Secondary Organic Aerosol Production Research Articles

Widespread 2013-2020 decreases and reduction challenges of organic aerosol in China

High concentrations of organic aerosol (OA) occur in Asian countries, leading to great health burdens. Clean air actions have resulted in significant emission reductions of air pollutants in China. However, long-term nation-wide trends in OA and their causes remain unknown. Here, we present both observational and model evidence demonstrating widespread decreases with a greater reduction in primary OA than in secondary OA (SOA) in China during the period of 2013 to 2020. Most of the decline is attributed to reduced residential fuel burning while the interannual variability in SOA may have been driven by meteorological variations. We find contrasting effects of reducing NOx and SO2 on SOA production which may have led to slight overall increases in SOA. Our findings highlight the importance of clean energy replacements in multiple sectors on achieving air-quality targets because of high OA precursor emissions and fluctuating chemical and meteorological conditions.

Influence of ozone pollution on the mixing state and formation of oxygenated organics containing single particles

The formation and aging processes of oxygenated organic molecules (OOMs) are important for understanding the formation mechanisms of secondary organic aerosols (SOAs) in the field. In this study, we investigated the mixing states of OOM particles by identifying several oxygenated species along with the distributions of secondary organic carbon (SOC) during both clean and ozone (O3)-polluted periods in July and September of 2022 in Guangzhou, China. OOM-containing particles accounted for 57 % and 49 % of the total detected single particles in July and September, respectively. Most of the OOM particles were internally mixed with sulfate and nitrate, while elemental carbon and hydrocarbon species were absent. Despite the higher SOC/OC ratio in September (81 %) than it in July (72 %), comparative investigations of the mass spectra, diurnal patterns, and distributions of OOM particles revealed the same composition and aging states of OOMs in two O3 pollution periods. As the O3 concentration increased from the clean to the polluted periods, the ratio of SOC to OC increased along with the relative abundance of secondary OOM particles among total OOM particles. In contrast, the relative abundance of OC-type OOM particles gradually decreased, indicating the conversion of hydrocarbon species into OOMs as the SOC/OC ratio increased. Both the bulk analysis of SOC from filter measurement and the mixing states of OOM particles suggested that OOM production and degree of oxidation were higher in the O3-polluted periods than in the clean periods. These results elucidate the effects of O3 pollution on the OOM formation process and offer new perspectives for the joint investigation of SOA production based on filter sampling and single-particle measurements.

Unraveling the influence of biogenic volatile organic compounds and their constituents on ozone and SOA formation within the Yellow River Basin, China

Biogenic volatile organic compounds (BVOC) assume a pivotal role during the formation stages of ozone (O3) and secondary organic aerosols (SOA), serving as their primary precursors. We used the latest MEGAN3.1 model, updated vegetation data and emission factors, combined with MODIS data analysis to simulate and estimate the integrated emissions of BVOC from nine provinces in China's Yellow River Basin in 2018. Following an extensive evaluation of the WRF-CMAQ model utilizing diverse parameters, the simulated and observed values had correlation coefficients between them that ranged from 0.94 to 0.99, implying a favorable outcome in terms of simulation efficacy. The findings from the simulation analysis reveal that the combined BVOC emissions from the nine provinces in the Yellow River Basin reached a total of 6.51 Tg in 2018. Among these provinces, Sichuan, Henan, and Shaanxi ranked highest, with emissions of 1.28 Tg, 1.04 Tg, and 0.96 Tg, respectively. BVOC emissions led to concentrations of 36.72 μg/m³ in the daily maximum 8-h ozone and 0.59 μg/m³ in the average SOA in nine provinces of the Yellow River Basin in July. Isoprene contributed the most to the O3 production with 6.31 μg/m3, and monoterpenes contributed the most to SOA production with 0.45 μg/m3. ΔSOA and ΔOzone are mainly distributed in the belts of central Sichuan Province, southern Shaanxi Province, western Henan Province, northern Qinghai Province, central Inner Mongolia, and southern Shanxi Province, and most of these areas are located 50 km around the Yellow River. O3 and SOA in Taiyuan, Xi'an, Chengdu, and Zhengzhou cities are strongly influenced by the generation of BVOCs. This study provides a reliable scientific basis for the prevention and control of air pollution in the Yellow River Basin.

