Study of Secondary Organic Aerosol Formation from Chlorine Radical-Initiated Oxidation of Volatile Organic Compounds in a Polluted Atmosphere Using a 3D Chemical Transport Model.

The impact of chlorine (Cl) chemistry on the formation of secondary organic aerosol (SOA) during a severe wintertime air pollution episode is investigated in this study. The Community Multiscale Air Quality (CMAQ) model v5.0.1 with a modified SAPRC-11 gas-phase mechanism and heterogeneous reactions for reactive chlorine species is updated to include the formation of chlorine radical (Cl•)-initiated SOA (Cl-SOA) from aromatic compounds, terpenes, and isoprene. Reported SOA yield data on Cl-SOA formation from environmental chamber studies are used to derive the mass yield and volatility data for the two-product equilibrium-partitioning model. The heterogeneous reaction of particulate chloride (pCl-) leads to a significant increase in the Cl• and hydroxyl radical (OH) concentrations throughout the domain. Monthly Cl-SOA concentrations range from 0.7 to 3.0 µg m-3, with increasing anthropogenic Cl emissions leading to higher Cl-SOA concentrations. Indirectly, this also leads to an increase of monthly SOA by up to 2.5-3.0 g µm-3 from the traditional OH oxidation pathways as well as the surface uptake of glyoxal and methylglyoxal. Increased OH concentrations, however, do not always lead to higher overall SOA concentrations in the entire domain. High OH reduces the lifetime of glyoxal/methylglyoxal (GLY/MGLY), making them less available to form SOA. In the Sichuan Basin (SCB) and part of Southwest China where high O3 concentrations meet high pCl emissions, a higher Cl•/OH ratio leads to net O3 loss from the Cl• + O3 reaction, thus reducing SOA formation from the O3 oxidation of volatile organic compounds (VOCs). Also, the competition between Cl• and OH for VOCs could lead to lower overall SOA because the molar yields of the semivolatile products in Cl-VOC reactions are lower than their OH + VOC reaction counterparts. When Cl• concentrations are further increased with higher emissions of Cl, precursor gases can be depleted and become the limiting factor in SOA formation. This study reveals the direct and indirect impacts of chlorine chemistry on SOA in polluted winter conditions, which are greatly affected by the Cl emissions, the ambient O3 level, and the availability of SOA precursors.

Importance of Wintertime Anthropogenic Glyoxal and Methylglyoxal Emissions in Beijing and Implications for Secondary Organic Aerosol Formation in Megacities.

Atmospheric glyoxal (GLY) and methylglyoxal (MGLY) are key precursors of secondary organic aerosol (SOA). However, anthropogenic emissions of GLY and MGLY and their contribution to surface GLY and MGLY concentrations, as well as the secondary organic aerosol (SOA) formation, are not well quantified. By developing an emission inventory of anthropogenic GLY and MGLY and improving the Community Multiscale Air Quality Model (CMAQ) with SOA formation from irreversible surface uptake of GLY and MGLY, as well as a precursor-origin resolved technique, we quantified the source contributions of GLY and MGLY and their impact on wintertime SOA formation in Beijing, China. The total emissions of GLY and MGLY in Beijing in 2017 are 1.1 × 104 kmol and 7.0 × 103 kmol, respectively. Anthropogenic primary emissions are found to be the dominant contributor to wintertime GLY and MGLY concentrations (∼74% for GLY and ∼63% for MGLY). Anthropogenic primary emissions of GLY and MGLY contributes to 30% of GLY/MGLY SOA daily average concentration and accounts for up to 45% of nighttime GLY/MGLY SOA in winter. The study suggests that the anthropogenic GLY and MGLY emissions, mainly from gasoline vehicles and cooking, are important for SOA formation and shall be strictly controlled in Chinese megacities.

Effect of current emission abatement strategies on air quality improvement in China: A case study of Baotou, a typical industrial city in Inner Mongolia.

