Predicting Atmospheric Water-Soluble Organic Mass Reversibly Partitioned to Aerosol Liquid Water in the Eastern United States.

Water-soluble organic matter (WSOM) formed through aqueous processes contributes substantially to total atmospheric aerosol, however, the impact of water evaporation on particle concentrations is highly uncertain. Herein, we present a novel approach to predict the amount of evaporated organic mass induced by sample drying using multivariate polynomial regression and random forest (RF) machine learning models. The impact of particle drying on fine WSOM was monitored during three consecutive summers in Baltimore, MD (2015, 2016, and 2017). The amount of evaporated organic mass was dependent on relative humidity (RH), WSOM concentrations, isoprene concentrations, and NOx/isoprene ratios. Different models corresponding to each class were fitted (trained and tested) to data from the summers of 2015 and 2016 while model validation was performed using summer 2017 data. Using the coefficient of determination (R2) and the root-mean-square error (RMSE), it was concluded that an RF model with 100 decision trees had the best performance (R2 of 0.81) and the lowest normalized mean error (NME < 1%) leading to low model uncertainties. The relative feature importance for the RF model was calculated to be 0.55, 0.2, 0.15, and 0.1 for WSOM concentrations, RH levels, isoprene concentrations, and NOx/isoprene ratios, respectively. The machine learning model was thus used to predict summertime concentrations of evaporated organics in Yorkville, Georgia, and Centerville, Alabama in 2016 and 2013, respectively. Results presented herein have implications for measurements that rely on sample drying using a machine learning approach for the analysis and interpretation of atmospheric data sets to elucidate their complex behavior.

Measurement artifacts in the dithiothreitol (DTT) oxidative potential assay caused by interactions between aqueous metals and phosphate buffer.

Metals in particulate matter (PM) are hypothesized to have enhanced toxicity based on their ability to catalyze reactive oxygen species (ROS) formation. Acellular assays are used to measure the oxidative potential (OP) of PM and its individual components. Many OP assays, including the dithiothreitol (DTT) assay, use a phosphate buffer matrix to simulate biological conditions (pH 7.4 and 37 °C). Prior work from our group observed transition metal precipitation in the DTT assay, consistent with thermodynamic equilibria. In this study, we characterized the effects of metal precipitation on OP measured by the DTT assay. Metal precipitation was affected by aqueous metal concentrations, ionic strength, and phosphate concentrations in ambient PM sampled in Baltimore, MD and a standard PM sample (NIST SRM-1648a, Urban Particulate Matter). Critically, differences in metal precipitation induced differing OP responses of the DTT assay as a function of phosphate concentration in all PM samples analyzed. These results indicate that comparison of DTT assay results obtained at differing phosphate buffer concentrations is highly problematic. Further, these results have implications for other chemical and biological assays that use phosphate buffer for pH control and their use to infer PM toxicity.

Aqueous processing of water-soluble organic compounds in the eastern United States during winter.

Aqueous multi-phase processes are significant contributors to organic aerosol (OA) mass in the atmosphere. This study characterizes the formation of water-soluble organic matter during the winter in the eastern United States through simultaneous measurements of water-soluble organic carbon in the gas and particle phases (WSOCg and WSOCp, respectively). The formation of secondary WSOCp occurred primarily through two pathways: (1) absorptive partitioning of oxygenated organics to the bulk OA and (2) aqueous phase processes. WSOCp formation through the former pathway was evident through the relationship between the fraction of total WSOC in the particle phase (Fp) and the total OA concentration. Conversely, evidence for nighttime aqueous WSOCp formation was based upon the strong enhancement in Fp with increasing relative humidity, indicating the uptake of WSOCg to aerosol liquid water (ALW). The Fp-RH relationship was only observed for temperatures between 0-10 °C, suggesting conditions for aqueous multi-phase processes were enhanced during these times. Temperature exhibited an inverse relationship with ALW and a proportional relationship with aerosol potassium. ALW and biomass burning precursors were both abundant in the 0-10 °C temperature range, facilitating aqueous WSOCp formation. To assess the impact of particle drying on the WSOCp concentrations, the particle measurements alternated between ambient and dried channels. No change was observed in the concentration of particles before and after drying, indicating that the WSOCp formed through the uptake of WSOCg into OA and ALW remained in the condensed phase upon particle drying at all temperature ranges. This work contributes to our understanding of sources, pathways, and factors affecting aqueous aerosol formation in the winter.