Characteristics and Source Apportionment of Volatile Organic Compounds in an Industrial Area at the Zhejiang–Shanghai Boundary, China

As “fuel” for atmospheric photochemical reactions, volatile organic compounds (VOCs) play a key role in the secondary generation of ozone (O3) and fine particulate matter (PM2.5, an aerodynamic diameter ≤ 2.5 μm). To determine the characteristics of VOCs in a high-level ozone period, comprehensive monitoring of O3 and its precursors (VOCs and NOx) was continuously conducted in an industrial area in Shanghai from 18 August to 30 September 2021. During the observation period, the average concentration of VOCs was 47.33 ppb, and alkanes (19.64 ppb) accounted for the highest proportion of TVOCs, followed by oxygenated volatile organic compounds (OVOCs) (13.61 ppb), alkenes (6.92 ppb), aromatics (4.65 ppb), halogenated hydrocarbons (1.60 ppb), and alkynes (0.91 ppb). Alkenes were the predominant components that contributed to the ozone formation potential (OFP), while aromatics such as xylene, toluene, and ethylbenzene contributed the most to the secondary organic aerosol production potential (SOAFP). During the study period, O3, NOx, and VOCs showed significant diurnal variations. Industrial processes were the main source of VOCs, and the second largest source of VOCs was vehicle exhaust. While the largest contribution to OFP was from vehicle exhaust, the second largest contribution was from liquid petroleum gas (LPG). High potential source contribution function (PSCF) values were observed in western and southeastern areas near the sampling sites. The results of a health risk evaluation showed that the Hazard Index was less than 1 and there was no non-carcinogenic risk, but 1,3-butadiene, benzene, chloroform, 1,2-dibromoethane, and carbon tetrachloride pose a potential carcinogenic risk to the population.

Process-evaluation of forest aerosol-cloud-climate feedback shows clear evidence from observations and large uncertainty in models

Natural aerosol feedbacks are expected to become more important in the future, as anthropogenic aerosol emissions decrease due to air quality policy. One such feedback is initiated by the increase in biogenic volatile organic compound (BVOC) emissions with higher temperatures, leading to higher secondary organic aerosol (SOA) production and a cooling of the surface via impacts on cloud radiative properties. Motivated by the considerable spread in feedback strength in Earth System Models (ESMs), we here use two long-term observational datasets from boreal and tropical forests, together with satellite data, for a process-based evaluation of the BVOC-aerosol-cloud feedback in four ESMs. The model evaluation shows that the weakest modelled feedback estimates can likely be excluded, but highlights compensating errors making it difficult to draw conclusions of the strongest estimates. Overall, the method of evaluating along process chains shows promise in pin-pointing sources of uncertainty and constraining modelled aerosol feedbacks.

Ozone stress response of leaf BVOC emission and photosynthesis in mountain birch (Betula pubescens spp. czerepanovii) depends on leaf age.

Oxidative stress from ozone (O3) causes plants to alter their emission of biogenic volatile organic compounds (BVOC) and their photosynthetic rate. Stress reactions from O3 on birch trees can result in prohibited plant growth and lead to increased BVOC emission rates as well as changes in their compound blend to emit more monoterpenes (MT) and sesquiterpenes (SQT). BVOCs take part in atmospheric reactions such as enhancing the production of secondary organic aerosols (SOA). As the compound blend and emission rate change with O3 stress, this can influence the atmospheric conditions by affecting the production of SOA. Studying the stress responses of plants provides important information on how these reactions might change, which is vital to making better predictions of the future climate. In this study, measurements were taken to find out how the leaves of mature mountain birch trees (Betula pubescens ssp. czerepanovii) respond to different levels of elevated O3 exposure insitu depending on leaf age. We found that leaves from both early and late summers responded with induced SQT emission after exposure to 120 ppb O3. Early leaves were, however, more sensitive to increased O3 concentrations, with enhanced emission of green leaf volatiles (GLV) and tendencies of both induced leaf senescence as well as poor recovery in the photosynthetic rate between exposures. Late leaves had more stable photosynthetic rates throughout the experiment and responded less to exposure at different O3 levels.