The national Air Pollution Prevention and Control Action Plan required significant decreases in PM2.5 levels over China. To explore more effective emission abatement strategies in industrial cities, a case study was conducted in Baotou to evaluate the current national control measures. The total emissions of SO2, NOX, PM2.5 and NMVOC (non-methane volatile organic compounds) in Baotou were 211.2Gg, 156.1Gg, 28.8Gg, and 48.5Gg, respectively in 2013, and they would experience a reduction of 30.4%, 26.6%, 15.1%, and 8.7%, respectively in 2017 and 39.0%, 32.0%, 24.4%, and 12.9%, respectively in 2020. The SO2, NOX and PM2.5 emissions from the industrial sector would experience a greater decrease, with reductions of 37%, 32.7 and 24.3%, respectively. From 2013 to 2020, the concentrations of SO2, NO2, and PM2.5 are expected to decline by approximately 30%, 10% and 14.5%, respectively. The reduction rate of SNA (sulfate, nitrate and ammonium) concentrations was significantly higher than that of PM2.5 in 2017, implying that the current key strategy toward controlling air pollutants from the industrial sector is more powerful for SNA. Although air pollution control measures implemented in the industrial sector could greatly reduce total emissions, constraining the emissions from lower sources such as residential coal combustion would be more effective in decreasing the concentration of PM2.5 from 2017 to 2020. These results suggest that even for a typical industrial city, the reduction of PM2.5 concentrations not only requires decreases in emissions from the industrial sector, but also from the low emission sources. The seasonal variation in sulfate concentration also showed that emission from coal-burning is the key factor to control during the heating season.

Deriving High-Resolution Emission Inventory of Open Biomass Burning in China based on Satellite Observations.

Open biomass burning plays an important role in atmospheric pollution and in climate change. However, the current emission inventory of open biomass burning is generally of highly uncertainty because of missing small fire data and limited resolution because of the lack of localized vegetation data. In this study, the MODIS (MODerate Resolution Imaging Spectroradiometer) burned area product MCD64Al combined with the active fire product MCD14 ML, as well as a high-resolution land cover data set, were applied to develop a high-resolution emission inventory of open biomass burning in China in 2013. Total CO, CH4, NOx, NMVOC (nonmethane volatile organic compounds), SO2, NH3, PM2.5, PM10, OC (organic carbon), BC (black carbon), and CO2 emissions were estimated to be 1.03 × 104, 666, 536, 1.91 × 103, 87, 138, 1.45 × 103, 2.09 × 103, 741, 137, and 2.45 × 105 Gg, respectively. The provinces that contributed the most emissions included Heilongjiang, Henan, Shandong, and Jilin. The major source for all pollutants was cropland burning, whereas Xizang, Xinjiang, and Heilongjiang had greater emissions from natural vegetation. The temporal distribution of average provincial emissions showed that the peaks were in June and October. This study updated the emission information that may support future research and policy-making on air pollution control and GHG emission abatement.

Chemical composition and source apportionment of PM10 and PM2.5 in different functional areas of Lanzhou, China.

To elucidate the air pollution characteristics of northern China, airborne PM10 (atmospheric dynamic equivalent diameter ≤ 10 µm) and PM2.5 (atmospheric dynamic equivalent diameter ≤ 2.5 µm) were sampled in three different functional areas (Yuzhong County, Xigu District and Chengguan District) of Lanzhou, and their chemical composition (elements, ions, carbonaceous species) was analyzed. The results demonstrated that the highest seasonal mean concentrations of PM10 (369.48 µg/m(3)) and PM2.5 (295.42 µg/m(3)) were detected in Xigu District in the winter, the lowest concentration of PM2.5 (53.15 µg/m(3)) was observed in Yuzhong District in the fall and PM10 (89.60 µg/m(3)) in Xigu District in the fall. The overall average OC/EC (organic carbon/elemental carbon) value was close to the representative OC/EC ratio for coal consumption, implying that the pollution of Lanzhou could be attributed to the burning of coal. The content of SNA (the sum of sulfate, nitrate, ammonium, SNA) in PM2.5 in Yuzhong County was generally lower than that at other sites in all seasons. The content of SNA in PM2.5 and PM10 in Yuzhong County was generally lower than that at other sites in all seasons (0.24-0.38), indicating that the conversion ratios from precursors to secondary aerosols in the low concentration area was slower than in the area with high and intense pollutants. Six primary particulate matter sources were chosen based on positive matrix factorization (PMF) analysis, and emissions from dust, secondary aerosols, and coal burning were identified to be the primary sources responsible for the particle pollution in Lanzhou.

