Secondary Organic Aerosol Concentrations Research Articles

A modeling study of secondary organic aerosol formation from sesquiterpenes using the STOCHEM global chemistry and transport model

AbstractSesquiterpenes are one of the precursors of secondary organic aerosol (SOA) which can be an important global sources of organic aerosol (OA). Updating the chemistry scheme in the global chemistry transport model by incorporating an oxidation mechanism for β‐caryophyllene (representing all sesquiterpenes), adding global sesquiterpene emissions of 29 Tg/yr, and revising global monoterpene emissions up to 162 Tg/yr [Guenther et al., 2012] led to an increase of SOA burden by 95% and SOA production rate by 106% relative to the base case described in Utembe et al. [2011]. Including the emissions of sesquiterpenes resulted in increase of SOA burden of 0.11 Tg and SOA production rate of 12.9 Tg/yr relative to the base case. The highest concentrations of sesquiterpene‐derived SOA (by up to 1.2 μg/m3) were found over central Africa and South America, the regions having high levels of biogenic emissions with significant biomass burning. In the updated model simulation, the multigeneration oxidation products from sesquiterpenes and monoterpenes transported above the boundary layer and condensed to the aerosol phase at higher altitude led to an increase of OA by up to 30% over the tropics and northern midlatitude to higher altitude. The model evaluation showed an underestimation of model OA mostly for the campaigns dominated by regional anthropogenic pollution. The increase of SOA production from sesquiterpenes reduced the discrepancies between modeled and observed OA concentrations over the remote and rural areas. The increase of SOA concentrations by up to 200% from preindustrial to present scenarios was found over the tropical oceans.

Characteristics and emission-reduction measures evaluation of PM2.5 during the two major events: APEC and Parade

The measurement of PM2.5 was conducted from 20th October to 25th November 2014, and 15th August to 15th September 2015 at Beijing, Shijiazhuang, and Tangshan, China, covering two important events of Asia-Pacific Economic Cooperation (APEC) and Grand Military Parade (Parade). A series of stringent emission-reduction measures were implemented in Beijing and neighbouring regions to ensure good air quality in Beijing. PM2.5 concentrations and major components (water-soluble ions and carbonaceous aerosol) were analyzed and compared between the two events. PM2.5 concentration during control demonstrated a decreasing trend with 51.6%–65.1% and 34.2%–64.7% compared to no control during APEC and Parade, respectively. The water-soluble ions and carbonaceous aerosol concentrations also decreased obviously. The lower secondary inorganic ions (SIA, including SO42−, NO3−, and NH4+) percentage (31.7%–38.3%) during control indicated the relative weak contribution of atmospheric chemical processes. Due to the unfavorable weather conditions and increased emissions during coal-fired heating, the relative higher organic carbon (OC), secondary organic carbon (SOC), secondary organic aerosol (SOA) concentrations during APEC was found. The WRF-CMAQ modeling system was also used to quantify the effect of emission-reduction and different sources controls on PM2.5. The results indicated PM2.5 decrease by 30.4% and 34.2% under control during APEC and Parade, respectively. We found that the local emission-reduction in Beijing contributes more to “APEC blue” (20.8%) and “Parade blue” (25.8%) in Beijing than those from neighbouring regions of Beijing, and meteorological condition was more beneficial for the improvement of the air quality during Parade than APEC. The emission source apportionment to PM2.5 in Beijing indicated that PM2.5 concentration increased by 13.5% and 14.7% during APEC and Parade, respectively if no vehicles emission-reduction measures were taken in Beijing.

Assessment of the impacts of aromatic VOC emissions and yields of SOA on SOA concentrations with the air quality model RAMS-CMAQ