Chemical Data Assimilation With Aqueous Chemistry in WRF‐Chem Coupled With WRFDA (V4.4.1)

AbstractThis study introduces a new chemistry option in the Weather Research and Forecasting model data assimilation (WRFDA) system, coupled with the WRF‐Chem model (Version 4.4.1), to incorporate aqueous chemistry (AQCHEM) in the assimilation of ground‐level chemical measurements. The new DA capability includes the integration of aqueous‐phase aerosols from the Regional Atmospheric Chemistry Mechanism (RACM) gas chemistry, the Modal Aerosol Dynamics Model for Europe (MADE) aerosol chemistry, and the Volatility Basis Set (VBS) for secondary organic aerosol production. The RACM‐MADE‐VBS‐AQCHEM scheme facilitates aerosol‐cloud‐precipitation interactions by activating aerosol particles in cloud water during the model simulation. With the goal of enhancing air quality forecasting in cloudy conditions, this new implementation is demonstrated in the weakly coupled three‐dimensional variational data assimilation (3D‐Var) system through regional air quality cycling over East Asia. Surface particulate matter (PM) concentrations and four gas species (SO2, NO2, O3, and CO) are assimilated every 6 hr for the month of March 2019. The results show that including aqueous‐phase aerosols in both the analysis and forecast can represent aerosol wet removal processes associated with cloud development and rainfall production. During a pollution event with high cloud cover, simulations without aerosols defined in cloud water exhibit significantly higher values for liquid water path, and surface PM10 (PM2.5) concentrations are overestimated by a factor of 10 (3) when wet scavenging processes dominate. On the contrary, AQCHEM proves to be helpful in simulating the wet deposition of aerosols, accurately predicting the evolution of surface PM concentrations without such overestimation.

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.

Updating Biogenic Volatile Organic Compound (BVOC) Emissions With Locally Measured Emission Factors in South China and the Effect on Modeled Ozone and Secondary Organic Aerosol Production

AbstractBiogenic volatile organic compounds (BVOCs) emitted from terrestrial plants contribute substantially to ozone (O3) and secondary organic aerosol (SOA) formation in the troposphere. Accurate estimation of BVOC emissions is highly challengeable with a variety of uncertainties, one of which is the use of default emission factors (EFs) particularly for underrepresented regions without local data. In this study, locally measured BVOC‐EFs in south China, a subtropical region with abundant vegetation, were used to update regional BVOC emissions as estimated by the Model of Emissions of Gases and Aerosols from Nature (MEGAN). These EFs were recently determined in situ with characterized dynamic chambers for the emissions of isoprene, monoterpenes, and sesquiterpenes from tree species. The Community Multiscale Air Quality (CMAQ) model was then employed to see how much the regional O3 and SOA production is altered with the updated BVOC emissions. Results revealed lower BVOC emission estimates in south China when using the localized EFs than the MEGAN default ones, particularly for sesquiterpenes with a notable average reduction rate of approximately 40%. Using the updated BVOC emissions improved model O3 predictions in all seasons when compared to surface O3 monitoring, yet the lower BVOC emissions resulted in modeled O3 and SOA concentrations decreased by up to −6 ppb and −1.5 μg m−3, respectively, throughout south China. This study highlights the significance of localized EFs in refining emission estimates and air quality predictions in regions with a wealth of vegetation.

Sources and secondary transformation potentials of aromatic hydrocarbons observed in a medium-sized city in yangtze river delta region: Emphasis on intermediate-volatility naphthalene

Aromatic hydrocarbons (AHs) play an important role in the formation of ground-level ozone (O3) and secondary organic aerosols (SOA). Although monoaromatics (MAHs) have been extensively measured in previous studies, the heavy AHs such as 2-ring naphthalene (Nap), a key surrogate of intermediate-volatility organic compounds (IVOCs), were overlooked. Additionally, field observations of AHs are mostly restricted to megacities and limited results have been reported in medium/small-sized cities, which could demonstrate different AH pollution characteristics due to the factors like emission source and population. In this study, sixteen MAHs as well as intermediate-volatility Nap were measured concurrently at six urban sites surrounded by traffic lanes, residential and commercial areas in Wuhu, a medium-sized city in Yangtze River Delta (YRD) region of China, in summertime. During the campaign, the mean concentration of seventeen AHs was 2.85 ± 0.55 ppb (mean ± 95% C.I.). Compared to megacities, lower MAHs but comparable or even relatively higher Nap levels were observed here during the sampling period. Source apportionment revealed that gasoline exhaust and industrial emission were the major contributors to AHs, accounting for 32.2%–52.6% and 20.3%–46.2%, respectively, followed by biomass/coal burning (8.5%–14.2%), solvent utilization (4.5%–13.1%) and diesel engine emission (3.0%–7.3%). Interestingly, diesel engine emission made the largest contribution to Nap, while industrial emission was less significant, suggesting that the source of IVOC species might be different from that of traditional VOCs. Among the measured AHs, Nap ranked the largest and the third-largest contributor to calculated SOA formation potentials (SOAFPs) and O3 formation potentials (OFPs), respectively. Our results highlight the role of intermediate-volatility Nap in the production of SOA and O3 and reveal different source contributions between Nap and MAHs. It is suggested to re-evaluate the contributions of various sources and precursors to urban O3 and SOA after considering the IVOCs.

