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Organic aerosols (OAs) are key components of atmospheric PM 2.5, which impose an important influence on citizens' health and atmospheric chemistry. However, the vertical distribution and evolutionary mechanisms of OAs have remained unclear, posing challenges for accurate model simulations. To compare the difference in molecular distribution and sources at different altitudes and explore the impact of vertical transport of mountain-valley breezes on OAs in mountainous regions, PM 2.5 samples were simultaneously collected at both the summit (SM: 1840 m a.s.l) and the foot Mt. Huang (FM: 480 m a.s.l.) in winter. All the 128 detected organic species presented higher concentrations at FM (237 ± 118 ng m−3) than at SM (89.3 ± 40.9 ng m−3), indicating that the influence of anthropogenic emissions on OAs became more significant as elevation decreased. Levoglucosan was the dominant single organic species and displayed robust correlations with most organic components at both sites, suggesting the notable effect of biomass burning on OAs in the alpine atmosphere. Molecular composition, characteristic ratios, and positive matrix factorization analysis suggested that OAs at SM were more aged and mostly derived from secondary oxidation and biomass burning from long-range transport. However, anthropogenic sources (e.g., biomass burning and fossil fuel combustion) contributed more to OAs at FM. Potential source contribution function results revealed that the northeastern part of Jiangxi Province and the nearby regions of the observation point are the potential source regions for OAs at FM, while long-range transport from northwestern Anhui Province and northern Jiangxi Province played a key role in OAs at SM. This study proved that OAs at FM could influence the mountaintop aerosols through the vertical transport of valley breeze in the day at Mt. Huang. These findings enhance our understanding of the vertical distribution of OAs and provide a scientific foundation for cross-regional atmospheric pollution control. [Display omitted] • The concentrations of all the 128 organic species decreased as elevation increased. • The effect of anthropogenic pollutants became more significant as elevation decreased. • The mountaintop organic aerosols (OAs) were more aged than those at the foot. • The mountaintop OAs in the day were strongly impacted by the valley wind in winter. [ABSTRACT FROM AUTHOR]

As a part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB-2018), shipborne measurements of aerosol properties were carried out to study the influence of South Asian outflow on cloud condensation nuclei (CCN) characteristics over the South East Arabian Sea (SEAS) and equatorial Indian Ocean (EIO) during winter, 2018. The poor association between CCN concentration and mass concentrations of the major aerosol species (organic matter, sulfate, and elemental carbon) is observed over SEAS, where airmass is mostly continental. In comparison, the CCN is well-correlated with aerosol mass concentrations over remote EIO. The dependence of CCN activation (at lower supersaturations) on the ultrafine particle fraction differs between SEAS and EIO depending on the mass concentration of organic aerosols. The coexistence of small and hydrophobic aerosols (organics) decreased the CCN activation ratio during ultrafine particle events. The hygroscopicity estimated using the chemical composition of aerosols is comparable to that estimated from CCN measurements over the Indian Ocean, except for the SEAS, where organic aerosols dominate. The effect of outflow aerosols on cloud droplet number concentration (CDNC) is simulated using a parcel model, which showed a good association (R ∼0.96) with CCN measurements over the EIO; however, the estimated CDNC did not follow the CCN variation over the SEAS region. This study highlights the importance of aerosol chemical composition, especially organics, in CCN activation and cloud droplet formation in the South Asian outflow over the Indian Ocean. • Hygroscopicity (κ) of continental aerosols over the Indian Ocean is examined. • Ultrafine particles and organics decrease the CCN activation of South Asian outflow. • CCN at 0.2% SS estimated from measured κ has positive bias over the Arabian Sea. • Estimated CDNC over near-coastal regions is mostly invariant with CCN concentration. [ABSTRACT FROM AUTHOR]

Abstract A high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed during wintertime (5 February to 15 March 2016) at a suburban site in Douai, northern France, in order to investigate the characteristics and sources of the organic matter (OM). The campaign average concentration of non-refractory submicron particulate matter (NR-PM 1) was 11.1 ± 9.3 μg m−3, and composed of 38% OM, 36% nitrate, 16% ammonium and 9% sulfate. The average values for the OM:OC, O:C and H:C ratios were 1.60 ± 0.15, 0.32 ± 0.11 and 1.55 ± 0.14, respectively, indicating a moderate level of aerosol oxidation. The positive matrix factorization (PMF) source apportionment method was applied to the high-resolution organic aerosol (OA) mass spectra, resulting in four factors: a hydrocarbon-like (HOA) factor; one associated with oxidized biomass burning (oBBOA); and two oxygenated factors (OOA) denoted as less oxidized (LO-OOA) and more oxidized (MO-OOA), with average contributions to OA of 20%, 28%, 17% and 35%, respectively. The oBBOA factor was found to be mainly local as shown by non-parametric wind regression (NWR) analysis, and to correlate well with relative humidity (RH), suggesting fast aqueous processing of locally emitted primary biomass burning emissions. During most part of the campaign, the sampling site was affected by different air masses. However, during the last period of the campaign (5–16 March 2016) the site was heavily impacted by air masses from Eastern Europe which were rich in secondary inorganic and organic aerosols. The H:C versus O:C (Van Krevelen, VK) diagram highlighted that the aerosol followed an oxidation process throughout the whole campaign, with an average slope of −1.05. The impact of continental air masses towards the end of the campaign confined the aerosol towards a narrower space in the VK diagram, suggesting a homogenization of the different aerosol sources due to OA ageing during transport. Several nocturnal NPF (new particle formation)-like events were observed and associated to the fast processing of BBOA emissions and the formation of ammonium nitrate. Graphical abstract Image 1 Highlights • Organic matter at a suburban site in winter investigated at high time resolution. • Aerosols were moderately oxidized with four sources of organic aerosol identified. • Oxidized biomass burning organic aerosols (BBOA) mostly local represented 28% of OA. • Several nocturnal NPF (new particle formation)-like events were observed. • It suggests a fast processing of BBOA and the possible role of ammonium nitrate. [ABSTRACT FROM AUTHOR]

