Organic Aerosol Oxidation Research Articles

Observational evidence reveals the significance of nocturnal chemistry in seasonal secondary organic aerosol formation

Oxidized Organic Aerosol (OOA), a major component of fine atmospheric particles, impacts climate and human health. Previous experiments and atmospheric models emphasize the importance of nocturnal OOA formation from NO3· oxidation of biogenic VOCs. This seasonal study extends the understanding by showing that nocturnal oxidation of biomass-burning emissions can account for up to half of total OOA production in fall and winter. It is the first to distinguish nocturnal OOA characteristics from daytime OOA across all seasons using bulk aerosol measurements. Summer observations of nocturnal OOA align well with regional chemistry transport model predictions, but discrepancies in other seasons reveal a common model deficiency in representing biomass-burning emissions and their nocturnal oxidation. This study underscores the significance of near-ground nocturnal OOA production, proposes a method to differentiate it using bulk aerosol measurements, and suggests model optimization strategies. These findings enhance the understanding and prediction of nighttime OOA formation.

Insights into the compositional differences of PM1 and PM2.5 from aerosol mass spectrometer measurements in Beijing, China

Aerosol mass spectrometers (AMS) have been widely used for characterization of submicron aerosol (PM1) species in the world. However, the chemical differences between PM1 and fine particles (PM2.5) in megacities are poorly understood. Here a PM1 high-resolution AMS with standard vaporizer (SV) and a PM2.5 time-of-flight aerosol chemical speciation monitor (ToF-ACSM) with capture vaporizer (CV) were deployed simultaneously in summer and winter in Beijing, China, and the chemical differences of non-refractory aerosol species between PM1 and PM2.5 were characterized. Our results showed that aerosol species in PM2.5 were overall highly correlated with those in PM1 (r2 > 0.8), yet the PM1/PM2.5 ratios varied largely for different aerosol species. Nitrate existed dominantly in PM1 (80–90%), while a considerable fraction of sulfate (30–45%) was observed in the range of 1–2.5 μm (PM1–2.5). Comparatively, PM1 organics showed higher fractions in PM2.5 in summer than winter (∼70%). The changes in PM1/PM2.5 ratios of secondary inorganic aerosol (SIA) depended strongly on relative humidity (RH) in summer with considerable decreases as the increase of RH, while such a RH dependence of PM1/PM2.5 ratios was not significant in winter due to the absence of high RH data. We further compared the source apportionment results of organic aerosol (OA) between PM1 and PM2.5. Similar to previous studies, the SV AMS reported ubiquitously higher primary OA than the CV ToF-ACSM, nearly by a factor of 2 for cooking OA, and the difference in secondary OA composition can be substantial, particularly in winter. With the simultaneous measurements of AMS and ACSM, a parameterized relationship between oxygen-to-carbon (O/C) ratio and CV f44 (fraction of m/z 44 in OA) in summer and winter was developed, which has a potential implication for studying the oxidation and hygroscopicity properties of OA considering the rapid increases in CV ToF-ACSM measurements worldwide.

Characterization of volatile organic compounds and submicron organic aerosol in a traffic environment

