Important Secondary Organic Aerosol Research Articles

Influence of updated isoprene oxidation mechanisms on the formation of intermediate and secondary products in MCM v3.3.1

In this study, we developed an observation based photochemical box model with the measurement data collected at a suburban site (i.e., the Xianghe, XH site) of the Beijing-Tianjin-Hebei (BTH) region and the latest version of the Master Chemical Mechanism (MCM v3.3.1), which newly incorporated 498 reaction for the additional 155 species with the oxidation of isoprene by the OH radical. The influence of updated isoprene degradation on the radical budgets, formation of intermediate and secondary products by comparing the simulation results in the original version (MCM v3.2) was firstly analyzed to estimate whether the updates of precursors could influence the model performance on its evolution pathways and related products. The results demonstrated that though the input of isoprene could significantly decrease or increase the concentrations of OH and HO2 radicals in the model simulation, respectively, the difference in the simulated concentrations of these radicals with isoprene oxidation before and after the updates was not statistically significant. For intermediate products, the simulated concentrations of formaldehyde and methacrolein (MACR) did not significantly alter before and after the updates, while the abundance of methyl vinyl ketone (MVK) reduced obviously (the maximum reduction of 31%) due to lower reaction coefficient between ISOPBO2 and NO and the lower branching ratios caused by the newly added pathways involving CISOPA and TISOPA in the updated scheme. On the other hand, the simulation concentrations of acrolein (ACR), glyoxal (Gly) and methylglyoxal (Mgly) increased significantly after the model updates. The dominant pathway for the increment of ACR was the decomposition of CH2CHCH2O produced by PE4E2CO, while those of Gly and Mgly were associated with radicals of C537O and PACLOOA that were both from the new intermediate radical of CISOPC. In addition, 39 isoprene oxidation products among the 155 new added species were identified to partition in the formation of secondary organic aerosol (SOA), which were coupled in the gas-particle partitioning module to optimize the formation of SOA, contributing 1.15 μg m−3 to the increase of SOA mean concentration. The updates of isoprene degradation further resulted in the significant change for its sensitivity in SOA formation, making it as the secondary important SOA precursor at the XH site. Last but not the least, the influence on photochemical ozone formation related to updates of isoprene degradation was ignorable. Overall, the above results confirmed the insufficient description of the degradation of precursors indeed limited the simulation of the intermediate and secondary products, highlighting the needs of exploring and parameterizing the precursor degradation processes for better understanding of their behaviors and SOA formation in the atmosphere.

Contribution of intermediate-volatility organic compounds from on-road transport to secondary organic aerosol levels in Europe

Abstract. Atmospheric organic compounds with an effective saturation concentration (C∗) at 298 K between 103 and 106 µg m−3 are called intermediate-volatility organic compounds (IVOCs), and they have been identified as important secondary organic aerosol (SOA) precursors. In this work, we simulate IVOCs emitted from on-road diesel and gasoline vehicles over Europe with a chemical transport model (CTM), utilizing a new approach in which IVOCs are treated as lumped species that preserve their chemical characteristics. This approach allows us to assess both the overall contribution of IVOCs to SOA formation and the role of specific compounds. For the simulated early-summer period, the highest concentrations of SOA formed from the oxidation of on-road IVOCs (SOA-iv) are predicted for major European cities, like Paris, Athens, and Madrid. In these urban environments, on-road SOA-iv can account for up to a quarter of the predicted total SOA. Over Europe, unspeciated cyclic alkanes in the IVOC range are estimated to account for up to 72 % of the total on-road SOA-iv mass, with compounds with 15 to 20 carbons being the most prominent precursors. The sensitivity of the predicted SOA-iv concentrations to the selected parameters of the new lumping scheme is also investigated. Active multigenerational aging of the secondary aerosol products has the most significant effect as it increases the predicted SOA-iv concentrations by 67 %.