Significant co-benefits of air pollutant and CO2 emission reduction from biomass energy utilization in power plants in China.

Replacing coal energy with biomass energy in power plants is important in reducing air pollutants and CO2 emissions in China. We first calculated the optimal economic transport radius (OETR) to evaluate the optimally available biomass (OAB) and the potentially available biomass (PAB) for the year 2018. The OAB and PAB of power plants are estimated to be 423-1013 Mt, with higher values found in the provinces with higher population and crop yields. Unlike crop and forestry residue, the PAB can access to OAB for waste, mainly because the waste is easier to collect and transfer to a power plant. When all the PAB were consumed, the NOx, SO2, PM10, PM2.5, and CO2 emissions would decrease by 41.7 kt, 115.3 kt, 117.6 kt, 26.0 kt, and 701.2 Mt, respectively. The scenario analysis results showed that the PAB will not be sufficient to meet the biomass power growth estimated for 2040, 2035, and 2030 under the baseline scenario (BS), policy scenario (PS), and reinforcement scenario (RS), respectively, whereas the CO2 emission will dramatically decrease by 1473 Mt in 2040 under the BS, 1271 Mt in 2035 under the PS, and 1096 Mt in 2030 under the RS. Our findings indicate that the abundant biomass resources in China will bring significant co-benefits, reducing air pollutants and CO2 emissions, if biomass energy can be applied in power plants. Furthermore, more advanced technologies, such as bioenergy with carbon capture and storage (BECCS), are expected to be used in power plants in the future, which will probably result in significantly lower CO2 emissions and promote the achievement of the CO2 emission peaking target and carbon neutrality. Our results provide useful information for developing a strategy for the coordinated reduction of air pollutants and CO2 emissions from power plants.

Identifying the dominant driver of elevated surface ozone concentration in North China plain during summertime 2012-2017.

The increasingly serious surface ozone (O3) pollution in North China Plain (NCP) has received wide attention. However, the contribution of the changes for each emission source to the elevated O3 concentration, as well as the direct and indirect effect of meteorological condition variation on increased O3 level have not been comprehensively analyzed. This study applied the Community Multiscale Air Quality (CMAQ) model coupled with the integrated source apportionment method (ISAM) to quantify changes in daily maximum 8-h average O3 concentration (MDA8 O3) under different air pollutants emissions and meteorological conditions during summertime 2012-2017. The results showed that incoordinate NOx/VOC emission control sustainably increased MDA8 O3 by 2.2-36.2 µg/m3 in the NCP, of which emission changes from industrial and transportation sectors were the predominant contributors (-0.6-19.5 µg/m3 for industrial sector and 1.2-18.1 µg/m3 for transportation, respectively). In contrast, MDA8 O3 decreased by 2.5-9.2 µg/m3 for the power plants. The effect of changes in meteorological condition on MDA8 O3 exhibited significantly spatial and temporal variation and unfavorable meteorological fields were shown in 2014, 2016, and 2017, which enhanced MDA8 O3 by -2.5-23.1, -5.3-20.7, and -7.2-25.8 µg/m3, respectively. In addition, the changed meteorological factors indirectly affected the biogenic emission thus prompting the increases of MDA8 O3 by -3.9-4.9 µg/m3 in the NCP during 2012-2017. The sensitive simulations suggested that more aggressive control measures about VOC reduction in industrial and transportation sectors should be implemented to further mitigate the O3 pollution under unfavorable meteorological condition.