The secondary organic aerosol (SOA) concentration is generally underestimated by models. Recent studies suggest that the underprediction is related to underestimations of aromatic volatile organic compound (VOC) emissions and SOA yields in current models. Here, the impacts of these two factors in China were investigated with the regional air quality modeling system RAMS-CMAQ, referring to field observations during the episode from October 14 to November 14, 2014. Comparisons between the observed and modeled SOA of four sensitivity simulation cases indicated the significant impacts of the two underestimated factors on the SOA output. By considering these two aspects, the simulated mean SOA concentrations significantly increased by nearly 4 times with a good representation of the intensively temporal variations of concentrations, which were largely controlled by photochemical processes rather than meteorological conditions. The improvement in SOA compensated for the underestimations by approximately 23.5% and contributed to the mean fraction of SOA to organic aerosol (OA) by increasing the fraction from less than 7% to more than 25%, which was closer to the observed result. These results suggested a more reasonable and more realistic representation of SOA formation in the model after allowing for the two factors. Due to the better simulation of SOA, predictions of OA were correspondingly improved when the correlation coefficient increased from 0.57 to 0.73 and other bias parameters were reduced, which indicated the improved ability of our model to trace the temporal variations of OA. Based on the improved simulation throughout the episode, the mean SOA concentration was obviously higher in eastern China than in the west. The highest concentration appeared in the Sichuan Basin and Pearl River Delta (PRD) areas, with values of 6–11 μg/m3 and 8–17 μg/m3, respectively. Over the wide regions of central and eastern China, the dominant component in SOA was formed from anthropogenic sources (ASOA), generally accounting for more than 60%.

Secondary organic aerosol formation from propylene irradiations in a chamber study

Some studies have shown that low-molecular-weight VOCs such as ethylene and acetylene can form SOA. However, so far propylene (C3H6) has not been studied. The current work systematically investigates irradiations of propylene in the presence of NOx (x = 1, 2) in a self-made indoor chamber. Only a small amount of secondary organic aerosols (SOA) was formed under 5% and 80% RH conditions without sodium chloride (NaCl) seed particles or in the presence of solid NaCl. When NaCl was in the form of droplets, liquid water content (LWC) increased from 34.5 to 169.8 μg m−3 under different initial NaCl concentrations, and correspondingly the amount of SOA linearly increased from 5.9 to 29.8 μg m−3 (SOA = 0.0164 × LWC+1.137, R2 = 0.97) at the C3H6/NOx ratio of 32.2–44.9 (ppbC/ppb). The initial C3H6/NOx concentration ratio considerably impacted the formation of SOA, in which the amount of SOA increased from 12.1 to 47.9 μg m−3 exponentially as the ratio decreased from 46.5 to 6.3 with an important point of the ratio value of 11. At the ratio of less than 11 in the regime under the control of C3H6, SOA concentrations decreased considerably with increasing ratio, whereas at the ratio value of larger than 11 in the NOx controlled regime, SOA slightly decreased with increasing ratio. From combination of the analysis of different functional groups of particles by IR spectra and ESI-Exactive-Orbitrap mass spectrometer, the constituents of SOA were identified to be hydroperoxides (e.g. HOCH2CCl(CH3)OOH), esters (e.g. CH2ClC(O)OCHClCHO), organic nitrates (e.g. HO2CH(CH2Cl)C(O)OCCl(CH2Cl)C(O)OCHClCH2ONO2), etc. Furthermore, a liquid-phase mechanism of SOA formation has been proposed in this study.

Emission characteristics of volatile organic compounds and their secondary organic aerosol formation potentials from a petroleum refinery in Pearl River Delta, China.

A campaign was carried out to measure the emission characteristics of volatile organic compounds (VOCs) in different areas of a petroleum refinery in the Pearl River Delta (PRD) region in China. In the refining area, 2-methylpentane, 2,3-dimethylbutane, methylcyclopentane, 3-methylhexane, and butane accounted for >50% of the total VOCs; in the chemical industry area, 2-methylpentane, p-diethylbenzene, 2,3-dimethylbutane, m-diethylbenzene and 1,2,4-trimethylbenzene were the top five VOCs detected; and in the wastewater treatment area, the five most abundant species were 2-methylpentane, 2,3-dimethylbutane, methylcyclopentane, 3-methylpentane and p-diethylbenzene. The secondary organic aerosol (SOA) formation potential was estimated using the fractional aerosol coefficients (FAC), secondary organic aerosol potential (SOAP), and SOA yield methods. The FAC method suggests that toluene, p-diethylbenzene, and p-diethylbenzene are the largest contributors to the SOA formation in the refining, chemical industry, and wastewater treatment areas, respectively. With the SOAP method, it is estimated that toluene is the largest contributor to the SOA formation in the refining area, but o-ethyltoluene contributes the most both in the chemical industry and wastewater treatment areas. For the SOA yield method, aromatics dominate the yields and account for nearly 100% of the total in the three areas. The SOA concentrations estimated of the refining, chemical industry and wastewater treatment areas are 30, 3835 and 137μgm-3, respectively. Despite the uncertainties and limitations associated with the three methods, the SOA yield method is suggested to be used for the estimation of SOA formation from the petroleum refinery. The results of this study have demonstrated that the control of VOCs, especially aromatics such as toluene, ethyltoluene, benzene and diethylbenzene, should be a focus of future regulatory measures in order to reduce PM pollution in the PRD region.