Secondary Organic Aerosol Formation from Semi-Volatile and Intermediate Volatility Organic Compounds in the Fall in Beijing

Intermediate volatility organic compounds (IVOCs) and semi-volatile organic compounds (SVOCs) have recently been proposed as important precursors of secondary organic aerosol (SOA). In the present work, 97 volatile organic compounds (VOCs) and 80 intermediate volatility and semi-volatile organic compounds (IVOCs and SVOCs) were measured by online gas chromatography-mass spectrometer/flame ionization detection (GC-MS/FID), and offline thermal desorption gas chromatography-mass spectrometer (TD-GC-MS), respectively. The average concentration of speciated VOCs, IVOCs, and SVOCs were 22.36 ± 9.02 μg m−3, 1.01 ± 0.32 μg m−3, and 0.10 ± 0.17 μg m−3. Alkanes and polycyclic aromatic hydrocarbons (PAHs) are the main compounds of total S/IVOCs. With the increase in molecular weight, the concentrations decreased in the gas phase, while increasing in the particle phase. Vehicular emission is the most significant source according to the carbon preference index (CPI) and the carbon of the most abundant alkane (Cmax). The yield method was used to estimate SOA from the oxidation of VOCs and S/IVOCs. The estimated SOA mass from IVOCs and SVOCs (0.70 ± 0.57 μg m−3) was comparable to that of VOCs (0.62 ± 0.61 μg m−3), and the oxidation of PAHs and alkanes took up 28.70 ± 8.26% and 51.97 ± 20.77% of the total SOA estimation, respectively. Compared to previous work, our study provided detailed molecular information of ambient S/IVOC species and elucidated their importance on SOA formation. Despite their low concentration, S/IVOCs species are important SOA precursors which shared comparable contribution compared with VOCs.

Smog chamber study of secondary organic aerosol formation from gas- and particle-phase naphthalene ozonolysis

Naphthalene (Nap), one of the most abundant PAHs in the atmosphere, has been shown to be an important secondary organic aerosol (SOA) precursor. In this study, the gas- and particle-phases (Suspended Nap-coated polystyrene latex spheres (PSLs) generated by vaporization condensation) Nap ozonolysis were investigated in a 1000 L volume cubic Teflon smog chamber. Gas-phase products and Nap-SOAs were observed from the gas- and particle-phase Nap ozonolysis via homogeneous and heterogeneous reactions, respectively. The time-dependent size distribution and mass concentration of Nap-SOAs were analyzed using a scanning mobility particle sizer and a laboratory-developed single-particle aerosol mass spectrometry (SPAMS) instrument. The chemical components of the homogeneous and heterogeneous reaction products were analyzed on-line by single photo ionization mass spectrometry (SPIMS), SPAMS, as well as by thermal desorption gas chromatograph-mass spectrometry. The results showed that the chemical components of the gas-and particle-phase Nap ozonolysis were essentially the same, with the dominant products being phthalaldehyde, phthalic anhydride, 2-carboxybenzaldehyde, and benzoic acid; Nap-SOA formation was attributed to the formation of new particles as well as the uptake of homogeneous and heterogeneous Nap oxidation products on the gas-phase or/and the particle surface. Based on the online measurement results, the reaction mechanism of Nap-ozonolysis also exhibits an initial carbon atom or double-bond attacks by O3, and kinetic analysis of Nap-coated PSLs ozonolysis shows a heterogeneous reaction rate constant of 2.55E-4 s−1 with R2 = 0.984. This study provides a systematic approach to characterize the chemical and physical properties of the gas- and particle-phase Nap ozonolysis in real time, which will deepen our understanding of the migration and transformation of PAHs in the atmosphere.

A lumped species approach for the simulation of secondary organic aerosol production from intermediate-volatility organic compounds (IVOCs): application to road transport in PMCAMx-iv (v1.0)

Abstract. Secondary organic aerosol (SOA) is formed in the atmosphere through the oxidation and condensation of organic compounds. Intermediate-volatility compounds (IVOCs), compounds with effective saturation concentration (C∗) at 298 K between 103 and 106 µg m−3, have high SOA yields and can be important SOA precursors. The first efforts to simulate IVOCs in chemical transport models (CTMs) used the volatility basis set (VBS), a highly parametrized scheme that oversimplifies their chemistry. In this work we propose a more detailed approach for simulating IVOCs in CTMs, treating them as lumped species that retain their chemical characteristics. Specifically, we introduce four new lumped species representing large alkanes, two lumped species representing polyaromatic hydrocarbons (PAHs) and one species representing large aromatics, all in the IVOC range. We estimate IVOC emissions from road transport using existing estimates of volatile organic compound (VOC) emissions and emission factors of individual IVOCs from experimental studies. Over the European domain, for the simulated period of May 2008, estimated IVOC emissions from road transport were about 21 Mmol d−1, a factor of 8 higher than emissions used in previous VBS applications. The IVOC emissions from diesel vehicles were significantly higher than those from gasoline ones. SOA yields under low-NOx and high-NOx conditions for the lumped IVOC species were estimated based on recent smog chamber studies. Large cyclic alkane compounds have both high yields and high emissions, making them an important, yet understudied, class of IVOCs.