Impacts of the differences in PM2.5 air quality improvement on regional transport and health risk in Beijing-Tianjin-Hebei region during 2013-2017.

Due to the implementation of different air pollution control measures in the Beijing-Tianjin-Hebei (BTH) region during 2013-2017, the air quality exhibited varied improvements in each province, indicating substantial changes in the interprovincial regional transport of PM2.5. In this study, we investigated these changes by using the Community Multiscale Air Quality (CMAQ) model coupled with the integrated source apportionment method (ISAM) during this period. The results showed that the concentrations of primary particles, SO42-, and NO3- decreased by 41.5, 40.8, and 1.8%, respectively due to the air pollutants emission reduction. Local air pollutant emissions were the predominant contributors of PM2.5 for each region in BTH, accounting for 41.3-47.6, 38.1-40.6, 50.6-53.6, and 54.0-57.1% of PM2.5 in Beijing, Tianjin, and northern and southern Hebei, respectively. Total PM2.5 has been mitigated by 7.1-12.3 and 5.1-11.7 µg/m3 from local and regional emission reduction, respectively in the BTH. Moreover, diverse local meteorological conditions variation increased the PM2.5 concentration by 5.3 µg/m3 in Tianjin and decreased it by 7.6, 2.0, and 4.9 µg/m3 in Beijing, and northern and southern Hebei, respectively. Estimation by integrated exposure-response function revealed that the number of premature deaths attributable to PM2.5 exposure decreased by approximately 3000 in the BTH region during 2013-2017. Additional policies that focus on PM2.5-O3 coordinated control and stringent regional joint air pollution regulation are required to substantially reduce the health impacts, especially in southern Hebei.

Policy-driven changes in the health risk of PM2.5 and O3 exposure in China during 2013-2018.

China issued a series of control measures to mitigate PM2.5 pollution, including long-term (i.e., Air Pollution Prevention and Control Action Plan, APPCAP) and short-term (emergency measures in autumn and winter) acts. However, the O3 concentration increased significantly as PM2.5 levels sharply decreased when these measures were implemented. Therefore, the policy-driven positive/negative health effects of PM2.5/O3 need to be comprehensively estimated. The health impact function (HIF) is applied to evaluate the health burden attributable to long- and short-term PM2.5 and O3 exposure. The results show that the PM2.5 concentration decreased by 42.95% in 74 cities, whereas O3 pollution is increased by 17.56% from 2013 to 2018. Compared with 2013, the number of premature deaths attributable to long- and short-term PM2.5 exposure decreased by almost 5.31 × 104 (95% confidence interval [CI]: 2.87 × 104-4.71 × 104) (10.13%) and 3.00 × 104 (95% CI: 1.66 × 104-4.39 × 104) (72.49%), respectively, in 2018. In contrast, O3-attributable deaths, increased by 1.98 × 104 (95% CI: 0.31 × 104-3.59 × 104) (130.57%) and 0.91 × 104 (95% CI: 0.50 × 104-1.33 × 104) (76.16%) for long- and short-term exposure, respectively. The number of avoidable deaths attributed to PM2.5 reduction is larger than the level of premature deaths related to increasing O3. Although annual mean PM2.5 concentrations have fallen rapidly, the benefits of reducing long-term exposure are limited, whereas the deaths associated with acute exposure decrease more significantly due to the reduction of heavy-pollution days by implementing emergency measures. The results show appreciable effectiveness in protecting human health and illustrate that synchronous control of PM2.5 and O3 pollution should be emphasized.

[Characteristics and Cause Analysis of Heavy Air Pollution in a Mountainous City During Winter].

