxidation of acetylene, when we updated yields of ozonolysis of alkenes and included OH oxidation of isoprene, vinyl alcohol chemistry, reaction of formaldehyde with HO2, oxidation of aromatics, and reaction of CH3O2 with OH, the model predictions for formic acid were improved by a factor of 6.4 in UBWOS 2013 and 4.5 in CalNex, respectively. A comparison of measured and modeled HCOOH/acetone ratios is used to evaluate the model performance for formic acid. We conclude that the modified chemical mechanism can explain 19 and 45% of secondary formation of formic acid in UBWOS 2013 and CalNex, respectively. The contributions from aqueous reactions in aerosol and heterogeneous reactions on aerosol surface to formic acid are estimated to be 0–6 and 0–5% in UBWOS 2013 and CalNex, respectively. We observe that air–snow exchange processes and morning fog events may also contribute to ambient formic acid concentrations during UBWOS 2013 (~ 20% in total). In total, 53–59 in UBWOS 2013 and 50–55% in CalNex of secondary formation of formic acid remains unexplained. More work on formic acid formation pathways is needed to reduce the uncertainties in the sources and budget of formic acid and to narrow the gaps between measurements and model results.

A variety of methods are used to measure the capability of particulate matter (PM) to catalytically generate reactive oxygen species (ROS) in vivo, also defined as the aerosol oxidative potential. A widely used measure of aerosol oxidative potential is the dithiothreitol (DTT) assay, which monitors the depletion of DTT (a surrogate for cellular antioxidants) as catalyzed by the redox-active species in PM. However, a major constraint in the routine use of the DTT assay for integrating it with large-scale health studies is its labor-intensive and time-consuming protocol. To specifically address this concern, we have developed a semi-automated system for quantifying the oxidative potential of aerosol liquid extracts using the DTT assay. The system, capable of unattended analysis at one sample per hour, has a high analytical precision (coefficient of variation of 15% for positive control, 4% for ambient samples) and reasonably low limit of detection (0.31 nmol min−1). Comparison of the automated approach with the manual method conducted on ambient samples yielded good agreement (slope = 1.08 ± 0.12, r2 = 0.92, N = 9). The system was utilized for the Southeastern Center for Air Pollution & Epidemiology (SCAPE) to generate an extensive data set on DTT activity of ambient particles collected from contrasting environments (urban, roadside, and rural) in the southeastern US. We find that water-soluble PM2.5 DTT activity on a per-air-volume basis was spatially uniform and often well correlated with PM2.5 mass (r = 0.49 to 0.88), suggesting regional sources contributing to the PM oxidative potential in the southeastern US. The correlation may also suggest a mechanistic explanation (oxidative stress) for observed PM2.5 mass-health associations. The heterogeneity in the intrinsic DTT activity (per-PM-mass basis) across seasons indicates variability in the DTT activity associated with aerosols from sources that vary with season. Although developed for the DTT assay, the instrument can also be used to determine oxidative potential with other acellular assays.

We assess the potential of the water-soluble fraction of atmospheric fine aerosols in the southeastern United States to generate reactive oxygen species (ROS) and identify major ROS-associated emission sources. ROS-generation potential of particles was quantified by the dithiothreitol (DTT) assay and involved analysis of fine particulate matter (PM) extracted from high-volume quartz filters (23 h integrated samples) collected at various sites in different environmental settings in the southeast, including three urban-Atlanta sites, in addition to a rural site. Paired sampling was conducted with one fixed site in Atlanta (Jefferson Street), representative of the urban environment, with the others rotating among different sites, for ~250 days between June 2012 and September 2013 (N=483). A simple linear regression between the DTT activity and aerosol chemical components revealed strong associations between PM ROS-generation potential and secondary organic aerosol (WSOC – water-soluble organic carbon) in summer, and biomass burning markers in winter. Redox-active metals were also somewhat correlated with the DTT activity, but mostly at urban and roadside sites. Positive matrix factorization (PMF) was applied to apportion the relative contribution of various sources to the ROS-generation potential of water-soluble PM2.5 in urban Atlanta. PMF showed that vehicular emissions contribute uniformly throughout the year (12–25%), while secondary oxidation processes dominated the DTT activity in summer (46%) and biomass burning in winter (47%). Road dust was significant only during drier periods (~12% in summer and fall). Source apportionment by chemical mass balance (CMB) was reasonably consistent with PMF, but with higher contribution from vehicular emissions (32%). Given the spatially large data set of PM sampled over an extended period, the study reconciles the results from previous work that showed only region- or season-specific aerosol components or sources contributing to PM ROS activity, possibly due to smaller sample sizes. The ubiquitous nature of the major sources of PM-associated ROS suggests widespread population exposures to aerosol components that have the ability to catalyze the production of oxidants in vivo.

