Source Of Secondary Organic Aerosol Research Articles

Ethylene, xylene, toluene and hexane are major contributors of atmospheric ozone in Hangzhou, China, prior to the 2022 Asian Games

The occurrence of volatile organic compounds (VOCs) in the atmosphere is a major health issue, notably in China, because those compounds are the precursors of tropospheric ozone and sources of secondary organic aerosols. Also, some compounds are carcinogenic. Atmospheric nitrogen oxides (NOx) are also a source of tropospheric ozone, thus creating diseases in human respiratory systems. Therefore, controlling VOCs and NOx is essential for the welfare of local citizens and incoming 2022 Asian Games in Hangzhou. Here, we recorded daily average concentrations for 55 different VOCs species and NOx in 2017. For each VOC, we calculated the ozone formation potential, the propene equivalent concentration and the secondary organic aerosol formation potential. We used the Weather Research and Forecast coupled with Community Multi-scale Air Quality models to assess the regional transport and local sector contributions for VOCs and NOx. Results show that ethylene contributed over 16% of total ozone generated from VOCs, followed by M/P-xylene and toluene. M/P-Xylene, toluene and n-hexane were the top 3 VOCs species that generated ozone fastest. Benzene, toluene and ethylbenzene generated most secondary organic aerosols. The annual regional transport rate is 45% for VOCs and 22% for NOx. Local industry produced 28% VOCs, while local traffic contributed 64% NOx. Overall, ground-level ozone could be reduced by controlling VOCs emissions first and then controlling NOx emissions.

Organic Aerosol Particle Chemical Properties Associated With Residential Burning and Fog in Wintertime San Joaquin Valley (Fresno) and With Vehicle and Firework Emissions in Summertime South Coast Air Basin (Fontana)

AbstractOrganic aerosol mass (OM) components were investigated at Fresno in winter and at Fontana in summer by positive matrix factorization of high‐resolution time‐of‐flight aerosol mass spectra and of Fourier Transform infrared spectra, as well as by k‐means clustering of light‐scattering (LS) aerosol single‐particle spectra. The results were comparable for all three methods at both sites, showing different contributions of primary and secondary organic aerosol sources to PM1. At Fresno biomass burning organic aerosol contributed 27% of OM on low‐fog days, and nitrate‐related oxidized OA (NOOA) accounted for 47% of OM on high‐fog days, whereas at Fontana very oxygenated organic aerosol (VOOA) components contributed 58–69% of OM. Amine and organosulfate fragment concentrations were between 2 and 3 times higher on high‐fog days than on low‐fog days at Fresno, indicating increased formation from fog‐related processes. NOOA and biomass burning organic aerosol components were largely on different particles than the VOOA components in Fresno, but in Fontana both NOOA and VOOA components were distributed on most particle types, consistent with a longer time for and a larger contribution from gas‐phase photochemical secondary organic aerosol formation in summer Fontana than winter Fresno. Uncommon trace organic fragments, elevated inorganic, and alcohol group submicron mass concentrations persisted at Fontana for more than 5 days after 4 July fireworks. These unique aerosol chemical compositions at Fresno and Fontana show substantial and extended air‐quality impacts from residential burning and fireworks.

Photochemical Cloud Processing of Primary Wildfire Emissions as a Potential Source of Secondary Organic Aerosol.

We investigated the gas-phase chemical composition of biomass burning (BB) emissions and their role in aqueous secondary organic aerosol (aqSOA) formation through photochemical cloud processing. A high-resolution time-of-flight chemical ionization mass spectrometer using iodide reagent ion chemistry detected more than 100 gas-phase compounds from the emissions of 30 different controlled burns during the 2016 Fire Influence on Regional and Global Environments Experiment (FIREX) at the Fire Science Laboratory. Compounds likely to partition to cloudwater were selected based on high atomic oxygen-to-carbon ratio and abundance. Water solubility was confirmed by detection of these compounds in water after mist chamber collection during controlled burns and analysis using ion chromatography and electrospray ionization interfaced to high-resolution time-of-flight mass spectrometry. Known precursors of aqSOA were found in the primary gaseous BB emissions (e.g., phenols, acetate, and pyruvate). Aqueous OH oxidation of the complex biomass burning mixtures led to rapid depletion of many compounds (e.g., catechol, levoglucosan, methoxyphenol) and formation of others (e.g., oxalate, malonate, mesoxalate). After 150 min of oxidation (approximatively 1 day of cloud processing), oxalate accounted for 13-16% of total dissolved organic carbon. Formation of known SOA components suggests that cloud processing of primary BB emissions forms SOA.