Characteristics of volatile organic compounds in the metropolitan city of Seoul, South Korea: Diurnal variation, source identification, secondary formation of organic aerosol, and health risk

Atmospheric volatile organic compounds (VOCs) in Seoul, the capital of South Korea, have attracted increased attention owing to their emission, secondary formation, and human health risk. In this study, we collected 24 hourly samples once a month at an urban site in Seoul for a year (a total of 288 samples) using a sequential tube sampler. Analysis results revealed that toluene (9.08 ± 8.99 μg/m3) exhibited the highest annual mean concentration, followed by ethyl acetate (5.55 ± 9.09 μg/m3), m,p-xylenes (2.79 ± 4.57 μg/m3), benzene (2.37 ± 1.55 μg/m3), ethylbenzene (1.81 ± 2.27 μg/m3), and o-xylene (0.91 ± 1.47 μg/m3), indicating that these compounds accounted for 77.8–85.6% of the seasonal mean concentrations of the total (Σ59) VOCs. The concentrations of the Σ59 VOCs were statistically higher in spring and winter than in summer and fall because of meteorological conditions, and the concentrations of individual VOCs were higher during the daytime than nighttime owing to higher human activities during the daytime. The conditional bivariate probability function and concentration weighted trajectory analysis results suggested that domestic effects (e.g., vehicular exhaust and solvents) exhibited a dominant effect on the presence of VOCs in Seoul, as well as long-range atmospheric transport of VOCs. Further, the most important secondary organic aerosol formation potential (SOAFP) compounds included benzene, toluene, ethylbenzene, and m,p,o-xylenes, and the total SOAFP of nine VOCs accounted for 5–29% of the seasonal mean PM2.5 concentrations. The cancer and non-cancer risks of the selected VOCs were below the tolerable (1 × 10−4) and acceptable (Hazard quotient: HQ < 1) levels, respectively. Overall, this study highlighted the feasibility of the sequential sampling of VOCs and hybrid receptor modeling to further understand the source–receptor relationship of VOCs.

Distribution characteristics of organosulfates (OSs) in PM2.5 in Tianjin, Northern China: Quantitative analysis of total and three OS species

Organosulfates (OSs) are important secondary organic aerosol (SOA) species in atmospheric fine particles (PM2.5) and can be considered as molecular indicators of SOA. To understand their seasonal and diurnal distribution characteristics and formation mechanism in northern China, PM2.5 samples collected in daytime and nighttime in winter and summer 2019 in Tianjin, China were studied for total OSs and three OS species (methyl sulfate (MS), glycolic acid sulfate (GAS), benzyl sulfate (BS)). The S contents of total OSs (SOSs) in winter and summer were 0.6 ± 1 μg m−3 and 0.4 ± 0.3 μg m−3, respectively, in PM2.5. BS found to be less abundant among the measured species, and accounted for only 0.8%–4.8% of methyl sulfate (MS), and 0.01%–0.3% of glycolic acid sulfate (GAS). Average content of GAS was higher in summer than in winter, while that of MS and BS were opposite. The fractions of MS, GAS, and BS in SOSs were higher in daytime than that in night during winter, despite their concentrations were higher in nighttime, indicating that the concentrations of unidentified OS species were much higher in nighttime than in daytime. Such diurnal variations implied that relative humidity (RH) played a major role in the formation processes of OSs, especially biogenic OSs and the acid catalyzed reaction of SO42− might be a main pathway of OSs formation during winter. High T, RH and O3 determined biological GAS in summer, while NO2 and SO2 determined anthropogenic OSs in winter. We also found that the fractions of SOSs in S contents of organic sulfur (SOS) and the S contents of MS + GAS+BS (SMS+GAS+BS) in SOSs were accounted for only less than 10% and 5%, respectively. Therefore, this study suggests the components of OS and OSs in PM2.5 have not been discovered fully yet and needs further research.

