Secondary Organic Aerosol Precursors Research Articles

Development of a multiphase chemical mechanism to improve secondary organic aerosol formation in CAABA/MECCA (version 4.7.0)

Abstract. During the last few decades, the impact of multiphase chemistry on secondary organic aerosols (SOAs) has been demonstrated to be the key to explaining laboratory experiments and field measurements. However, global atmospheric models still show large biases when simulating atmospheric observations of organic aerosols (OAs). Major reasons for the model errors are the use of simplified chemistry schemes of the gas-phase oxidation of vapours and the parameterization of heterogeneous surface reactions. The photochemical oxidation of anthropogenic and biogenic volatile organic compounds (VOCs) leads to products that either produce new SOA or are taken up by existing aqueous media like cloud droplets and deliquescent aerosols. After partitioning, aqueous-phase processing results in polyols, organosulfates, and other products with a high molar mass and oxygen content. In this work, we introduce the formation of new low-volatility organic compounds (LVOCs) to the multiphase chemistry box model CAABA/MECCA. Most notable are the additions of the SOA precursors, limonene and n-alkanes (5 to 8 C atoms), and a semi-explicit chemical mechanism for the formation of LVOCs from isoprene oxidation in the gas and aqueous phases. Moreover, Henry's law solubility constants and their temperature dependences are estimated for the partitioning of organic molecules to the aqueous phase. Box model simulations indicate that the new chemical scheme predicts the enhanced formation of LVOCs, which are known for being precursor species to SOAs. As expected, the model predicts that LVOCs are positively correlated to temperature but negatively correlated to NOx levels. However, the aqueous-phase processing of isoprene epoxydiols (IEPOX) displays a more complex dependence on these two key variables. Semi-quantitative comparison with observations from the SOAS campaign suggests that the model may overestimate methylbutane-1,2,3,4-tetrol (MeBuTETROL) from IEPOX. Further application of the mechanism in the modelling of two chamber experiments, one in which limonene is consumed by ozone and one in which isoprene is consumed by NO3 shows a sufficient agreement with experimental results within model limitations. The extensions in CAABA/MECCA are transferred to the 3D atmospheric model MESSy for a comprehensive evaluation of the impact of aqueous- and/or aerosol-phase chemistry on SOA at a global scale in a follow-up study.

Modeling the influence of carbon branching structure on secondary organic aerosol formation via multiphase reactions of alkanes

Abstract. Branched alkanes represent a significant proportion of hydrocarbons emitted in urban environments. To accurately predict the secondary organic aerosol (SOA) budgets in urban environments, these branched alkanes should be considered as SOA precursors. However, the potential to form SOA from diverse branched alkanes under varying environmental conditions is currently not well understood. In this study, the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model is extended to predict SOA formation via the multiphase reactions of various branched alkanes. Simulations with the UNIPAR model, which processes multiphase partitioning and aerosol-phase reactions to form SOA, require a product distribution predicted from an explicit gas kinetic mechanism, whose oxygenated products are applied to create a volatility- and reactivity-based αi species array. Due to a lack of practically applicable explicit gas mechanisms, the prediction of the product distributions of various branched alkanes was approached with an innovative method that considers carbon lengths and branching structures. The αi array of each branched alkane was primarily constructed using an existing αi array of the linear alkane with the nearest vapor pressure. Generally, the vapor pressures of branched alkanes and their oxidation products are lower than those of linear alkanes with the same carbon number. In addition, increasing the number of alkyl branches can also decrease the ability of alkanes to undergo autoxidation reactions that tend to form low-volatility products and significantly contribute to alkane SOA formation. To account for this, an autoxidation reduction factor, as a function of the degree and position of branching, was applied to the lumped groups that contain autoxidation products. The resulting product distributions were then applied to the UNIPAR model for predicting branched-alkane SOA formation. The simulated SOA mass was compared to SOA data generated under varying experimental conditions (i.e., NOx levels, seed conditions, and humidity) in an outdoor photochemical smog chamber. Branched-alkane SOA yields were significantly impacted by NOx levels but insignificantly impacted by seed conditions or humidity. The SOA formation from branched and linear alkanes in diesel fuel was simulated to understand the relative importance of branched and linear alkanes with a wide range of carbon numbers. Overall, branched alkanes accounted for a higher proportion of SOA mass than linear alkanes due to their higher contribution to diesel fuel.