Abstract. Urban air consists of a complex mixture of gaseous and particulate species from anthropogenic and biogenic sources that are further processed in the atmosphere. This study investigated the characteristics and sources of volatile organic compounds (VOCs) and submicron organic aerosol (OA) in a traffic environment in Helsinki, Finland, in late summer. The anthropogenic VOCs (aVOCs; aromatic hydrocarbons) and biogenic VOCs (bVOCs; terpenoids) relevant for secondary-organic-aerosol formation were analyzed with an online gas chromatograph mass spectrometer, whereas the composition and size distribution of submicron particles was measured with a soot particle aerosol mass spectrometer. This study showed that aVOC concentrations were significantly higher than bVOC concentrations in the traffic environment. The largest aVOC concentrations were measured for toluene (campaign average of 1630 ng m−3) and p/m xylene (campaign average of 1070 ng m−3), while the dominating bVOC was α-pinene (campaign average of 200 ng m−3). For particle-phase organics, the campaign-average OA concentration was 2.4 µg m−3. The source apportionment analysis extracted six factors for OA. Three OA factors were related to primary OA sources – traffic (24 % of OA, two OA types) and a coffee roastery (7 % of OA) – whereas the largest fraction of OA (69 %) consisted of oxygenated OA (OOA). OOA was divided into less oxidized semi-volatile OA (SV-OOA; 40 % of OA) and two types of low-volatility OA (LV-OOA; 30 %). The focus of this research was also on the oxidation potential of the measured VOCs and the association between VOCs and OA in ambient air. Production rates of the oxidized compounds (OxPR) from the VOC reactions revealed that the main local sources of the oxidation products were O3 oxidation of bVOCs (66 % of total OxPR) and OH radical oxidation of aVOCs and bVOCs (25 % of total OxPR). Overall, aVOCs produced a much smaller portion of the oxidation products (18 %) than bVOCs (82 %). In terms of OA factors, SV-OOA was likely to originate from biogenic sources since it correlated with an oxidation product of monoterpene, nopinone. LV-OOA consisted of highly oxygenated long-range or regionally transported OA that had no correlation with local oxidant concentrations as it had already spent several days in the atmosphere before reaching the measurement site. In general, the main sources were different for VOCs and OA in the traffic environment. Vehicle emissions impacted both VOC and OA concentrations. Due to the specific VOCs attributed to biogenic emissions, the influence of biogenic emissions was more clearly detected in the VOC concentrations than in OA. In contrast, the emissions from the local coffee roastery had a distinctive mass spectrum for OA, but they could not be seen in the VOC measurements due to the measurement limitations for the large VOC compounds. Long-range transport increased the OA concentration and oxidation state considerably, while its effect was observed less clearly in the VOC measurements due to the oxidation of most VOC in the atmosphere during the transport. Overall, this study revealed that in order to properly characterize the impact of different emission sources on air quality, health, and climate, it is of importance to describe both gaseous and particulate emissions and understand how they interact as well as their phase transfers in the atmosphere during the aging process.

Molecular tracers, mass spectral tracers and oxidation of organic aerosols emitted from cooking and fossil fuel burning sources

Secondary organic aerosol (SOA) composes a substantial fraction of atmospheric particles, yet the formation and aging mechanism of SOA remains unclear. Here we investigate the initial oxidation of primary organic aerosol (POA) and further aging of SOA in winter Beijing by using aerosol mass spectrometer (AMS) measurements along with offline molecular tracer analysis. Multilinear engine (ME-2) source apportionment was conducted to capture the characteristic of source-related SOA, and connect them with specific POA. Our results show that urban cooking and fossil fuel burning sources contribute significantly (17 % and 20 %) to total organic aerosol (OA) in winter Beijing. Molecular tracer analysis by two-dimensional gas chromatography-time-of-flight mass spectrometer (GC × GC-ToF-MS) reveals that cooking SOA (CSOA) is produced through both photooxidation and aqueous-phase processing, while less oxidized SOA (LO-SOA) is the photooxidation product of fossil fuel burning OA (FFOA) and may experience aqueous-phase aging to form more-oxidized oxygenated OA (MO-OOA). Furthermore, CHOm/z 69 and CHOm/z 85 are mass spectral tracers indicating the initial photooxidation, while CHO2+ and C2H2O2+ imply further aqueous-phase aging of OA. Tracer analysis indicates that the formation of diketones is involved in the initial photooxidation of POA, while the formation of glyoxal and diacids is involved in the further aqueous-phase aging of SOA.

Impact of aging on the sources, volatility, and viscosity of organic aerosols in Chinese outflows