Primary and Secondary Organic Aerosol Formation from Asphalt Pavements.

Asphalt is ubiquitous across cities and a source of organic compounds spanning a wide range of volatility and may be an overlooked source of urban organic aerosols. The emission rate and composition depend strongly on temperature, but emissions have been observed at both application temperatures and surface temperatures during warm sunny days. Here we report primary organic aerosol (POA) emissions and secondary organic aerosol (SOA) production from asphalt. We reheated real-world asphalt samples to application-relevant temperatures (∼130 °C) and typical summertime road-surface temperatures (∼55 °C) and then flushed the emitted vapors into an environmental oxidation chamber containing ammonium sulfate seed particles. SOA was then formed following the photo-oxidation of emissions under high-NOx conditions typical of urban atmospheres. We find that POA only forms at application temperature as it does not require further oxidation, whereas SOA forms under both conditions; with the resulting POA and SOA both being semi-volatile. While total OA formation rates were substantially greater under the limited time spent under application conditions, SOA formation from passive asphalt heating presents a potential long-term source, as heating continues for the lifetime of the road surface. This suggests that persistent asphalt solar heating is likely a considerable and continued source of summertime SOA in urban environments.

Source apportionment of volatile organic compounds during paddy-residue burning season in north-west India reveals large pool of photochemically formed air toxics

Paddy-residue burning is associated with poor air quality in north-west India during October–November every year. However, till date a quantitative study of its contribution to ambient volatile organic compounds (VOCs) using highly time-resolved measurements within the region has been lacking. Several VOCs like benzene are carcinogenic and also fuel formation of secondary pollutants such as secondary organic aerosol (SOA) and ozone. Here, we undertake quantitative source-apportionment using a PMF source-receptor model on a high-quality in-situ measured dataset of 54 VOCs in Punjab, India, and validate the model results using source profiles. The contribution of the seven most dominant sources to the total VOC mass concentrations were: daytime photochemistry and biogenic VOCs (BVOCs) (26%), followed by solid-fuel usage and waste-disposal (18%), traffic (two-wheeler 14% and four-wheeler 10%), photochemically aged biomass burning (17%), industries and solvent usage (9%), and fresh paddy residue burning (6%). Ozone production potential was dominated by solid fuel usage and waste disposal (25%), followed by traffic (two-wheeler 11% and four-wheeler 12%), BVOCs and photooxidation products (21%), photochemically aged biomass burning (16%), industries & solvent usage (9%) and fresh paddy residue burning (6%). SOA production was dominated by traffic (two-wheeler 26% and four-wheeler 28%) followed by solid fuel usage and waste disposal (22%), photochemically aged biomass burning emissions (15%) with minor contribution from industries & solvents (6%), fresh paddy residue burning (2%) and photochemistry and biogenic VOCs (1%). Comparisons with global emission inventories REASv3.2.1 and EDGARv4.3.2, showed both overestimate the industry and solvent source. Further, EDGARv4.3.2 underestimated the traffic source whereas paddy residue burning emissions are absent in REASv3.2.1. Although the overall mass contribution of paddy-residue burning emissions isn't high, our results show that health-relevant compounds emitted directly and formed photochemically from biomass burning sources active at this time are majorly responsible for the unhealthy air.

