Aqueous-phase Secondary Organic Aerosol Research Articles

Modeling Novel Aqueous Particle and Cloud Chemistry Processes of Biomass Burning Phenols and Their Potential to Form Secondary Organic Aerosols.

Phenols emitted from biomass burning contribute significantly to secondary organic aerosol (SOA) formation through the partitioning of semivolatile products formed from gas-phase chemistry and multiphase chemistry in aerosol liquid water and clouds. The aqueous-phase SOA (aqSOA) formed via hydroxyl radical (•OH), singlet molecular oxygen (1O2*), and triplet excited states of organic compounds (3C*), which oxidize dissolved phenols in the aqueous phase, might play a significant role in the evolution of organic aerosol (OA). However, a quantitative and predictive understanding of aqSOA has been challenging. Here, we develop a stand-alone box model to investigate the formation of SOA from gas-phase •OH chemistry and aqSOA formed by the dissolution of phenols followed by their aqueous-phase reactions with •OH, 1O2*, and 3C* in cloud droplets and aerosol liquid water. We investigate four phenolic compounds, i.e., phenol, guaiacol, syringol, and guaiacyl acetone (GA), which represent some of the key potential sources of aqSOA from biomass burning in clouds. For the same initial precursor organic gas that dissolves in aerosol/cloud liquid water and subsequently reacts with aqueous phase oxidants, we predict that the aqSOA formation potential (defined as aqSOA formed per unit dissolved organic gas concentration) of these phenols is higher than that of isoprene-epoxydiol (IEPOX), a well-known aqSOA precursor. Cloud droplets can dissolve a broader range of soluble phenols compared to aqueous aerosols, since the liquid water contents of aerosols are orders of magnitude smaller than cloud droplets. Our simulations suggest that highly soluble and reactive multifunctional phenols like GA would predominantly undergo cloud chemistry within cloud layers, while gas-phase chemistry is likely to be more important for less soluble phenols. But in the absence of clouds, the condensation of low-volatility products from gas-phase oxidation followed by their reversible partitioning to organic aerosols dominates SOA formation, while the SOA formed through aqueous aerosol chemistry increases with relative humidity (RH), approaching 40% of the sum of gas and aqueous aerosol chemistry at 95% RH for GA. Our model developments of biomass-burning phenols and their aqueous chemistry can be readily implemented in regional and global atmospheric chemistry models to investigate the aqueous aerosol and cloud chemistry of biomass-burning organic gases in the atmosphere.

Direct formation of HONO through aqueous-phase photolysis of organic nitrates

Abstract. Organic nitrates (RONO2) are secondary compounds whose fate is closely related to the transport and removal of NOx in the atmosphere. Despite their ubiquitous presence in submicron aerosols, the photochemistry of RONO2 has only been investigated in the gas phase, leaving their reactivity in condensed phases poorly explored. This work aims to address this gap by investigating, for the first time, the reaction products and the mechanisms of aqueous-phase photolysis of four RONO2 (i.e., isopropyl nitrate, isobutyl nitrate, α-nitrooxy acetone, and 1-nitrooxy-2-propanol). The results show that the reactivity of RONO2 in the aqueous phase differs significantly from that in the gas phase. In contrast to the gas phase, where RONO2 release NOx upon photolysis, the aqueous-phase photolysis of RONO2 leads primarily to the direct formation of nitrous acid (HONO or HNO2), which was confirmed by quantum chemistry calculations. Hence, the aqueous-phase photolysis of RONO2 represents both a NOx sink and a source of atmospheric nitrous acid, a significant precursor of ⋅ OH and ⋅ NO. These secondary radicals (⋅ OH and ⋅ NO) are efficiently trapped in the aqueous phase, leading to the formation of HNO3 and functionalized RONO2. This reactivity can thus potentially contribute to the aging of secondary organic aerosol (SOA) and serves as an additional source of aqueous-phase SOA.