The characteristics of non-refractory submicron aerosols (NR-PM 1), focusing on organic aerosols, are examined over a tropical coastal location, Thumba, in southern peninsular India. The measurements were carried out using an aerosol chemical speciation monitor during 2017–2021. The results indicated organic aerosols as the major component (67–85%) in the NR-PM 1 mass loading during all seasons. Positive matrix factorization (PMF) analysis on the OA mass spectra yielded two factors resolving Hydrocarbon-like organic aerosols (HOA) and Oxygenated organic aerosols (OOA), with the major contribution to OA coming from OOA (mass fraction >50%) during all the seasons. Examination of the diurnal variations revealed the daytime enhancement in OOA and sulfate mass fractions, indicating photochemical production overwhelming the dilution at the surface due to local atmospheric boundary layer expansion. The OOA mass concentration showed a good correlation (R > 0.5) with odd oxygen (O 3 +NO 2) and with an increase in oxygen-to-carbon ratio (ΔO:C = 0.11 to 0.35 with Δodd oxygen = 60 μg m−3 for different seasons) (i.e., increased oxidation degree of OA) signifying the dominance of photochemistry and subsequent enhanced atmospheric oxidative capacity in OOA formation. Interestingly, the aerosol liquid water content (ALWC) doubled during the nighttime (>40 μg m−3) compared to the daytime (<20 μg m−3) in different seasons, indicating the combined impact of varying submicron aerosol composition and ambient relative humidity. Since highly humid (relative humidity >85%) conditions prevail over this tropical coastal station, especially during the nighttime, the role of aqueous-phase formation of OA is also examined. It was found that less oxidized/less-hygroscopic OA were prevalent during the nighttime, resulting in lower aqueous-phase OA processing. During all the seasons, the hygroscopicity of organics (к organics) reached as high as > 0.22 and was mainly driven by photochemical processing rather than aqueous-phase chemistry. These results highlight the importance of the OA formation pathways in affecting their hygroscopic properties over the tropical atmosphere. • Organics are the dominant NR-PM 1 species in all seasons. • Oxygenated species dominated (>50%) the overall OA. • Photochemical reactions dominated OOA production. • The hygroscopicity of organics is mainly driven by photochemical processing. [ABSTRACT FROM AUTHOR]

Organic aerosols (OAs) account typically for 20–90% of fine particles (PM 2.5 ) in the lower troposphere. They contribute to a wide range of environmental problems, from local issues (e.g., urban haze) to global problems (e.g., climate change). Huge efforts have been dedicated to studying the composition, abundance, spatial and temporal distribution and sources of organic aerosols in China. This review aims to summarize recent studies on characteristics and sources of OAs and assesses the current state of understanding of the organic aerosol pollution in China. The OA constitutes ∼20–45% of the PM 2.5 with an annual mean value of 5.2–44.5 μg/m 3 at sites across China, which is about five times higher than that reported in North America and Europe. There are thousands of different organic compounds in atmospheric aerosols, but only about 5–10% of them have been identified and quantified. OAs show pronounced spatial variations with much higher concentrations in Northern than Southern China. Seasonally, the highest OA concentrations are observed in the winter, whereas the lowest are in the summer. This is due to higher emission rates from anthropogenic sources (e.g., biomass and coal combustion) and poorer dispersion conditions in the winter. Approximately 60–80% of total OA is apportioned by receptor modeling (such as Chemical Mass Balance, CMB) and tracer-yield method, based on the source profiles of primary organic aerosols (POA) and secondary OA (SOA) derived from local emission sources. A number of OA sources are identified, including motor vehicles, industrial emissions, biomass combustion, food cooking, and coal combustion for POA and anthropogenic and biogenic emissions for SOA. Industrial emissions and motor vehicular exhaust are the dominant sources of organic aerosols in the industrialized areas of Northern China, as well as the Pearl River Delta and Eastern China, whereas in other urban areas, residential coal combustion and motor vehicular exhaust are the dominant sources in winter and summer respectively. [ABSTRACT FROM AUTHOR]

The effects of aromatic carbonyls (ACs), polycyclic aromatic hydrocarbons (PAHs) and phenols (PHs) on the photolysis of nitroaromatic compounds (ArNO 2) in the solid phase were explored. The reactive nitrogen gas formation by the photolysis process was investigated in a cylindrical reactor coupled to a NO x analyzer. These organics had an enhanced or inhibited influence on the nitrous acid (HONO) and NO formations by the photolysis of 1-nitropyrene (NPy) and 5-nitrosalicylic acid (NSA), depending on the hydrogen abstraction and donation abilities of organics. Based on the high-performance liquid chromatography (HPLC) results, methyl-2-benzoylbenzoate (MBB), fluoranthene (FLA) and syringaldehyde (SG) showed a promoted effect on the photodegradation of NPy and NSA, as shown by an increase in the photolysis frequency of ArNO 2 (J ArNO 2) by a factor of 1.19–2.76. Based on experimental results, the HONO source strength by the photolysis of ArNO 2 in organic aerosols was estimated to be lower than 1 ppt h−1. • Organics enhanced or impeded the HONO generation by the nitroaromatics photolysis. • The nitroaromatics photolysis frequency increased by 0.19–1.76 times with organics. • The HONO source strength of nitroaromatics photolysis was lower than 1 ppt h−1. [ABSTRACT FROM AUTHOR]

Organic aerosol (OA) is one of the major components of fine atmospheric aerosol particles. Recent field campaigns and modeling studies in Colombia have demonstrated the presence of a large OA fraction in urban and rural aerosols. In this work we focus on constraining the sources associated with OA and its precursors over the city of Bogotá, Colombia. We used PM 2.5 chemical speciation data from field campaigns carried out during 2018 to evaluate the ability of a regional transport model to reproduce and explain the observed OA. The samples were collected at three sites during high- and low-aerosol concentration seasons in the region. We used The Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) with two different chemical mechanisms and aerosol schemes, RACM/MADE-VBS and MOZART/MOSAIC, in conjunction with the detailed in-situ aerosol chemical speciation observations to analyze the seasonality of OA in the city of Bogotá, and to establish the most relevant sources. Simulations were carried out for the same periods for which data is available, spanning a high biomass burning (BB) season (February 2018) and a low biomass burning season (September 2018). We demonstrated that long-range transport of BB emissions from Northern Amazonia can episodically increase aerosol loading during September, while BB activity in the Grasslands of the Orinoco River Basin are the main source during February. The two aerosol schemes utilized in this work reproduce the observed seasonal variations in total fine aerosol concentration, with a difference between February and September of 10 μ g m−3. The comparison between aerosol schemes demonstrated that MOZART/MOSAIC consistently predicted more OA with a larger SOA:OA ratio than in the RACM/MADE-VBS experiment. SOA dominates the OA fraction by 66% for RACM/MADE-VBS and 74% for MOZART/MOSAIC during February and 69% for RACM/MADE-VBS and 71% for MOZART/MOSAIC during September. These differences between organic aerosol burden between the mechanisms used in this study may be attributed to the different treatment of SOA gas/particle partitioning in the schemes. • WRF-Chem simulations and in-situ PM 2.5 chemical composition data were used to understand the contribution of OA over Bogotá. • Biomass burning plumes from Northern Amazonia and from the Orinoco River Grassland can severely impact the region. • Field data shows that OA contributes over 40% to PM 2.5 in the city. • The model underpredicts organic aerosols, with the MOZART/MOSAIC scheme predicting higher POA and SOA than RACM/MADE-VBS. [ABSTRACT FROM AUTHOR]