We report measurements of ambient amines and ammonia with a fast response chemical ionization mass spectrometer (CIMS) in a southeastern US forest and a moderately polluted midwestern site during the summer. At the forest site, mostly C3-amines (from pptv to tens of pptv) and ammonia (up to 2 ppbv) were detected, and they both showed temperature dependencies. Aerosol-phase amines measured thermal-desorption chemical ionization mass spectrometer (TDCIMS) showed a higher mass fraction in the evening with cooler temperatures and lower in the afternoon with warmer temperatures, a trend opposite to the gas-phase amines. Concentrations of aerosol-phase primary amines measured with Fourier transform infrared spectroscopy (FTIR) from micron and submicron particles were 2 orders of magnitude higher than the gas-phase amines. These results indicate that gas to particle conversion is one of the major processes that control the ambient amine concentrations at this forest site. Temperature dependencies of C3-amines and ammonia also imply reversible processes of evaporation of these nitrogen-containing compounds from soil surfaces in daytime and deposition to soil surfaces at nighttime. During the transported biomass burning plume events, various amines (C1–C6) appeared at the pptv level, indicating that biomass burning is a substantial source of amines in the southeastern US. At the moderately polluted Kent site, there were higher concentrations of C1- to C6-amines (pptv to tens of pptv) and ammonia (up to 6 ppbv). C1- to C3-amines and ammonia were well correlated with the ambient temperature. C4- to C6-amines showed frequent spikes during the nighttime, suggesting that they were emitted from local sources. These abundant amines and ammonia may in part explain the frequent new particle formation events reported from Kent. Higher amine concentrations measured at the polluted site than at the rural forested site highlight the importance of constraining anthropogenic emission sources of amines.

We present in situ measurements of particle-phase liquid water. Measurements were conducted from 3 June to 15 July 2013 during the Southern Oxidant and Aerosol Study (SOAS) in the southeastern US. The region is photochemically active, humid, dominated by biogenic emissions, impacted by anthropogenic pollution, and known to contain high concentrations of organic aerosol mass. Measurements characterized mobility number size distributions of ambient atmospheric aerosols in three states: unperturbed, dry, and dry-humidified. Unperturbed measurements describe the aerosol distribution at ambient temperature and relative humidity. For the dry state, the sample was routed through a cold trap upstream of the inlet then reheated, while for the dry-humidified state the sample was rehumidified after drying. The total volume of water and semi-volatile compounds lost during drying was quantified by differencing dry and unperturbed volumes from the integrated size spectra, while semi-volatile volumes lost during drying were quantified differencing unperturbed and dry-humidified volumes. Results indicate that particle-phase liquid water was always present. Throughout the SOAS campaign, median water mass concentrations at the relative humidity (RH) encountered in the instrument typically ranged from 1 to 5 μg m−3 but were as high as 73 μg m−3. On non-raining days, morning time (06:00–09:00) median mass concentrations exceeded 15 μg m−3. Hygroscopic growth factors followed a diel cycle and exceed 2 from 07:00 to 09:00 local time. The hygroscopicity parameter kappa ranged from 0.14 to 0.46 and hygroscopicity increased with increasing particle size. An observed diel cycle in kappa is consistent with changes in aerosol inorganic content and a dependency of the hygroscopicity parameter on water content. Unperturbed and dry-humidified aerosol volumes did not result in statistically discernible differences, demonstrating that drying did not lead to large losses in dry particle volume. We anticipate that our results will help improve the representation of aerosol water content and aqueous-phase-mediated partitioning of atmospheric water-soluble gases in photochemical models.