Secondary organic aerosol formation from ethylene ozonolysis in the presence of sodium chloride

Secondary organic aerosol (SOA) formation from the gas-phase ozonolysis of ethylene without irradiation was studied at different levels of relative humidity (RH) in both absence and presence of sodium chloride (NaCl) in a Teflon bag reactor. Results show that a small amount of SOA was formed from the ethylene ozonolysis in the absence of NaCl. When NaCl was in the form of liquid droplets, much more SOA could be formed. With the initial concentrations of 1 ppm and 0.5 ppm for ethylene and ozone, SOA concentrations of 3.0 to 8.6μg/m3 were obtained after 5-h reactions under RH levels of 62% to 88% in the presence of NaCl seed particles, which were about 3~9 times higher than the results from the experiments without NaCl. The yield of SOA also increased with increasing RH, with the value being 3.0% at 88% RH in the presence of NaCl. Addition of the scavenger of OH radicals (n-hexane, ~900 ppm) into the reaction system at 86% RH resulted in a decrease of the SOA yield by 21%. The liquid water content in aerosols was a key factor to SOA formation in the presence of different seed particles, including NaCl and Na2SO4. An analysis of the SOA with a Fourier-transform (FT) IR spectrometer shows that the particles formed from the ethylene ozonolysis were organic compounds that contained the functional groups of O–H, C=O, C–O, C–Cl and C–OH. The heterogeneous aqueous reaction is probably an effective pathway to form SOA from the ethylene ozonolysis, which should be considered in the atmosphere.

Modeling biogenic and anthropogenic secondary organic aerosol in China

Abstract. A revised Community Multi-scale Air Quality (CMAQ) model with updated secondary organic aerosol (SOA) yields and a more detailed description of SOA formation from isoprene oxidation was applied to study the spatial and temporal distribution of SOA in China in the entire year of 2013. Predicted organic carbon (OC), elemental carbon and volatile organic compounds agreed favorably with observations at several urban areas, although the high OC concentrations in wintertime in Beijing were under-predicted. Predicted summer SOA was generally higher (10–15 µg m−3) due to large contributions of isoprene (country average, 61 %), although the relative importance varies in different regions. Winter SOA was slightly lower and was mostly due to emissions of alkane and aromatic compounds (51 %). Contributions of monoterpene SOA was relatively constant (8–10 %). Overall, biogenic SOA accounted for approximately 75 % of total SOA in summer, 50–60 % in autumn and spring, and 24 % in winter. The Sichuan Basin had the highest predicted SOA concentrations in the country in all seasons, with hourly concentrations up to 50 µg m−3. Approximately half of the SOA in all seasons was due to the traditional equilibrium partitioning of semivolatile components followed by oligomerization, while the remaining SOA was mainly due to reactive surface uptake of isoprene epoxide (5–14 %), glyoxal (14–25 %) and methylglyoxal (23–28 %). Sensitivity analyses showed that formation of SOA from biogenic emissions was significantly enhanced due to anthropogenic emissions. Removing all anthropogenic emissions while keeping the biogenic emissions unchanged led to total SOA concentrations of less than 1 µg m−3, which suggests that manmade emissions facilitated biogenic SOA formation and controlling anthropogenic emissions would result in reduction of both anthropogenic and biogenic SOA.

On the implications of aerosol liquid water and phase separation for organic aerosol mass.