Changes in primary and secondary aerosols during a controlled Chinese New Year

Large reductions in anthropogenic emissions during the Chinese New Year (CNY) holiday in Beijing have been well reported. However, the changes during the CNY of 2021 are different because most people stayed in Beijing to control the spread of coronavirus disease (COVID-19). Here a high-resolution aerosol mass spectrometer (HR-AMS) was deployed for characterization of the changes in size-resolved aerosol composition and sources during the CNY. We found that the reductions in traffic-related NOx and fossil fuel-related organic aerosol (OA), and cooking OA (1.3–12.7%) during the CNY of 2021 were much smaller than those in previous CNY holidays of 2013, 2015, and 2020. In contrast, the mass concentrations of secondary aerosol species except nitrate showed ubiquitous increases (17.6–30.4%) during the CNY of 2021 mainly due to a 4-day severe haze episode. OA composition also changed substantially during the CNY of 2021. In particular, we observed a large increase by nearly a factor of 2 in oxidized primary OA likely from biomass burning, and a decrease of 50.1% in aqueous-phase secondary OA. A further analysis of the severe haze episode during the CNY illustrated a rapid transition of secondary formation from photochemical to aqueous-phase processing followed by a scavenging process, leading to significant changes in aerosol composition, size distributions, and oxidation degree of OA. A parameterization relationship between oxygen-to-carbon (O/C) and f44 (fraction of m/z 44 in OA) from a collocated capture vaporizer aerosol chemical speciation monitor (CV-ACSM) was developed, which has a significant implication for characterization of OA evolution and the impacts on hygroscopicity due to the rapidly increased deployments of CV-ACSM worldwide.

The effect of clouds and precipitation on the aerosol concentrations and composition in a boreal forest environment

Abstract. Atmospheric aerosol particle concentrations are strongly affected by various wet processes, including below and in-cloud wet scavenging and in-cloud aqueous-phase oxidation. We studied how wet scavenging and cloud processes affect particle concentrations and composition during transport to a rural boreal forest site in northern Europe. For this investigation, we employed air mass history analysis and observational data. Long-term particle number size distribution (∼15 years) and composition measurements (∼8 years) were combined with air mass trajectories with relevant variables from reanalysis data. Some such variables were rainfall rate, relative humidity, and mixing layer height. Additional observational datasets, such as temperature and trace gases, helped further evaluate wet processes along trajectories with mixed effects models. All chemical species investigated (sulfate, black carbon, and organics) exponentially decreased in particle mass concentration as a function of accumulated precipitation along the air mass route. In sulfate (SO4) aerosols, clear seasonal differences in wet removal emerged, whereas organics (Org) and equivalent black carbon (eBC) exhibited only minor differences. The removal efficiency varied slightly among the different reanalysis datasets (ERA-Interim and Global Data Assimilation System; GDAS) used for the trajectory calculations due to the difference in the average occurrence of precipitation events along the air mass trajectories between the reanalysis datasets. Aqueous-phase processes were investigated by using a proxy for air masses travelling inside clouds. We compared air masses with no experience of approximated in-cloud conditions or precipitation during the past 24 h to air masses recently inside non-precipitating clouds before they entered SMEAR II (Station for Measuring Ecosystem–Atmosphere Relations). Significant increases in SO4 mass concentration were observed for the latter air masses (recently experienced non-precipitating clouds). Our mixed effects model considered other contributing factors affecting particle mass concentrations in SMEAR II: examples were trace gases, local meteorology, and diurnal variation. This model also indicated in-cloud SO4 production. Despite the reanalysis dataset used in the trajectory calculations, aqueous-phase SO4 formation was observed. Particle number size distribution measurements revealed that most of the in-cloud SO4 formed can be attributed to particle sizes larger than 200 nm (electrical mobility diameter). Aqueous-phase secondary organic aerosol (aqSOA) formation was non-significant.