Precipitation of aqueous transition metals in particulate matter during the dithiothreitol (DTT) oxidative potential assay.

Transition metals in particulate matter (PM) are hypothesized to have enhanced toxicity based on their oxidative potential (OP). The acellular dithiothreitol (DTT) assay is widely used to measure the OP of PM and its chemical components. In our prior study, we showed that the DTT assay (pH 7.4, 0.1 M phosphate buffer, 37 °C) provides favorable thermodynamic conditions for precipitation of multiple metals present in PM. This study utilizes multiple techniques to characterize the precipitation of aqueous metals present at low concentrations in the DTT assay. Metal precipitation was identified using laser particle light scattering analysis, direct chemical measurement of aqueous metal removal, and microscopic imaging. Experiments were run with aqueous metals from individual metal salts and a well-characterized urban PM standard (NIST SRM-1648a, Urban Particulate Matter). Our results demonstrated rapid precipitation of metals in the DTT assay. Metal precipitation was independent of DTT but dependent on metal concentration. Metal removal in the chemically complex urban PM samples exceeded the thermodynamic predictions and removal seen in single metal salt experiments, suggesting co-precipitation and/or adsorption may have occurred. These results have broad implications for other acellular assays that study PM metals using phosphate buffer, and subsequently, the PM toxicity inferred from these assays.

Fate of transition metals in PO4-based in vitro assays: equilibrium modeling and macroscopic studies.

Transition metals are thought to be among the most toxic components in atmospheric particulate matter (PM) due to their role in catalyzing reactive oxygen species (ROS) formation. We show that precipitation of the transition metals Fe(ii), Fe(iii), and Mn(ii) are thermodynamically favored in phosphate-based assays used to measure the oxidative potential (OP) - a surrogate for toxicity - of PM. Fe and Mn precipitation is likely to occur at extremely low metal concentrations (<0.5 µM), levels that are imperceptible to the naked eye. The concentration of each metal (other than Cu) in aqueous PM filter extracts often exceeds the solubility limit in OP assays, indicating favorable thermodynamic conditions for precipitation. Macroscopic experimental results at higher metal concentrations (>100 µM) with visible precipitates provide quasi-validation of the thermodynamic modeling. Oxidation of Fe(ii) to Fe(iii) is likely to be rapid in all in vitro OP assays, transforming Fe to a much less soluble form. Fe precipitates are likely to increase the rate of precipitation of other metals and possibly induce co-precipitation. These results have direct relevance for all PO4-based assays; the implications for studies of PM toxicity are discussed.

Mulitphase Atmospheric Chemistry in Liquid Water: Impacts and Controllability of Organic Aerosol.