The air quality in the outflow from Fort Worth, TX was studied in June 2011 at a location surrounded by oil and gas development in the Barnett Shale. The objectives of this study were to understand the major sources of volatile organic compounds (VOCs) and organic aerosols and explore the potential influence each VOC source had on ozone and secondary organic aerosol formation. Measurements of VOCs were apportioned between six factors using Positive Matrix Factorization (PMF): Natural Gas (25 ± 2%; ±99% CL); Fugitive Emissions (15 ± 2%); Internal Combustion Engines (15 ± 2%); Biogenic Emissions (7 ± 1%); Industrial Emissions/Oxidation 1(8 ± 1%); and Oxidation 2 (18 ± 2%). Reactivity calculations suggest the Biogenic and Oxidation 2 factors were the most likely VOC sources to influence local ozone. However, enough OH reactivity was calculated for factors related to the oil and gas development that they could incrementally increase O 3 . Three organic aerosol (OA) types were identified with PMF applied to high-resolution time-of-flight aerosol mass spectrometry measurements: hydrocarbon-like OA (HOA; 11% of mass) and two classes of oxidized OA (semi- and less-volatile OOA, SV and LV; 45% and 44%, respectively). The HOA correlated with the Internal Combustion Engine VOC factor indicating that a large fraction of the HOA was emitted by gasoline and diesel motors. The SV-OOA correlated with the oxidized VOC factors during most of the study, whereas a correlation between LV-OOA and the oxidized VOC factors was only observed during part of the study. It is hypothesized that SV-OOA and the oxidized VOC factors correlated reasonably well because these factors likely were separated by at most only a few oxidation generations on the oxidation pathway of organic compounds. [ABSTRACT FROM AUTHOR]

In this study the long-term trends of non-refractory submicrometer aerosol (NR-PM 1 ) composition and mass concentration measured by an Aerosol Chemical Speciation Monitor (ACSM) at the Atmospheric Radiation Measurement (ARM) program's Southern Great Plains (SGP) site are discussed. NR-PM 1 data was recorded at ∼30 min intervals over a period of 19 months between November 2010 and June 2012. Positive Matrix Factorization (PMF) was performed on the measured organic mass spectral matrix using a rolling window technique to derive factors associated with distinct sources, evolution processes, and physiochemical properties. The rolling window approach also allows us to capture the dynamic variations of the chemical properties in the organic aerosol (OA) factors over time. Three OA factors were obtained including two oxygenated OA (OOA) factors, differing in degrees of oxidation, and a biomass burning OA (BBOA) factor. Back trajectory analyses were performed to investigate possible sources of major NR-PM 1 species at the SGP site. Organics dominated NR-PM 1 mass concentration for the majority of the study with the exception of winter, when ammonium nitrate increases due to transport of precursor species from surrounding urban and agricultural areas and also due to cooler temperatures. Sulfate mass concentrations have little seasonal variation with mixed regional and local sources. In the spring BBOA emissions increase and are mainly associated with local fires. Isoprene and carbon monoxide emission rates were obtained by the Model of Emissions of Gases and Aerosols from Nature (MEGAN) and the 2011 U.S. National Emissions Inventory to represent the spatial distribution of biogenic and anthropogenic sources, respectively. The combined spatial distribution of isoprene emissions and air mass trajectories suggest that biogenic emissions from the southeast contribute to SOA formation at the SGP site during the summer. [ABSTRACT FROM AUTHOR]

An intensive field campaign was carried out in Hefei, a city in the Yangtze River Delta in eastern China, in the summer of 2020 to examine the composition and sources of organic aerosols (OA). Time-resolved filter samples were collected and quantified to obtain the concentrations for organic functional groups (OFGs), including alkane, hydroxyl, carboxylic acid, non-acid carbonyl, amine, and aromatic groups using Fourier transform infrared (FTIR). Inorganic aerosol components and gas-phase pollutants were also measured to facilitate the data analysis. The OA concentration ranged from 2.8 to 41.8 μg m−3, with a mean value (±standard deviation) of 12.5 (±5.8) μg m−3, which accounted for 53.6% of the PM 2.5 mass. The most abundant OFG in OA was the hydroxyl group (35.2%), followed by alkane (24.0%) and carboxylic (22.4%) groups. The positive matrix factorization (PMF) was applied to the IR spectra of OA that identified three major sources, i.e., aged fossil fuel combustion, biomass burning, and vegetative detritus/dust. Their campaign-average contributions to OA were 38.3%, 28.3%, and 33.4%, respectively. The OFG composition of the aged fossil fuel combustion component was similar to the OFG composition of oxidized alkane products, indicating that alkanes may be important precursors of OA at the sampling site. Cluster and air mass trajectory analyses showed that the OFG concentration was highest when the air masses originated from the eastern Yangtze River Delta, suggesting the influence of the transported industrial emissions on the particulate pollution at the sampling site. • Organic functional group composition of PM 2.5 was quantified using FTIR in eastern China for the first time. • PMF identified aged fossil fuel combustion, biomass burning, and vegetative detritus/dust as sources of organic aerosols. • High particulate pollution was associated with air masses originating from the industrial areas of the Yangtze River Delta. [ABSTRACT FROM AUTHOR]