Impacts of marine organic emissions on low-level stratiform clouds – a large eddy simulator study

Abstract. The goal of this study is to investigate the role of organic aerosols emitted with sea spray or formed from marine gas phase emissions of volatile organic compounds (VOCs) in influencing the stability of stratiform marine clouds. We aim to point out the processes and drivers that could be relevant for global climate and should thus be considered in large-scale models. We employ a large eddy simulator coupled with an aerosol–cloud microphysical model together with different parameterizations for emission of sea salt, primary organic aerosol, and VOCs from sea surface and formation of secondary organic aerosol (SOA), to simulate the conditions of the second Dynamics and Chemistry of Marine Stratocumulus observational campaign characterized by low-level stratocumulus clouds transitioning from closed cells to drizzling open cell structure. We find that the inclusion of sea spray emissions can both extend and shorten the transitioning timescale between closed and open cells based on the parameterization employed. Fine sea spray provides extra cloud condensation nuclei (CCN) and delays the onset of drizzle as the collision–coalescence process is slowed down due to smaller cloud droplet mean size. The coarse mode has an opposite effect due to giant CCN (GCCN) speeding up the drizzle formation through the enhanced collision–coalescence processes. The balance between two processes depends on the model parameterization employed. Compared to differences between different sea spray parameterizations, the sensitivity of the clouds to the variations in organic fraction of sea spray and hygroscopicity of the emitted particles is relatively limited. However, our results show that it is important to account for the size dependence of the sea spray organic fraction as attributing organic emissions to coarse mode noticeably reduces the GCCN effect. In addition, including the secondary organic aerosol formation from VOCs can potentially have a noticeable impact, but only when emitting the highest observed fluxes of monoterpenes. This impact is also highly sensitive on the size distribution of the background aerosol population. SOA production from isoprene is visible only if aqueous phase SOA production pathways are included, and even then, the effect is lower than from monoterpenes.

The importance of hydroxymethanesulfonate (HMS) in winter haze episodes in North China Plain

Hydroxymethanesulfonate (HMS), a key marker species of aqueous-phase processing, plays a significant role in sulfur budget in atmosphere. Here we have a comprehensive characterization of HMS at urban and rural sites in North China Plain (NCP) by using the real-time measurements from a high-resolution aerosol mass spectrometer (AMS) and a single-particle AMS together with offline filter analysis. Our results showed much higher winter concentration of HMS at the rural site (average±1σ: 2.58 ± 2.56 μg m−3) than that (1.70 ± 2.68 μg m−3) in Beijing due to the more frequent fog events, low particle acidity and high concentration of precursors. The HMS on average contributed 6.3% and 5.2% to organic aerosol (OA), and 16% and 12% to the total particulate sulfur, at the rural and urban sites, respectively. HMS was highly correlated with aqueous-phase secondary OA and sulfate, and its contribution to the total particulate sulfur increased significantly as a function of relative humidity demonstrating the effective HMS production from aqueous-phase processing. Single-particle analysis showed that HMS-containing particles were mainly mixed with amine-related compounds. In addition, we found that organosulfur compounds (OS) estimated from sulfur-containing fragments of AMS correlated well with HMS at both urban and rural sites. While OS at the rural site was dominated by HMS, other types of OS were also important in urban area. The high HMS also affected the estimation of particle acidity using the AMS measured and predicted ammonium, particularly during severe haze episodes. Overall, our results demonstrated the importance of HMS in winter in NCP, and it could be more important in total particulate sulfur budget as the continuous decrease in sulfate in the future.

Atmospheric Chemistry of Allylic Radicals from Isoprene: A Successive Cyclization-Driven Autoxidation Mechanism.

