Sulfate aerosols have profound impacts on the climate, ecosystem, visibility, and public health, but the sulfate formation pathway remains elusive. In the present study, a source-oriented WRF-Chem model is applied to simulate a persistent air pollution episode from 4 to 15 July 2015 in Beijing–Tianjin–Hebei (BTH), China, to study the contributions of four pathways to sulfate formation. When comparing simulations to measurements in BTH, the index of agreement (IOA) of meteorological parameters, air pollutants, and aerosol species generally exceeds 0.6. On average in BTH, the heterogeneous reaction of SO2 involving aerosol water and the SO2 oxidation by OH constitutes the two most important sulfate sources, with a contribution of about 35 %–38 % and 33 %–36 %, respectively. Primary sulfate emissions account for around 22 %–24 % of the total sulfate concentration. SO2 oxidation by stabilized Criegee intermediates (sCIs) also plays an appreciable role in sulfate formation, with a contribution of around 9 % when an upper limit of the reaction rate constant of sCIs with SO2 (κsCI+SO2=3.9×10-11 cm3 s−1) and a lower limit of the reaction rate constant of sCIs with H2O (κsCI+H2O=1.97×10-18 cm3 s−1) are used. Sensitivity studies reveal that there are still large uncertainties in the sulfate contribution of SO2 oxidation by sCIs. The sulfate contribution of the reaction is decreased to less than 3 % when κSCI+SO2 is decreased to 6.0×10-13 cm3 s−1. Furthermore, when κsCI+H2O is increased to 2.38×10-15 cm3 s−1 based on the reported ratio of κSCI+H2O to κSCI+SO2 (6.1×10-5), the sulfate contribution becomes insignificant at less than 2 %. Further studies need to be conducted to better determine κsCI+SO2 and κsCI+H2O to evaluate the effects of sCI chemistry on sulfate formation.

Secondary organic aerosol enhanced by increasing atmospheric oxidizing capacity in 1 Beijing-Tianjin-Hebei (BTH), China 2 3 Tian Feng1,2,6, Shuyu Zhao1,3, Naifang Bei4, Jiarui Wu1,3, Suixin Liu1,3, Xia Li1,3, Lang Liu1,3, Yang Qian7, 4 Qingchuan Yang3, Yichen Wang3, Weijian Zhou1,5,6, Junji Cao1,3,5, Guohui Li1,3,5* 5 6 1State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of 7 Sciences, Xi’an, China 8 2Department of Geography & Spatial Information Techniques, Ningbo University, Ningbo, China 9 3Key Laboratory of Aerosol Chemistry and Physics, Institute of Earth Environment, Chinese Academy of 10 Sciences, Xi’an, China 11 4School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an, China 12 5CAS Center for Excellence in Quaternary Science and Global Change, Xi’an, China 13 6Xi’an Accelerator Mass Spectrometry Center, Xi’an, China 14 7State Key Laboratory of Environmental Criteria and Risk Assessment & Environmental Standards Institute, 15 Chinese Research Academy of Environmental Sciences, Beijing, China 16 17 Correspondence to: Guohui Li (ligh@ieecas.cn) 18 19 Abstract. The implementation of the Air Pollution Prevention and Control Action Plan in 20 China since 2013 has profoundly altered the ambient pollutants in the Beijing-Tianjin-Hebei 21 region (BTH). Here we show observations of substantially increased O3 concentrations 22 (about 30%) and a remarkable increase in the ratio of organic carbon (OC) to elemental 23 carbon (EC) in BTH during the autumn from 2013 to 2015, revealing an enhancement in 24 atmospheric oxidizing capacity (AOC) and secondary organic aerosol (SOA) formation. To 25 explore the impacts of increasing AOC on the SOA formation, a severe air pollution episode 26 from 3 to 8 October 2015 with high O3 and PM2.5 concentrations is simulated using the 27 WRF-Chem model. The model performs reasonably well in simulating the spatial 28 distributions of PM2.5 and O3 concentrations over BTH and the temporal variations of PM2.5, 29 O3, NO2, OC, and EC concentrations in Beijing compared to measurements. Sensitivity 30 studies show that the change in AOC substantially influences the SOA formation in BTH. A 31 sensitivity case characterized by a 31% O3 decrease (or 36% OH decrease) reduces the SOA 32 level by about 30% and the SOA fraction in total organic aerosol by 17% (from 0.52 to 0.43, 33 dimensionless). Spatially, the SOA decrease caused by reduced AOC is ubiquitous in BTH, 34 but the spatial relationship between SOA concentrations and the AOC is dependent on the 35 SOA precursor distribution. Studies on SOA formation pathways further show that, when the 36 AOC is reduced, the SOA from oxidation and partitioning of semi-volatile POA and 37 co-emitted intermediate volatile organic compounds (IVOCs) decreases remarkably, followed 38 by those from anthropogenic and biogenic VOCs. Meanwhile, the SOA decrease in the 39 irreversible uptake of glyoxal and methylglyoxal on aerosol surfaces is negligible. 40