Light absorbing organic carbon, often called brown carbon, has the potential to significantly contribute to the visible light-absorption budget, particularly at shorter wavelengths. Currently, the relative contributions of particulate brown carbon to light absorption, as well as the sources of brown carbon, are poorly understood. With this in mind size-resolved direct measurements of brown carbon were made at both urban (Atlanta), and rural (Yorkville) sites in Georgia. Measurements in Atlanta were made at both a representative urban site and a road-side site adjacent to a main highway. Fine particle absorption was measured with a multi-angle absorption photometer (MAAP) and seven-wavelength Aethalometer, and brown carbon absorption was estimated based on Mie calculations using direct size-resolved measurements of chromophores in solvents. Size-resolved samples were collected using a cascade impactor and analyzed for water-soluble organic carbon (WSOC), organic and elemental carbon (OC and EC), and solution light-absorption spectra of water and methanol extracts. Methanol extracts were more light-absorbing than water extracts for all size ranges and wavelengths. Absorption refractive indices of the organic extracts were calculated from solution measurements for a range of wavelengths and used with Mie theory to predict the light absorption by fine particles comprised of these components, under the assumption that brown carbon and other aerosol components were externally mixed. For all three sites, chromophores were predominately in the accumulation mode with an aerodynamic mean diameter of 0.5 μm, an optically effective size range resulting in predicted particle light absorption being a factor of 2 higher than bulk solution absorption. Mie-predicted brown carbon absorption at 350 nm contributed a significant fraction (20 to 40%) relative to total light absorption, with the highest contributions at the rural site where organic to elemental carbon ratios were highest. Brown carbon absorption, however, was highest by the roadside site due to vehicle emissions. The direct size-resolved measurement of brown carbon in solution definitively shows that it is present and optically important in the near-UV range in both a rural and urban environment during the summer when biomass burning emissions are low. These results allow estimates of brown carbon aerosol absorption from direct measurements of chromophores in aerosol extracts.

Biomass burning, the largest global source of elemental carbon (EC) and primary organic carbon (OC), is strongly associated with many subjects of great scientific concern, such as secondary organic aerosol and brown carbon which exert important effects on the environment and on climate in particular. This study investigated the relationships between levoglucosan and other biomass burning tracers (i.e., water soluble potassium and mannosan) based on both ambient samples collected in Beijing and source samples. Compared with North America and Europe, Beijing was characterized by high ambient levoglucosan concentrations and low winter to summer ratios of levoglucosan, indicating significant impact of biomass burning activities throughout the year in Beijing. Comparison of levoglucosan and water soluble potassium (K+) levels suggested that it was acceptable to use K+ as a biomass burning tracer during summer in Beijing, while the contribution of fireworks to K+ could be significant during winter. Moreover, the levoglucosan to K+ ratio was found to be lower during the typical summer period (0.21 ± 0.16) compared with the typical winter period (0.51 ± 0.15). Levoglucosan correlated strongly with mannosan (R2 = 0.97) throughout the winter and the levoglucosan to mannosan ratio averaged 9.49 ± 1.63, whereas levoglucosan and mannosan exhibited relatively weak correlation (R2 = 0.73) during the typical summer period when the levoglucosan to mannosan ratio averaged 12.65 ± 3.38. Results from positive matrix factorization (PMF) model analysis showed that about 50% of the OC and EC in Beijing were associated with biomass burning processes. In addition, a new source identification method was developed based on the comparison of the levoglucosan to K+ ratio and the levoglucosan to mannosan ratio among different types of biomass. Using this method, the major source of biomass burning aerosol in Beijing was suggested to be the combustion of crop residuals, while the contribution from softwood burning was also non-negligible, especially in winter.