The atmospheric chemistry of isoprene has broad implications for regional air quality and the global climate. Allylic radicals, taking 13-17% yield in the isoprene oxidation by •Cl, can contribute as much as 3.6-4.9% to all possible formed intermediates in local regions at daytime. Considering the large quantity of isoprene emission, the chemistry of the allylic radicals is therefore highly desirable. Here, we investigated the atmospheric oxidation mechanism of the allylic radicals using quantum chemical calculations and kinetics modeling. The results indicate that the allylic radicals can barrierlessly combine with O2 to form peroxy radicals (RO2•). Under ≤100 ppt NO and ≤50 ppt HO2• conditions, the formed RO2• mainly undergo two times "successive cyclization and O2 addition" to finally form the product fragments 2-alkoxy-acetaldehyde (C2H3O2•) and 3-hydroperoxy-2-oxopropanal (C3H4O4). The presented reaction illustrates a novel successive cyclization-driven autoxidation mechanism. The formed 3-hydroperoxy-2-oxopropanal product is a new isomer of the atmospheric C3H4O4 family and a potential aqueous-phase secondary organic aerosol precursor. Under >100 ppt NO condition, NO can mediate the cyclization-driven autoxidation process to form C5H7NO3, C5H7NO7, and alkoxy radical-related products. The proposed novel autoxidation mechanism advances our current understanding of the atmospheric chemistry of both isoprene and RO2•.

Diol it up: The influence of NaCl on methylglyoxal surface adsorption and hydration state at the air-water interface.

Methylglyoxal (MG)-an atmospherically important α-dicarbonyl implicated in aqueous-phase secondary organic aerosol formation-is known to be surface-active. Due to the presence of carbonyl moieties, MG can hydrate to form geminal diols in solution. Recently, it has been shown that MG exists predominantly as a monohydrate at the neat air-water interface. However, inorganic aerosol constituents have the potential to "salt-out" MG to the interface, shift its hydration equilibria, and catalyze self- and cross-oligomerization reactions. Here, we study the influence of the non-reactive salt, sodium chloride (NaCl), on the MG's surface adsorption and hydration state using vibrational sum frequency spectroscopy. The presence of NaCl is found to enhance MG's surface activity but not to the extent that water is fully excluded from the interface. Perturbations in the interfacial water structure are attributed to shifts in MG's hydration equilibrium at higher ionic strengths. Evidence of surface-active MG oligomer species is presented, but such oligomers are not thought to contribute significantly to the interfacial population. This work builds on the published studies on MG in pure water and gives insight into the interface's perturbation by NaCl, which has important implications for understanding MG's atmospheric fate.

Photochemical Aqueous-Phase Reactions Induce Rapid Daytime Formation of Oxygenated Organic Aerosol on the North China Plain.

Secondary organic aerosol (SOA) constitutes a large fraction of organic aerosol worldwide, however, the formation mechanisms in polluted environments remain poorly understood. Here we observed fast daytime growth of oxygenated organic aerosol (OOA) (with formation rates up to 10 μg m-3 h-1) during low relative humidity (RH, daytime average 38 ± 19%), high RH (53 ± 19%), and fog periods (77 ± 13%, fog occurring during nighttime with RH reaching 100%). Evidence showed that photochemical aqueous-phase SOA (aqSOA) formation dominantly contributed to daytime OOA formation during the periods with nighttime fog, while both photochemical aqSOA and gas-phase SOA (gasSOA) formation were important during other periods with the former contributing more under high RH and the latter under low RH conditions, respectively. Compared to daytime photochemical aqSOA production, dark aqSOA formation was only observed during the fog period and contributed negligibly to the increase in OOA concentrations due to fog scavenging processes. The rapid daytime aging, as indicated by the rapid decrease in m,p-xylene/ethylbenzene ratios, promoted the daytime formation of precursors for aqSOA formation, e.g., carbonyls such as methylglyoxal. Photooxidants related to aqSOA formation such as OH radical and H2O2 also bear fast daytime growth features even under low solar radiative conditions. The simultaneous increases in ultraviolet radiation, photooxidant, and aqSOA precursor levels worked together to promote the daytime photochemical aqSOA formation. We also found that biomass burning emissions can promote photochemical aqSOA formation by adding to the levels of aqueous-phase photooxidants and aqSOA precursors. Therefore, future mitigation of air pollution in a polluted environment would benefit from stricter control on biomass burning especially under high RH conditions.