Secondary organic aerosol (SOA) yields from NO<sub>3</sub> radical + isoprene based on nighttime aircraft power plant plume transects

Abstract. Nighttime reaction of nitrate radicals (NO3) with biogenic volatile organic compounds (BVOC) has been proposed as a potentially important but also highly uncertain source of secondary organic aerosol (SOA). The southeastern United States has both high BVOC and nitrogen oxide (NOx) emissions, resulting in a large model-predicted NO3-BVOC source of SOA. Coal-fired power plants in this region constitute substantial NOx emissions point sources into a nighttime atmosphere characterized by high regionally widespread concentrations of isoprene. In this paper, we exploit nighttime aircraft observations of these power plant plumes, in which NO3 radicals rapidly remove isoprene, to obtain field-based estimates of the secondary organic aerosol yield from NO3 + isoprene. Observed in-plume increases in nitrate aerosol are consistent with organic nitrate aerosol production from NO3 + isoprene, and these are used to determine molar SOA yields, for which the average over nine plumes is 9 % (±5 %). Corresponding mass yields depend on the assumed molecular formula for isoprene-NO3-SOA, but the average over nine plumes is 27 % (±14 %), on average larger than those previously measured in chamber studies (12 %–14 % mass yield as ΔOA ∕ ΔVOC after oxidation of both double bonds). Yields are larger for longer plume ages. This suggests that ambient aging processes lead more effectively to condensable material than typical chamber conditions allow. We discuss potential mechanistic explanations for this difference, including longer ambient peroxy radical lifetimes and heterogeneous reactions of NO3-isoprene gas phase products. More in-depth studies are needed to better understand the aerosol yield and oxidation mechanism of NO3 radical + isoprene, a coupled anthropogenic–biogenic source of SOA that may be regionally significant.

Synergistic O3 + OH oxidation pathway to extremely low-volatility dimers revealed in β-pinene secondary organic aerosol

Dimeric compounds contribute significantly to the formation and growth of atmospheric secondary organic aerosol (SOA) derived from monoterpene oxidation. However, the mechanisms of dimer production, in particular the relevance of gas- vs. particle-phase chemistry, remain unclear. Here, through a combination of mass spectrometric, chromatographic, and synthetic techniques, we identify a suite of dimeric compounds (C15-19H24-32O5-11) formed from concerted O3 and OH oxidation of β-pinene (i.e., accretion of O3- and OH-derived products/intermediates). These dimers account for an appreciable fraction (5.9-25.4%) of the β-pinene SOA mass and are designated as extremely low-volatility organic compounds. Certain dimers, characterized as covalent dimer esters, are conclusively shown to form through heterogeneous chemistry, while evidence of dimer production via gas-phase reactions is also presented. The formation of dimers through synergistic O3 + OH oxidation represents a potentially significant, heretofore-unidentified source of low-volatility monoterpene SOA. This reactivity also suggests that the current treatment of SOA formation as a sum of products originating from the isolated oxidation of individual precursors fails to accurately reflect the complexity of oxidation pathways at play in the real atmosphere. Accounting for the role of synergistic oxidation in ambient SOA formation could help to resolve the discrepancy between the measured atmospheric burden of SOA and that predicted by regional air quality and global climate models.