An online, semi-continuous instrument to measure fine particle (PM2.5) reactive oxygen species (ROS) was developed based on the fluorescent probe 2'7'-dichlorofluorescin (DCFH). Parameters that influence probe response were first characterized to develop an optimal method for use in a field instrument. The online method used a mist chamber scrubber to collect total (gas plus particle) ROS components (ROSt) alternating with gas phase ROS (ROSg) by means of an inline filter. Particle phase ROS (ROSp) was determined by the difference between ROSt and ROSg. The instrument was deployed in urban Atlanta, Georgia, USA, and at a rural site during various seasons. Concentrations from the online instrument generally agreed well with those from an intensive filter measurement of ROSp. Concentrations of the ROSp measurements made with this instrument were lower than reported in other studies, often below the instrument's average limit of detection (0.15 nmol H2O2 equivalents m−3). Mean ROSp concentrations were 0.26 nmol H2O2 equivalents m−3 at the Atlanta urban sites compared to 0.14 nmol H2O2 equivalents m−3 at the rural site.

The NASA DC-8 aircraft characterized the aerosol properties, chemical composition, and cloud condensation nuclei (CCN) concentrations of the summertime Arctic during the 2008 NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign. Air masses characteristic of fresh and aged biomass burning, boreal forest, Arctic background, and anthropogenic industrial pollution were sampled. Observations were spatially extensive (50–85° N and 40–130° W) and exhibit significant variability in aerosol and CCN concentrations. The chemical composition was dominated by highly oxidized organics (66–94% by volume), with a water-soluble mass fraction of more than 50%. The aerosol hygroscopicity parameter, κ, ranged between κ = 0.08–0.32 for all air mass types. Industrial pollution had the lowest κ of 0.08 ± 0.01, while the Arctic background had the highest and most variable κ of 0.32 ± 0.21, resulting from a lower and more variable organic fraction. Both fresh and aged (long-range transported) biomass burning air masses exhibited remarkably similar κ (0.18 ± 0.13), consistent with observed rapid chemical and physical aging of smoke emissions in the atmosphere, even in the vicinity of fresh fires. The organic hygroscopicity (κorg) was parameterized by the volume fraction of water-soluble organic matter (εWSOM), with a κ = 0.12, such that κorg = 0.12εWSOM. Assuming bulk (size-independent) composition and including the κorg parameterization enabled CCN predictions to within 30% accuracy for nearly all environments sampled. The only exception was for industrial pollution from Canadian oil sands exploration, where an external mixture and size-dependent composition was required. Aerosol mixing state assumptions (internal vs. external) in all other environments did not significantly affect CCN predictions; however, the external mixing assumption provided the best results, even though the available observations could not determine the true degree of external mixing and therefore may not always be representative of the environments sampled. No correlation was observed between κorg and O : C. A novel correction of the CCN instrument supersaturation for water vapor depletion, resulting from high concentrations of CCN, was also employed. This correction was especially important for fresh biomass burning plumes where concentrations exceeded 1.5×104 cm−3 and introduced supersaturation depletions of &geq;25%. Not accounting for supersaturation depletion in these high concentration environments would therefore bias CCN closure up to 25% and inferred κ by up to 50%.