Abstract: Sugars are important water-soluble organic constituents of atmospheric particulate matter (PM). In order to better understand the sources and seasonal variations of sugars in aerosols, primary saccharides (fructose, glucose, sucrose, and trehalose) and sugar alcohols (arabitol and mannitol), together with levoglucosan, have been studied in ambient aerosols at Gosan, Jeju Island in the western North Pacific, the downwind region of the Asian outflow, using gas chromatography–mass spectrometry. The results showed that the sugar composition varied seasonally with a total concentration range of 6.8–1760 ng m−3 (mean 246 ng m−3). The total identified sugars had the highest concentration in April, the spring bloom season at Jeju Island, when sucrose contributed up to 80% of the total sugars. The dominance of sucrose was also detected in pollen samples, suggesting that pollen can contribute significantly to sucrose in aerosols during the spring bloom. The seasonal variation of trehalose is consistent with those of non-sea-salt Ca2+ and δ13C of total carbon with elevated levels during the Asian dust storm events. This study indicates that sugar compounds in atmospheric PM over East Asia can be derived from biomass burning, Asian dust, and primary biological aerosols such as fungal spores and pollen. Furthermore, this study supports the idea that sucrose could be used as a tracer for airborne pollen grains, and trehalose as a tracer for Asian dust outflow. [Copyright &y& Elsevier]

Abstract: Organic aerosol chemical markers from normalized concentrations of independent measurements of mass fragments (using Aerosol Mass Spectrometry, AMS) are compared to bond-based functional groups (from Fourier Transform Infrared spectroscopy, FTIR) during eight field projects in the western hemisphere. Several field projects show weak correlations between alcohol group fractions and m/z 60 fractions, consistent with the organic hydroxyl groups and the fragmentation of saccharides, but the weakness of the correlations indicate chemical differences among the relationships for ambient aerosols in different regions. Carboxylic acid group fractions and m/z 44 fractions are correlated weakly for three projects, with correlations expected for aerosols dominated by di-acid compounds since their fragmentation is typically dominated by m/z 44. Despite differences for three projects with ratios of m/z 44 to m/z 57 fragments less than 10, five projects showed a linear trend between the project-average m/z 44 to m/z 57 ratio and the ratio of acid and alkane functional groups. While this correlation explains only a fraction of the fragment and bond variability measured, the consistency of this relationship at multiple sites indicates a general agreement with the interpretation of the relative amount of m/z 44 as a carboxylic acid group marker and m/z 57 as an alkane group marker. [Copyright &y& Elsevier]

Abstract: The high-molecular weight water-soluble organic compounds present in atmospheric aerosols underwent functional-group characterization using liquid chromatography tandem mass spectrometry (LC-MS/MS), with a focus on understanding the chemical structure and origins of humic-like substances (HULIS) in the atmosphere. Aerosol samples were obtained from several locations in North America at times when primary sources contributing to organic aerosol were well-characterized: Riverside, CA, Fresno, CA, urban and peripheral Mexico City, Atlanta, GA, and Bondville, IL. Chemical analysis targeted identification and quantification of functional groups, such as aliphatic, aromatic, and bulk carboxylic acids, organosulfates, and carbohydrate-like substances that comprise species with molecular weights (MW) 200–600 amu. Measured high-MW functional groups were compared to modeled primary sources with the purpose of identifying associations between aerosol sources, high-MW aerosol species, and HULIS. Mobile source emissions were linked to high-molecular weight carboxylic acids, especially aromatic acids, biomass burning was associated with carboxylic acids and carbohydrate-like substances, and secondary organic aerosol (SOA) correlated well with the total amount of HULIS measured, whereas organosulfates showed no correlation with aerosol sources and exhibited unique spatial trends. These results suggested the importance of motor vehicles, biomass burning, and SOA as important sources of precursors to HULIS. Structural characteristics of atmospheric HULIS were compared to terrestrial humic and fulvic acids and revealed striking similarities in chemical structure, with the exception of organosulfates which were unique to atmospheric HULIS. [Copyright &y& Elsevier]

Abstract: We present highly time-resolved measurements of organic molecular markers in downtown Pittsburgh, which are used to investigate sources contributing to atmospheric aerosols in the area. Two-hour average concentrations of condensed-phase and semivolatile organic species were measured using a Thermal Desorption Aerosol GC/MS (TAG). Concentrations for mobile source markers like hopanes had regular diurnal and day-of-week patterns. Pairing high time-resolved measurements with meteorological data helped identify contributions from known point sources for markers correlated with wind direction. Black carbon (BC), volatile organic compounds (VOCs) and organic molecular markers were apportioned to sources using the Chemical Mass Balance (CMB) and Positive Matrix Factorization (PMF) receptor models. Diesel and gasoline mobile source factors were identified as the main sources of BC in the downtown Pittsburgh area, contributing 67% and 20% of the study-average BC. 13% of the BC was associated with a source factor tentatively identified as an industrial or regional source. The high time resolution of the TAG has the potential to provide important new insight into source apportionment efforts using organic molecular marker measurements. [Copyright &y& Elsevier]

Mesoscale variation of the chemical composition of submicron aerosol concentrations and their influence on cloud condensation nuclei (CCN) activation have been examined over a tropical coastal location in peninsular India during the winter season. The land-sea breeze circulation, boundary layer evolution, and photochemical processes significantly influence the diurnal variation of the aerosol chemical composition over the region. Broadly, the organic aerosols (mean ∼37.50 ± 27.00 μg m−3) dominated the submicron mass loading with a mass fraction (MF) of 0.66, followed by sulfate (∼10.92 ± 6.13 μg m−3; MF ∼0.19), ammonium (3.46 ± 1.71 μg m−3; MF∼0.06) and a small contribution from nitrate (1.7 ± 1.5 μg m−3; MF ∼0.03). The average mass concentration of organic carbon (OC) is 8.84 ± 6.42 μg m−3 with a mean organics/OC ratio of ∼3, indicating the presence of aged, highly oxidized organics. The diurnal variation of all the aerosol species, except sulfate, depicted daytime-low and nighttime-high. The photochemical formation of sulfate aerosols overwhelms the reduction in concentration due to boundary layer mixing and sea-land breeze circulation during the daytime. The decrease in the organics/sulfate ratio during the daytime has significant implications on the CCN concentration and its activation properties. The average CCN number concentration and activation ratio (ratio of CCN to total aerosol number concentration) at 0.38% supersaturation during the study period are ∼3756 ± 1609 cm−3 and 0.52 ± 0.17, respectively. The diurnal variation of CCN concentrations and activation ratio is distinctly different. The change in the mode diameter of the number size distribution between sea-land breeze was found to be less (∼8 nm) compared to the large variation in chemical composition. This suggested the importance of mesoscale variations of aerosols, especially the contrasting diurnal variation of organics and sulfate aerosols in CCN activation. • Mesoscale variation of aerosol chemical composition influenced the CCN activation. • The organics and sulfate aerosols showed contrasting diurnal variation. • OC/EC ratio values were lower during daytime compared to nighttime values. • High values of organics to OC ratio indicated the presence of aged organics. • CCN concentrations and activation ratio showed contrasting diurnal variation. [ABSTRACT FROM AUTHOR]