Taking the typical heavy air pollution process in Yangquan from December 26, 2018 to January 20, 2019 as an example, the characteristics and cause analysis of heavy air pollution in a mountainous city in winter were analyzed in this study. The results showed that fine particle mass (PM2.5) was the primary pollutant during the heavy pollution period. The water-soluble ions and carbonaceous components were the main components of PM2.5. The secondary ions of SO42-, NO3-, and NH4+ had the lager contribution to water-soluble ions (87.7%), and the secondary organic carbon (SOC) was the main component of the carbonaceous components (71.6%). The concentration of the secondary ions during the heavy pollution period increased by 5.3 times compared to levels before the heavy pollution period, and was an important component resulting in the fast increase of PM2.5. An analysis of meteorological conditions showed that PM2.5 and its main components had a significantly positive relationship with humidity and a significantly negative relationship with wind speed. And that pollution became stronger with an increase in humidity and a decrease in wind speed. The typical meteorological characteristics of mountainous cities are high relative humidity and large temperature variations, which can accelerate the formation of secondary pollutants and are the main reasons for the rapid aggravation of PM2.5. In addition, the lower average wind speed caused by the relatively closed terrain in mountainous cities makes the diffusion conditions of air pollutants relatively poor, which is one of the reasons for the accumulation of pollutants. The source apportionment results showed that the secondary sources (46.0%) were the most important source of PM2.5, followed by coal combustion (32.6%), vehicle exhaust (19.8%), and fugitive dust (1.6%). Therefore, mountainous cities should pay more attention to controlling secondary components, especially secondary ions.

Impacts of chlorine chemistry and anthropogenic emissions on secondary pollutants in the Yangtze river delta region.

Multiphase chemistry of chlorine is coupled into a 3D regional air quality model (CMAQv5.0.1) to investigate the impacts on the atmospheric oxidation capacity, ozone (O3), as well as fine particulate matter (PM2.5) and its major components over the Yangtze River Delta (YRD) region. The developed model has significantly improved the simulated hydrochloric acid (HCl), particulate chloride (PCl), and hydroxyl (OH) and hydroperoxyl (HO2) radicals. O3 is enhanced in the high chlorine emission regions by up to 4% and depleted in the rest of the region. PM2.5 is enhanced by 2-6%, mostly due to the increases in PCl, ammonium, organic aerosols, and sulfate. Nitrate exhibits inhomogeneous variations, by up to 8% increase in Shanghai and 2-5% decrease in most of the domain. Radicals show different responses to the inclusion of the multiphase chlorine chemistry during the daytime and nighttime. Both OH and HO2 are increased throughout the day, while nitrate radicals (NO3) and organic peroxy radicals (RO2) show an opposite pattern during the daytime and nighttime. Higher HCl and PCl emissions can further enhance the atmospheric oxidation capacity, O3, and PM2.5. Therefore, the anthropogenic chlorine emission inventory must be carefully evaluated and constrained.

Nitrate dominates the chemical composition of PM2.5 during haze event in Beijing, China.