Aircraft observations of water-soluble dicarboxylic acids in the aerosols over China

Abstract. Vertical profiles of dicarboxylic acids, related organic compounds and secondary organic aerosol (SOA) tracer compounds in particle phase have not yet been simultaneously explored in East Asia, although there is growing evidence that aqueous-phase oxidation of volatile organic compounds may be responsible for the elevated organic aerosols (OA) in the troposphere. Here, we found consistently good correlation of oxalic acid, the most abundant individual organic compounds in aerosols globally, with its precursors as well as biogenic-derived SOA compounds in Chinese tropospheric aerosols by aircraft measurements. Anthropogenically derived dicarboxylic acids (i.e., C5 and C6 diacids) at high altitudes were 4–20 times higher than those from surface measurements and even occasionally dominant over oxalic acid at altitudes higher than 2 km, which is in contrast to the predominance of oxalic acid previously reported globally including the tropospheric and surface aerosols. This indicates an enhancement of tropospheric SOA formation from anthropogenic precursors. Furthermore, oxalic acid-to-sulfate ratio maximized at altitudes of ∼ 2 km, explaining aqueous-phase SOA production that was supported by good correlations with predicted liquid water content, organic carbon and biogenic SOA tracers. These results demonstrate that elevated oxalic acid and related SOA compounds from both the anthropogenic and biogenic sources may substantially contribute to tropospheric OA burden over polluted regions of China, implying aerosol-associated climate effects and intercontinental transport.

Aqueous-phase oxidation of green leaf volatiles by hydroxyl radical as a source of SOA: Product identification from methyl jasmonate and methyl salicylate oxidation

Green leaf volatiles (GLVs) are a group of biogenic volatile organic compounds (BVOCs) released into the atmosphere by vegetation. BVOCs produce secondary organic aerosol (SOA) via gas-phase reactions, but little is known of their aqueous-phase oxidation as a source of SOA. GLVs can partition into atmospheric water phases, e.g., fog, mist, dew or rain, and be oxidized by hydroxyl radicals (˙OH). These reactions in the liquid phase also lead to products that have higher molecular weights, increased polarity, and lower vapor pressures, ultimately forming SOA after evaporation of the droplet. To examine this process, we investigated the aqueous, ˙OH-mediated oxidation of methyl jasmonate (MeJa) and methyl salicylate (MeSa), two GLVs that produce aqueous-phase SOA. High performance liquid chromatography/electrospray ionization mass spectrometry (HPLC-ESI-MS) was used to monitor product formation. The oxidation products identified exhibit higher molecular mass than their parent GLV due to either dimerization or the addition of oxygen and hydroxyl functional groups. The proposed structures of potential products are based on mechanistic considerations combined with the HPLC/ESI-MS data. Based on the structures, the vapor pressure and the Henry's law constant were estimated with multiple methods (SPARC, SIMPOL, MPBPVP, Bond and Group Estimations). The estimated vapor pressures of the products identified are significantly (up to 7 orders of magnitude) lower than those of the associated parent compounds, and therefore, the GLV oxidation products may remain as SOA after evaporation of the water droplet. The contribution of the identified oxidation products to SOA formation is estimated based on measured HPLC-ESI/MS responses relative to previous aqueous SOA mass yield measurements.

FT-IR quantification of the carbonyl functional group in aqueous-phase secondary organic aerosol from phenols