Criegee Intermediate Reactions with Carboxylic Acids: A Potential Source of Secondary Organic Aerosol in the Atmosphere

Trace atmospheric concentrations of carboxylic acids have a potent effect upon the environment, where they modulate aqueous chemistry and perturb Earth’s radiative balance. Halogenated carboxylic acids are produced by the tropospheric oxidation of halocarbons and are considered persistent pollutants because of their weak tropospheric and aqueous sinks. However, recent studies reported rapid reactions between selected carboxylic acids and Criegee intermediates, which may provide an efficient gas-phase removal process. Accordingly, absolute rate coefficients of two Criegee intermediates, CH2OO and (CH3)2COO, with a suite of carboxylic acids (HCOOH, CH3COOH, CClF2COOH, CF3CF2COOH, and pyruvic acid) were measured with a view to develop a structure–activity relationship (SAR). This SAR is based upon the dipole-capture model and predicts the reactivity of many further combinations of Criegee intermediates and carboxylic acids. Complementary synchrotron-based photoionization mass spectrometry measurements demons...

Physico-chemical properties of green leaf volatiles (GLV) for ascertaining atmospheric fate and transport in fog

Green Leaf Volatiles (GLVs) is a class of biogenically emitted oxygenated hydrocarbons that have been identified as a potential source of Secondary Organic Aerosols (SOA) via aqueous oxidation. The physico-chemical properties of GLVs are vital to understanding their fate and transport in the atmosphere via fog processing, but few experimental data are available. We studied the aqueous solubility, 1-octanol/water partition coefficient, and Henry\'s law constant (KH) of five GLVs at 25 C: methyl jasmonate, methyl salicylate, 2-methyl-3-buten-2-ol, cis-3-hexen-1-ol, and cis-3-hexenyl acetate. Henry\'s law constant was also measured at temperatures and ionic strengths typical of fog. Experimental values are compared to scarcely-available literature values, as well as estimations using group and bond contribution methods, property-specific correlations and molecular dynamics simulations. From these values, the partition coefficients to the air-water interface were also calculated. The large Henry\'s law constant of methyl jasmonate (8091+/-1121 Mxatm-1) made it the most significant GLV for aqueous phase photochemistry. The HENRYWIN program\'s bond contribution method from the Estimation Programs Interface Suite (EPI Suite) produced the best estimate of the Henry\'s constant for GLVs. Estimations of 1-octanol/water partition coefficient and solubility are best when correlating an experimental value of one to find the other. Finally, the scavenging efficiency was calculated for each GLV indicating aqueous phase processing will be most important for methyl jasmonate.

The impact of biogenic, anthropogenic, and biomass burning volatile organic compound emissions on regional and seasonal variations in secondary organic aerosol

Abstract. The global secondary organic aerosol (SOA) budget is highly uncertain, with global annual SOA production rates, estimated from global models, ranging over an order of magnitude and simulated SOA concentrations underestimated compared to observations. In this study, we use a global composition-climate model (UKCA) with interactive chemistry and aerosol microphysics to provide an in-depth analysis of the impact of each VOC source on the global SOA budget and its seasonality. We further quantify the role of each source on SOA spatial distributions, and evaluate simulated seasonal SOA concentrations against a comprehensive set of observations. The annual global SOA production rates from monoterpene, isoprene, biomass burning, and anthropogenic precursor sources is 19.9, 19.6, 9.5, and 24.6 Tg (SOA) a−1, respectively. When all sources are included, the SOA production rate from all sources is 73.6 Tg (SOA) a−1, which lies within the range of estimates from previous modelling studies. SOA production rates and SOA burdens from biogenic and biomass burning SOA sources peak during Northern Hemisphere (NH) summer. In contrast, the anthropogenic SOA production rate is fairly constant all year round. However, the global anthropogenic SOA burden does have a seasonal cycle which is lowest during NH summer, which is probably due to enhanced wet removal. Inclusion of the new SOA sources also accelerates the ageing by condensation of primary organic aerosol (POA), making it more hydrophilic, leading to a reduction in the POA lifetime. With monoterpene as the only source of SOA, simulated SOA and total organic aerosol (OA) concentrations are underestimated by the model when compared to surface and aircraft measurements. Model agreement with observations improves with all new sources added, primarily due to the inclusion of the anthropogenic source of SOA, although a negative bias remains. A further sensitivity simulation was performed with an increased anthropogenic SOA reaction yield, corresponding to an annual global SOA production rate of 70.0 Tg (SOA) a−1. Whilst simulated SOA concentrations improved relative to observations, they were still underestimated in urban environments and overestimated further downwind and in remote environments. In contrast, the inclusion of SOA from isoprene and biomass burning did not improve model–observations biases substantially except at one out of two tropical locations. However, these findings may reflect the very limited availability of observations to evaluate the model, which are primarily located in the NH mid-latitudes where anthropogenic emissions are high. Our results highlight that, within the current uncertainty limits in SOA sources and reaction yields, over the NH mid-latitudes, a large anthropogenic SOA source results in good agreement with observations. However, more observations are needed to establish the importance of biomass burning and biogenic sources of SOA in model agreement with observations.