To better understand the current physical and chemical properties of East Asian aerosols, an intensive observation of atmospheric particles was conducted at Gosan site, Jeju Island, South Korea during 2005 spring. Total suspended particle (TSP) samples were collected using pre-combusted quartz filters and a high-volume air sampler with the time intervals ranging from 3h to 48h. The kinds and amount of various organic compounds were measured in the samples using gas chromatography–mass spectrometry. Among the 99 target compounds detected, saccharides (average, 130±14ngm−3), fatty acids (73±7ngm−3), alcohols (41±4ngm−3), n-alkanes (32±3ngm−3), and phthalates (21±2ngm−3) were found to be major compound classes with polyols/polyacids, lignin and resin products, PAHs, sterols and aromatic acids being minor. Compared to the previous results reported for 2001 late spring samples, no significant changes were found in the levels of their concentrations and compositions for 4 years, although the economy in East Asia, especially in China, has sharply expanded from 2001 to 2005. During the campaign at Gosan site, we encountered two distinct dust storm episodes with high TSP concentrations. The first dust event occurred on March 28, which was characterized by a predominance of secondary organic aerosols. The second event that occurred on the next day (March 29) was found to be characterized by primary organic aerosols associated with forest fires in Siberia/northeastern China. A significant variation in the molecular compositions, which was found within a day, suggests that the compositions of East Asian aerosols are heterogeneous due to multi-contributions from different source regions together with different pathways of long-range atmospheric transport of particles. [Copyright &y& Elsevier]

Real-time analyzers of selected chemical components (sulfate, nitrate, Black Carbon) and integrative aerosol parameters (particulate matter and light scattering coefficient) were implemented for a 2-week campaign (November–December 2005) in a suburban area of Clermont-Ferrand (France) in order to document fast changes in the chemical composition of submicron aerosols. Measurements of particulate organic matter (POM) were not available in the field but were indirectly estimated from time-resolved (3-min) reconstruction of the light scattering coefficient. This methodology offered the opportunity to investigate almost real-time and artifact-free POM concentrations even at low concentrations (typically below 0.1μgm−3). The overall uncertainties associated with this POM calculation were of the order of 20%, which are comparable to those commonly referred in literature for POM calculation or measurements. A chemical mass balance (CMB) of PM1 was performed using the derived POM concentrations and showed a very good correlation (slope=0.93; r 2=0.91, N=663) with real-time PM1 measurements obtained from R&P TEOM-FDMS, demonstrating the consistency of our approach. Important diurnal variations were observed in POM concentrations, with a dominant contribution of POM from fossil fuel origin during daytime and a dominant contribution of POM from residential wood burning at night. POM was calculated to contribute as much as 70% of PM1 during our study, pointing out the major role of carbonaceous aerosols at this period of the year at our residential area. [Copyright &y& Elsevier]

To assess the contribution of sources to fine particulate organic carbon (OC) at four sites in North Carolina, USA, a molecular marker chemical mass balance model (MM-CMB) was used to quantify seasonal contributions for 2 years. The biomass burning contribution at these sites was found to be 30–50% of the annual OC concentration. In order to provide a better understanding of the uncertainty in MM-CMB model results, a biomass burning profile sensitivity test was performed on the 18 seasonal composites. The results using reconstructed emission profiles based on published profiles compared well, while model results using a single source test profile resulted in biomass burning contributions that were more variable. The biomass burning contribution calculated using an average regional profile of fireplace emissions from five southeastern tree species also compared well with an average profile of open burning of pine-dominated forest from Georgia. The standard deviation of the results using different source profiles was a little over 30% of the annual average biomass contributions. Because the biomass burning contribution accounted for 30–50% of the OC at these sites, the choice of profile also impacted the motor vehicle source attribution due to the common emission of elemental carbon and polycyclic aromatic hydrocarbons. The total mobile organic carbon contribution was less effected by the biomass burning profile than the relative contributions from gasoline and diesel engines. [Copyright &y& Elsevier]

Organic materials make up a significant fraction of ambient particulate mass. It is important to quantify their contributions to the total aerosol mass for the identification of aerosol sources and subsequently formulating effective control measures. The organic carbon (OC) mass can be determined by an aerosol carbon analyzer; however, there is no direct method for the determination of the mass of organic compounds, which also contain N, H, and O atoms in addition to C. The often-adopted approach is to estimate the organic mass (OM) from OC multiplying by a factor. However, this OC-to-OM multiplier was rarely measured for a lack of appropriate methods for OM. We report here a top-down approach to determine OM by coupling thermal gravimetric and chemical analyses. OM is taken to be the mass difference of a filter before and after heating at 550°C in air for 4h minus mass losses due to elemental carbon (EC), volatile inorganic compounds (e.g., NH4NO3), and loss of aerosol-associated water that arise from the heating treatment. The losses of EC and inorganic compounds are determined through chemical analysis of the filter before and after the heating treatment. We analyzed 37 ambient aerosol samples collected in Hong Kong during the winter of 2003, spring of 2004, and summer of 2005. A value of 2.1±0.3 was found to be the appropriate factor to convert OC to OM in these Hong Kong aerosol samples. If the dominant air mass is classified into two categories, then an OM-to-OC ratio of 2.2 was applicable to aerosols dominated by continent-originated air mass, and 1.9 was applicable to aerosols dominated by marine air mass. [Copyright &y& Elsevier]