Aerosol composition and mixing state near anthropogenic sources can be highly variable and can challenge predictions of cloud condensation nuclei (CCN). The impacts of chemical composition on CCN activation kinetics is also an important, but largely unknown, aspect of cloud droplet formation. Towards this, we present in-situ size-resolved CCN measurements carried out during the 2008 summertime August Mini Intensive Gas and Aerosol Study (AMIGAS) campaign in Atlanta, GA. Aerosol chemical composition was measured by two particle-into-liquid samplers measuring water-soluble inorganic ions and total water-soluble organic carbon. Size-resolved CCN data were collected using the Scanning Mobility CCN Analysis (SMCA) method and were used to obtain characteristic aerosol hygroscopicity distributions, whose breadth reflects the aerosol compositional variability and mixing state. Knowledge of aerosol mixing state is important for accurate predictions of CCN concentrations and that the influence of an externally-mixed, CCN-active aerosol fraction varies with size from 31% for particle diameters less than 40 nm to 93% for accumulation mode aerosol during the day. Assuming size-dependent aerosol mixing state and size-invariant chemical composition decreases the average CCN concentration overprediction (for all but one mixing state and chemical composition scenario considered) from over 190–240% to less than 20%. CCN activity is parameterized using a single hygroscopicity parameter, κ, which averages to 0.16 ± 0.07 for 80 nm particles and exhibits considerable variability (from 0.03 to 0.48) throughout the study period. Particles in the 60–100 nm range exhibited similar hygroscopicity, with a κ range for 60 nm between 0.06–0.076 (mean of 0.18 ± 0.09). Smaller particles (40 nm) had on average greater κ, with a range of 0.20–0.92 (mean of 0.3 ± 0.12). Analysis of the droplet activation kinetics of the aerosol sampled suggests that most of the CCN activate as rapidly as calibration aerosol, suggesting that aerosol composition exhibits a minor (if any) impact on CCN activation kinetics.

We present simultaneous fast, in-situ measurements of formaldehyde and glyoxal from two rural campaigns, BEARPEX 2009 and BEACHON-ROCS, both located in Pinus Ponderosa forests with emissions dominated by biogenic volatile organic compounds (VOCs). Despite considerable variability in the formaldehyde and glyoxal concentrations, the ratio of glyoxal to formaldehyde, RGF, displayed a very regular diurnal cycle over nearly 2 weeks of measurements. The only deviations in RGF were toward higher values and were the result of a biomass burning event during BEARPEX 2009 and very fresh anthropogenic influence during BEACHON-ROCS. Other rapid changes in glyoxal and formaldehyde concentrations have hardly any affect on RGF and could reflect transitions between low and high NO regimes. The trend of increased RGF from both anthropogenic reactive VOC mixtures and biomass burning compared to biogenic reactive VOC mixtures is robust due to the short timescales over which the observed changes in RGF occurred. Satellite retrievals, which suggest higher RGF for biogenic areas, are in contrast to our observed trends. It remains important to address this discrepancy, especially in view of the importance of satellite retrievals and in situ measurements for model comparison. In addition, we propose that RGF represents a useful metric for biogenic or anthropogenic reactive VOC mixtures and, in combination with absolute concentrations of glyoxal and formaldehyde, furthermore represents a useful metric for the extent of anthropogenic influence on overall reactive VOC processing via NOx. In particular, RGF yields information about not simply the VOCs dominating reactivity in an airmass, but the VOC processing itself that is directly coupled to ozone and secondary organic aerosol production.

Secondary organic aerosol (SOA) in the southeastern US is investigated by analyzing the spatial-temporal distribution of water-soluble organic carbon (WSOC) and other PM2.5 components from 900 archived 24-h Teflon filters collected at 15 urban or rural EPA Federal Reference Method (FRM) network sites throughout 2007. Online measurements of WSOC at an urban/rural-paired site in Georgia in the summer of 2008 are contrasted to the filter data. Based on FRM filters, excluding biomass-burning events (levoglucosan < 50 ng m−3), WSOC and sulfate were highly correlated with PM2.5 mass (r2~0.7). Both components comprised a large mass fraction of PM2.5 (13% and 31%, respectively, or ~25% and 50% for WSOM and ammonium sulfate). Sulfate and WSOC both tracked ambient temperature throughout the year, suggesting the temperature effects were mainly linked to faster photochemistry and/or synoptic meteorology and less due to enhanced biogenic hydrocarbon emissions. FRM WSOC, and to a lesser extent sulfate, were spatially homogeneous throughout the region, yet WSOC was moderately enhanced (27%) in locations of greater predicted isoprene emissions in summer. A Positive Matrix Factorization (PMF) analysis identified two major source types for the summer WSOC; 22% of the WSOC were associated with ammonium sulfate, and 56% of the WSOC were associated with brown carbon and oxalate. A small urban excess of FRM WSOC (10%) was observed in the summer of 2007, however, comparisons of online WSOC measurements at one urban/rural pair (Atlanta/Yorkville) in August 2008 showed substantially greater difference in WSOC (31%) relative to the FRM data, suggesting a low bias for urban filters. The measured Atlanta urban excess, combined with the estimated boundary layer heights, gave an estimated Atlanta daily WSOC production rate in August of 0.55 mgC m−2 h−1 between mid-morning and mid-afternoon. This study characterizes the regional nature of fine particles in the southeastern US, confirming the importance of SOA and the roles of both biogenic and anthropogenic emissions.