Exploring the ionic strength effects on the photochemical degradation of pyruvic acid in atmospheric deliquescent aerosol particles

There is increasing evidence that aqueous-phase atmospheric chemistry is an important source of secondary organic aerosols (SOA), but the related processes are currently not adequately represented in atmospheric chemistry models. Here we show that the absorption spectrum of pyruvic acid (PA) exhibits both an increase of the absorption intensity and a red shift of 13 nm while going from a dilute aqueous phase to a solution containing the inert salt sodium perchlorate (5M NaClO4). If this phenomenon turns out to be more general, many compounds that do not absorb actinic light in clouds and fog could become light absorbers at elevated salt concentrations in aerosol deliquescent particles. Compared to the direct photolysis of PA in dilute aqueous solution, the photolysis rate is increased by three times at high ionic strength (5M NaClO4). Such a considerable enhancement can be rationalized in the framework of the Debye-McAulay approach for reactions of ionic + neutral (or neutral + neutral) species, considering that the PA direct photolysis likely involves interaction between the photogenerated triplet state and water. This is, to our knowledge, the first report of a significant effect of the ionic strength on the rate of an atmospheric photochemical reaction. The phenomenon has important implications for the fate of PA and, potentially, of other organic compounds in atmospheric aerosol deliquescent particles.

An exhaustive classification for the seasonal variation of organic peaks in the atmospheric fine particles obtained by a gas chromatography/mass spectrometry

Abstract For organic compounds in fine particles ( PM 2.5 ), the knowledge on their sources and formation processes are still rather limited because of labile characteristics. To assess the contribution of primary and secondary organic aerosols, it is necessary to utilize the behavior of organic markers. In this study, cluster analysis was applied to the seasonal variations of peaks in chromatogram for unquantitative organic compounds in PM 2.5 obtained by a gas chromatography/mass spectrometry (GC/MS) analysis after derivatization method. We used the PM 2.5 samples collected at Maebashi in Japan for four seasons. Based on cluster analysis, total 115 peaks were divided into nine clusters. The combination effects of the primary and secondary particles transported plume from urban atmosphere were found in two clusters for spring and summer. For dicarboxylic acids, C3 − C5 diacids were in same cluster, whereas C2 and C6 diacids were separated with the other cluster. These results show different trends of secondary formation for dicarboxylic acids. This approach might help to find a group of markers in organic aerosols and to classify potential source of secondary organic aerosols.

Significant Production of Secondary Organic Aerosol from Emissions of Heated Cooking Oils

Cooking emissions have been identified as a major source of primary organic aerosol (POA) in urban environments. Cooking may also be a potential source of secondary organic aerosol (SOA) because of the abundant emissions of non-methane organic gases. We studied SOA formation from the photooxidation of emissions from seven vegetable oils heated at 200 °C under high-NOx conditions in a smog chamber. After the samples had been aged under an OH exposure of 1.0 × 1010 molecules cm–3 s, the SOA formation rate was generally 1 order of magnitude higher than the POA emission rate. We determined that alkenals, which are not traditional SOA precursors in chemical transport models, accounted for 5–34% of the observed SOA. The unexplained SOA may be attributed to the oxidation of primary semivolatile and intermediate-volatility organic compounds (SVOCs and IVOCs, respectively), which were estimated to contribute an additional 9–106% of the observed SOA assuming the volatility distribution of heated cooking oils is the...