Size‐Dependent Nighttime Formation of Particulate Secondary Organic Nitrates in Urban Air

AbstractParticulate secondary organic nitrates play a key role in understanding secondary organic aerosol production, ozone formation, and the atmospheric nitrogen cycle. However, the formation of particulate secondary organic nitrates in ambient air remains poorly understood. In this study, the nighttime formation processes of particulate secondary organic nitrates were investigated based on size‐resolved aerosols measured in urban air of China with a soot particle long‐time‐of‐flight aerosol mass spectrometer. The results show a bimodal size distribution of particulate secondary organic nitrates, peaking at ∼350 nm in condensation mode (100–400 nm) and ∼750 nm in droplet mode (400–2,500 nm), respectively. The nighttime formation processes of particulate secondary organic nitrates in the two size modes were respectively governed by temperature‐dependent condensation and aqueous‐phase processing. In particular, the mass concentration of particulate secondary organic nitrates in condensation mode was positively correlated with nitrate radical production and negatively correlated with temperature, suggesting that the formation processes were associated with the gas‐particle conversion of nitrate radical oxidation products. In contrast, the enhanced particulate secondary organic nitrates in droplet mode were predominantly contributed by aqueous‐phase processing, as indicated by the strong positive correlation with aerosol liquid water content and typical fragment ions from aqueous processing products (r = 0.52–0.59, P < 0.01). Our results highlight the potential of a size‐dependent mechanism to elucidate the formation processes of particulate secondary organic nitrates.

Characteristics of intermediate volatility organic compounds emitted from inland vessels with different influential factors and implication of reduction emissions

Ships could emit an abundance intermediate volatility organic compounds (IVOCs). In recent years, many studies on the emission characteristics of IVOCs have focused on the burning of heavy fuel oil by ocean-going ships; however, few have focused on inland vessels which have a more significant impact on air quality and human health owing to their closer proximity to cities than ocean-going ships. In this study, the IVOC emission factors (EFIVOCs) of three inland vessels were determined using a dilution sampling system considering different influencing factors (ship age and operating conditions). The results showed that the EFIVOCs values ranged from 869.9 to 7607 mg/kg fuel, with an average of 4128 ± 2703 mg/kg fuel. In addition, the age of the vessel was found to have a dramatic effect on emissions with the average EFIVOCs of inland vessels aged >10 years was 4300 ± 4319, 5769, and 6484 ± 1586 mg/kg fuel under cruising, idling, and maneuvering conditions, respectively, while that of vessels <10 years old was 1180 ± 328.3 mg/kg fuel when maneuvering. The percentages of emission factors for unresolved complex mixture (UCM), normal alkanes (n-alkanes), branched alkanes (b-alkanes), and polycyclic aromatic hydrocarbons (PAHs) from inland vessels were 82.1 ± 2.6 %, 5.2 ± 0.9 %, 10.6 ± 2.0 % and 2.0 ± 0.6 % of the total IVOCs, respectively. The secondary organic aerosols (SOA) production of inland vessels was estimated to be 1212 ± 801.7 mg/kg fuel, which was substantially higher than those of diesel vehicles, non-road construction machinery, and gasoline vehicles reported by other researches. Moreover, based on the ship movement and measured EFIVOCs data, the IVOCs emission inventory of inland vessels in Jiangsu Province and China in 2016 was 4.2 ± 2.8 and 32.0 ± 21.0 Gg respectively, which was comparable to those from diesel vehicle emissions.

Low-NO-like Oxidation Pathway Makes a Significant Contribution to Secondary Organic Aerosol in Polluted Urban Air.

Anthropogenic pollutants can greatly mediate formation pathways and chemical compositions of secondary organic aerosol (SOA) in urban atmospheres. We investigated the molecular tracers for different types of SOA in PM2.5 under varying NO/NO2 conditions in Guangzhou using source analysis of particle-phase speciated organics obtained from an iodide chemical ionization mass spectrometer with a Filter Inlet for Gases and AEROsols (FIGAERO-I-CIMS). Results show that low-NO-like pathways (when NO/NO2 < 0.2) explained ∼75% of the total measured FIGAERO-OA during regional transport periods, which was enriched in more-oxidized C4-C6 non-nitrogenous compounds over ozone accumulation. Daytime high-NO chemistry played larger roles (38%) in local pollution episodes, with organic nitrates (ONs) and nitrophenols increasing with enhanced aerosol water content and nitrate fraction. Nighttime NO3-initiated oxidation, characterized by monoterpene-derived ONs, accounted for comparable percentages (10-12%) of FIGAERO-OA for both two periods. Furthermore, the presence of organosulfates (OSs) improves the understanding of the roles of aqueous-phase processes in SOA production. Carbonyl-derived OSs exhibited a preferential formation under conditions of high aerosol acidity and/or abundant sulfate, which correlated well with low-NO-like SOA. Our results demonstrate the importance of NO/NO2 ratios in controlling SOA compositions, as well as interactions between water content, aerosol acidity, and inorganic salts in gas-to-particle partitioning of condensable organics.