Recent findings suggest that secondary organic aerosols (SOA) formed from aqueous-phase reactions of some organic species, including phenols, contribute significantly to particulate mass in the atmosphere. In this study, we employ a Fourier transform infrared (FT-IR) spectroscopic technique to identify and quantify the functional group makeup of phenolic SOA. Solutions containing an oxidant (hydroxyl radical or 3,4-dimethoxybenzaldehyde) and either one phenol (phenol, guaiacol, or syringol) or a mixture of phenols mimicking softwood or hardwood emissions were illuminated to make SOA, atomized, and collected on a filter. We produced laboratory standards of relevant organic compounds in order to develop calibrations for four functional groups: carbonyls (CO), saturated C–H, unsaturated C–H and O–H. We analyzed the SOA samples with transmission FT-IR to identify and determine the amounts of the four functional groups. The carbonyl functional group accounts for 3–12% of the SOA sample mass in single phenolic SOA samples and 9–14% of the SOA sample mass in mixture samples. No carbonyl functional groups are present in the initial reactants. Varying amounts of each of the other functional groups are observed. Comparing carbonyls measured by FT-IR (which could include aldehydes, ketones, esters, and carboxylic acids) with eight small carboxylic acids measured by ion chromatography indicates that the acids only account for an average of 20% of the total carbonyl reported by FT-IR.

Aqueous oxidation of green leaf volatiles by hydroxyl radical as a source of SOA: Kinetics and SOA yields

Green leaf volatiles (GLVs) are a class of oxygenated hydrocarbons released from vegetation, especially during mechanical stress or damage. The potential for GLVs to form secondary organic aerosol (SOA) via aqueous-phase reactions is not known. Fog events over vegetation will lead to the uptake of GLVs into water droplets, followed by aqueous-phase reactions with photooxidants such as the hydroxyl radical (OH). In order to determine if the aqueous oxidation of GLVs by OH can be a significant source of secondary organic aerosol, we studied the partitioning and reaction of five GLVs: cis-3-hexen-1-ol, cis-3-hexenyl acetate, methyl salicylate, methyl jasmonate, and 2-methyl-3-butene-2-ol. For each GLV we measured the kinetics of aqueous oxidation by OH, and the corresponding SOA mass yield. The second-order rate constants for GLVs with OH were all near diffusion controlled, (5.4–8.6) × 109 M−1 s−1 at 298 K, and showed a small temperature dependence, with an average activation energy of 9.3 kJ mol−1 Aqueous-phase SOA mass yields ranged from 10 to 88%, although some of the smaller values were not statistically different from zero. Methyl jasmonate was the most effective aqueous-phase SOA precursor due to its larger Henry's law constant and high SOA mass yield (68 ± 8%). While we calculate that the aqueous-phase SOA formation from the five GLVs is a minor source of aqueous-phase SOA, the availability of other GLVs, other oxidants, and interfacial reactions suggest that GLVs overall might be a significant source of SOA via aqueous reactions.

Quantitative IR Spectrum and Vibrational Assignments for Glycolaldehyde Vapor: Glycolaldehyde Measurements in Biomass Burning Plumes

Glycolaldehyde (GA, 2-hydroxyethanal, C2H4O2) is a semivolatile molecule of atmospheric importance, recently proposed as a precursor in the formation of aqueous-phase secondary organic aerosol (SOA). There are few methods to measure GA vapor, but infrared spectroscopy has been used successfully. Using vetted protocols we have completed the first assignment of all fundamental vibrational modes and also derived quantitative IR absorption band strengths using both neat and pressure-broadened GA vapor. Even though GA is problematic due to its propensity to both dimerize and condense, our intensities agree well with the few previously published values. Using the ν10 band Q-branch at 860.51 cm(-1), we have also determined GA mixing ratios in biomass burning plumes generated by field and laboratory burns of fuels from the southeastern and southwestern United States, including the first IR field measurements of GA in smoke. The GA emission factors were anti-correlated with modified combustion efficiency confirming release of GA from smoldering combustion. The GA emission factors (grams of GA emitted per kilogram of biomass burned on a dry mass basis) had a low dependence on fuel type consistent with the production mechanism being pyrolysis of cellulose. GA was emitted at 0.23 ± 0.13% of CO from field fires, and we calculate that it accounts for ∼18% of the aqueous-phase SOA precursors that we were able to measure.