Molecular composition of secondary organic aerosol from styrene under different NOx and humidity conditions

Styrene is highly reactive in the atmosphere, which has been identified as a hazardous air pollutant by the Clean Air Act, and as an important secondary organic aerosol (SOA) precursor. In this study, the SOA formation from styrene under different NOx and relative humidity (RH) conditions were explored in an indoor chamber. The chemical composition of SOA was characterized using a Fourier transform infrared spectroscopy (FTIR) and a modified atmospheric pressure chemical ionization mass spectrometry (APCI-MS). The results show that the SOA yields decrease from 11.4% to 7.2% at about 70 μg m−3 SOA as RH increases from 8.5% to 62.4% in the H2O2 system; the SOA yields decrease from 1.1% to 0.7% at about 10 μg m−3 SOA as RH increases from 8.4% to 60.9% in the NOx system. In the H2O2 system, C7 and C8 series compounds are major components of SOA at both RHs. The total signal ratio of less oxidized C7 and C8 compounds (nO < 4) to the total SOA at 8.8% RH is 2.2 times higher than that at 63.3% RH. The OSc values of SOA were determined to be −0.2 at 8.8% RH and 0.04 at 63.3% RH based on MS data. It is proposed that high RH can accelerate the heterogeneous uptake of H2O2 onto particles and promote the further oxidation of less oxidized compounds in the particle phase. In the NOx system, SOA formation is significantly suppressed by the competitive reaction of RO2 with NO. The FTIR and MS results show that the main composition of SOA is organonitrates. At 8.3% RH, OH, O3 and NO3 are oxidants in the styrene-NOx irritation system. Among these oxidants, NO3 plays a minor role due to its photolysis. While at 60.9% RH, SOA formation from O3 is significantly inhibited by the reaction of stabilized Criegee intermediates (SCIs) with H2O, and SOA from the NO3 pathway is suppressed by the heterogeneous uptake of N2O5 to humid particles.

Precursors and Pathways Leading to Enhanced Secondary Organic Aerosol Formation during Severe Haze Episodes.

Molecular analyses help to investigate the key precursors and chemical processes of secondary organic aerosol (SOA) formation. We obtained the sources and molecular compositions of organic aerosol in PM2.5 in winter in Beijing by online and offline mass spectrometer measurements. Photochemical and aqueous processing were both involved in producing SOA during the haze events. Aromatics, isoprene, long-chain alkanes or alkenes, and carbonyls such as glyoxal and methylglyoxal were all important precursors. The enhanced SOA formation during the severe haze event was predominantly contributed by aqueous processing that was promoted by elevated amounts of aerosol water for which multifunctional organic nitrates contributed the most followed by organic compounds having four oxygen atoms in their formulae. The latter included dicarboxylic acids and various oxidation products from isoprene and aromatics as well as products or oligomers from methylglyoxal aqueous uptake. Nitrated phenols, organosulfates, and methanesulfonic acid were also important SOA products but their contributions to the elevated SOA mass during the severe haze event were minor. Our results highlight the importance of reducing nitrogen oxides and nitrate for future SOA control. Additionally, the formation of highly oxygenated long-chain molecules with a low degree of unsaturation in polluted urban environments requires further research.

Naphthalene‐Derived Secondary Organic Aerosols Interfacial Photosensitizing Properties

AbstractWe investigated the photosensitizing properties of secondary organic aerosol (SOA) formed during the hydroxyl radical (OH) initiated oxidation of naphthalene. This SOA was injected into an aerosol flow tube and exposed to UV radiation and gaseous volatile organic compounds or sulfur dioxide (SO2). The aerosol particles were observed to grow in size by photosensitized uptake of d‐limonene and β‐pinene. In the presence of SO2, a photosensitized production (0.2–0.3 µg m−3 h−1) of sulfate was observed at all relative humidity (RH) levels. Some sulfate also formed on particles in the dark, probably due to the presence of organic peroxides. The dark and photochemical pathways exhibited different trends with RH, unraveling different contributions from bulk and surface chemistry. As naphthalene and other polycyclic aromatics are important SOA precursors in the urban and suburban areas, these dark and photosensitized reactions are likely to play an important role in sulfate and SOA formation.

SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and NH&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; emissions enhance organosulfur compounds and fine particle formation from the photooxidation of a typical aromatic hydrocarbon

Abstract. Aromatic hydrocarbons can dominate the volatile organic compound budget in the urban atmosphere. Among them, 1,2,4-trimethylbenzene (TMB), mainly emitted from solvent use, is one of the most important secondary organic aerosol (SOA) precursors. Although atmospheric SO2 and NH3 levels can affect secondary aerosol formation, the influenced extent of their impact and their detailed driving mechanisms are not well understood. The focus of the present study is to examine the chemical compositions and formation mechanisms of SOA from TMB photooxidation influenced by SO2 and/or NH3. Here, we show that SO2 emission could considerably enhance aerosol particle formation due to SO2-induced sulfate generation and acid-catalyzed heterogeneous reactions. Orbitrap mass spectrometry measurements revealed the generation of not only typical TMB products but also hitherto unidentified organosulfates (OSs) in SO2-added experiments. The OSs designated as being of unknown origin in earlier field measurements were also detected in TMB SOA, indicating that atmospheric OSs might also be originated from TMB photooxidation. For NH3-involved experiments, results demonstrated a positive correlation between NH3 levels and particle volume as well as number concentrations. The effects of NH3 on SOA composition were slight under SO2-free conditions but stronger in the presence of SO2. A series of multifunctional products with carbonyl, alcohols, and nitrate functional groups were tentatively characterized in NH3-involved experiments based on infrared spectra and mass spectrometry analysis. Plausible formation pathways were proposed for detected products in the particle phase. The volatility distributions of products, estimated using parameterization methods, suggested that the detected products gradually condense onto the nucleation particles to contribute to aerosol formation and growth. Our results suggest that strict control of SO2 and NH3 emissions might remarkably reduce organosulfates and secondary aerosol burden in the atmosphere. Updating the aromatic oxidation mechanism in models could result in more accurate treatment of particle formation for urban regions with considerable SO2, NH3, and aromatics emissions.

Volatile Organic Compounds Monitored Online at Three Photochemical Assessment Monitoring Stations in the Pearl River Delta (PRD) Region during Summer 2016: Sources and Emission Areas

Volatile organic compounds (VOCs) were monitored online at three photochemical assessment monitoring stations (MDS, WQS and HGS) in the Pearl River Delta region during the summer of 2016. Measured levels of VOCs at the MDS, WQS and HGS sites were 34.78, 8.54 and 8.47 ppbv, respectively, with aromatics and alkenes as major ozone precursors and aromatics as major precursors to secondary organic aerosol (SOA). The positive matrix factorization (PMF) model revealed that VOCs at the sites mainly came from vehicle exhaust, petrochemical industry, and solvent use. Vehicle exhaust and industrial processes losses contributed most to ozone formation potentials (OFP) of VOCs, while industrial processes losses contributed most to SOA formation potentials of VOCs. Potential source contribution function (PSCF) analysis revealed a north-south distribution for source regions of aromatics occurring at MDS with emission sources in Guangzhou mainly centered in the Guangzhou central districts, and source regions of aromatics at WQS showed an east-west distribution across Huizhou, Dongguan and east of Guangzhou, while that at HGS showed a south-north distribution across Guangzhou, Foshan, Zhaoqing and Yangjiang. This study demonstrates that multi-point high time resolution data can help resolve emission sources and locate emission areas of important ozone and SOA precursors.

Humidity-Dependent Viscosity of Secondary Organic Aerosol from Ozonolysis of β-Caryophyllene: Measurements, Predictions, and Implications

To predict important secondary organic aerosol (SOA) properties, information on viscosity or diffusion rates within SOA is needed. Ozonolysis of β-caryophyllene is an important SOA source; however,...

Chemical characterization of secondary organic aerosol at a rural site in the southeastern US: insights from simultaneous high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and FIGAERO chemical ionization mass spectrometer (CIMS) measurements

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

Secondary organic aerosol reduced by mixture of atmospheric vapours.

Secondary organic aerosol contributes to the atmospheric particle burden with implications for air quality and climate. Biogenic volatile organic compounds such as terpenoids emitted from plants are important secondary organic aerosol precursors with isoprene dominating the emissions of biogenic volatile organic compoundsglobally. However, theparticle mass from isopreneoxidation is generally modest compared to that of other terpenoids. Here we show that isoprene, carbon monoxide and methane can each suppress the instantaneous mass and the overall mass yield derived from monoterpenes in mixtures of atmospheric vapours. We find that isoprene 'scavenges' hydroxyl radicals, preventing their reaction with monoterpenes, and the resulting isoprene peroxy radicals scavenge highly oxygenated monoterpene products. These effects reduce the yield of low-volatility products that would otherwise form secondary organic aerosol. Global model calculations indicate that oxidant and product scavenging can operate effectively in the real atmosphere. Thus highly reactive compounds (such as isoprene) that produce a modest amount of aerosol are not necessarily net producers of secondary organic particle mass and their oxidation in mixtures of atmospheric vapours can suppress both particle number and mass of secondary organic aerosol. We suggest that formation mechanisms of secondary organic aerosol in the atmosphere need to be considered more realistically, accounting for mechanistic interactions between the products of oxidizing precursor molecules (as is recognized to be necessary when modelling ozone production).