Organic compounds and liquid water are major aerosol constituents in the southeast United States (SE US). Water associated with inorganic constituents (inorganic water) can contribute to the partitioning medium for organic aerosol when relative humidities or organic matter to organic carbon (OM/OC) ratios are high such that separation relative humidities (SRH) are below the ambient relative humidity (RH). As OM/OC ratios in the SE US are often between 1.8 and 2.2, organic aerosol experiences both mixing with inorganic water and separation from it. Regional chemical transport model simulations including inorganic water (but excluding water uptake by organic compounds) in the partitioning medium for secondary organic aerosol (SOA) when RH > SRH led to increased SOA concentrations,· particularly at night. Water uptake to the organic phase resulted in even greater SOA concentrations as a result of a positive feedback in which water uptake increased SOA, which further increased aerosol water and organic aerosol. Aerosol properties· such as the OM/OC and hygroscopicity parameter (κorg), were captured well by the model compared with measurements during the Southern Oxidant and Aerosol Study (SOAS) 2013. Organic nitrates from monoterpene oxidation were predicted to be the least water-soluble semivolatile species in the model, but most biogenically derived semivolatile species in the Community Multiscale Air Quality (CMAQ) model were highly water soluble and expected to contribute to water-soluble organic carbon (WSOC). Organic aerosol and SOA precursors were abundant at night, but additional improvements in daytime organic aerosol are needed to close the model–measurement gap. When taking into account deviations from ideality, including both inorganic (when RH > SRH) and organic water in the organic partitioning medium reduced the mean bias in SOA for routine monitoring networks and improved model performance compared to observations from SOAS. Property updates from this work will be released in CMAQ v5.2.

A Chamber Study of Secondary Organic Aerosol (SOA) Formed by Ozonolysis of d-Limonene in the Presence of NO

Experiments were conducted in a dynamic chamber system to measure the formation of secondary organic aerosol (SOA) and particle-bound reactive oxygen species (ROS) produced from limonene ozonolysis in the presence of NO by varying the ratio of O_3 to NO. A diffusion cell system was used to produce the constant input of limonene in to the chamber, and six sets of experiments were conducted. The concentration of SOA mass and ROS produced were measured at steady-state. ROS, including peroxides, peroxy radicals and ions, was determined using dichlorofluorescin (DCFH) and converted to equivalent H_2O_2 concentration. The particle mass was measured using a tapered element oscillating microbalance (TEOM) and a scanning mobility particle sizer (SMPS) was used to obtain particle volume distributions. The results showed that the SOA mass concentration ranged from 30.3 to 157.3 μg m^(-3), and the ROS concentration ranged from 6.1 to 29.4 nmol m^(-3) of H_2O_2. For the different combinations of NO_x and O_3, the concentration ratio of [O_3]/[NO] around 1 was found to produce highest SOA mass and ROS, which is 157.3 μg m^(-3) and 29.4 nmol m^(-3). The SOA density was estimated by comparing the mass concentrations with the volume concentrations ranged from 1.21 to 1.48 g cm^(-3). The highest SOA density (1.48 g cm^(-3)) occurred with the lowest concentration ratio of [O_3]/[NO]. Compared with other monoterpene and linalool where each has one unsaturated carbon bond in other studies, limonene which has two unsaturated carbon bonds. Thus, it is the most efficient in generating the SOA and ROS concentrations in prior experiments without NO present.

Description and evaluation of the Community Multiscale Air Quality (CMAQ) modeling system version 5.1.