Technical note: Characterization of a single-beam gradient force aerosol optical tweezer for droplet trapping, phase transition monitoring, and morphology studies

Abstract. Single particle analysis is essential for a better understanding of the particle transformation process and to predict its environmental impact. In this study, we developed an aerosol optical tweezer (AOT) Raman spectroscopy system to investigate the phase state and morphology of suspended aerosol droplets in real time. The system comprises four modules: optical trapping, reaction, illumination and imaging, and detection. The optical trapping module utilizes a 532 nm laser and a 100 × oil immersion objective to stably trap aerosol droplets within 30 s. The reaction module allows us to adjust relative humidity (RH) and introduce reaction gases into the droplet levitation chamber, facilitating experiments to study liquid–liquid phase transitions. The illumination and imaging module employs a high-speed camera to monitor the trapped droplets, while the detector module records Raman scattering light. We trapped sodium chloride (NaCl) and 3-methyl glutaric acid (3-MGA) mixed droplets to examine RH-dependent morphology changes. Liquid–liquid phase separation (LLPS) occurred when RH was decreased. Additionally, we introduced ozone and limonene/pinene to generate secondary organic aerosol (SOA) particles in situ, which collided with the trapped droplet and dissolved in it. To determine the trapped droplet's characteristics, we utilized an open-source program based on Mie theory to retrieve diameter and refractive index from the observed whispering gallery modes (WGMs) in Raman spectra. It is found that mixed droplets formed core–shell morphology when RH was decreased, and the RH dependence of the droplets' phase transitions generated by different SOA precursors varied. Our AOT system serves as an essential experimental platform for in situ assessment of morphology and phase state during dynamic atmospheric processes.

Biogenic emission as a potential source of atmospheric aromatic hydrocarbons: Insights from a cyanobacterial bloom-occurring eutrophic lake

As important precursors of ozone (O3) and secondary organic aerosol (SOA), reactive aromatic hydrocarbons (AHs) have typically been classified as anthropogenic air pollutants. However, biogenic emission can also be a potential source of atmospheric AHs. Herein, field observations in a eutrophic lake were combined with laboratory incubation experiments to investigate the biogenic AH emission. Field work showed that the water-air fluxes of AHs measured at sites with high cyanobacteria abundance could reach an order of magnitude greater than those at sites with low cyanobacteria abundance, suggesting that cyanobacteria could be the important contributor to measured AHs. Laboratory incubation experiments further confirmed the AH emission of cyanobacteria and revealed that the emission could change significantly over the lifespan of cyanobacteria and varied to their growing conditions. By combining field observations and laboratory incubation experiments, it has been suggested that the emission of different AH species from cyanobacteria could be modulated by variable biogeochemical mechanisms and that the biochemical process of toluene could be different from that of other AHs. This study investigates AH emissions from inland aquatic ecosystem and suggests that biogenic emission could be a potential source of atmospheric AHs.

Seasonal estimates of ozone and secondary organic aerosol formation from volatile organic compounds in a rural atmosphere of India

Volatile Organic Compounds (VOCs) serve as precursors for tropospheric ozone (O3) and Secondary Organic Aerosol (SOA) formation. The formation of O3 and SOA are the indicators of the oxidative capacity specific to a given chemical environment. The current study investigates the oxidative capacity of the relatively less explored tropical rural atmosphere. This study is accomplished by measuring the concentrations of various VOCs and combining them with OH loss rates to estimate the potentials for O3 and SOA formation (OFP and SOAP, respectively). Continuous diel VOC measurement data from Gadanki (13.5°N, 79.2°E), Peninsular India, encompassing four distinct seasons and comprising over 4000 samples, have been utilized to estimate OFP and SOAP and their variations across different seasons. Additionally, efforts have been made to comprehend the contribution of different VOC sources to O3 and SOA formation. The results indicate that, 1, 3, 6-trimethyl benzene (20.09 %) among the VOCs and aromatics (44.37%) among the VOC groups exhibit the highest OFP at the observational site. Among seasons, the post-monsoon period exhibits the highest OFP (31.94%). The increased presence of biogenic VOCs, such as ethylene, propylene, and 1-butene during monsoon, likely due to the increased vegetation cover can be attributed for the elevated OFP. Similarly, n-dodecane (43.22%) and the VOC group of alkanes (50.79%) show the highest SOAP. The summer season has the highest SOAP (29.7%), owing to the enhanced concentrations and photochemistry initiated by OH radicals. Within the PMF-modelled sources, biomass-burning VOCs make a substantial contribution to both OFP and SOAP, distinguishing the rural atmosphere from its urban counterpart, where traffic emissions predominantly influence OFP and SOAP.