Liquid water is a dominant and critical tropospheric constituent. Over polluted land masses low level cumulus clouds interact with boundary layer aerosol. The planetary boundary layer (PBL) is the lowest atmospheric layer and is directly influenced by Earth's surface. Water-aerosol interactions are critical to processes that govern the fate and transport of trace species in the Earth system and their impacts on air quality, radiative forcing, and regional hydrological cycling. In the PBL, air parcels rise adiabatically from the surface, and anthropogenically influenced hygroscopic aerosols take up water and serve as cloud condensation nuclei (CCN) to form clouds. Water-soluble gases partition to liquid water in wet aerosols and cloud droplets and undergo aqueous-phase photochemistry. Most cloud droplets evaporate, and low volatility material formed during aqueous phase chemistry remains in the condensed phase and adds to aerosol mass. The resulting cloud-processed aerosol has different physicochemical properties compared to the original CCN. Organic species that undergo multiphase chemistry in atmospheric liquid water transform gases to highly concentrated, nonideal ionic aqueous solutions and form secondary organic aerosol (SOA). In recent years, SOA formation modulated by atmospheric waters has received considerable interest.Key uncertainties are related to the chemical nature of hygroscopic aerosols that become CCN and their interaction with organic species. Gas-to-droplet or gas-to-aqueous aerosol partitioning of organic compounds is affected by the intrinsic chemical properties of the organic species in addition to the pre-existing condensed phase. Environmentally relevant conditions for atmospheric aerosol are nonideal. Salt identity and concentration, in addition to aerosol phase state, can dramatically affect organic gas miscibility for many compounds, in particular when ionic strength and salt molality are outside the bounds of limiting laws. For example, Henry's law and Debye-Hückel theory are valid only for dilute aqueous systems uncharacteristic of real atmospheric conditions. Chemical theory is incomplete, and at ambient conditions, this chemistry plays a determining role in total aerosol mass and particle size, controlling factors for air quality and climate-relevant aerosol properties.Accurate predictive skill to understand the impacts of societal choices and policies on air quality and climate requires that models contain correct chemical mechanisms and appropriate feedbacks. Globally, SOA is a dominant contributor to the atmospheric organic aerosol burden, and most mass can be traced back to precursor gas-phase volatile organic compounds (VOCs) emitted from the biosphere. However, organic aerosol concentrations in the Amazon Rainforest, the largest emitter of biogenic VOCs, are generally lower than in U.S. national parks. The Interagency Monitoring of Protected Visual Environments (IMPROVE) air quality network, with sites located predominantly in national parks, provides the longest continuous record of organic aerosol measurements in the U.S. Analysis of IMPROVE data provides a useful chemical climatology of changing air resources in response to environmental rules and shifting economic trends. IMPROVE data provides an excellent test bed for case studies to assess model skill to accurately predict changes in organic aerosol concentrations in the context of a changing climate.

No evidence for brown carbon formation in ambient particles undergoing atmospherically relevant drying.

Recent laboratory studies have reported the formation of light-absorbing organic carbon compounds (brown carbon, BrC) in particles undergoing drying. Atmospheric particles undergo cycles of humidification and drying during vertical transport and through daily variations in temperature and humidity, which implies particle drying could potentially be an important source of BrC globally. In this work, we investigated BrC formation in ambient particles undergoing drying at a site in the eastern United States during summer. Aerosol BrC concentrations were linked to secondary organic aerosol (SOA) formation, consistent with seasonal expectations for this region. Measurements of water-soluble organic aerosol concentrations and light absorption (365 nm) were alternated between an unperturbed channel and a channel that dried particles to 41% or 35% relative humidity (RH), depending on the system configuration. The RH maintained in the dry channels was below most ambient RH levels observed throughout the study. We did not observe BrC formation in particles that were dried to either RH level. The results were consistent across two summers, spanning ∼5 weeks of measurements that included a wide range of RH conditions and organic and inorganic aerosol loadings. This work suggests that mechanisms aside from humidification-drying cycles are more important contributors to ambient particle BrC loadings. The implications of this work on the atmospheric budget of BrC are discussed.

The Acidity of Atmospheric Particles and Clouds.

Acidity, defined as pH, is a central component of aqueous chemistry. In the atmosphere, the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semi-volatile gases such as HNO3, NH3, HCl, and organic acids and bases as well as chemical reaction rates. It has implications for the atmospheric lifetime of pollutants, deposition, and human health. Despite its fundamental role in atmospheric processes, only recently has this field seen a growth in the number of studies on particle acidity. Even with this growth, many fine particle pH estimates must be based on thermodynamic model calculations since no operational techniques exist for direct measurements. Current information indicates acidic fine particles are ubiquitous, but observationally-constrained pH estimates are limited in spatial and temporal coverage. Clouds and fogs are also generally acidic, but to a lesser degree than particles, and have a range of pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient ammonia. Historical measurements indicate that cloud and fog droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for aerosol particles indicates acidity may be relatively constant due to the semi-volatile nature of the key acids and bases and buffering in particles. This paper reviews and synthesizes the current state of knowledge on the acidity of atmospheric condensed phases, specifically particles and cloud droplets. It includes recommendations for estimating acidity and pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale.

Additional Benefits of Federal Air-Quality Rules: Model Estimates of Controllable Biogenic Secondary Organic Aerosol.