Simulating secondary organic aerosol from anthropogenic and biogenic precursors: comparison to outdoor chamber experiments, effect of oligomerization on SOA formation and reactive uptake of aldehydes

Abstract. New parameterizations for the formation of organic aerosols were developed. These parameterizations cover secondary organic aerosol (SOA) formation from biogenic and anthropogenic precursors, NOx dependency, oligomerization and the reactive uptake of pinonaldehyde. These parameterizations were implemented in a box model in which the condensation and/or evaporation of semi-volatile organic compounds was simulated by the Secondary Organic Aerosol Processor (SOAP) model to take the dynamic evolution of concentrations into account.The parameterizations were tested against several experiments carried out in previous studies in the EUPHORE outdoor chamber. Two datasets of experiments were used: the anthropogenic experiments (in which SOA is formed mainly from a mixture of toluene, 1,3,5-trimethylbenzene and o-xylene) and the biogenic experiments (in which SOA is formed mainly from α-pinene and limonene).When assuming no wall deposition of organic vapors, satisfactory results (bias lower than 20 %) were obtained for the biogenic experiments and for most of the anthropogenic experiments. However, a decrease of SOA concentrations (up to 30 %) was found when taking wall deposition of organic vapors into account (with the parameters of Zhang et al., 2014). The anthropogenic experiments seem to indicate a complex NOx dependency that could not be reproduced by the model. Oligomerization was found to have a strong effect on SOA composition (oligomers were estimated to account for up to 78 % of the SOA mass) and could therefore have a strong effect on the formation of SOA. The uptake of pinonaldehyde (which is a high-volatility semi-volatile organic compound, SVOC) onto acidic aerosol was found to be too slow to be significant under atmospheric conditions (no significant amount of SOA formed after 3 days of evolution), indicating that the parameterization of Pun and Seigneur (2007) used in some air quality models may lead to an overestimation of SOA concentrations. The uptake of aldehydes could nevertheless be an important SOA formation pathway for less volatile or more reactive aldehydes than pinonaldehyde.Regarding viscosity, a low effect of viscosity on SOA concentrations was estimated by the model, although a decrease of SVOC evaporation was found when taking it into account, as well as a lower sensitivity of concentrations to changes of temperature during the experiments.

Mitigation of Secondary Organic Aerosol Formation from Log Wood Burning Emissions by Catalytic Removal of Aromatic Hydrocarbons.

Log wood burning is a significant source of volatile organic compounds including aromatic hydrocarbons (ArHC). ArHC are harmful, are reactive in the ambient atmosphere, and are important secondary organic aerosol (SOA) precursors. Consequently, SOA represents a major fraction of the sub-micron organic aerosol pollution from log wood burning. ArHC reduction is thus critical in the mitigation of adverse health and environmental effects of log wood burning. In this study, two Pt-based catalytic converters were prepared and tested for the mitigation of real-world log wood burning emissions, including ArHC and SOA formation, as well as toxic carbon monoxide and methane, a greenhouse gas. Substantial removal of mono- and polycyclic ArHC and phenolic compounds was achieved with both catalysts operated at realistic chimney temperatures (50% conversion was achieved at 200 and 300 °C for non-methane hydrocarbons in our experiments for Pt/Al2O3 and Pt/CeO2-Al2O3, respectively). The catalytically cleaned emissions exhibited a substantially reduced SOA formation already at temperatures as low as 185-310 °C. This reduces the sub-micron PM burden of log wood burning significantly. Thus, catalytic converters can effectively reduce primary and secondary log wood burning pollutants and, thereby, their adverse health impacts and environmental effects.