Online molecular characterisation of organic aerosols in an atmospheric chamber using extractive electrospray ionisation mass spectrometry

Abstract. The oxidation of biogenic volatile organic compounds (VOCs) represents a substantial source of secondary organic aerosol (SOA) in the atmosphere. In this study, we present online measurements of the molecular constituents formed in the gas and aerosol phases during α-pinene oxidation in the Cambridge Atmospheric Simulation Chamber (CASC). We focus on characterising the performance of extractive electrospray ionisation (EESI) mass spectrometry (MS) for particle analysis. A number of new aspects of EESI-MS performance are considered here. We show that relative quantification of organic analytes can be achieved in mixed organic–inorganic particles. A comprehensive assignment of mass spectra for α-pinene derived SOA in both positive and negative ion modes is obtained using an ultra-high-resolution mass spectrometer. We compare these online spectra to conventional offline ESI-MS spectra and find good agreement in terms of the compounds identified, without the need for complex sample work-up procedures. Under our experimental conditions, EESI-MS signals arise only from particle-phase analytes. High-time-resolution (7 min) EESI-MS spectra are compared with simulations from the near-explicit Master Chemical Mechanism (MCM) for a range of reaction conditions. We show that MS peak abundances scale with modelled concentrations for condensable products (pinonic acid, pinic acid, OH-pinonic acid). Relative quantification is achieved throughout SOA formation as the composition, size and mass (5–2400 µg m−3) of particles is evolving. This work provides a robust demonstration of the advantages of EESI-MS for chamber studies over offline ESI-MS (time resolution, relative quantification) and over hard online techniques (molecular information).

Investigating the role of chemical and physical processes on organic aerosol modelling with CAMx in the Po Valley during a winter episode

Traditional aerosol mechanisms underestimate the observed organic aerosol concentration, especially due to the lack of information on secondary organic aerosol (SOA) formation and processing. In this study we evaluate the chemical and transport model CAMx during a one-month in winter (February 2013) over a 5 km resolution domain, covering the whole Po valley (Northern Italy). This works aims at investigating the effects of chemical and physical atmospheric processing on modelling results and, in particular, to evaluate the CAMx sensitivity to organic aerosol (OA) modelling schemes: we will compare the recent 1.5D-VBS algorithm (CAMx-VBS) with the traditional Odum 2-product model (CAMx-SOAP). Additionally, the thorough diagnostic analysis of the reproduction of meteorology, precursors and aerosol components was intended to point put strength and weaknesses of the modelling system and address its improvement. Firstly, we evaluate model performance for criteria PM concentration. PM10 concentration was underestimated both by CAMx-SOAP and even more by CAMx-VBS, with the latter showing a bias ranging between −4.7 and −7.1 μg m−3. PM2.5 model performance was to some extent better than PM10, showing a mean bias ranging between −0.5 μg m−3 at rural sites and −5.5 μg m−3 at urban and suburban sites. CAMx performance for OA was clearly worse than for the other PM compounds (negative bias ranging between −40% and −75%). The comparisons of model results with OA sources (identified by PMF analysis) shows that the VBS scheme underestimates freshly emitted organic aerosol while SOAP overestimates. The VBS scheme correctly reproduces biomass burning (BBOA) contributions to primary OA concentrations (POA). In contrast VBS slightly underestimates the contribution from fossil-fuel combustion (HOA), indicating that POA emissions related to road transport are either underestimated or associated to higher volatility classes. The VBS scheme under-predictes the SOA too, but to a lesser extent than CAMx-SOAP. SOA underestimation can be related to corresponding underestimation of either aging processes or precursor emissions. This indicates that improvements in the emission inventories for semi- and intermediate-volatility organic compounds are needed for further progress in this area. Finally, the comparison between modelled and observed SOA sources points out the urgency to include processing of OA in particle water phase into SOA formation mechanisms, to reconcile model results and observations.