Aerosol–cloud condensation nuclei (CCN) closure was studied in a semi-rural location 80km north of Toronto, Canada at the Centre for Atmospheric Research Experiments outside of Egbert, Ontario during the fall of 2005. This site is subject to both polluted air from southern Ontario and clean air from the north. The purpose of the investigation was to evaluate the degree to which closure is attained at a supersaturation of 0.32% when size-resolved aerosol compositions from an Aerodyne Quadrupole Aerosol Mass Spectrometer are made alongside measurements of CCN number density and aerosol size distribution. Attention was given to assessing the sensitivity of closure to assumptions made concerning the water solubility and surface tension of the organic fraction of the aerosol in the Köhler analysis. By assuming that the organics are insoluble and that the growing droplet has the surface tension of water, a good overall degree of closure is attained throughout the analysis time period, with the predicted numbers of CCN within 15% of the modelled numbers, which is within our estimated systematic uncertainties. However, for the specific periods during which the organic content of the aerosol is high, the degree of closure is significantly lower. Sensitivity analyses indicate that some degree of organic water solubility and/or surface tension reduction is necessary to achieve the best agreement and least variance between the modelled and measured numbers of CCN. A general conclusion is that significant uncertainties arise in predicting CCN levels only when the level of soluble inorganic species is below approximately 25% by mass. [Copyright &y& Elsevier]

Abstract: Field observations of atmospheric aerosols have established that organic compounds compose a large fraction of the atmospheric aerosol mass. However, the physical/chemical pathway by which organic compounds are incorporated into atmospheric aerosols remains unclear. The potential role of acid-catalyzed reactions of organic compounds on acidic aerosols has been explored as a possible chemical pathway for the incorporation of organic material into aerosols. In the present study, ultraviolet–visible (UV–vis) spectroscopy was used to monitor the kinetics of formation of the products of the acid-catalyzed aldol condensation reaction of a range of aliphatic aldehydes (C2–C8). The experiments were carried out at various sulfuric acid concentrations and a range of temperatures in order to estimate the rate constants of such reactions on sulfuric acid aerosols under tropospheric conditions. The rate constants were generally found to decrease as the chain length of the aliphatic aldehyde increased (except for acetaldehyde, which had an unusually small rate constant), increase as a function of sulfuric acid concentration as predicted by excess acidity theory, and showed normal Arrhenius behavior as a function of temperature. While the kinetic data are generally consistent with previous laboratory reports of aldehyde reactivity in various sulfuric acid media, the aldol condensation reactions involving aliphatic aldehydes do not appear fast enough to be responsible for significant transfer of organic material into atmospheric aerosols. [Copyright &y& Elsevier]

Heterogeneous reactions of organic aerosols with atmospheric oxidants are important processes that affect the hygroscopicity and cloud condensation nuclei (CCN) activities of atmospheric aerosols. An electrodynamic balance (EDB) coupled with Raman spectroscopy is a particularly attractive platform for studying atmospheric reactions since it allows long-duration (days) particle levitation and reactions at atmospherically relevant low-oxidant concentrations can be investigated. In this study, we demonstrated the use of an EDB/Raman system to investigate the heterogeneous reactions of oleic acid particles with ozone (240–280ppb) under ambient temperatures (22–24°C) and dry conditions (relative humidity <5%) over a period of 20h. The Raman signatures of the ozone-processed oleic acid particles indicate the formation of oxidation products predominately consisting of peroxidic compounds (Oof peroxides and/or ozonides), carbonyl (Cdroxyl (Oonal characteristics, which are consistent with the predictions of the Criegee mechanism as well as the results reported in the literature. We also confirmed that the Raman signatures of the reacted particles at atmospheric and much higher (>10ppm) ozone concentrations are practically the same, which provides assurance to the use of elevated ozone concentrations in reaction studies of the oleic acid–ozone system in the literature. The ratio of the percentage of mass loss (due to evaporation of volatile organic products) to the percentage of oleic acid conversion was estimated to be 0.05. Furthermore, the oxidation products that remained in the particle phase were more hygroscopic than were their hydrophobic parent molecules. [Copyright &y& Elsevier]

The Interagency Monitoring of Protected Visual Environments (IMPROVE) protocols for reconstructing the ambient light extinction coefficient (b ext) from measured aerosol species are the basis for evaluating compliance under the Regional Haze Rule. Aerosol mass composition and optical properties have been measured as part of the IMPROVE program since 1988, providing a long-term data set of aerosol properties at 38 sites around the US. This data set is used to evaluate assumptions made in calculating reconstructed mass and b ext by applying statistical analysis techniques. In particular, the molecular weight to carbon weight ratio used to compute particulate organic matter is investigated. An annual average value of 1. 7±0. 2 for the IMPROVE sites, compared to the value of 1. 4 currently assumed in the IMPROVE algorithm, is derived. Regression analysis also indicates that fine soil mass concentrations are underestimated by roughly 20% on average. Finally, aerosol mass scattering and extinction efficiencies assumed in the IMPROVE reconstructed b ext protocol are examined. Fine mode (D p<2. 5μm) mass scattering efficiencies have a functional dependence on mass concentrations at many sites, and use of a mass-concentration-dependent adjustment factor to refine the assumed efficiencies provides for closer agreement between measured and reconstructed b ext. [Copyright &y& Elsevier]

Abstract: PM2.5 aerosols were collected in Nanjing, a typical mega-city in China, during summer and winter 2004 and were characterized for aromatic and cyclic compounds using a GC/MS technique to understand the air pollution problem. They include polycyclic aromatic hydrocarbons (PAHs), hopanes, phthalates and hydroxy-PAHs (OH-PAHs). PAHs, hopanes and OH-PAHs presented higher concentrations in winter (26–178, 3.0–18, and 0.013–0.421ngm−3, respectively) than in summer (12–96, 1.6–11, and 0.029–0.171ngm−3, respectively) due to an enhanced coal burning for house heating and atmospheric inversion layers developed in the cold season. In contrast, phthalates are more abundant in summer (109–368ngm−3, average 230ngm−3) than in winter (33–390ngm−3, average 170ngm−3) due to an enhanced evaporation from plastics during the hot season and the subsequent deposition on the pre-existing particles. Generally, all the identified compounds showed higher concentrations in nighttime than in daytime due to inversion layers and increased emissions from heavy-duty trucks at night. PAHs, hopanes and phthalates in Nanjing aerosols are 5–100 times more abundant than those in Los Angeles, USA, indicating a serious air pollution problem in the city. Concentrations of OH-PAHs are 1–3 orders of magnitude less than their parent PAHs and comparable to those reported from other international cities. Source identification using diagnostic ratios of the organic tracers suggests that PAHs in Nanjing urban area are mainly derived from coal burning, whereas hopanes are more attributable to traffic emissions. [Copyright &y& Elsevier]