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

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

Effects of acrylamide on indoor air based on the reactions with O3 and •OH: Mechanism, kinetics, and toxicity evaluation

Acrylamide (AM), a hazardous chemical prevalent in daily food processing, exerts carcinogenic, teratogenic and mutagenic impacts. In the ambient environment, O3 and •OH stand as pervasive oxidants. However, the reaction mechanism of AM with atmospheric oxidants O3 and •OH is still unclear. This study delves into the oxidation behavior of AM with O3/•OH in both gas phase and aqueous droplets, unveiling their environmental implications through theoretical calculations. The intricate reaction mechanisms and rate constants (k) governing the interplay of AM with O3 and •OH are clarified. Notably, the energy barrier of AM reaction with O3 surpasses that of AM with •OH. Impressively, the gas phase k values of the two systems (kO3+AM = 3.52 × 10−16 cm3 molecule−1 s−1, k•OH+AM = 6.64 × 10−12 cm3 molecule−1 s−1) are 2–3 times higher than those observed in aqueous reactions. An increase in temperature enhances the reaction rate constant between O3 and AM, but decreases that of •OH with AM. In the field of reaction kinetics, the half-lives of AM degradation initiated by O3 and •OH are 0.55 h and 1.16 h in the gas phase, and 1.56 h and 2.04 h in aqueous droplets. The presence of aqueous droplets may exhibit the transformation potential of AM, which is more likely to accumulate in dry food processing. Furthermore, resultant products typically exhibit reduced toxicity compared to AM. However, in this target reaction, the formation of organic acids, glycolaldehyde, 2-oxoacetamide and multi-functional compounds assumes significance, serving as vital precursors of indoor secondary organic aerosol.

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

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

The newest emission inventory of anthropogenic full-volatility organic in Central China

Organic aerosol (OA) is the largest and most complicated component of aerosols that has significant impacts on human health and global climate. However, current models cannot accurately predict the concentrations and sources of primary and secondary OA due to the oversimplification of traditional organic emission inventories. Traditional emission inventories do not consider important precursors of secondary organic aerosols (SOA), such as semi-volatile and intermediate-volatility organic compounds (SVOCs and IVOCs). To improve the accuracy of OA prediction and source identification, it is essential to refine precursor emissions. In this study, a full-volatility organic compounds emission inventory was established for the Central China, with Henan Province as a representative in 2020. The anthropogenic emissions of ultralow volatility organic compounds (xLVOCs), SVOCs, IVOCs, and VOCs were 53.9 kt, 55.3 kt, 182.1 kt, and 586.3 kt, respectively, effectively filling the gap of 181.3 kt in traditional organic compound emission inventory. Residential biomass burning was identified as the primary contribution source for xLVOCs and SVOCs, accounting for 70.1% and 42.2%, respectively. Industrial process was the main contributor for IVOCs and VOCs, accounting for 31.7% and 44.0% of their emissions, respectively. Meanwhile, the emission of volatile organic compound from different sources showed an increasing trend as volatility increased. The gas-phase emission was 212.1 kt, primarily from industrial process (33.9% contribution), while the particle-phase emission was 79.2 kt, mainly from domestic biomass combustion (65.0% contribution). xLVOCs and SVOCs emissions were mainly concentrated in cities with agricultural activities, such as Zhoukou, while IVOCs and VOCs were concentrated in highly industrialized and urbanized areas, such as Zhengzhou. It is hoped that the incompleteness of emission factors (EFs) for full-volatility organic compounds, which mainly contributes to the uncertainty of this study's results, can be further improved to better understand source contributions.

Uncovering the dominant contribution of intermediate volatility compounds in secondary organic aerosol formation from biomass-burning emissions.