Abstract. To investigate the impact of aging on the sources, volatility, and viscosity of organic aerosol (OA) in Chinese outflows, a high-resolution time-of-flight aerosol mass spectrometer (HR-AMS) coupled with a thermodenuder (TD) was deployed in the spring of 2018 in Dongying, which is a regional receptor site of metropolitan emissions in the North China Plain (NCP). The average mass concentration of PM1 is 31.5±22.7 µg m−3, which is mainly composed of nitrate (33 %) and OA (25 %). The source apportionment results show that the OA is mainly contributed by oxygenated OA (OOA) from secondary sources, including background-OOA (33 %) representing a background concentration of OA (2.6 µg m−3) in the NCP area, and transported-OOA (33 %) oxidized from urban emissions. The other two factors include aged hydrocarbon-liked OA (aged-HOA, 28 %) from transported vehicle emissions and biomass burning OA (BBOA, 5 %) from local open burning. The volatility of total OA (average C*=3.2×10-4 µg m−3) in this study is generally lower than that reported in previous field studies, which is mainly due to the high OA oxidation level resulting from aging processes during transport. The volatilities of OA factors follow the order of background-OOA (average C*=2.7×10-5 µg m−3) < transported-OOA (3.7×10-4µgm-3)< aged-HOA (8.1×10-4µgm-3)< BBOA (0.012 µg m−3). Extremely low volatilities in ambient air indicate that oligomers may exist in aged plumes. The viscosity estimation suggests that the majority of ambient OA in this study behaves as semisolid (60 %), liquifies at higher relative humidity (RH) (21 %), and solidifies (19 %) during noon when the RH is low and the oxidation level is high. Finally, the estimated mixing time of molecules in 200 nm OA varies dramatically from minutes at night to years in the afternoon, emphasizing the need to consider its dynamic kinetic limits when modeling OA. In general, the overall results of this study improve our understanding of the impact of aging on OA volatility and viscosity.

Spatio-temporal variation of C-PM2.5 (composition based PM2.5) sources using PMF*PMF (double-PMF) and single-combined PMF technique on real-time non-refractory, BC and elemental measurements during post-monsoon and winter at two sites in Delhi, India

Delhi's post-monsoon and winter haze reduces visibility, causes health hazards, and interrupts usual routines. Multiple source apportionment (SA) studies recently apportioned sources and discussed organic and elemental PM source characteristics independently. To design effective pollution control strategies, it's important to obtain a comprehensive picture of particulate matter (PM2.5) sources by doing SA on consolidated data that combines organics, inorganics, black carbon (BC), and metals. In this study, we tried to improve the understanding of PM2.5 sources by performing double-PMF (D-PMF) analysis on a consolidated dataset encompassing non-refractory PM2.5 factors, elements, and BC. Real-time instrumentation (HR-ToF-AMS, Xact, and Aethalometer) were used at two urban sites (IITD and IITMD) in Delhi during post-monsoon and winter seasons. The hourly average C-PM2.5 (Composition based PM2.5 is sum of NR-PM2.5, BC and metals) at IITD for the overall study period (1st Oct 2019 to 8th Jan 2020) was 91.9 ± 56.5 μg m−3, while at IITMD for the overall sampling period (31st Oct 2019 to 31st Dec 2019) was 105.2 ± 61.2 μg m−3. The sources identified by the double-PMF at both sites were road dust, traffic, secondary nitrate, secondary sulfate, oxidized organic aerosol (OOA), biomass burning (BB), and industrial. In contrast, fireworks were only resolved at IITD, and secondary chloride at IITMD. At IITD, the temporal variation of sources was noted between the post-monsoon, Diwali, and winter seasons. The secondary nitrate (17%, 15.8 ± 9.4 μg m−3) and BB (39.4%, 36.6 ± 43.6 μg m−3) were dominant during winter suggest the rapid-nighttime oxidation with favourable low temperature condition and increased heating activities, while OOA (33.4%, 17.2 ± 8.0 μg m−3) and secondary sulfate (12.7%, 6.5 ± 4.4 μg m−3) were dominating during the post-monsoon at IITD due to the high photochemical oxidation and long-range transport. The fireworks (22.2%, 26.6 ± 43.8 μg m−3) contributed a major fraction of sources during Diwali at IITD. For the concurrent measurement period (14th Nov 2019 to 31st Dec 2019) at both the sites, the mean hourly average concentration (C-PM2.5) was 102.2 ± 53.2 μg m−3 at IITD and 99.7 ± 59.6 μg m−3 at the IITMD site. Secondary nitrate and secondary sulfate were prevalent at IITMD, while OOA and traffic dominated at IITD. D-PMF improved interpretation of sources such as the SO4_OA and NO3_OA contributions to industrial sources (Pb-rich and Zn-rich) indicate the possibility of sulfate and nitrate accumulation on metal-rich particles (for example, the formation of ZnSO4 and PbSO4). The signals of NO3_OA in the fireworks factor are due to the gunpowder (KNO3), while HOA and BBOA suggest the role of burn-related organics. The elemental ratios K/Pb (17 at IITD) and K/As (137 at IITMD) in the secondary sulfate source from D-PMF further highlighted the role of coal combustion. The D-PMF results were in excellent agreement with single PMF on combined data (SC-PMF) done as a part of an extended study.