Water-soluble inorganic ions (WSI), a major component of PM2.5, often increased rapidly during the haze event in Beijing. Sulfate (SO42-), Nitrate (NO3-), and Ammonium (NH4+) are three main components of WSI. Since year 2015, sulfate concentrations in PM2.5 has gradually decreased owing to the effective control of SO2 emissions. However, the contribution of nitrate to PM2.5 has significantly increased during haze events in Beijing at the same time. In this study, a highly time-resolved online analyzer (Monitor for Aerosols and Gases, MARGA) was employed to measure the WSI in PM2.5 in Beijing from 5 February to 15 November 2017. Three typical haze events during this sampling period were investigated. During heavy pollution episodes in winter, nitrate concentrations increased from 7.5 µg/m3 to 45.6 µg/m3 (45.0% of WSI), while sulfate increased from 4.2 µg/m3 to 20.1 µg/m3 (19.8% of WSI). This indicated that nitrate is more important than sulfate as a driver for the growth of PM2.5 during the period of heavy air pollution in winter. Nitrate also dominates the increase of WSI in the pollution episodes in autumn, with an average concentration of 52.5 µg/m3, and contributed up to 67% of WSI. The average concentration ratio of NH4+ to SO42- was higher in autumn (1.02) than that in summer (0.74) and close to that in winter (1.00). This is mainly because the emission control of coal combustion in Beijing and surrounding areas results in an NH3-rich and SO2-lean atmosphere, which promoted the formation of ammonium nitrate. Our study indicates that nitrate has become the most important component of WSI in PM2.5 and is driving the rapid growth of PM2.5 concentrations during heavy pollution episodes in Beijing. Therefore, more efforts shall be made to reduce the nitrogen oxide and ammonia emissions in Beijing and surrounding areas.

Significant impact of heterogeneous reactions of reactive chlorine species on summertime atmospheric ozone and free-radical formation in north China.

Heterogeneous reactions of N2O5, O3, OH, ClONO2, HOCl, ClNO2, and NO2, with chlorine-containing particles are incorporated in the Community Multiscale Air Quality (CMAQ) model to evaluate the impact of heterogeneous reactions of reactive chlorine species on ozone and free radicals. Changes of summertime ozone and free radical concentrations due to the additional heterogeneous reactions in north China were quantified. These heterogeneous reactions increased the O3, OH, HO2 and RO2 concentrations by up to 20%, 28%, 36% and 48% for some regions in the Beijing-Tianjin-Hebei (BTH) area. These areas typically have a larger amount of NOx emissions and a lower VOC/NOx ratio. The zero-out method evaluates that the photolysis of ClNO2 and Cl2 are the major contributors (42.4% and 57.6%, respectively) to atmospheric Cl in the early morning hours but the photolysis of Cl2 is the only significant contributor after 10:00 am. The results highlight that heterogeneous reactions of reactive chlorine species are important to atmospheric ozone and free-radical formation. Our study also suggests that the on-going NOx emission controls in the NCP region with a goal to reduce both O3 and secondary nitrate can also have the co-benefit of reducing the formation Cl from ClNO2 and Cl2, which may also lead to lower secondary organic aerosol formation and thus the control of summertime PM2.5 in the region.

[Pollution characteristics of microbial aerosols generated from a municipal sewage treatment plant].

To characterize the pollution characteristics of microbial aerosols emitted from municipal sewage treatment plants, microbial aerosols were sampled with an Andersen 6-stage impactor at different treatment units of a Xi'an sewage treatment plant between June 2011 and July 2011. The plate-culture and colony-counting methods were employed to determine the concentrations, particle size distributions and median diameters of the airborne bacteria, fungi and actinomycetes. The results showed that the highest concentrations of bacteria (7 866 CFU x m(-3) +/- 960 CFU x m(-3)) and actinomycetes (2 139 CFU x m(-3) +/- 227 CFU x m(-3)) were found in the sludge-dewatering house while the highest fungi concentration (2156 CFU x m(-3) +/- 119 CFU x m(-3)) in the oxidation ditch. The airborne bacteria, fungi and actinomycetes all showed a skewed distribution in particle size. The peaks of bacteria and fungi were in the size range of 2.1-3.3 microm, whereas the peak of airborne actinomycetes was between 1. 1-2.1 microm in size. In general, the order of the median diameters of different microbial aerosols generated from the sewage treatment plant was airborne bacteria > airborne fungi > airborne actinomycetes. In addition, the spatial variation characteristics of microbial aerosols showed that the larger the particle size of the microorganism, the faster the reducing rate of the aerosol concentration. The variations in the reducing rate of concentration with particle sizes can be ordered as airborne bacteria > airborne fungi > airborne actinomycetes.