The Community Multiscale Air Quality (CMAQ) model is a comprehensive multipollutant air quality modeling system developed and maintained by the US Environmental Protection Agency’s (EPA) Office of Research and Development (ORD). Recently, version 5.1 of the CMAQ model (v5.1) was released to the public, incorporating a large number of science updates and extended capabilities over the previous release version of the model (v5.0.2). These updates include the following: improvements in the meteorological calculations in both CMAQ and the Weather Research and Forecast (WRF) model used to provide meteorological fields to CMAQ, updates to the gas and aerosol chemistry, revisions to the calculations of clouds and photolysis, and improvements to the dry and wet deposition in the model. Sensitivity simulations isolating several of the major updates to the modeling system show that changes to the meteorological calculations result in enhanced afternoon and early evening mixing in the model, periods when the model historically underestimates mixing. This enhanced mixing results in higher ozone (O3) mixing ratios on average due to reduced NO titration, and lower fine particulate matter (PM2.5) concentrations due to greater dilution of primary pollutants (e.g., elemental and organic carbon). Updates to the clouds and photolysis calculations greatly improve consistency between the WRF and CMAQ models and result in generally higher O3 mixing ratios, primarily due to reduced cloudiness and attenuation of photolysis in the model. Updates to the aerosol chemistry result in higher secondary organic aerosol (SOA) concentrations in the summer, thereby reducing summertime PM2.5 bias (PM2.5 is typically underestimated by CMAQ in the summer), while updates to the gas chemistry result in slightly higher O3 and PM2.5 on average in January and July. Overall, the seasonal variation in simulated PM2.5 generally improves in CMAQv5.1 (when considering all model updates), as simulated PM2.5 concentrations decrease in the winter (when PM2.5 is generally overestimated by CMAQ) and increase in the summer (when PM2.5 is generally underestimated by CMAQ). Ozone mixing ratios are higher on average with v5.1 vs. v5.0.2, resulting in higher O3 mean bias, as O3 tends to be overestimated by CMAQ throughout most of the year (especially at locations where the observed O3 is low); however, O3 correlation is largely improved with v5.1. Sensitivity simulations for several hypothetical emission reduction scenarios show that v5.1 tends to be slightly more responsive to reductions in NOx (NO + NO2), VOC and SOx (SO2 + SO4) emissions than v5.0.2, representing an improvement as previous studies have shown CMAQ to underestimate the observed reduction in O3 due to large, widespread reductions in observed emissions.

Impact of global climate change on ozone, particulate matter, and secondary organic aerosol concentrations in California: A model perturbation analysis

Air quality simulations are performed to determine the impact of changes in future climate and emissions on regional air quality in the South Coast Air Basin (SoCAB) of California. The perturbation parameters considered in this study include (1) temperature, (2) absolute humidity, (3) biogenic VOC emissions due to temperature changes, and (4) boundary conditions. All parameters are first perturbed individually. In addition, the impact of simultaneously perturbing more than one parameter is analyzed. Air quality is simulated with meteorology representative of a summertime ozone pollution episode using both a baseline 2005 emissions inventory and a future emissions projection for the year 2023. Different locations within the modeling domain exhibit varying degrees of sensitivity to the perturbations considered. Afternoon domain wide average ozone concentrations are projected to increase by 13–18% as a result of changes in future climate and emissions. Afternoon increases at individual locations range from 10 to 36%. The change in afternoon particulate matter (PM) levels is a strong function of location in the basin, ranging from −7.1% to +4.7% when using 2005 emissions and −8.6% to +1.7% when using 2023 emissions. Afternoon secondary organic aerosol (SOA) concentrations for the entire domain are projected to decrease by over 15%, and the change in SOA levels is not a strong function of the emissions inventory utilized. Temperature increases play the dominant role in determining the overall impact on ozone, PM, and SOA concentrations in both the individual and combined perturbation scenarios.

SOA in newly decorated residential buildings

Secondary organic aerosols (SOA) are products of the reaction of reactive organic gases (ROGs) and ozone. Most SOA are in the nanometer or submicron range, and thus SOA may be harmful to human health. Concentrations of ROGs may be high in newly decorated rooms, which may result in high SOA concentrations, especially during seasons with high ozone concentration. The objective of this study was to investigate the concentrations of SOA and the extent to which SOA contribute to indoor PM2.5 concentrations in newly decorated residential buildings. We measured concentrations of ROGs in 24 rooms of eight newly decorated (within the preceding three months) residential buildings in Beijing, China. Using previously measured indoor ozone decay rates, air change rates, and corresponding monitored ozone concentrations, we estimated the indoor ozone concentrations for the newly decorated rooms. Subsequently SOA concentrations were calculated using an aerosol mass fraction (AMF) model. We further estimated the fractions of SOA concentrations in indoor PM2.5. The SOA concentrations in 18 of the rooms ranged from 0.88 to 18.18 μg/m3, contributing to between 3.77% and 78.57% of indoor PM2.5 concentrations. The other six rooms had much lower SOA concentrations, less than 0.1 μg/m3, contributing to less than 0.39% of indoor PM2.5 concentrations. The results indicate that the concentrations and contribution of SOA in most newly decorated residential rooms are much higher than those in rooms with long-term occupancy, while some newly decorated rooms can still have low SOA concentrations.