Reducing secondary organic aerosol formation from gasoline vehicle exhaust

On-road gasoline vehicles are a major source of secondary organic aerosol (SOA) in urban areas. We investigated SOA formation by oxidizing dilute, ambient-level exhaust concentrations from a fleet of on-road gasoline vehicles in a smog chamber. We measured less SOA formation from newer vehicles meeting more stringent emissions standards. This suggests that the natural replacement of older vehicles with newer ones that meet more stringent emissions standards should reduce SOA levels in urban environments. However, SOA production depends on both precursor concentrations (emissions) and atmospheric chemistry (SOA yields). We found a strongly nonlinear relationship between SOA formation and the ratio of nonmethane organic gas to oxides of nitrogen (NOx) (NMOG:NOx), which affects the fate of peroxy radicals. For example, changing the NMOG:NOx from 4 to 10 ppbC/ppbNOx increased the SOA yield from dilute gasoline vehicle exhaust by a factor of 8. We investigated the implications of this relationship for the Los Angeles area. Although organic gas emissions from gasoline vehicles in Los Angeles are expected to fall by almost 80% over the next two decades, we predict no reduction in SOA production from these emissions due to the effects of rising NMOG:NOx on SOA yields. This highlights the importance of integrated emission control policies for NOx and organic gases.

Quantum Chemical Investigation on Photochemical Reactions of Nonanoic Acids at Air-Water Interface.

Photoinduced chemical reactions of organic compounds at the marine boundary layer have recently attracted significant experimental attention because this kind of photoreactions has been proposed to have substantial impact on local new particle formation and their photoproducts could be a source of secondary organic aerosols. In this work, we have employed first-principles density functional theory method combined with cluster models to systematically explore photochemical reaction pathways of nonanoic acids (NAs) to form volatile saturated and unsaturated C9 and C8 aldehydes at air-water interfaces. On the basis of the results, we have found that the formation of C9 aldehydes is not initiated by intermolecular Norrish type II reaction between two NAs but by intramolecular T1 C-O bond fission of NA generating acyl and hydroxyl radicals. Subsequently, saturated C9 aldehydes are formed through hydrogenation reaction of acyl radical by another intact NA. Following two dehydrogenation reactions, unsaturated C9 aldehydes are generated. In parallel, the pathway to C8 aldehydes is initiated by T1 C-C bond fission of NA, which generates octyl and carboxyl radicals; then, an octanol is formed through recombination reaction of octyl with hydroxyl radical. In the following, two dehydrogenation reactions result into an enol intermediate from which saturated C8 aldehydes are produced via NA-assisted intermolecular hydrogen transfer. Finally, two dehydrogenation reactions generate unsaturated C8 aldehydes. In these reactions, water and NA molecules are found to play important roles. They significantly reduce relevant reaction barriers. Our work has also explored oxygenation reactions of NA with molecular oxygen and radical-radical dimerization reactions.

Emission of volatile organic compounds and production of secondary organic aerosol from stir-frying spices

Cooking is an important source of volatile organic compounds (VOCs) and a potential source of secondary organic aerosol (SOA) both indoors and outdoors. In this study, VOC emissions from heating corn oil and stir-frying spices (i.e. garlic, ginger, myrcia and zanthoxylum piperitum (Sichuan pepper)) were characterized using an on-line membrane inlet vacuum ultraviolet single-photon ionization time-of-flight mass spectrometer (VUV-SPI-TOFMS). VOC emissions from heating corn oil were dominated by aldehydes, which were enhanced by factors of one order of magnitude when stir-frying spices. Stir-frying any of the spices studied generated large amounts of methylpyrrole (m/z 81). In addition, stir-frying garlic produced abundant dihydrohydroxymaltol (m/z 144) and diallyldisulfide (DADS) (m/z 146), while stir-frying ginger, myrcia and zanthoxylum piperitum produced abundant monoterpenes (m/z 136) and terpenoids (m/z 152, 154). SOA formed from emissions of stir-frying spices through reactions with excess ozone in a flow reactor as well as primary organic aerosol (POA) emissions were characterized using a scanning mobility particle sizer (SMPS) and a high-resolution time-of-flight aerosol mass spectrometer (HR-TOF-AMS). Stir-frying garlic and ginger generated similar POA concentrations to those from heating corn oil while stir-frying myrcia and zanthoxylum piperitum generated double the amount of emissions. No SOA was observed from stir-frying garlic and ginger. The rates of SOA production from stir-frying myrcia and zanthoxylum piperitum were 1.8μgmin−1gspice−1 and 8.7μgmin−1gspice−1, equivalent to 13.4% and 53.1% of their own POA emission rates, respectively. Therefore, the contribution of stir-frying spices to ambient organic aerosol levels is likely dominated by POA. The rates of total terpene emission from stir-frying myrcia and zanthoxylum piperitum were estimated to be 5.1μgmin−1gspice−1 and 24.9μgmin−1gspice−1, respectively. Our results suggest that stir-frying spices could be an important source of terpenes in indoor environments in Hong Kong, at least during cooking.