Soluble iron in fine atmospheric particles has been identified as a public health concern by participating in reactions that generate reactive oxygen species (ROS). The mineralogy and oxidation state (speciation) of iron have been shown to influence fractional iron solubility (soluble iron/total iron). In this study, iron speciation was determined in single particles at urban and rural sites in Georgia USA using synchrotron-based techniques, such as X-ray Absorption Near-Edge Structure (XANES) spectroscopy and microscopic X-ray fluorescence measurements. Soluble and total iron content (soluble + insoluble iron) of these samples was measured using spectrophotometry and synchrotron-based techniques, respectively. These bulk measurements were combined with synchrotron-based measurements to investigate the relationship between iron speciation and fractional iron solubility in ambient aerosols. XANES measurements indicate that iron in the single particles was present as a mixture of Fe(II) and Fe(III), with Fe(II) content generally between 5 and 35% (mean: ~25%). XANES and elemental analyses (e.g. elemental molar ratios of single particles based on microscopic X-ray fluorescence measurements) indicate that a majority (74%) of iron-containing particles are best characterized as Al-substituted Fe-oxides, with a Fe/Al molar ratio of 4.9. The next most abundant group of particles (12%) was Fe-aluminosilicates, with Si/Al molar ratio of 1.4. No correlation was found between fractional iron solubility (soluble iron/total iron) and the abundance of Al-substituted Fe-oxides and Fe-aluminosilicates present in single particles at any of the sites during different seasons, suggesting solubility largely depended on factors other than differences in major iron phases.

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.

This paper compares measurements of gaseous and particulate emissions from a wide range of biomass-burning plumes intercepted by the NASA DC-8 research aircraft during the three phases of the ARCTAS-2008 experiment: ARCTAS-A, based out of Fairbanks, Alaska, USA (3 April to 19 April 2008); ARCTAS-B based out of Cold Lake, Alberta, Canada (29 June to 13 July 2008); and ARCTAS-CARB, based out of Palmdale, California, USA (18 June to 24 June 2008). Approximately 500 smoke plumes from biomass burning emissions that varied in age from minutes to days were segregated by fire source region and urban emission influences. The normalized excess mixing ratios (NEMR) of gaseous (carbon dioxide, acetonitrile, hydrogen cyanide, toluene, benzene, methane, oxides of nitrogen and ozone) and fine aerosol particulate components (nitrate, sulfate, ammonium, chloride, organic aerosols and water soluble organic carbon) of these plumes were compared. A detailed statistical analysis of the different plume categories for different gaseous and aerosol species is presented in this paper. The comparison of NEMR values showed that CH4 concentrations were higher in air-masses that were influenced by urban emissions. Fresh biomass burning plumes mixed with urban emissions showed a higher degree of oxidative processing in comparison with fresh biomass burning only plumes. This was evident in higher concentrations of inorganic aerosol components such as sulfate, nitrate and ammonium, but not reflected in the organic components. Lower NOx NEMRs combined with high sulfate, nitrate and ammonium NEMRs in aerosols of plumes subject to long-range transport, when comparing all plume categories, provided evidence of advanced processing of these plumes.