Atmospheric models that accurately describe the fate and transport of trace species for the right reasons aid in the development of effective air-quality management strategies that safeguard human health. Controllable emissions facilitate the formation of biogenic secondary organic aerosol (BSOA) to enhance the atmospheric fine particulate matter (PM2.5) burden. Previous modeling with the EPA's Community Multiscale Air Quality (CMAQ) model predicted that anthropogenic primary organic aerosol (POA) emissions had the greatest impact on BSOA. That experiment included formation processes involving semivolatile partitioning but not aerosol liquid water (ALW), a ubiquitous PM constituent. We conduct 17 summertime CMAQ simulations with updated chemistry and evaluate changes in BSOA due to the removal of individual pollutants and source sectors for the contiguous U.S. CMAQ predicts SO2 from electricity generating units, and mobile source NOX emissions have the largest impacts on BSOA. The removal of anthropogenic NOX, SO2, and POA emissions during the simulation reduces the nationally averaged BSOA by 23, 14, and 8% and PM2.5 by 9.2, 14, and 5.3%, respectively. ALW mass concentrations decrease by 10 and 35% in response to the removal of NOX and SO2 emissions. This work contributes chemical insight into ancillary benefits of Federal NOX and SO2 rules that concurrently reduce organic PM2.5 mass.

The effects of isoprene and NO x on secondary organic aerosols formed through reversible and irreversible uptake to aerosol water.

Isoprene oxidation produces water-soluble organic gases capable of partitioning to aerosol liquid water. The formation of secondary organic aerosols through such aqueous pathways (aqSOA) can take place either reversibly or irreversibly; however, the split between these fractions in the atmosphere is highly uncertain. The aim of this study was to characterize the reversibility of aqSOA formed from isoprene at a location in the eastern United States under substantial influence from both anthropogenic and biogenic emissions. The reversible and irreversible uptake of water-soluble organic gases to aerosol water was characterized in Baltimore, Maryland, USA, using measurements of particulate water-soluble organic carbon (WSOCp) in alternating dry and ambient configurations. WSOCp evaporation with drying was observed systematically throughout the late spring and summer, indicating reversible aqSOA formation during these times. We show through time lag analyses that WSOCp concentrations, including the WSOCp that evaporates with drying, peak 6 to 11h after isoprene concentrations, with maxima at a time lag of 9h. The absolute reversible aqSOA concentrations, as well as the relative amount of reversible aqSOA, increased with decreasing NO x /isoprene ratios, suggesting that isoprene epoxydiol (IEPOX) or other low-NO x oxidation products may be responsible for these effects. The observed relationships with NO x and isoprene suggest that this process occurs widely in the atmosphere, and is likely more important in other locations characterized by higher isoprene and/or lower NO x levels. This work underscores the importance of accounting for both reversible and irreversible uptake of isoprene oxidation products to aqueous particles.

Effect of the Urban Heat Island on Aerosol pH.

The urban heat island (UHI) is a widely observed phenomenon whereby urban environments have higher temperatures and different relative humidities than surrounding suburban and rural areas. Temperature (T) and relative humidity (RH) strongly affect the partitioning of semivolatile species found in the atmosphere, such as nitric acid, ammonia, and water. These species are inherently tied to aerosol pH, which is a key parameter driving some atmospheric chemical processes and environmental effects of aerosols. In this study, we characterized the effect of the UHI on aerosol pH in Baltimore, MD, and Chicago, IL. The T and RH differences that define the UHI lead to substantial differences in aerosol liquid water (ALW) content. The ALW differences produce urban aerosol pH that is systematically lower (more acidic) than rural aerosol pH for identical atmospheric composition. The UHI in Baltimore and Chicago are most intense during the summer and at night, with urban-rural aerosol pH differences in excess of 0.8 and 0.65 pH units, respectively. The UHI has been observed in cities of all sizes: the similarity of our results for cities with different climatologies and aerosol compositions suggests that these results have broad implications for chemistry occurring in and around urban atmospheres globally.

Intermediate Volatility Organic Compound Emissions from On-Road Gasoline Vehicles and Small Off-Road Gasoline Engines.