Organic vapors from biomass burning are a major source of secondary organic aerosols (SOAs). Previous smog chamber studies found that the SOA contributors in biomass-burning emissions are mainly volatile organic compounds (VOCs). While intermediate volatility organic compounds (IVOCs) are efficient SOA precursors and contribute a considerable fraction of biomass-burning emissions, their contribution to SOA formation has not been directly observed. Here, by deploying a newly-developed oxidation flow reactor to study SOA formation from wood burning, we find that IVOCs can contribute ∼70% of the formed SOA, i.e. >2 times more than VOCs. This previously missing SOA fraction is interpreted to be due to the high wall losses of semi-volatile oxidation products of IVOCs in smog chambers. The finding in this study reveals that SOA production from biomass burning is much higher than previously thought, and highlights the urgent need for more research on the IVOCs from biomass burning and potentially other emission sources.

Floating reactive barriers to mitigate secondary organic aerosol formation from oil sands tailings ponds

Oil sands operations to recover bitumen in Alberta, Canada generate large volumes of contaminated water which are stored in tailings ponds. These ponds emit large quantities of intermediate / semi- volatile organic compounds (IVOC/SVOCs) which upon emission can react or partition to form secondary organic aerosols (SOAs), a significant issue concerning human and environmental health. These ponds are one of the largest sources of SOAs in North America, and the responsible VOCs (polycyclic aromatic hydrocarbons (PAHs), alkanes, single ring aromatics) are difficult to treat. Passive technologies that can reduce aqueous emissions and degrade volatiles in-situ are desired. A promising technology is buoyant photocatalysts (BPCs), which float on the surface of water bodies and absorb sunlight to oxidize contaminants. In this work, a glass microsphere TiO2-coated BPC is studied, which forms a semi-porous floating reactive barrier on the surface of water, intercepting VOC emissions and photocatalytically oxidizing them in water before they can be released to air. Based on a group of F2 – F3 hydrocarbons, 16 PAHs (C10 – C22) and 8 alkanes (C11-C18), this BPC technology reduced 89% of these SOA precursor emissions while degrading 84% of the compounds in the system under 2 days of simulated sunlight. In a synthetic tailings pond environment, BPCs were demonstrated to block and treat VOCs of particular concern well (phenanthrene, pyrene, chrysene), while further preventing emissions from free phase hydrocarbons. In context of tailings pond SOA precursor emissions, application of this floating reactive barrier technology to all ponds could reduce the environmental impact of oil sands operations by preventing release of between 2400 to 11000 tonnes of SOAs per year. The proven ability to intercept and treat VOCs in water extends the potential of this technology past the oil sands industry, providing one of the first options for passive fugitive emission management for various industrial water bodies. SynopsisBuoyant Photocatalysts permanently prevent aqueous gas emissions and likely subsequent secondary organic aerosol formation by preventing release while oxidizing aqueous volatiles.

Insights into secondary organic aerosol formation from the day- and nighttime oxidation of polycyclic aromatic hydrocarbons and furans in an oxidation flow reactor

Abstract. Secondary organic aerosols (SOAs) formed by oxidation of typical precursors largely emitted by biomass burning, such as polycyclic aromatic hydrocarbons (PAHs) and furans, are still poorly characterized. We evaluated and compared the formation yields, effective density (ρeff), absorption Ångström exponent (α), and mass absorption coefficient (MAC) of laboratory-generated SOAs from three furan compounds and four PAHs. SOAs were generated in an oxidation flow reactor under day- (OH radicals) or nighttime (NO3 radicals) conditions. The ρeff, formation yields, α, and MAC of the generated SOAs varied depending on the precursor and oxidant considered. The ρeff of SOAs formed with OH and NO3 tended to increase with particle size before reaching a “plateau”, highlighting potential differences in SOA chemical composition and/or morphology, according to the particle size. Three times lower SOA formation yields were obtained with NO3 compared with OH. The yields of PAH SOAs (18 %–76 %) were five to six times higher than those obtained for furans (3 %–12 %). While furan SOAs showed low or negligible light absorption properties, PAH SOAs had a significant impact in the UV–visible region, implying a significant contribution to atmospheric brown carbon. No increase in the MAC values was observed from OH to NO3 oxidation processes, probably due to a low formation of nitrogen-containing chromophores with NO3 only (without NOx). The results obtained demonstrated that PAHs are significant SOA precursors emitted by biomass burning, through both, day- and nighttime processes, and have a substantial impact on the aerosol light absorption properties.