Abstract: While it is well established that organics compose a large fraction of the atmospheric aerosol mass, the mechanisms through which organics are incorporated into atmospheric aerosols are not well understood. Acid-catalyzed reactions of compounds with carbonyl groups have recently been suggested as important pathways for transfer of volatile organics into acidic aerosols. In the present study, we use the aerodyne aerosol mass spectrometer (AMS) to probe the uptake of gas-phase hexanal into ammonium sulfate and sulfuric acid aerosols. While both deliquesced and dry non-acidic ammonium sulfate aerosols showed no organic uptake, the acidic aerosols took up substantial amounts of organic material when exposed to hexanal vapor. Further, we used 1H-NMR, Fourier transform infrared (FTIR) spectroscopy and GC–MS to identify the products of the acid-catalyzed reaction of hexanal in acidic aerosols. Both aldol condensation and hemiacetal products were identified, with the dominant reaction products dependent upon the initial acid concentration of the aerosol. The aldol condensation product was formed only at initial concentrations of 75–96wt% sulfuric acid in water. The hemiacetal was produced at all sulfuric acid concentrations studied, 30–96wt% sulfuric acid in water. Aerosols up to 88.4wt% organic/11.1wt% H2SO4/0.5wt% water were produced via these two dimerization reaction pathways. The UV-VIS spectrum of the isolated aldol condensation product, 2-butyl 2-octenal, extends into the visible region, suggesting these reactions may impact aerosol optical properties as well as aerosol composition. In contrast to previous suggestions, no polymerization of hexanal or its products was observed at any sulfuric acid concentration studied, from 30 to 96wt% in water. [Copyright &y& Elsevier]

Abstract: The Pacific 2001 Air Quality Study was undertaken to characterize the physical and chemical properties of particulate matter in the Lower Fraser Valley (LFV) airshed of British Columbia. Aerosol transport in the LFV was observed to occur through several regional scale mechanisms, including seabreeze–landbreeze flows, upslope–downslope flows and north–south valley flows. Enhanced aerosol concentrations were observed in the eastern portion of the LFV, the north–south tributary valleys and the Gulf/San Juan islands convergence zone. Particle composition was found to vary according to proximity to emission sources relative to surface flows. Approximately half of the particle mass was comprised of organic carbon, with the rest being inorganic species. Sodium nitrate was an important component of the coarse particle fraction, while ammonium sulphate was concentrated in the fine fraction. A broad suite of organic substances were detected, and both anthropogenic and biogenic sources were found to contribute to secondary organic aerosol formation. Particle formation and growth was observed to occur via nucleation, condensation and coagulation. In general, the western part of the LFV airshed was dominated by combustion-related compounds, sea salt chemistry and organics (both anthropogenic and biogenic), while the eastern part of the airshed was dominated by NH3 chemistry and biogenic organics. The extent of processing of air masses was found to increase in a west to east direction, although the occasional accumulation of photochemically aged pollutants in the Gulf/San Juan islands convergence zone was observed to reverse this gradient. NH3 was found not to be a limiting species in the formation of fine mode inorganic aerosols, suggesting that NH3 emission reductions would have to be substantial in order to confer a significant improvement in PM and visibility. This coupled with the non-linear relationships between NO x , VOC and secondary particulates, suggests that PM and visibility improvement in the LFV would most likely require concurrent reductions in NH3, SO x , NO x and VOC emissions. [Copyright &y& Elsevier]

Abstract: The ability of mixed ammonium sulfate/organic acid particles to act as cloud condensation nuclei (CCN) has been studied in the laboratory using a continuous flow, thermal-gradient diffusion chamber operated at supersaturations between 0.3% and 0.6%. The organic acids studied were malonic acid, azelaic acid, hexanoic acid, cis-pinonic acid, oleic acid and stearic acid, and the particles were largely prepared by condensation of the organic vapor onto a dry ammonium sulfate core. For malonic acid and hexanoic acid, the mixed particles activated as predicted by a simple Köhler theory model where both species are assumed to be fully soluble and the droplet has the surface tension of water. Three low-solubility species, cis-pinonic acid, azelaic acid and oleic acid, are well modeled where the acid was assumed to be either partially or fully insoluble. Interestingly, although thin coats of stearic acid behaved in a manner similar to that displayed by oleic and cis-pinonic acid, we observed that thick coats led to a complete deactivation of the ammonium sulfate, presumably because the water vapor could not diffuse through the solid stearic acid. We observed no CCN behavior that could be clearly attributed to a lowering of the surface tension of the growing droplet by the presence of the organic constituents, some of which are highly surface active. [Copyright &y& Elsevier]

Two Aerodyne Aerosol Mass Spectrometers (AMS) were deployed at three sites representing urban, semi-rural and rural areas during the Pacific 2001 experiment in the Lower Fraser Valley (LFV), British Columbia, Canada in August 2001. The AMS provides on-line quantitative measurements of the size and chemical composition of the non-refractory fraction of submicron aerosol particles. A significant accumulation mode with a peak around 400–500nm was observed at all sites that was principally composed of sulphate, organics, ammonium and some nitrate. Another significant mode with a peak below 200nm was also observed at the urban site and when urban plumes affected the other sites. This paper focuses on the variability of the organic particulate composition and size distribution as a function of location and photochemical activity with a particular emphasis on the urban and rural areas. The small organic mode at the urban site was well correlated with gas phase CO, 1,3-butadiene, benzene and toluene with Pearson''s r values of 0.76, 0.71, 0.79 and 0.69, respectively, suggesting that combustion-related emissions are likely to be the main source of the small organic mode at this site. The mass spectra of the urban organic particulate are similar to those of internal combustion engine lubricating oil, and of diesel exhaust aerosol particles, implying that they were composed of a mixture of n-alkanes, branched alkanes, cycloalkanes, and aromatics. In contrast, organic particulate at the rural site was dominated by shorter chain oxidized organic compounds. Correlations between the two organic modes and gas phase compounds at the rural site indicated that a significant part of the small mode originated from combustion sources, while the large accumulation organic mode appeared to be the result of photochemical processing. Processing of organic particulate during a relatively high O3 episode at the rural site appeared to increase the modal diameter of the accumulation mode from about 400 to 600nm and almost doubled its mass loading. [Copyright &y& Elsevier]