Phase State and Relative Humidity Regulate the Heterogeneous Oxidation Kinetics and Pathways of Organic-Inorganic Mixed Aerosols.

Inorganic species always coexist with organic materials in atmospheric particles and may influence the heterogeneous oxidation of organic aerosols. However, very limited studies have explored the role of the inorganics in the chemical evolution of organic species in mixed aerosols. This study examines the heterogeneous oxidation of glutaric acid-ammonium sulfate and 1,2,6-hexanetriol-ammonium sulfate aerosols by hydroxyl radicals (OH) under varied organic mass fractions (forg) and relative humidity in a flow tube reactor. Coupling the oxidation kinetics and product measurements with kinetic model simulations, we found that under both low relative humidity (RH, 30-35%) and high RH conditions (85%), the decreased forg from 0.7 to 0.2 accelerates the oxidation of the organic materials by a factor of up to 11. We suggest that the faster oxidation kinetics under low-RH conditions is due to full or partial phase separation, with the organics greatly enriched at the particle outer region, while enhanced "salting-out" of the organics and OH adsorption caused by higher inorganics could explain the observations under high-RH conditions. Analysis of the oxidation products reveals that the dilution of organics by the inorganic salts and corresponding water uptake under high-RH conditions will favor alkoxy radical fragmentation by a factor of 3-4 and inhibit its secondary chain propagation chemistry. Our results suggest that atmospheric organic aerosol oxidation lifetime and composition are strongly impacted by the coexistent inorganic salts.

Understanding the Evolution of Smoke Mass Extinction Efficiency Using Field Campaign Measurements.

Aerosol mass extinction efficiency (MEE) is a key aerosol property used to connect aerosol optical properties with aerosol mass concentrations. Using measurements of smoke obtained during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign we find that mid-visible smoke MEE can change by a factor of 2-3 between fresh smoke (<2hr old) and one-day-old smoke. While increases in aerosol size partially explain this trend, changes in the real part of the aerosol refractive index (real(n)) are necessary to provide closure assuming Mie theory. Real(n) estimates derived from multiple days of FIREX-AQ measurements increase with age (from 1.40 - 1.45 to 1.5-1.54 from fresh to one-day-old) and are found to be positively correlated with organic aerosol oxidation state and aerosol size, and negatively correlated with smoke volatility. Future laboratory, field, and modeling studies should focus on better understanding and parameterizing these relationships to fully represent smoke aging.

Cloud processing and weeklong ageing affect biomass burning aerosol properties over the south-eastern Atlantic

Southern Africa produces a third of global biomass burning emissions, which have a long atmospheric lifetime and influence regional radiation balance and climate. Here, we use airmass trajectories to link different aircraft observations to investigate the evolution of biomass-burning aerosols during their westward transport from Southern Africa over the south-eastern Atlantic, where a semi-permanent stratocumulus cloud deck is located. Our results show secondary organic aerosol formation during the initial 3 days of transport, followed by decreases in organic aerosol via photolysis before reaching equilibrium. Aerosol absorption wavelength dependency decreases with ageing, due to an increase in particle size and photochemical bleaching of brown carbon. Cloud processing, including aqueous-phase reaction and scavenging, contributes to the oxidation of organic aerosols, while it strongly reduces large diameter particles and single-scattering albedo of biomass burning aerosols. Together, these processes resulted in a marine boundary layer with fewer yet more oxidized and absorbing aerosols.