Sulfate radical-initiated formation of isoprene-derived organosulfates in atmospheric aerosols

Recent studies show that isoprene-derived organosulfates are an important fraction of ambient secondary organic aerosol (SOA), adding up to 20% to the organic mass. Organosulfates with m/z of 199 and 183 relating to C4 compounds are found in ambient and laboratory generated SOA and a sulfate radical induced oxidation of methacrolein (MACR) and methyl vinyl ketone (MVK) has been shown to be a possible formation mechanism. In the present study, experiments on the sulfate radical-induced oxidation of methacrolein and methyl vinyl ketone were performed in bulk aqueous phase, as well as in an aerosol chamber, and finally compared with ambient PM10 samples collected at a rural East German village during the summer 2008, to investigate their relevance in aqueous phase SOA formation. Samples from aqueous phase experiments and extracts from filters were analysed with UPLC/(-)ESI-IMS-QTOFMS. All the samples showed the abundance of highly oxidised organosulfates with m/z 153, 155, 167, 183 and 199 corresponding to the species found in ambient particle samples. In the bulk phase studies with laser-induced sulfate radical formation, the signal intensities increased with increasing number of laser pulses, indicating the sulfate radical-induced formation of these organosulfates. Additionally, the chamber experiments showed a particle mass growth of about 10 microg m(-3) and 4 microg m(-3) for experiments on the reactive uptake of MACR and MVK with a sulfate radical precursor (K2S2O8) in the seed particles. Correlations of the C2 to C5 organosulfate species (including the m/z 215, C5H11O7S-), detected in the ambient samples were found to be very strong (r > 0.8), indicating that these compounds are formed from similar mechanisms and under equal environmental conditions. This study shows that sulfate radical-induced oxidation in the aqueous particle phase provides a reasonable explanation for the formation of these organosulfates from methacrolein and methyl vinyl ketone.

Aqueous-Phase Secondary Organic Aerosol and Organosulfate Formation in Atmospheric Aerosols: A Modeling Study

We have examined aqueous-phase secondary organic aerosol (SOA) and organosulfate (OS) formation in atmospheric aerosols using a photochemical box model with coupled gas-phase chemistry and detailed aqueous aerosol chemistry. SOA formation in deliquesced ammonium sulfate aerosol is highest under low-NO(x) conditions, with acidic aerosol (pH = 1) and low ambient relative humidity (40%). Under these conditions, with an initial sulfate loading of 4.0 μg m(-3), 0.9 μg m(-3) SOA is predicted after 12 h. Low-NO(x) aqueous-aerosol SOA (aaSOA) and OS formation is dominated by isoprene-derived epoxydiol (IEPOX) pathways; 69% or more of aaSOA is composed of IEPOX, 2-methyltetrol, and 2-methyltetrol sulfate ester. 2-Methyltetrol sulfate ester comprises >99% of OS mass (66 ng m(-3) at 40% RH and pH 1). In urban (high-NO(x)) environments, aaSOA is primarily formed via reversible glyoxal uptake, with 0.12 μg m(-3) formed after 12 h at 80% RH, with 20 μg m(-3) initial sulfate. OS formation under all conditions studied is maximum at low pH and lower relative humidities (<60% RH), i.e., when the aerosol is more concentrated. Therefore, OS species are expected to be good tracer compounds for aqueous aerosol-phase chemistry (vs cloudwater processing).

Modeling secondary organic aerosol formation from xylene and aromatic mixtures using a dynamic partitioning approach incorporating particle aqueous-phase chemistry (II)