Aerosol composition data, simultaneously collected from collocated samplers at an urban, a rural and a background site in the Tennessee Valley, have been compared for all seasons in 2001. Consistent with previous data, organic aerosols and sulfates are the two largest contributors to fine mass throughout the year at all three sites. Levels of major constituents were not significantly different at the rural and background sites during any season, but levels of organic and elemental carbon were higher at the urban site during fall and winter periods. Seasonal trends at all sites showed maxima in summer for sulfate and significant nitrate levels only in winter, but no significant seasonal trend was observed for organic aerosol mass at any site. Year-to-year variability in aerosol composition at the background site is small compared to day-to-day variability within seasons. The appropriate factor for converting organic carbon to organic mass (conventionally *1.4), used in calculating aerosol chemical composition, may vary with season—larger in warm seasons, smaller in cool seasons. [Copyright &y& Elsevier]

We report quartz crystal microbalance (QCM) measurements of the room temperature uptake of water by thin (<1 μm) organic films. The mass of water taken up by films of dodecane, 1-octanol, octanoic acid, 1,5-pentanediol, 1,8-octanediol and malonic acid was measured as a function of the ambient relative humidity (RH). All compounds tested here displayed some sorption of water. Uptake by dodecane is probably due to surface adsorption; in the other films, water dissolves into the film material. Malonic acid and 1,8 octanediol show deliquescence-like behaviour at relative humitidies near 72% and 95%, respectively. In general, the higher the oxidation state of the film compound, the greater is the amount of water sorbed. An absorptive partitioning model, using UNIFAC calculations of activity coefficients, yields semiquantitative agreement with the experimental results at low relative humidities for all compounds except dodecane. Model estimates of water uptake were generally higher than measurements at low RH and lower than measurements at high RH. Model-measurement deviations displayed a similar nonlinear behaviour with changes in RH for all compounds. The differences between the modelled and measured uptakes yield insight into the limitations of currently available model parameters. [Copyright &y& Elsevier]

Understanding the drivers for high ozone (O 3) and atmospheric particulate matter (PM) concentrations is a pressing issue in urban air quality, as this understanding informs decisions for control and mitigation of these key pollutants. The Houston, TX metropolitan area is an ideal location for studying the intersection between O 3 and atmospheric secondary organic carbon (SOC) production due to the diversity of source types (urban, industrial, and biogenic) and the on- and off-shore cycling of air masses over Galveston Bay, TX. Detailed characterization of filter-based samples collected during Deriving Information on Surface Conditions from Column and VERtically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) Houston field experiment in September 2013 were used to investigate sources and composition of organic carbon (OC) and potential relationships between daily maximum 8 h average O 3 and PM. The current study employed a novel combination of chemical mass balance modeling defining primary (i.e. POC) versus secondary (i.e. SOC) organic carbon and radiocarbon (14C) for apportionment of contemporary and fossil carbon. The apportioned sources include contemporary POC (biomass burning [BB], vegetative detritus), fossil POC (motor vehicle exhaust), biogenic SOC and fossil SOC. The filter-based results were then compared with real-time measurements by aerosol mass spectrometry. With these methods, a consistent urban background of contemporary carbon and motor vehicle exhaust was observed in the Houston metropolitan area. Real-time and filter-based characterization both showed that carbonaceous aerosols in Houston was highly impacted by SOC or oxidized OC, with much higher contributions from biogenic than fossil sources. However, fossil SOC concentration and fractional contribution had a stronger correlation with daily maximum 8 h average O 3 , peaking during high PM and O 3 events. The results indicate that point source emissions processed by on- and off-shore wind cycles likely contribute to peak events for both PM and O 3 in the greater Houston metropolitan area. • Contemporary carbon was largest contributor to total organic carbon in Houston. • Secondary organic carbon contributed an average of 67% of total organic carbon. • Secondary fossil carbon was variable ranging from 3 to 27% of total organic carbon. • During peak ozone event, secondary fossil carbon was well correlated with ozone. • Houston was impacted by sea breeze re-circulation during high pollutant day. [ABSTRACT FROM AUTHOR]

We examined the characteristics and source apportionment of organic aerosols in ambient PM 2.5 samples collected during the late autumn in Changchun, Northeast China. 8 compound classes (>90 individual species) were detected in the aerosol samples, including biomass burning tracers, aliphatic lipids (fatty acids and fatty alcohols), secondary oxidation products, polycyclic aromatic hydrocarbons (PAHs), sugar compounds, hopanes and biogenic secondary organic aerosol (SOA) tracers. The concentrations of total quantified organic species ranged from 138.2 to 6.8 × 103 ng m−3, among which levoglucosan was the most abundant compound. Biomass burning tracers (anhydrosugars, lignin and resin acids) were the most abundant compounds, followed by fatty acids, secondary oxidation products, PAHs, sugar compounds and fatty alcohols. Biogenic SOA tracers and hopanes were less abundant. The homohopane index [defined as 31abS/(31abS + 31abR)] was 0.5, indicating a potential contribution of traffic emission. PAHs showed a dominance of benzo(b)fluoranthene (BbF), and the diagnostic ratios implicated a substantial contribution of petroleum combustion as well as coal combustion. 2-methylglyceric acid to 2-methyltetrols ratio (2.2) indicated that NOx influence isoprene oxidation products formation. Furthermore, the average ratio of cis-pinonic acid plus pinic acid to 3-hydroxyglutaric acid (31.4) revealed the much fresher α / β -pinene oxidation products to some extent. A good correlation was found between β -caryophyllinic acid and levoglucosan (r = 0.61), suggesting that β -caryophyllene can mainly be generated by biomass burning. The biogenic secondary organic carbon (SOC) was underestimated by the tracer-based method, which only occupied 0.4% of the organic carbon (OC). In contrast, PMF model indicated that emissions from fossil fuel combustion and biomass burning were most important, accounting for 42.4% and 33.6%, respectively, followed by biogenic SOA emission (17.0%) and fungal spore derived source (7.0%), suggesting biogenic aerosol is a nonnegative contributor. Fossil fuel combustion (42.4%) as well as biomass burning (33.6%) were most important contributors of organic aerosols in a typical city in Northeast China. Image 1 • The chemical and source characteristics of organic aerosols in Northeast China was studied. • 90 individual species were quantified with biomass burning tracers as most abundant compounds. • NOx influence SOA formation from isoprene. • Fossil combustion (42.4%) and biomass burning (33.6%) were most important contributors of OA. • Biogenic aerosol is a non-negligible contributor. [ABSTRACT FROM AUTHOR]