Comprehensive characterization of speciated volatile organic compounds (VOCs), gas-phase and particle-phase intermediate- and semi-volatile volatility organic compounds (I/S-VOCs) from Chinese diesel trucks

We established the comprehensive emission profiles of organic compounds for typical Chinese diesel trucks. The profiles cover the entire volatility range, including speciated volatile organic compounds (VOCs), intermediate-volatility organic compounds (IVOCs), and semi-volatile organic compounds (SVOCs). The VOCs and I/SVOCs were analyzed by one-dimensional gas chromatography quadrupole mass spectrometry (GC qMS) and two-dimensional gas chromatography time-of-flight mass spectrometry (GC × GC-ToF-MS) separately. The impacts of starting mode and aftertreatment technology on the VOC, gaseous and particulate I/SVOC emissions, and the gas-particle partitioning were investigated. The emission factor (EF) of gas phase I/SVOCs was approximately 10 times higher than that of particle phase I/SVOCs and the chemical compositions and volatility distributions varied greatly. VOC, IVOC, and SVOC emissions significantly decreased when vehicles were equipped with advanced aftertreatment technologies. Diesel particulate filters (DPF) can remove >71 % VOC, 74 % gaseous, and 88 % particulate I/SVOCs, many of which are significant secondary organic aerosol (SOA) precursors. The chemical compositions and volatility distributions of the gaseous I/SVOCs and unburned diesel fuel were similar, revealing that diesel fuel is the main origin of the gaseous I/SVOCs. The I/SVOC emission profiles covering the whole volatility range, i.e., log10C* = −3 to 10 (C*: effective saturation concentration, μg m−3) were established.

Oligomer formation from cross-reaction of Criegee intermediates in the styrene-isoprene-O3 mixed system

Alkene ozonolysis can produce stabilized Criegee intermediates (SCIs), which play a key role in oligomers' formation. Though styrene and isoprene coexist in the ambient atmosphere as important anthropogenic and biogenic secondary organic aerosol (SOA) precursors, respectively, their cross-reactions have not received attention. This study investigated the interactions of SCIs from styrene and isoprene ozonolysis for the first time. The high-resolution Orbitrap mass spectrometer was used to determine the unique ion mass spectra of the isoprene-styrene-O3 mixture. The results show that the signal intensities of new ions account for >8.4% of total ions in the mass spectra of the styrene-isoprene-O3 mixed system. Styrene and isoprene ozonolysis can produce characteristic C7–SCI and C4–SCI, respectively. C7–SCI and C4–SCI can be involved in the cross-reactions, and the results of tandem mass spectra directly confirmed both C7–SCI and C4–SCI as chain units. The O/C and H/C ratios of cross-products are in the range of 0.38–1.07 and 1.00–1.50, respectively, which are consistent with cross-reaction products. Adding a C7–SCI unit reduces the oligomer's volatility by 1.3–1.4 orders of magnitude lower than adding a C4–SCI unit. Thus, C4–SCI can compete with C7–SCI to react with styrene-derived RO2/RC(O)OH to produce more volatile cross-products, while the less volatile cross-products can be formed when isoprene-derived RO2/RC(O)OH reacted with C7–SCI instead of C4–SCI. The SOA yield of the mixed system is lower than that of the single styrene-O3 system but higher than that of the single isoprene-O3 system. Ambient particles were also collected, and 5 possible SCI-related cross-products were identified. This study illustrates the effects of SCI-related cross-reactions on SOA components and physicochemical properties, providing a basis for future research on SCI-related cross-reactions that frequently occur in the ambient atmosphere.

Contribution of fossil and biomass-derived secondary organic carbon to winter water-soluble organic aerosols in Delhi, India