Dynamometer experiments were conducted to characterize the intermediate volatility organic compound (IVOC) emissions from a fleet of on-road gasoline vehicles and small off-road gasoline engines. IVOCs were quantified through gas chromatography/mass spectrometry analysis of adsorbent samples collected from a constant volume sampler. The dominant fraction (>80%, on average) of IVOCs could not be resolved on a molecular level. These unspeciated IVOCs were quantified as two chemical classes (unspeciated branched alkanes and cyclic compounds) in 11 retention-time-based bins. IVOC emission factors (mg kg-fuel(-1)) from on-road vehicles varied widely from vehicle to vehicle, but showed a general trend of lower emissions for newer vehicles that met more stringent emission standards. IVOC emission factors for 2-stroke off-road engines were substantially higher than 4-stroke off-road engines and on-road vehicles. Despite large variations in the magnitude of emissions, the IVOC volatility distribution and chemical characteristics were consistent across all tests and IVOC emissions were strongly correlated with nonmethane hydrocarbons (NMHCs), primary organic aerosol and speciated IVOCs. Although IVOC emissions only correspond to approximately 4% of NMHC emissions from on-road vehicles over the cold-start unified cycle, they are estimated to produce as much or more SOA than single-ring aromatics. Our results clearly demonstrate that IVOCs from gasoline engines are an important class of SOA precursors and provide observational constraints on IVOC emission factors and chemical composition to facilitate their inclusion into atmospheric chemistry models.

Drying-Induced Evaporation of Secondary Organic Aerosol during Summer.

This study characterized the effect of drying on the concentration of atmospheric secondary organic aerosol (SOA). Simultaneous measurements of water-soluble organic carbon in the gas (WSOCg) and particle (WSOCp) phases were carried out in Baltimore, MD during the summertime. To investigate the effect of drying on SOA, the WSOCp measurement was alternated through an ambient channel (WSOCp) and a "dried" channel (WSOCp,dry) maintained at ∼35% relative humidity (RH). The average mass ratio between WSOCp,dry and WSOCp was 0.85, showing that significant evaporation of the organic aerosol occurred due to drying. The average amount of evaporated water-soluble organic matter (WSOM = WSOC × 1.95) was 0.6 µg m(-3); however, the maximum evaporated WSOM concentration exceeded 5 µg m(-3), demonstrating the importance of this phenomenon. The systematic difference between ambient and dry channels indicates a significant and persistent source of aqueous SOA formed through reversible uptake processes. The wide-ranging implications of the work are discussed, and include: new insight into atmospheric SOA formation; impacts on particle measurement techniques; a newly identified bias in PM2.5 measurements using the EPA's Federal Reference and Equivalent Methods (FRM and FEM); atmospheric model evaluations; and the challenge in relating ground-based measurements to remote sensing of aerosol properties.

Intermediate Volatility Organic Compound Emissions from On-Road Diesel Vehicles: Chemical Composition, Emission Factors, and Estimated Secondary Organic Aerosol Production.

Emissions of intermediate-volatility organic compounds (IVOCs) from five on-road diesel vehicles and one off-road diesel engine were characterized during dynamometer testing. The testing evaluated the effects of driving cycles, fuel composition and exhaust aftertreatment devices. On average, more than 90% of the IVOC emissions were not identified on a molecular basis, instead appearing as an unresolved complex mixture (UCM) during gas-chromatography mass-spectrometry analysis. Fuel-based emissions factors (EFs) of total IVOCs (speciated + unspeciated) depend strongly on aftertreatment technology and driving cycle. Total-IVOC emissions from vehicles equipped with catalyzed diesel particulate filters (DPF) are substantially lower (factor of 7 to 28, depending on driving cycle) than from vehicles without any exhaust aftertreatment. Total-IVOC emissions from creep and idle operations are substantially higher than emissions from high-speed operations. Although the magnitude of the total-IVOC emissions can vary widely, there is little variation in the IVOC composition across the set of tests. The new emissions data are combined with published yield data to investigate secondary organic aerosol (SOA) formation. SOA production from unspeciated IVOCs is estimated using surrogate compounds, which are assigned based on gas-chromatograph retention time and mass spectral signature of the IVOC UCM. IVOCs contribute the vast majority of the SOA formed from exhaust from on-road diesel vehicles. The estimated SOA production is greater than predictions by previous studies and substantially higher than primary organic aerosol. Catalyzed DPFs substantially reduce SOA formation potential of diesel exhaust, except at low speed operations.