Secondary organic aerosol formation from in situ OH, O<sub>3</sub>, and NO<sub>3</sub> oxidation of ambient forest air in an oxidation flow reactor

Abstract. Ambient pine forest air was oxidized by OH, O3, or NO3 radicals using an oxidation flow reactor (OFR) during the BEACHON-RoMBAS (Bio–hydro–atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen – Rocky Mountain Biogenic Aerosol Study) campaign to study biogenic secondary organic aerosol (SOA) formation and organic aerosol (OA) aging. A wide range of equivalent atmospheric photochemical ages was sampled, from hours up to days (for O3 and NO3) or weeks (for OH). Ambient air processed by the OFR was typically sampled every 20–30 min, in order to determine how the availability of SOA precursor gases in ambient air changed with diurnal and synoptic conditions, for each of the three oxidants. More SOA was formed during nighttime than daytime for all three oxidants, indicating that SOA precursor concentrations were higher at night. At all times of day, OH oxidation led to approximately 4 times more SOA formation than either O3 or NO3 oxidation. This is likely because O3 and NO3 will only react with gases containing C = C bonds (e.g., terpenes) to form SOA but will not react appreciably with many of their oxidation products or any species in the gas phase that lacks a C = C bond (e.g., pinonic acid, alkanes). In contrast, OH can continue to react with compounds that lack C = C bonds to produce SOA. Closure was achieved between the amount of SOA formed from O3 and NO3 oxidation in the OFR and the SOA predicted to form from measured concentrations of ambient monoterpenes and sesquiterpenes using published chamber yields. This is in contrast to previous work at this site (Palm et al., 2016), which has shown that a source of SOA from semi- and intermediate-volatility organic compounds (S/IVOCs) 3.4 times larger than the source from measured VOCs is needed to explain the measured SOA formation from OH oxidation. This work suggests that those S/IVOCs typically do not contain C = C bonds. O3 and NO3 oxidation produced SOA with elemental O : C and H : C similar to the least-oxidized OA observed in local ambient air, and neither oxidant led to net mass loss at the highest exposures, in contrast to OH oxidation. An OH exposure in the OFR equivalent to several hours of atmospheric aging also produced SOA with O : C and H : C values similar to ambient OA, while higher aging (days–weeks) led to formation of SOA with progressively higher O : C and lower H : C (and net mass loss at the highest exposures). NO3 oxidation led to the production of particulate organic nitrates (pRONO2), while OH and O3 oxidation (under low NO) did not, as expected. These measurements of SOA formation provide the first direct comparison of SOA formation potential and chemical evolution from OH, O3, and NO3 oxidation in the real atmosphere and help to clarify the oxidation processes that lead to SOA formation from biogenic hydrocarbons.

Isomerization of Second-Generation Isoprene Peroxy Radicals: Epoxide Formation and Implications for Secondary Organic Aerosol Yields

We report chamber measurements of secondary organic aerosol (SOA) formation from isoprene photochemical oxidation, in which radical concentrations were systematically varied and the molecular composition of semi- to low-volatility gases and SOA were measured online. Using a detailed chemical kinetics box model, we find that to explain the behavior of low-volatility products and SOA mass yields relative to input H2O2 concentrations, the second-generation dihydroxy hydroperoxy peroxy radical (C5H11O6·) must undergo an intramolecular H-shift with a net forward rate constant of order 0.1 s-1 or higher. This finding is consistent with quantum chemical calculations that suggest a net forward rate constant of 0.3-0.9 s-1. Furthermore, these calculations suggest that the dominant product of this isomerization is a dihydroxy hydroperoxy epoxide (C5H10O5), which is expected to have a saturation vapor pressure ∼2 orders of magnitude higher, as determined by group-contribution calculations, than the dihydroxy dihydroperoxide, ISOP(OOH)2(C5H12O6), a major product of the peroxy radical reacting with HO2. These results provide strong constraints on the likely volatility distribution of isoprene oxidation products under atmospheric conditions and, thus, on the importance of nonreactive gas-particle partitioning of isoprene oxidation products as an SOA source.