Unexpected moderate contribution of intermediate volatility organic compounds from gasoline vehicle emission to secondary organic aerosol formation in summer of Beijing

Intermediate volatility organic compounds (IVOCs) have been proposed to be important contributors to secondary organic aerosol (SOA) formation. In this study, we characterized ambient IVOCs in Beijing and performed a comprehensive closure study of ambient SOA production using top-down (photochemical-age-based parameterization method to apportion OA) and bottom-up (estimation of SOA from detailed speciation of IVOCs and VOCs) approaches. The IVOCs in Beijing were more oxidized and had a high abundance (62.5 ± 45.2 μg m−3), significantly higher than in the US. Unlike the US, the IVOCs in Beijing shows a hybrid characteristic of gasoline and vehicle exhausts and are influenced by complicated sources. A ∼ 80% SOA mass closure could be reached between the top-down and bottom-up methods, in which IVOCs was the dominant contributor (75.5%). Furthermore, we established a parametric method (SOA/CO vs. photochemical age) to estimate the contribution of vehicular SOA to ambient OA. Unexpectedly, the contributions of gasoline vehicles to ambient OA are only 7.3% for POA and 13.0% for SOA. Our results imply that IVOCs from other sources, e.g., diesel vehicles and cooking emissions, may be important precursors to secondary organic aerosol in Beijing. In addition, this study highlights the importance of comprehensively performing the molecular identification of IVOCs from different emissions in China. Plain language summarySecondary organic aerosol (SOA), formed by multigenerational oxidation of gas-phase compounds, represents a significant fraction of organic aerosol. However, current models often underestimate the measured SOA mass. One of the ubiquitous reasons is the missing measurement of the intermediate volatility organic compounds (IVOCs). Here we quantified and qualified the IVOCs in Beijing and performed a comprehensive SOA study using bottom-up and top-down methods. Besides, we estimate the SOA formation by gasoline vehicles. Unexpectedly, the contributions of gasoline vehicles to ambient OA are only 7.3% for POA and 13.0% for SOA, highlighting the importance of studying IVOCs from other sources, e.g., diesel exhausts and cooking emissions.

Anthropogenic amplification of biogenic secondary organic aerosol production

Abstract. Biogenic secondary organic aerosols (SOAs) contribute to a large fraction of fine aerosols globally, impacting air quality and climate. The formation of biogenic SOA depends on not only emissions of biogenic volatile organic compounds (BVOCs) but also anthropogenic pollutants including primary organic aerosol, sulfur dioxide (SO2), and nitrogen oxides (NOx). However, the anthropogenic impact on biogenic SOA production (AIBS) remains unclear. Here we use the decadal trend and variability in observed organic aerosol (OA) in the southeast US, combined with a global chemistry–climate model, to better constrain AIBS. We show that the reduction in SO2 emissions can only explain 40 % of the decreasing decadal trend of OA in this region, constrained by the low summertime month-to-month variability in surface OA. We hypothesize that the rest of the OA decreasing trend is largely due to a reduction in NOx emissions. By implementing a scheme for monoterpene SOA with enhanced sensitivity to NOx, our model can reproduce the decadal trend and variability in OA in this region. Extending to a centennial scale, our model shows that global SOA production increases by 36 % despite BVOC reductions from the preindustrial period to the present day, largely amplified by AIBS. Our work suggests a strong coupling between anthropogenic and biogenic emissions in biogenic SOA production that is missing from current climate models.