Seasonal characteristics and provenance of organic aerosols in the urban atmosphere of Liaocheng in the North China Plain: Significant effect of biomass burning

To better understand the seasonal characteristics of urban organic aerosol (OA) in the North China Plain (NCP), PM2.5 samples in the urban atmosphere of Liaocheng were collected and analyzed. The molecular distribution of the organic markers in the urban atmosphere of Liaocheng reveals that n-alkanes (39.3%) was the most abundant species all year round, followed by saccharides (28.2%), phthalic acids (Ph, 20.8%), biogenic secondary organic aerosol (BSOA) tracers (9.4%), and polycyclic aromatic hydrocarbon (PAHs, 2.3%). PM2.5, organic carbon (OC), elemental carbon (EC), and primary organic markers exhibit the highest concentrations in winter, due largely to the increased biomass burning and coal combustion for house heating in local and surrounding regions. However, the concentration and relative abundance of BSOA are significantly higher in summer than other seasons, induced by the more favorable meteorological conditions that would promote the emissions of biogenic volatile organic compounds (BVOCs) and the secondary production of BSOA. The ratios of OC/EC and 3-methyl-1,2,3-butanetricarboxylic acid to cis-pinic acid plus cis-pinonic acid (MBTCA/(PA + PNA) are higher in the warm seasons than those in the cold seasons, indicating that the oxidation of OA is sensitive to air temperature. Compared to 2017, the concentration level of PAHs during wintertime decreased by 40.8%, confirming that the stringent regulation of coal burning is effective. The highest concentration of high molecular weight (HMW) n-alkanes and three anhydrosugars in winter, and the close correlation of levoglucosan with HMW n-alkanes suggests that the impact of biomass burning was more significant in winter. The same seasonal characteristic of the ratios of high-/low-NOx products with NOx and the strong correlation of high-/low-NOx products with levoglucosan indicate that the formation of isoprene SOA (SOAI) tracers was significantly influenced by anthropogenic emissions. The molecular compositions, the distributions of fire spots, backward trajectories of air masses, and correlation analysis suggest that air pollution events in spring were primarily resulted from biomass burning and secondary oxidation, while pollution events in winter were largely driven by the increased combustion sources, and promoted aqueous secondary formation. Our results suggest that the reduction of biomass and coal combustion should be taken into account to improve the urban air quality in the NCP.

Measurement report: On the contribution of long-distance transport to the secondary aerosol formation and aging

Abstract. To investigate the physio-chemical properties of aerosol transported from major pollution regions in China, observations were conducted ∼ 200 m above the ground at the junction location of the North China Plain and Fenwei Basin, which are two regions of top priority for China's Blue Sky Campaign. We identified three pollution transport sectors including those from Beijing–Tianjin–Hebei (BTH), urban Guanzhong Basin (GZB) and northern China and one clean transport sector from the rural Guanzhong Basin region. Secondary inorganic aerosol (SIA) constituted a major fraction (39 %–46 %) in all pollution transport sectors, with a high sulfur oxidation ratio (0.44–0.58) and a high nitrogen oxidation ratio (0.24–0.29), suggesting efficient formation of secondary inorganic aerosol during regional transport. More oxidized oxygenated organic aerosol (MO-OOA) played a dominant role in the source of organic aerosol in all sectors including the clean one, accounting for 42 %–58 % of total organic aerosol. Elemental analysis (O and C) shows that aerosol particles at this receptor site were much more oxidized than in urban regions, pointing that long-range transport contributed markedly to the organic aerosol oxidation and aging. Case studies of pollution events with high sulfate, nitrate and more-oxidized oxygenated organic aerosol production rate indicate the strong formation efficiency of secondary aerosol during regional transport in the Beijing–Tianjin–Hebei transport sector.

Real-time single particle characterization of oxidized organic aerosols in the East China Sea