Delhi, among the world's most polluted megacities, is a hotspot of particulate matter emissions, with high contribution from organic aerosol (OA), affecting health and climate in the entire northern India. While the primary organic aerosol (POA) sources can be effectively identified, an incomplete source apportionment of secondary organic aerosol (SOA) causes significant ambiguity in the management of air quality and the assessment of climate change. Present study uses positive matrix factorization analysis on the water-soluble organic aerosol (WSOA) data from the offline-aerosol mass spectrometry (AMS). It revealed POA as the dominant source of WSOA, with biomass-burning OA (31–34 %) and solid fuel combustion OA (∼21 %) being two major contributors. Here we use water-solubility fingerprints to track the SOA precursors, such as oxalates or organic nitrates, instead of identifying them based on their O:C ratio. Non-fossil precursors dominate in more oxidized oxygenated organic carbon (MO-OOC) (∼90 %), a proxy for aged secondary organic carbon (SOC), by coupling offline-AMS with 14C measurements. On the contrary, the oxidation of fossil fuel emissions produces a large quantity of fresh fossil SOC, which accounts for ∼75 % of less oxidized oxygenated organic carbon (LO-OOC). Our study reveals that apart from major POA contributions, large fractions of fossil (10–14 %) and biomass-derived SOA (23–30 %) contribute significantly to the total WSOA load, having impact on climate and air quality of the Delhi megacity. Our study reveals that large-scale unregulated biomass burning was not only found to dominate in POA but was also observed to be a significant contributor to SOA with implications on human health, highlighting the need for effective control strategies.

Contribution of Carbonyl Chromophores in Secondary Brown Carbon from Nighttime Oxidation of Unsaturated Heterocyclic Volatile Organic Compounds.

The light absorption properties of brown carbon (BrC), which are linked to molecular chromophores, may play a significant role in the Earth's energy budget. While nitroaromatic compounds have been identified as strong chromophores in wildfire-driven BrC, other types of chromophores remain to be investigated. Given the electron-withdrawing nature of carbonyls ubiquitous in the atmosphere, we characterized carbonyl chromophores in BrC samples from the nighttime oxidation of furan and pyrrole derivatives, which are important but understudied precursors of secondary organic aerosols primarily found in wildfire emissions. Various carbonyl chromophores were characterized and quantified in BrC samples, and their ultraviolet-visible spectra were simulated by using time-dependent density functional theory. Our findings suggest that chromophores with carbonyls bonded to nitrogen (i.e., imides and amides) derived from N-containing heterocyclic precursors substantially contribute to BrC light absorption. The quantified N-containing carbonyl chromophores contributed to over 40% of the total light absorption at wavelengths below 350 nm and above 430 nm in pyrrole BrC. The contributions of chromophores to total light absorption differed significantly by wavelength, highlighting their divergent importance in different wavelength ranges. Overall, our findings highlight the significance of carbonyl chromophores in secondary BrC and underscore the need for further investigation.

Volatile oxidation products and secondary organosiloxane aerosol from D5 + OH at varying OH exposures

Abstract. Siloxanes are composed of silicon, oxygen, and alkyl groups and are emitted from consumer chemicals. Despite being entirely anthropogenic, siloxanes are being detected in remote regions and are ubiquitous in indoor and urban environments. Decamethylcyclopentasiloxane (D5) is one of the most common cyclic congeners, and smog chamber and oxidation flow reactor (OFR) experiments have found D5 + OH to form secondary organosiloxane aerosol (SOSiA). However, there is uncertainty about the reaction products and the reported SOSiA mass yields (YSOSiA) appear inconsistent. To quantify small volatile oxidation products (VOPs) and to consolidate the YSOSiA in the literature, we performed experiments using a potential aerosol mass OFR while varying D5 concentration, humidity, and OH exposure (OHexp). We use a proton transfer reaction time-of-flight mass spectrometer to quantify D5, HCHO, and HCOOH and to detect other VOPs, which we tentatively identify as siloxanols and siloxanyl formates. We determine molar yields of HCHO and HCOOH between 52 %–211 % and 45 %–127 %, respectively. With particle size distributions measured with a scanning mobility particle sizer, we find YSOSiA to be < 10 % at OHexp < 1.3 × 1011 s cm−3 and ∼ 20 % at OHexp, corresponding to that of the lifetime of D5 at atmospheric OH concentrations. We also find that YSOSiA is dependent on both organic aerosol mass loading and OHexp. We use a kinetic box model of SOSiA formation and oxidative aging to explain the YSOSiA values found in this study and the literature. The model uses a volatility basis set (VBS) of the primary oxidation products as well as an aging rate coefficient in the gas phase, kage,gas, of 2.2×10-12 cm3 s−1 and an effective aging rate coefficient in the particle phase, kage,particle, of 2.0 × 10−12 cm3 s−1. The combination of a primary VBS and OH-dependent oxidative aging predicts SOSiA formation much better than a standard-VBS parameterization that does not consider aging (root mean square error = 42.6 vs. 96.5). In the model, multi-generational aging of SOSiA products occurred predominantly in the particle phase. The need for an aging-dependent parameterization to accurately model SOSiA formation shows that concepts developed for secondary organic aerosol precursors, which can form low-volatile products at low OHexp, do not necessarily apply to D5 + OH. The resulting yields of HCHO and HCOOH and the parameterization of YSOSiA may be used in larger-scale models to assess the implications of siloxanes for air quality.