Effects of stabilized Criegee Intermediates (sCI) on the sulfate formation: A sensitivity 1 analysis during summertime in Beijing-Tianjin-Hebei (BTH), China 2 3 Lang Liu1,3,4, Naifang Bei2, Jiarui Wu1,3, Suixin Liu1,3, Jiamao Zhou1,3, Xia Li1,3, Qingchuan Yang1, Tian Feng1, 4 Junji Cao1,3, Xuexi Tie1, Guohui Li1,3* 5 6 1Key Lab of Aerosol Chemistry and Physics, SKLLQG, Institute of Earth Environment, Chinese Academy of 7 Sciences, Xi’an, 710061, China 8 2School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, 9 China 10 3CAS Center for Excellence in Quaternary Science and Global Change, Xi'an, 710061, China 11 4University of Chinese Academy of Sciences, Beijing, 100049, China 12 13 Correspondence to: Guohui Li (ligh@ieecas.cn) 14 15 Abstract: Sulfate aerosols exert profound impacts on climate, ecosystem, visibility, and 16 public health, but the sulfate formation pathway remains elusive. In the present study, a 17 source-oriented WRF-Chem model is applied to simulate a persistent air pollution episode 18 from 04 to 15 July 2015 in Beijing-Tianjin-Hebei (BTH), China to study contributions of four 19 pathways to the sulfate formation. When comparing simulations to measurements in BTH, 20 the index of agreement (IOA) of meteorological parameters, air pollutants and aerosol species 21 generally exceeds 0.6. On average in BTH, the heterogeneous reaction of SO2 involving 22 aerosol water and the SO2 oxidation by OH constitutes the two most important sulfate 23 sources, with a contribution of about 35~38% and 33~36% respectively. The primary 24 emission accounts for around 22~24% of sulfate concentrations due to high SO2 emissions. 25 The SO2 oxidation by stabilized Criegee Intermediates (sCI) also plays an appreciable role in 26 the sulfate formation, with a contribution of around 9% when an upper limit of the reaction 27 rate constant of sCI with SO2 (κ"#$%&'(=3.9×10 -11 cm3 s-1) and a lower limit of the reaction 28 rate constant of sCI with H2O (κ"#$%)('=1.97×10 -18 cm3 s-1) are used. Sensitivity studies 29 reveal that there still exist large uncertainties in the sulfate contribution of the SO2 oxidation 30 by sCI. The sulfate contribution of the reaction is decreased to less than 3% when κ&#$%&'( 31 is decreased to 6.0×10-13 cm3 s-1. Furthermore, when κ"#$%)(' is increased to 2.38×10 -15 cm3 32 s-1 based on the reported ratio of κ&#$%)(' to κ&#$%&'( (6.1×10 -5), the sulfate contribution 33 becomes insignificant, less than 2%. Further studies need to be conducted to better determine 34 κ"#$%&'( and κ"#$%)(' to evaluate effects of the sCI chemistry on the sulfate formation. 35

The implementation of the Air Pollution Prevention and Control Action Plan in China since 2013 has profoundly altered the ambient pollutants in the Beijing–Tianjin–Hebei (BTH) region. Here we show observations of substantially increased O3 concentrations (about 30 %) and a remarkable increase in the ratio of organic carbon (OC) to elemental carbon (EC) in BTH during the autumn from 2013 to 2015, revealing an enhancement in atmospheric oxidizing capacity (AOC) and secondary organic aerosol (SOA) formation. To explore the impacts of increasing AOC on the SOA formation, a severe air pollution episode from 3 to 8 October 2015 with high O3 and PM2.5 concentrations is simulated using the WRF-Chem model. The model performs reasonably well in simulating the spatial distributions of PM2.5 and O3 concentrations over BTH and the temporal variations in PM2.5, O3, NO2, OC, and EC concentrations in Beijing compared to measurements. Sensitivity studies show that the change in AOC substantially influences the SOA formation in BTH. A sensitivity case characterized by a 31 % O3 decrease (or 36 % OH decrease) reduces the SOA level by about 30 % and the SOA fraction in total organic aerosol by 17 % (from 0.52 to 0.43, dimensionless). Spatially, the SOA decrease caused by reduced AOC is ubiquitous in BTH, but the spatial relationship between SOA concentrations and the AOC is dependent on the SOA precursor distribution. Studies on SOA formation pathways further show that when the AOC is reduced, the SOA from oxidation and partitioning of semivolatile primary organic aerosol (POA) and co-emitted intermediate volatile organic compounds (IVOCs) decreases remarkably, followed by those from anthropogenic and biogenic volatile organic compounds (VOCs). Meanwhile, the SOA decrease in the irreversible uptake of glyoxal and methylglyoxal on the aerosol surfaces is negligible.