Knowledge of the chemical characteristics and sources of organic aerosols (OA) over marine is needed for evaluating their effects on climate change and air quality. Here, a quadrupole aerosol chemical speciation monitor (Q-ACSM) and a single-particle aerosol mass spectrometry (SPAMS) were synchronously employed to investigate the chemical composition, mixing state, and oxidation degree of oxidized organic aerosols (OOA) in PM1 over the East China Sea (ECS) from 3 to 27 June 2017. Both aerosol mass spectrometers demonstrated that a higher oxidation state of OOA in aerosol particles could be generated during marine air mass-dominated periods (MDP) than that generated during land air mass-dominated periods (LDP). Two OOA factors including semi-volatile oxidized organic aerosol (SV-OOA) and low-volatility oxidized organic aerosol (LV-OOA) were distinguished based on Q-ACSM. Fifty-seven percent of the total detected particles with obvious signals of organic markers were identified as oxidized organic carbon (OOC) particles via SPAMS and further divided into lower oxidized organic carbon (LOOC) particles and more oxidized organic carbon (MOOC) particles. All OOC-containing particles were clustered into seven particle subgroups. The EC and K subgroups dominated the LOOC and MOOC particles, respectively, during periods controlled by land air masses, indicating that notable OOC formation was influenced by continental sources. OOA with higher oxygen states were found to dominate near ports. This suggested that OOA chemical characteristics over the ESC are seriously affected by continental, ship, and port emissions, which should be synergistically considered in evaluating their effects on solar radiation transfer and cloud processes.

Unraveling the sources of atmospheric organic aerosols over the Arabian Sea: Insights from the stable carbon and nitrogen isotopic composition

The isotopic composition of stable carbon (δ13C) and nitrogen (δ15N) in marine aerosols influenced by the continental outflows are useful proxies for understanding the aging and secondary formation processes. Every winter, the haze pollutants transported from South Asia significantly affect the chemical composition of marine atmospheric boundary layer of the Arabian Sea. Here, we assessed the δ13C of total carbon (TC) and δ15N of total nitrogen (TN) in marine aerosols collected over the Arabian Sea during a winter cruise (6–24 December 2018). TC (2.1–13.4 μg m−3) is strongly correlated with TN (0.9–5.0 μg m−3), likely because of their common source-emissions, biomass burning and fossil-fuel combustion in the Indo-Gangetic Plain and South Asia (corroborated by backward-air mass trajectories and satellite fire counts). Besides, the linear relationship between the mass ratios of water-soluble organic carbon (WSOC) to TC (0.04–0.65) and δ13CTC (−25.1‰ to −22.9‰) underscores the importance of aging process. This means oxidation of organic aerosols during transport not only influences the WSOC levels but also affects their δ13CTC. Likewise, the prevalent inverse linear relationship between the equivalent mass ratio of (NH4+/non-sea-salt- or nss-SO42−) and δ15NTN (+15.3‰ to +25.1‰) emphasizes the overall significance of neutralization reactions between major acidic ([nss-SO42−] ≫ [NO3−]) and alkaline species (NH4+) in aerosols. Higher δ15NTN values in winter than the spring inter-monsoon clearly emphasizes the significance of the anthropogenic combustion sources (i.e., biomass burning) in the South Asian outflow. A comparison of δ13CTC and δ15NTN with the source emissions revealed that crop-residue burning emissions followed by the coal fired power plants mostly dictate the atmospheric abundance of organic aerosols in the wider South Asian outflow.

Highly oxidized organic aerosols in Beijing: Possible contribution of aqueous-phase chemistry

The oxidation state of organic aerosol (OA) substantially influences the optical and hygroscopic properties, toxicity, and atmospheric lifetime of OA. Comprehensive field measurements have been carried out in Beijing to understand the possible mechanism relevant to the evolution of OA oxidation state in the ambient atmosphere. We found that OA was more oxidized in pollution events in autumn than in winter, evidenced by the higher ratios of OA/OC (2.50 ± 0.69) and f44/f43 (1.90 ± 0.54) in autumn than the corresponding values (1.80 ± 0.39 and 1.64 ± 0.45) in winter. Plumes with highly oxidized OA were identified to be transported from the southern regions of Beijing and strongly correlated with the high concentration of aerosol water content (AWC), which was dominantly affected by the high concentration of ammonium nitrate and the high RH. This suggests that aqueous-phase chemistry, as regulated by ammonium nitrate and RH, plays a crucial role in the formation of highly oxidized OA in Beijing.