On the importance of the model representation of organic aerosol in simulations of the direct radiative effect of Siberian biomass burning aerosol in the eastern Arctic

Shortcomings and uncertainties in the model representation of atmospheric transformations (the aging) of organic aerosol (OA) have long been identified as one of the potential sources of considerable uncertainty in OA simulations with both global and regional models. However, the impact of this uncertainty on predictions of radiative and climate effects of both anthropogenic and biomass burning (BB) aerosol yet needs to be understood. This study examines the importance of the model representation of OA for simulating the direct radiative effect (DRE) of Siberian BB aerosol in the eastern Arctic. We employ a regional coupled chemistry-meteorology model and a global fire emission database to simulate the optical properties and DRE of BB aerosol emitted from intense Siberian fires in July 2016 and compare the DRE estimates that were obtained using two alternative representations of Siberian BB OA. One of them is a “default” OA representation that predicts very little secondary OA (SOA), and another involves a simple original OA parameterization that has been developed previously within the volatility basis set (VBS) framework and features a strong production of SOA. The simulations of the aerosol optical properties are evaluated against satellite observations of the aerosol optical depth (AOD) in Siberia and the Arctic as well as against values of the single scattering albedo derived from in situ observations of the aerosol absorption and scattering coefficients at four Arctic sites. While the simulations with the default OA representation are found to strongly underestimate AOD both in Siberia and the eastern Arctic, the use of the VBS parameterization considerably improves the agreement between the AOD simulations and observations in both regions. Simulations of the single scattering albedo are found to be overall rather adequate with both representations. Differences in the OA representations are found to result in major differences in the estimates of the DRE of Siberian BB aerosol in the eastern Arctic. Specifically, although the simulations with both representations predict that the DRE is predominantly negative at the top of the atmosphere (TOA), the magnitude of the mean DRE is found to be more than twice as large (6.0 W m−2) with the VBS parameterization than with the default OA representation (2.8 W m−2). An even larger difference (by a factor of 3.5) is found between the estimates of the DRE over the snow- or ice-covered areas. The different treatments of the BB OA evolution are associated also with considerably different contributions of black and brown carbon to the DRE estimates. Overall, our results indicate that model estimates of the DRE of Siberian BB aerosol in the eastern Arctic are strongly sensitive to the assumptions regarding the evolution of OA in Siberian BB plumes and that the SOA formation in these plumes is one of the major factors determining the magnitude of the radiative effects of Siberian BB aerosol in the real atmosphere.

Volatility of Secondary Organic Aerosol from β-Caryophyllene Ozonolysis over a Wide Tropospheric Temperature Range.

We investigated secondary organic aerosol (SOA) from β-caryophyllene oxidation generated over a wide tropospheric temperature range (213-313 K) from ozonolysis. Positive matrix factorization (PMF) was used to deconvolute the desorption data (thermograms) of SOA products detected by a chemical ionization mass spectrometer (FIGAERO-CIMS). A nonmonotonic dependence of particle volatility (saturation concentration at 298 K, C298K*) on formation temperature (213-313 K) was observed, primarily due to temperature-dependent formation pathways of β-caryophyllene oxidation products. The PMF analysis grouped detected ions into 11 compound groups (factors) with characteristic volatility. These compound groups act as indicators for the underlying SOA formation mechanisms. Their different temperature responses revealed that the relevant chemical pathways (e.g., autoxidation, oligomer formation, and isomer formation) had distinct optimal temperatures between 213 and 313 K, significantly beyond the effect of temperature-dependent partitioning. Furthermore, PMF-resolved volatility groups were compared with volatility basis set (VBS) distributions based on different vapor pressure estimation methods. The variation of the volatilities predicted by different methods is affected by highly oxygenated molecules, isomers, and thermal decomposition of oligomers with long carbon chains. This work distinguishes multiple isomers and identifies compound groups of varying volatilities, providing new insights into the temperature-dependent formation mechanisms of β-caryophyllene-derived SOA particles.

Marked impacts of transient conditions on potential secondary organic aerosol production during rapid oxidation of gasoline exhausts

Vehicle emission is a major source of atmospheric secondary organic aerosols (SOA). Driving condition is a critical influencing factor for vehicular SOA production, but few studies have revealed the dependence on rapid-changing real-world driving conditions. Here, a fast-response oxidation flow reactor system is developed and deployed to quantify the SOA formation potential under transient driving conditions. Results show that the SOA production factor varies by orders of magnitude, e.g., 20–1500 mg kg-fuel−1 and 12–155 mg kg-fuel−1 for China V and China VI vehicles, respectively. High speed, acceleration, and deceleration are found to considerably promote SOA production due to higher organic gaseous emissions caused by unburned fuel emission or incomplete combustion. In addition, China VI vehicles significantly reduce SOA formation potential, yield, and acceleration and deceleration peaks. Our study provides experimental insight and parameterization into vehicular SOA formation under transient driving conditions, which would benefit high time-resolved SOA simulations in the urban atmosphere.