Multiple state-of-the-art instruments sampled ambient aerosol in Riverside, California during the 2005 Study of Organic Aerosols at Riverside (SOAR) to investigate the chemical composition and potential sources of fine particles (PMf) in the inland region of Southern California. In this paper, we briefly summarize the spatial, meteorological and gas-phase conditions during SOAR-1 (15 July–15 August), provide detailed intercomparisons of high-resolution aerosol mass spectrometer (HR-AMS) measurements against complementary measurements, and report the average composition of PMf including the composition of the organic fraction measured by the HR-AMS. Daily meteorology and gas-phase species concentrations were highly consistent, displaying clear diurnal cycles and weekday/weekend contrast. HR-AMS measurements of non-refractory submicron (NR-PM1) mass are consistent and highly correlated with those from a filter dynamics measurement system tapered-element oscillating microbalance (TEOM), while the correlation between HR-AMS and heated TEOM measurements is lower due to loss of high volatility species including ammonium nitrate from the heated TEOM. Speciated HR-AMS measurements are also consistent with complementary measurements as well as with measurements from a collocated compact AMS while HR-AMS OC is similar to standard semi-continuous Sunset measurements within the combined uncertainties of both instruments. A correction intended to account for the loss of semi-volatile OC from the Sunset, however, yields measurements ~30% higher than either HR-AMS or standard Sunset measurements. On average, organic aerosol (OA) was the single largest component of PMf. OA composition was investigated using both elemental analysis and positive matrix factorization (PMF) of HR-AMS OA spectra. Oxygen is the main heteroatom during SOAR-1, with O/C exhibiting a diurnal minimum of 0.28 during the morning rush hour and maximum of 0.42 during the afternoon. O/C is broadly anti-correlated with H/C, while N/C and S/C (excluding organonitrate (ON) and organosulfate (OS) functionalities) are far lower than O/C at about 0.015 and ~0.001, respectively. When ON and OS estimates are included O/C, N/C, and S/C increase by factors of 1.21, 2, and 30, respectively, while H/C changes are insignificant. The increase in these ratios implies that ON accounts for ~1/2 of the organic nitrogen while OS dominate organic sulfur at this location. Accounting for the estimated ON and OS also improves the agreement between anions and cations measured by HR-AMS by ~8%, while amines have only a very small impact (1%) on this balance. Finally, a number of primary and secondary OA components were resolved by PMF. Among these a hydrocarbon-like OA and two minor, local OA components, one of which was associated with amines, were attributed to primary emissions and contributed a minor fraction (~20%) of OA mass. The remaining OA mass was attributed to a number of secondary oxidized OA (OOA) components including the previously-identified low-volatility and semi-volatile OOA components. In addition, we also report for the first time the presence of two additional OOA components.

Biomass burning (BB) is a large source of primary and secondary organic aerosols (POA and SOA). This study addresses the physical and chemical evolution of BB organic aerosols. Firstly, the evolution and lifetime of BB POA and SOA signatures observed with the Aerodyne Aerosol Mass Spectrometer are investigated, focusing on measurements at high-latitudes acquired during the 2008 NASA ARCTAS mission, in comparison to data from other field studies and from laboratory aging experiments. The parameter f60, the ratio of the integrated signal at m/z 60 to the total signal in the organic component mass spectrum, is used as a marker to study the rate of oxidation and fate of the BB POA. A background level of f60~0.3% ± 0.06% for SOA-dominated ambient OA is shown to be an appropriate background level for this tracer. Using also f44 as a tracer for SOA and aged POA and a surrogate of organic O:C, a novel graphical method is presented to characterise the aging of BB plumes. Similar trends of decreasing f60 and increasing f44 with aging are observed in most field and lab studies. At least some very aged BB plumes retain a clear f60 signature. A statistically significant difference in f60 between highly-oxygenated OA of BB and non-BB origin is observed using this tracer, consistent with a substantial contribution of BBOA to the springtime Arctic aerosol burden in 2008. Secondly, a summary is presented of results on the net enhancement of OA with aging of BB plumes, which shows large variability. The estimates of net OA gain range from ΔOA/ΔCO(mass) = −0.01 to ~0.05, with a mean ΔOA/POA ~19%. With these ratios and global inventories of BB CO and POA a global net OA source due to aging of BB plumes of ~8 ± 7 Tg OA yr−1 is estimated, of the order of 5 % of recent total OA source estimates. Further field data following BB plume advection should be a focus of future research in order to better constrain this potentially important contribution to the OA burden.