Intermediate-volatility organic compounds: a large source of secondary organic aerosol.

Secondary organic aerosol (SOA) is a major component of atmospheric fine particle mass. Intermediate-volatility organic compounds (IVOCs) have been proposed to be an important source of SOA. We present a comprehensive analysis of atmospheric IVOC concentrations and their SOA production using measurements made in Pasadena, California during the California at the Nexus of Air Quality and Climate Change (CalNex) study. The campaign-average concentration of primary IVOCs was 6.3 ± 1.9 µg m(-3) (average ± standard deviation), which is comparable to the concentration of organic aerosol but only 7.4 ± 1.2% of the concentration of speciated volatile organic compounds. Only 8.6 ± 2.2% of the mass of the primary IVOCs was speciated. Almost no weekend/weekday variation in the ambient concentration of both speciated and total primary IVOCs was observed, suggesting that petroleum-related sources other than on-road diesel vehicles contribute substantially to the IVOC emissions. Primary IVOCs are estimated to produce about 30% of newly formed SOA in the afternoon during CalNex, about 5 times that from single-ring aromatics. The importance of IVOCs in SOA formation is expected to be similar in many urban environments.

Unspeciated organic emissions from combustion sources and their influence on the secondary organic aerosol budget in the United States.

Secondary organic aerosol (SOA) formed from the atmospheric oxidation of nonmethane organic gases (NMOG) is a major contributor to atmospheric aerosol mass. Emissions and smog chamber experiments were performed to investigate SOA formation from gasoline vehicles, diesel vehicles, and biomass burning. About 10-20% of NMOG emissions from these major combustion sources are not routinely speciated and therefore are currently misclassified in emission inventories and chemical transport models. The smog chamber data demonstrate that this misclassification biases model predictions of SOA production low because the unspeciated NMOG produce more SOA per unit mass than the speciated NMOG. We present new source-specific SOA yield parameterizations for these unspeciated emissions. These parameterizations and associated source profiles are designed for implementation in chemical transport models. Box model calculations using these new parameterizations predict that NMOG emissions from the top six combustion sources form 0.7 Tg y(-1) of first-generation SOA in the United States, almost 90% of which is from biomass burning and gasoline vehicles. About 85% of this SOA comes from unspeciated NMOG, demonstrating that chemical transport models need improved treatment of combustion emissions to accurately predict ambient SOA concentrations.

Gas-particle partitioning of primary organic aerosol emissions: (2) diesel vehicles.

Experiments were performed to investigate the gas-particle partitioning of primary organic aerosol (POA) emissions from two medium-duty (MDDV) and three heavy-duty (HDDV) diesel vehicles. Each test was conducted on a chassis dynamometer with the entire exhaust sampled into a constant volume sampler (CVS). The vehicles were operated over a range of driving cycles (transient, high-speed, creep/idle) on different ultralow sulfur diesel fuels with varying aromatic content. Four independent yet complementary approaches were used to investigate POA gas-particle partitioning: artifact correction of quartz filter samples, dilution from the CVS into a portable environmental chamber, heating in a thermodenuder, and thermal desorption/gas chromatography/mass spectrometry (TD-GC-MS) analysis of quartz filter samples. During tests of vehicles not equipped with diesel particulate filters (DPF), POA concentrations inside the CVS were a factor of 10 greater than ambient levels, which created large and systematic partitioning biases in the emissions data. For low-emitting DPF-equipped vehicles, as much as 90% of the POA collected on a quartz filter from the CVS were adsorbed vapors. Although the POA emission factors varied by more than an order of magnitude across the set of test vehicles, the measured gas-particle partitioning of all emissions can be predicted using a single volatility distribution derived from TD-GC-MS analysis of quartz filters. This distribution is designed to be applied directly to quartz filter data that are the basis for existing emissions inventories and chemical transport models that have implemented the volatility basis set approach.

Volatility of organic molecular markers used for source apportionment analysis: measurements and implications for atmospheric lifetime.