Global Anthropogenic Emissions of Full-Volatility Organic Compounds.

Traditional global emission inventories classify primary organic emissions into nonvolatile organic carbon and volatile organic compounds (VOCs), excluding intermediate-volatility and semivolatile organic compounds (IVOCs and SVOCs, respectively), which are important precursors of secondary organic aerosols. This study establishes the first global anthropogenic full-volatility organic emission inventory with chemically speciated or volatility-binned emission factors. The emissions of extremely low/low-volatility organic compounds (xLVOCs), SVOCs, IVOCs, and VOCs in 2015 were 13.2, 10.1, 23.3, and 120.5 Mt, respectively. The full-volatility framework fills a gap of 18.5 Mt I/S/xLVOCs compared with the traditional framework. Volatile chemical products (VCPs), domestic combustion, and on-road transportation sources were dominant contributors to full-volatility emissions, accounting for 30, 30, and 12%, respectively. The VCP and on-road transportation sectors were the main contributors to IVOCs and VOCs. The key emitting regions included Africa, India, Southeast Asia, China, Europe, and the United States, among which China, Europe, and the United States emitted higher proportions of IVOCs and VOCs owing to the use of cleaner fuel in domestic combustion and more intense emissions from VCPs and on-road transportation activities. The findings contribute to a better understanding of the impact of organic emissions on global air pollution and climate change.

Atmospheric impact of isoprene-derived Criegee intermediates and isoprene hydroxy hydroperoxide on sulfate aerosol formation in the Asian region

This study revealed the importance of two isoprene-derived pathways in the formation of sulfate aerosol (SO42−(p)), which are related to the gas-phase reactions of Criegee intermediates (CIs) and liquid-phase reactions of isoprene hydroxy hydroperoxide (ISOPOOH), in the atmosphere in the Asian region, using the community multiscale air quality (CMAQ) modeling system. The results showed that the incorporation of both isoprene-derived CIs and ISOPOOH chemistry increased the SO42−(p) concentration in specific domains of Asia, particularly isoprene hotspots, with up to 1% of the total SO42−(p) concentration compared to that obtained by the basic chemical transport model. The impact of ISOPOOH to SO42−(p) formation exhibited a more localized pattern compared to that of CIs, attributed to the tendency of ISOPOOH to condense in low-NO regions. SO42−(p) formation from CIs affected the compositions of nitrate (NO3−), ammonium (NH4+), and secondary organic aerosols (SOA) in the particle phase, and in most of the analyzed sites, those compositions increased from 0.5% to 2.5%. In contrast, SO42−(p) formation from ISOPOOH exhibits a negative impact on SOA, resulting in a potential reduction of up to 5% due to the loss of SOA precursor by incorporating the ISOPOOH uptake reaction. The sensitivity analysis results showed that most of the SO42−(p) was generated by the gas-phase reaction of SO2 with OH; however, the CIs and ISOPOOH also affected the SO42−(p) formation more than the liquid-phase SO42−(p) formations by H2O2, O3, O2, etc.

Impurity contribution to ultraviolet absorption of saturated fatty acids.

Saturated fatty acids are abundant organic compounds in oceans and sea sprays. Their photochemical reactions induced by solar radiation have recently been found as an abiotic source of volatile organic compounds, which serve as precursors of secondary organic aerosols. However, photoabsorption of wavelengths longer than 250 nanometers in liquid saturated fatty acids remains unexplained, despite being first reported in 1931. Here, we demonstrate that the previously reported absorption of wavelengths longer than 250 nanometers by liquid nonanoic acid [CH3(CH2)7COOH)] originates from traces of impurities (0.1% at most) intrinsically contained in nonanoic acid reagents. Absorption cross sections of nonanoic acid newly obtained here indicate that the upper limit of its photolysis rate is three to five orders of magnitude smaller than those for atmospherically relevant carbonyl compounds.