Model analysis of secondary organic aerosol over China with a regional air quality modeling system (RAMS-CMAQ)

The regional air quality modeling system RAMS-CMAQ was updated to incorporate secondary organic aerosol (SOA) production from isoprene and sesquiterpene and to account for the SOA production rate dependence on NOx and SOA aging. The system was then used to simulate spatiotemporal distributions of SOA concentration and its major constituents over China in winter. Modeled monthly mean SOA concentrations were high in central and eastern China and low in western regions. The highest SOA appeared in regions from Beijing–Tianjin–Hebei (BTH) to the middle reaches of the Yangtze River and areas from Sichuan Basin to the southwest border of China, where SOA contributions were less than 10% of the organic aerosol (OA). The lowest concentration was in the Qinghai–Tibet Plateau, accounting for 20%–30% of OA. It is notable that contributions from anthropogenic precursors to SOA were significant in winter, especially the wide areas of central and eastern China with contributions generally varying from 50% to 80% of the total SOA. Beijing was used as an example location representative of the heavily polluted BTH area for analysis of major components of SOA. Though the modeled concentration of SOA was still underestimated compared to the observations, it still showed that xylene and toluene were the two greatest contributors to anthropogenic SOA, which was in agreement with the observations. SOA produced from monoterpene was the greatest contributor to biogenic SOA due to the high mass yield of monoterpene, followed by isoprene. More than 57% of SOAs were aged, which may increase the extinction effect of SOA.

Impacts of historical climate and land cover changes on fine particulate matter (PM<sub>2.5</sub>) air quality in East Asia between 1980 and 2010

Abstract. To examine the effects of changes in climate, land cover and land use (LCLU), and anthropogenic emissions on fine particulate matter (PM2.5) between the 5-year periods 1981–1985 and 2007–2011 in East Asia, we perform a series of simulations using a global chemical transport model (GEOS-Chem) driven by assimilated meteorological data and a suite of land cover and land use data. Our results indicate that climate change alone could lead to a decrease in wintertime PM2.5 concentration by 4.0–12.0 µg m−3 in northern China, but to an increase in summertime PM2.5 by 6.0–8.0 µg m−3 in those regions. These changes are attributable to the changing chemistry and transport of all PM2.5 components driven by long-term trends in temperature, wind speed and mixing depth. The concentration of secondary organic aerosol (SOA) is simulated to increase by 0.2–0.8 µg m−3 in both summer and winter in most regions of East Asia due to climate change alone, mostly reflecting higher biogenic volatile organic compound (VOC) emissions under warming. The impacts of LCLU change alone on PM2.5 (−2.1 to +1.3 µg m−3) are smaller than that of climate change, but among the various components the sensitivity of SOA and thus organic carbon to LCLU change (−0.4 to +1.2 µg m−3) is quite significant especially in summer, which is driven mostly by changes in biogenic VOC emissions following cropland expansion and changing vegetation density. The combined impacts show that while the effect of climate change on PM2.5 air quality is more pronounced, LCLU change could offset part of the climate effect in some regions but exacerbate it in others. As a result of both climate and LCLU changes combined, PM2.5 levels are estimated to change by −12.0 to +12.0 µg m−3 across East Asia between the two periods. Changes in anthropogenic emissions remain the largest contributor to deteriorating PM2.5 air quality in East Asia during the study period, but climate and LCLU changes could lead to a substantial modification of PM2.5 levels.