Molecular markers are organic species used to define fingerprints for source apportionment of ambient fine particulate matter. Traditionally, these markers have been assumed to be stable in the atmosphere. This work investigates the gas-particle partitioning of eight organic species used as molecular markers in receptor models for biomass burning (levoglucosan), motor vehicles (5α-cholestane, n-hexacosane, n-triacontane, 1,2-benz[a]anthracene, coronene), and meat cooking (cholesterol, oleic acid). Experiments were conducted using a thermodenuder to measure the evaporation of single component particles. The data were analyzed using the integrated volume method to determine saturation concentrations and enthalpies of vaporization for each compound. The results indicate that appreciable quantities (>10%) of most of these markers exist in the gas phase under typical atmospheric conditions. Therefore, these species should be considered semivolatile. Predictions from a chemical kinetics model indicate that gas-particle partitioning has important effects on the atmospheric lifetime of these species. The atmospheric decay of semivolatile compounds proceeds much more rapidly than nonvolatile compounds because gas-phase oxidation induces evaporation of particle-phase material. Therefore, both gas-particle partitioning and chemical reactions need to be accounted for when semivolatile molecular markers are used for source apportionment studies.

Photo-oxidation of low-volatility organics found in motor vehicle emissions: production and chemical evolution of organic aerosol mass.

Recent research has proposed that low-volatility organic vapors are an important class of secondary organic aerosol (SOA) precursors. Mixtures of low-volatility organics were photo-oxidized in a smog chamber under low- and high-NO(x) conditions. Separate experiments addressed emission surrogates (diesel fuel and motor oil) and single components (n-pentacosane). Both diesel fuel and motor oil are major components of exhaust from diesel engines. Diesel fuel is a complex mixture of intermediate volatility organic compounds (IVOCs), whereas motor oil is a complex mixture of semivolatile organic compounds (SVOCs). IVOCs exist exclusively in the vapor phase, while SVOCs exist in both the aerosol and vapor phase. Oxidation of SVOC vapors (motor oil and n-pentacosane) creates substantial SOA, but this SOA is largely offset by evaporation of primary organic aerosol (POA). The net effect is a cycling or pumping of SVOCs between the gas and particle phases, which creates more oxygenated organic aerosol (OA) but little new OA mass. Since gas-phase reactions are much faster than heterogeneous ones, the processing of SVOC vapors likely contributes to the production of highly oxidized OA. The interplay between gas-particle partitioning and chemistry also blurs traditional definitions of POA and SOA. Photo-oxidation of diesel fuel (IVOCs) rapidly creates substantial new OA mass, similar to published aging experiments with dilute diesel exhaust. However, aerosol mass spectrometer (AMS) data indicated that the SOA formed from emission surrogates is less oxidized than either the oxygenated organic aerosol (OOA) measured in the atmosphere or SOA formed from the photo-oxidation of dilute diesel exhaust. Therefore, photo-oxidation of IVOCs helps explain the substantial SOA mass produced from aging diesel exhaust, but some component is missing from these emission surrogate experiments that leads to the rapid production of highly oxygenated SOA.

Effective rate constants and uptake coefficients for the reactions of organic molecular markers (n-alkanes, hopanes, and steranes) in motor oil and diesel primary organic aerosols with hydroxyl radicals.

Hydroxyl radical (OH) uptake by organic aerosols, followed by heterogeneous oxidation, happens nearly at the collision frequency. Oxidation complicates the use of organic molecular markers such as hopanes for source apportionment, since receptor models assume markers are stable during transport. We report the oxidation kinetics of organic molecular markers (C(25)-C(32) n-alkanes, hopanes and steranes) in motor oil and primary organic aerosol emitted from a diesel engine at atmospherically relevant conditions inside a smog chamber. A thermal desorption aerosol gas chromatograph/mass spectrometer (TAG) and Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) were used to measure the changes in molecular comosition and bulk primary organic aerosol. From the measured changes in molecular composition, we calculated effective OH rate constants, effective relative rate constants, and effective uptake coefficients for molecular markers. Oxidation rates varied with marker volatility, with more volatile markers being oxidized at rates much faster than could be explained from heterogeneous oxidation. This rapid oxidation can be explained by significant gas-phase OH oxidation that dominates heterogeneous oxidation, resulting in overall oxidation lifetimes of 1 day or less. Based on our results, neglecting oxidation of molecular markers used for source apportionment could introduce significant error, since many common markers such as norhopane appear to be semivolatile under atmospheric conditions.