Compositions and Sources of Summertime Dicarboxylic Acids and Related SOA in PM2.5 from Mt. Taishan

To identify the compositions and sources of dicarboxylic acids and related SOA in Mt. Taishan, PM2.5 samples were collected from July to August, 2004, and analyzed for dicarboxylic acids and related compounds, the tracers of biogenic secondary organic aerosol (SOA) (Isopene, α-/β-Pinene and β-caryophyllene SOA tracers), water-soluble organic carbon (WSOC) and inorganic ions. The results showed that total dicarboxylic acids were (376±189) ng·m-3, and C2 was the most abundant dicarboxylic acid, followed by C3, C4 and C9. The concentrations of diacids were more abundant than those in marine regions, but lower than those in urban and other mountainous areas, indicating that the atmosphere in Mt. Taishan was less influenced by anthropogenic pollution. C2/C4, C3/C4 and F/M ratios indicated that diacids were mostly produced by more intensive photochemical oxidation. The relative abundance of C9 to the total diacids, C9/C6 and C9/Ph ratios were higher than those in urban, marine and mountainous regions, further suggesting that SOA in Mt. Taishan atmosphere were mostly derived from biogenic sources rather than anthropogenic sources. Compared to the budgets of model and correlation analysis, it suggested that diacids and related SOA in Mt. Taishan were mostly derived from photochemical oxidation of local biogenic sources.

Heterogeneous Reactions of Limonene on Mineral Dust: Impacts of Adsorbed Water and Nitric Acid.

Biogenic volatile organic compounds (BVOCs), including the monoterpene limonene, are a major source of secondary organic aerosol (SOA). While gas-phase oxidation initiates the dominant pathway for BVOC conversion to SOA, recent studies have demonstrated that biogenic hydrocarbons can also directly react with acidic droplets. To investigate whether mineral dust may facilitate similar reactive uptake of biogenic hydrocarbons, we studied the heterogeneous reaction of limonene with mineral substrates using condensed-phase infrared spectroscopy and identified the formation of irreversibly adsorbed organic products. For kaolinite, Arizona Test Dust, and silica at 30% relative humidity, GC-MS identified limonene-1,2-diol as the dominant product with total organic surface concentrations on the order of (3-5) × 1018 molecules m-2. Experiments with 18O-labeled water support a mechanism initiated by oxidation of limonene by surface redox sites forming limonene oxide followed by water addition to the epoxide to form limonenediol. Limonene uptake on α-alumina, γ-alumina, and montmorillonite formed additional products in high yield, including carveol, carvone, limonene oxide, and α-terpineol. To model tropospheric processing of mineral aerosol, we also exposed each mineral substrate to gaseous nitric acid prior to limonene uptake and identified similar surface adsorbed products that were formed at rates 2 to 5 times faster than without nitrate coatings. The initial rate of reaction was linearly dependent on gaseous limonene concentration between 5 × 1012 and 5 × 1014 molecules cm-3 (0.22-20.5 ppm) consistent with an Eley-Rideal-type mechanism in which gaseous limonene reacts directly with reactive surface sites. Increasing relative humidity decreased the amount of surface adsorbed products indicating competitive adsorption of surface adsorbed water. Using a laminar flow tube reactor we measured the uptake coefficient for limonene on kaolinite at 25% RH to range from γ = 5.1 × 10-6 to 9.7 × 10-7. After adjusting for reactive surface areas, we estimate uptake coefficients for limonene on HNO3-processed mineral aerosol on the order of (1-6) × 10-6. Although this heterogeneous reaction will not impact the atmospheric lifetime of gaseous limonene, it does provide a new pathway for mineral aerosol to acquire secondary organic matter from biogenic hydrocarbons, which in turn will alter the physical properties of mineral dust.