Molecular Tracers, Mass Spectral Tracers and Oxidation of Organic Aerosols Emitted from Cooking and Fossil Fuel Burning Sources

Molecular Tracers, Mass Spectral Tracers and Oxidation of Organic Aerosols Emitted from Cooking and Fossil Fuel Burning Sources

Can Online Aerosol Mass Spectrometry Analysis Classify Secondary Organic Aerosol (SOA) and Oxidized Primary Organic Aerosol (OPOA)? A Case Study of Laboratory and Field Studies of Indonesian Biomass Burning

Organic aerosol (OA) makes up a significant fraction of ambient particulate matter, including those emitted during the wildfire. Oxidized organic aerosol (OOA), obtained by factor analysis, dominat...

Impact of the Atmospheric Photochemical Evolution of the Organic Component of Biomass Burning Aerosol on Its Radiative Forcing Efficiency: A Box Model Analysis

We present the first box model simulation results aimed at identification of possible effects of the atmospheric photochemical evolution of the organic component of biomass burning (BB) aerosol on the aerosol radiative forcing (ARF) and its efficiency (ARFE). The simulations of the dynamics of the optical characteristics of the organic aerosol (OA) were performed using a simple parameterization developed within the volatility basis set framework and adapted to simulate the multiday BB aerosol evolution in idealized isolated smoke plumes from Siberian fires (without dilution). Our results indicate that the aerosol optical depth can be used as a good proxy for studying the effect of the OA evolution on the ARF, but variations in the scattering and absorbing properties of BB aerosol can also affect its radiative effects, as evidenced by variations in the ARFE. Changes in the single scattering albedo (SSA) and asymmetry factor, which occur as a result of the BB OA photochemical evolution, may either reduce or enhance the ARFE as a result of their competing effects, depending on the initial concentration OA, the ratio of black carbon to OA mass concentrations and the aerosol photochemical age in a complex way. Our simulation results also reveal that (1) the ARFE at the top of the atmosphere is not significantly affected by the OA oxidation processes compared to the ARFE at the bottom of the atmosphere, and (2) the dependence of ARFE in the atmospheric column and on the BB aerosol photochemical ages almost mirrors the corresponding dependence of SSA.

Contrasting effects of secondary organic aerosol formations on organic aerosol hygroscopicity

Abstract. Water uptake abilities of organic aerosol under sub-saturated conditions play critical roles in direct aerosol radiative effects and atmospheric chemistry; however, field characterizations of the organic aerosol hygroscopicity parameter κOA under sub-saturated conditions remain limited. In this study, a field campaign was conducted to characterize κOA at a relative humidity of 80 % with hourly time resolution for the first time in the Pearl River Delta region of China. Observation results show that, during this campaign, secondary organic aerosol (SOA) dominated total organic aerosol mass (mass fraction &gt; 70 % on average), which provides a unique opportunity to investigate influences of SOA formation on κOA. Results demonstrate that the commonly used organic aerosol oxidation level parameter O/C was weakly correlated with κOA and failed to describe the variations in κOA. However, the variations in κOA were well reproduced by mass fractions of organic aerosol factor resolved based on aerosol mass spectrometer measurements. The more oxygenated organic aerosol (MOOA) factor, exhibiting the highest average O/C (∼ 1) among all organic aerosol factors, was the most important factor driving the increase in κOA and was commonly associated with regional air masses. The less oxygenated organic aerosol (LOOA; average O/C of 0.72) factor revealed strong daytime production, exerting negative effects on κOA. Surprisingly, the aged biomass burning organic aerosol (aBBOA) factor also formed quickly during daytime and shared a similar diurnal pattern with LOOA but had much lower O/C (0.39) and had positive effects on κOA. The correlation coefficient between κOA and mass fractions of aBBOA and MOOA in total organic aerosol mass reached above 0.8. The contrasting effects of LOOA and aBBOA formation on κOA demonstrate that volatile organic compound (VOC) precursors from diverse sources and different SOA formation processes may result in SOA with different chemical composition, functional properties and microphysical structure, consequently exerting distinct influences on κOA and rendering single oxidation level parameters (such as O/C) unable to capture those differences. Aside from that, distinct effects of aBBOA on κOA were observed during different episodes, suggesting that the hygroscopicity of SOA associated with similar sources might also differ much under different emission and atmospheric conditions. Overall, these results highlight that it is imperative to conduct more research on κOA characterization under different meteorological and source conditions and examine its relationship with VOC precursor profiles and formation pathways to formulate a better characterization and develop more appropriate parameterization approaches in chemical and climate models.