Simulations of organic aerosol concentrations during springtime in the Guanzhong Basin, China

Abstract. The organic aerosol (OA) concentration is simulated in the Guanzhong Basin, China from 23 to 25 April 2013 utilizing the WRF-CHEM model. Two approaches are used to predict OA concentrations: (1) a traditional secondary organic aerosol (SOA) module; (2) a non-traditional SOA module including the volatility basis-set modeling method in which primary organic aerosol (POA) is assumed to be semivolatile and photochemically reactive. Generally, the spatial patterns and temporal variations of the calculated hourly near-surface ozone and fine particle matters agree well with the observations in Xi'an and surrounding areas. The model also yields reasonable distributions of daily PM2.5 and elemental carbon (EC) compared to the filter measurements at 29 sites in the basin. Filter-measured organic carbon (OC) and EC are used to evaluate OA, POA, and SOA using the OC ∕ EC ratio approach. Compared with the traditional SOA module, the non-traditional module significantly improves SOA simulations and explains about 88 % of the observed SOA concentration. Oxidation and partitioning of POA treated as semivolatile constitute the most important pathway for the SOA formation, contributing more than 75 % of the SOA concentrations in the basin. Residential emissions are the dominant anthropogenic OA source, constituting about 50 % of OA concentrations in urban and rural areas and 30 % in the background area. The OA contribution from transportation emissions decreases from 25 % in urban areas to 20 % in the background area, and the industry emission OA contribution is less than 6 %.

Spatiotemporal Distribution and Source Attribution of SOA in China

Secondary organic aerosol (SOA) is an important pollutant due to its significant impact on atmospheric pollution, climate change and human health. Understanding the source and distribution of SOA is the prerequisite of air pollution control. This study utilized the global chemical transport model MOZART4 (Model for Ozone and Related Chemical Tracers, Version 4) coupled with the parameterizations of SOA multiphase formation, to simulate the spatial and temporal distribution of SOA in China, and analyzed the source contributions of SOA from the aspects of different precursors and regions. Our results indicate that the majority of SOA production happened over the east and the south China due to the availability of SOA precursors, with the summertime production a factor of 2.5 higher than that in winter. Anthropogenic emissions played a dominant role in SOA production, accounting for 55% of SOA source in China, with the rest from biogenic sources (45%). The domestic emissions contributed 77% of SOA production in China, mainly influencing the SOA concentration distribution near the surface of eastern China, especially over North China Plain. Foreign emissions mainly dominated the SOA abundance in western China, especially at higher altitudes mainly due to the rare domestic emission over these areas and the long-range transportation of SOA.

Global combustion sources of organic aerosols: model comparison with 84 AMS factor-analysis data sets

Abstract. Emissions of organic compounds from biomass, biofuel, and fossil fuel combustion strongly influence the global atmospheric aerosol load. Some of the organics are directly released as primary organic aerosol (POA). Most are emitted in the gas phase and undergo chemical transformations (i.e., oxidation by hydroxyl radical) and form secondary organic aerosol (SOA). In this work we use the global chemistry climate model ECHAM/MESSy Atmospheric Chemistry (EMAC) with a computationally efficient module for the description of organic aerosol (OA) composition and evolution in the atmosphere (ORACLE). The tropospheric burden of open biomass and anthropogenic (fossil and biofuel) combustion particles is estimated to be 0.59 and 0.63 Tg, respectively, accounting for about 30 and 32 % of the total tropospheric OA load. About 30 % of the open biomass burning and 10 % of the anthropogenic combustion aerosols originate from direct particle emissions, whereas the rest is formed in the atmosphere. A comprehensive data set of aerosol mass spectrometer (AMS) measurements along with factor-analysis results from 84 field campaigns across the Northern Hemisphere are used to evaluate the model results. Both the AMS observations and the model results suggest that over urban areas both POA (25–40 %) and SOA (60–75 %) contribute substantially to the overall OA mass, whereas further downwind and in rural areas the POA concentrations decrease substantially and SOA dominates (80–85 %). EMAC does a reasonable job in reproducing POA and SOA levels during most of the year. However, it tends to underpredict POA and SOA concentrations during winter indicating that the model misses wintertime sources of OA (e.g., residential biofuel use) and SOA formation pathways (e.g., multiphase oxidation).