The influence of UV-light irradiation and stable Criegee intermediate scavengers on secondary organic aerosol formation from isoprene ozonolysis

The formation of secondary organic aerosol (SOA) from isoprene ozonolysis was investigated using a FEP Teflon reactor with and without the presence of H2O, CO, SO2 and UV-light irradiation (365 nm) to reveal their possible influence on SOA formation. Compared with the base experiment (isoprene + O3) under dark condition, the presence of 2000 ppm H2O and 1000 ppm CO could remarkably suppress SOA formation from isoprene ozonolysis, with SOA yields reduced from 2.96% to 1.47% and 2.08%, respectively. The evident reduction of SOA yields from isoprene ozonolysis with the presence of H2O and CO was mainly ascribed to the suppression of Stable Criegee Intermediates (SCIs). In contrast, the particle yield from isoprene ozonolysis with the presence of SO2 pronouncedly increased from 2.96% to 57.9%, suggesting that the oxidation of SO2 by SCIs made great contribution to the particle formation. The SOA or particle yields from all the reaction systems with and without presence of H2O, CO and SO2 reduced more than 27% under the UV-light irradiation with respect to dark condition, which were mainly attributed to the SCIs photolysis. The contribution of OH channel to SOA formation from isoprene ozonolysis was estimated to be less than 30%. Considering the 50% reduction of SOA yield from isoprene ozonolysis under light irradiation, the disappearance of the component with low O:C and H:C and high molecule weight in the SOA sample collected under light irradiation indicated the polymerization of unsaturated SCIs might be an important SOA formation channel besides the oligomerization of SCIs, HCHO, and MACR from isoprene ozonolysis under dark condition.

Identification and Quantification of 4-Nitrocatechol Formed from OH and NO3 Radical-Initiated Reactions of Catechol in Air in the Presence of NOx: Implications for Secondary Organic Aerosol Formation from Biomass Burning.

Catechol (1,2-benzenediol) is emitted from biomass burning and produced from a reaction of phenol with OH radicals. It has been suggested as an important secondary organic aerosol (SOA) precursor, but the mechanisms of gas-phase oxidation and SOA formation have not been investigated in detail. In this study, catechol was reacted with OH and NO3 radicals in the presence of NOx in an environmental chamber to simulate daytime and nighttime chemistry. These reactions produced SOA with exceptionally high mass yields of 1.34 ± 0.20 and 1.50 ± 0.20, respectively, reflecting the low volatility and high density of reaction products. The dominant SOA product, 4-nitrocatechol, for which an authentic standard is available, was identified through thermal desorption particle beam mass spectrometry and Fourier transform infrared spectroscopy and was quantified in filter samples by liquid chromatography using UV detection. Molar yields of 4-nitrocatechol were 0.30 ± 0.03 and 0.91 ± 0.06 for reactions with OH and NO3 radicals, and thermal desorption measurements of volatility indicate that it is semivolatile at typical atmospheric aerosol loadings, consistent with field studies that have observed it in aerosol particles. Formation of 4-nitrocatechol is initiated by abstraction of a phenolic H atom by an OH or NO3 radical to form a β-hydroxyphenoxy/o-semiquinone radical, which then reacts with NO2 to form the final product.

Estimating the Secondary Organic Aerosol Concentration and Source Apportionment During the Summer and Winter in the Nanjing Industrial District

Volatile organic compounds (VOCs) were determined by GC5000, an automatic on-line Gas Chromatography-Flame Ionization Detector. Elemental carbon (EC) and organic carbon (OC) were determined by the thermal/optical method using DRI-2001A during the periods of June 15th-July 15th 2015 and December 16th 2015-January 15th 2016. The concentration of secondary organic aerosol(SOA) was estimated by fractional aerosol coefficients (FAC) and EC tracer method. The source apportionment relied on the positive matrix factorization model (PMF). There were several conclusions:First, aromatic hydrocarbon was the main substance causing the SOA pollution in the Nanjing Industrial district, the contributions of aromatic hydrocarbon to SOA during summer and winter were 80.39% and 94.63%, respectively. The main contributers were benzene, toluene, ethylbenzene, m,p-xylene and o-xylene (BTEX). In the summer, SOA concentration ranged from 5.84-20.88 μg·m-3 with an average of 12.15 μg·m-3 and in the winter ranged from 2.17-17.73 μg·m-3 in which the average concentration was 6.91 μg·m-3. Secondly, SOA concentration decreased when wind and precipitation increased. By using the PMF model, a total of 7sources of SOA were determined in summer and 6 were determined in winter. There were 3 main sources in summer, including painting, petroleum processing and petrochemical industry, and the contributions to SOA were 0.65 μg·m-3, 0.21 μg·m-3, 0.18 μg·m-3, respectively. In winter, the most important SOA pollution was from painting, in which the contribution was 0.94 μg·m-3.