Primary emissions and secondary production of organic aerosols from heated animal fats

Cooking is an important source of primary organic aerosol (POA) in urban areas, and it may also generate abundant non-methane organic gases (NMOGs), which form oxidized organic aerosol (OOA) after atmospheric oxidation. Edible fats play an important role in a balanced diet and are part of various types of cooking. We conducted laboratory studies to examine the primary emissions of POA and NMOGs and OOA formation using an oxidation flow reactor (OFR) for three animal fats (i.e., lard, beef and chicken fats) heated at two different temperatures (160 and 180 °C). Positive matrix factorization (PMF) revealed that OOA formed together with POA loss after photochemical aging, suggesting the conversion of some POA to OOA. The maximum OOA production rates (PRs) from heated animal fats, occurring under OH exposures (OHexp) of 8.3–15 × 1010 molecules cm−3 s, ranged from 8.9 to 24.7 μg min−1, 1.6–14.5 times as high as initial POA emission rates (ERs). NMOG emissions from heated animal fats were dominated by aldehydes, which contributed 14–71% of the observed OOA. We estimated that cooking-related OOA could contribute to as high as ~10% of total organic aerosol (OA) in an urban area in Hong Kong, where cooking OA (COA) dominated the POA. This study provides insights into the potential contribution of cooking to urban OOA, which might be especially pronounced when cooking contributions dominate the primary emissions.

On the similarities and differences between the products of oxidation of hydrocarbons under simulated atmospheric conditions and cool flames

Abstract. Atmospheric oxidation chemistry and, more specifically, photooxidation show that the long-term oxidation of organic aerosol (OA) progressively erases the initial signature of the chemical compounds and can lead to a relatively uniform character of oxygenated organic aerosol (OOA). This uniformity character observed after a long reaction time seems to contrast with the great diversity of reaction mechanisms observed in the early stages of oxidation. The numerous studies carried out on the oxidation of terpenes, and more particularly on limonene for its diversity of reaction sites (endo- and oxocyclic), allow this evolution to be studied. We have selected, for their diversity of experimental conditions, nine studies of limonene oxidation at room temperature over long reaction times to be compared to the present data set obtained at elevated temperature and short reaction time in order to investigate the similarities in terms of reaction mechanisms and chemical species formed. Here, the oxidation of limonene–oxygen–nitrogen mixtures was studied using a jet-stirred reactor at elevated temperature and atmospheric pressure. Samples of the reacting mixtures were collected and analyzed by high-resolution mass spectrometry (Orbitrap) after direct injection or after separation by reverse-phase ultra-high-pressure liquid chromatography and soft ionization, i.e., (+/-) HESI and (+/-) APCI. Unexpectedly, because of the diversity of experimental conditions in terms of continuous-flow tank reactor, concentration of reactants, temperature, reaction time, mass spectrometry techniques, and analysis conditions, the results indicate that among the 1138 presently detected molecular formulae, many oxygenates found in earlier studies of limonene oxidation by OH and/or ozone are also produced under the present conditions. Among these molecular formulae, highly oxygenated molecules and oligomers were detected in the present work. The results are discussed in terms of reaction pathways involving the initial formation of peroxy radicals (RO2), isomerization reactions yielding keto-hydroperoxides, and other oxygenated intermediates and products up to C25H32O17, products which could derive from RO2 autoxidation via sequential H shift and O2 addition (C10H14O3,5,7,9,11) and products deriving from the oxidation of alkoxy radicals (produced by RO2 self-reaction or reaction with HO2) through multiple H shifts and O2 additions (C10H14O2,4,6,8,10). The oxidation of RO2, with possible occurrence of the Waddington mechanism and of the Korcek mechanism, involving H shifts is also discussed. The present work demonstrates similitude between the oxidation products and oxidation pathways of limonene under simulated atmospheric conditions and in those encountered during the self-ignition of hydrocarbons at elevated temperatures. These results complement those recently reported by Vereecken and Nozière and confirm for limonene the existence of an oxidative chemistry of the alkylperoxy radical beyond 450 K based on the H shift (Nozière and Vereecken, 2019; Vereecken and Nozière, 2020).