Rethinking the global secondary organic aerosol (SOA) budget: stronger production, faster removal, shorter lifetime

Abstract. Recent laboratory studies suggest that secondary organic aerosol (SOA) formation rates are higher than assumed in current models. There is also evidence that SOA removal by dry and wet deposition occurs more efficiently than some current models suggest and that photolysis and heterogeneous oxidation may be important (but currently ignored) SOA sinks. Here, we have updated the global GEOS-Chem model to include this new information on formation (i.e., wall-corrected yields and emissions of semi-volatile and intermediate volatility organic compounds) and on removal processes (photolysis and heterogeneous oxidation). We compare simulated SOA from various model configurations against ground, aircraft and satellite measurements to assess the extent to which these improved representations of SOA formation and removal processes are consistent with observed characteristics of the SOA distribution. The updated model presents a more dynamic picture of the life cycle of atmospheric SOA, with production rates 3.9 times higher and sinks a factor of 3.6 more efficient than in the base model. In particular, the updated model predicts larger SOA concentrations in the boundary layer and lower concentrations in the upper troposphere, leading to better agreement with surface and aircraft measurements of organic aerosol compared to the base model. Our analysis thus suggests that the long-standing discrepancy in model predictions of the vertical SOA distribution can now be resolved, at least in part, by a stronger source and stronger sinks leading to a shorter lifetime. The predicted global SOA burden in the updated model is 0.88 Tg and the corresponding direct radiative effect at top of the atmosphere is −0.33 W m−2, which is comparable to recent model estimates constrained by observations. The updated model predicts a population-weighed global mean surface SOA concentration that is a factor of 2 higher than in the base model, suggesting the need for a reanalysis of the contribution of SOA to PM pollution-related human health effects. The potential importance of our estimates highlights the need for more extensive field and laboratory studies focused on characterizing organic aerosol removal mechanisms and rates.

Simulating secondary organic aerosol from missing diesel-related intermediate-volatility organic compound emissions during the Clean Air for London (ClearfLo) campaign

Abstract. We present high-resolution (5 km × 5 km) atmospheric chemical transport model (ACTM) simulations of the impact of newly estimated traffic-related emissions on secondary organic aerosol (SOA) formation over the UK for 2012. Our simulations include additional diesel-related intermediate-volatility organic compound (IVOC) emissions derived directly from comprehensive field measurements at an urban background site in London during the 2012 Clean Air for London (ClearfLo) campaign. Our IVOC emissions are added proportionally to VOC emissions, as opposed to proportionally to primary organic aerosol (POA) as has been done by previous ACTM studies seeking to simulate the effects of these missing emissions. Modelled concentrations are evaluated against hourly and daily measurements of organic aerosol (OA) components derived from aerosol mass spectrometer (AMS) measurements also made during the ClearfLo campaign at three sites in the London area. According to the model simulations, diesel-related IVOCs can explain on average ∼ 30 % of the annual SOA in and around London. Furthermore, the 90th percentile of modelled daily SOA concentrations for the whole year is 3.8 µg m−3, constituting a notable addition to total particulate matter. More measurements of these precursors (currently not included in official emissions inventories) is recommended. During the period of concurrent measurements, SOA concentrations at the Detling rural background location east of London were greater than at the central London location. The model shows that this was caused by an intense pollution plume with a strong gradient of imported SOA passing over the rural location. This demonstrates the value of modelling for supporting the interpretation of measurements taken at different sites or for short durations.

Effect of Organic Coatings, Humidity and Aerosol Acidity on Multiphase Chemistry of Isoprene Epoxydiols.

Multiphase chemistry of isomeric isoprene epoxydiols (IEPOX) has been shown to be the dominant source of isoprene-derived secondary organic aerosol (SOA). Recent studies have reported particles composed of ammonium bisulfate (ABS) mixed with model organics exhibit slower rates of IEPOX uptake. In the present study, we investigate the effect of atmospherically relevant organic coatings of α-pinene (AP) SOA on the reactive uptake of trans-β-IEPOX onto ABS particles under different conditions and coating thicknesses. Single particle mass spectrometry was used to characterize in real-time particle size, shape, density, and quantitative composition before and after reaction with IEPOX. We find that IEPOX uptake by pure sulfate particles is a volume-controlled process, which results in particles with uniform concentration of IEPOX-derived SOA across a wide range of sizes. Aerosol acidity was shown to enhance IEPOX-derived SOA formation, consistent with recent studies. The presence of water has a weaker impact on IEPOX-derived SOA yield, but significantly enhanced formation of 2-methyltetrols, consistent with offline filter analysis. In contrast, IEPOX uptake by ABS particles coated with AP-derived SOA is lower compared to that of pure ABS particles, strongly dependent on particle composition, and therefore on particle size.