Formation of secondary organic aerosol (SOA) is simulated for 14 outdoor smog chamber experiments using condensed gas-phase regulatory mechanisms and a new SOA framework. This framework is based on empirical parameterizations of independent chamber experiments and includes role of glyoxal and methylglyoxal in formation of particle aqueous-phase. To evaluate for regulatory applications, the chamber experiments include an urban non-SOA VOC mixture and NOx, with either injections of o/p-xylenes or toluene. The experiments are performed under varying conditions of relative humidity (RH) and in the presence of low initial background seed. Gas-particle partitioning of semi-volatile products into particle organic-phase is modeled using a dynamic partitioning approach with reactive uptake coefficient as the principal transport and kinetic parameter. Aqueous-phase SOA is predicted using formulations that describe the irreversible loss of both glyoxal and methylglyoxal to particle aqueous-phase. The predicted SOA mass in the new framework is evaluated using two regulatory gas-phase mechanisms – CB05 or SAPRC07 and, two regulatory parameterization schemes to predict semi-volatile product formation – an Odum-type two-product model and volatility basis-set (VBS). Predictions from the new SOA framework reproduce SOA mass within the uncertainty range of observations, irrespective of the choice of gas-phase mechanism and SOA parameterization scheme (root mean square error [RMSE] range of 0.18–3.08 μg m−3). Further, model results suggest strong possibility of dominance of bulk-process under low seed conditions and surface-uptake process under high seed for aqueous-phase SOA formation. Sensitivity analysis to the hygroscopic nature of aqueous-phase SOA indicates an uncertainty of a factor of 2 in bulk-process and surface-uptake rates. In summary, the results strongly point to considering mass-transfer and kinetic limitations in regulatory air quality models at low ambient seed concentrations and highlight the importance of aqueous-phase SOA for aromatics under high-RH conditions.

Airborne and ground-based measurements of the trace gases and particles emitted by prescribed fires in the United States

Abstract. We have measured emission factors for 19 trace gas species and particulate matter (PM2.5) from 14 prescribed fires in chaparral and oak savanna in the southwestern US, as well as conifer forest understory in the southeastern US and Sierra Nevada mountains of California. These are likely the most extensive emission factor field measurements for temperate biomass burning to date and the only published emission factors for temperate oak savanna fuels. This study helps to close the gap in emissions data available for temperate zone fires relative to tropical biomass burning. We present the first field measurements of the biomass burning emissions of glycolaldehyde, a possible precursor for aqueous phase secondary organic aerosol formation. We also measured the emissions of phenol, another aqueous phase secondary organic aerosol precursor. Our data confirm previous observations that urban deposition can impact the NOx emission factors and thus subsequent plume chemistry. For two fires, we measured both the emissions in the convective smoke plume from our airborne platform and the unlofted residual smoldering combustion emissions with our ground-based platform. The smoke from residual smoldering combustion was characterized by emission factors for hydrocarbon and oxygenated organic species that were up to ten times higher than in the lofted plume, including high 1,3-butadiene and isoprene concentrations which were not observed in the lofted plume. This should be considered in modeling the air quality impacts for smoke that disperses at ground level. We also show that the often ignored unlofted emissions can significantly impact estimates of total emissions. Preliminary evidence suggests large emissions of monoterpenes in the residual smoldering smoke. These data should lead to an improved capacity to model the impacts of biomass burning in similar temperate ecosystems.

Partitioning phase preference for secondary organic aerosol in an urban atmosphere

Secondary organic aerosol (SOA) comprises a significant portion of atmospheric particular matter. The impact of particular matter on both human health and global climate has long been recognized. Despite its importance, there are still many unanswered questions regarding the formation and evolution of SOA in the atmosphere. This study uses a modeling approach to understand the preferred partitioning behavior of SOA species into aqueous or organic condensed phases. More specifically, this work uses statistical analyses of approximately 24,000 data values for each variable from a state of the art 3D airshed model. Spatial and temporal distributions of fractions of SOA residing in the aqueous phase (fAQ) in the South Coast Air Basin of California are presented. Typical values of fAQ within the basin near the surface range from 5 to 80%. Results show that the likelihood of large fAQ values is inversely proportional to the total SOA loading. Analysis of various meteorological parameters indicates that large fAQ values are predicted because modeled aqueous-phase SOA formation is less sensitive than that of organic-phase SOA to atmospheric conditions that are not conducive to SOA formation. There is a diurnal variation of fAQ near the surface: It tends to be larger during daytime hours than during nighttime hours. Results also indicate that the largest fAQ values are simulated in layers above ground level at night. In summary, one must consider SOA in both organic and aqueous phases for proper regional and global SOA budget estimation.