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Aromatic hydrocarbons are one of the major precursors of atmospheric brown carbon (BrC) and both abundantly co-exist with NO x in the urban atmosphere especially in winter haze period. However, the impact of NOx on the formation of BrC derived from aromatic hydrocarbons is still not fully understood. In this study, the yield and light absorption of secondary organic aerosols (SOA) from toluene photooxidation under various nitrogen oxides (NO 2 ) levels were investigated by using a 5 m 3 photooxidation smog chamber. A trend of increase at first and then decrease in the SOA yield with an increasing NO 2 concentration was observed. The acid-catalyzed heterogeneous reactions lead to the increase of SOA yield in the low-NO 2 regime. The formation of low-volatility species might be suppressed at high-NO 2 conditions is responsible for the decreased SOA yield. In contrast, light absorption and mass absorption coefficient (MAC) of the toluene-derived SOA continuously increased with the increasing NO 2 concentrations. HR-ToF-AMS results showed that nitrogen-containing organic compounds (NOCs) are the main species that lead to the increase of the SOA light absorption. The ratio of CHN family to the total NOCs, which are derived from the nitro compounds, also increased dominantly with the increasing NO 2 levels and accounted for more than half of the total NOCs when the NO 2 concentration increased to 495 ppbv, indicating that nitro compounds rather than organic nitrates are the major light-absorbing species and preferably formed in the toluene oxidation process., Competing Interests: Declaration of competing interest We declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Extremely high levels of atmospheric sulfate aerosols have still frequently occurred in China especially in winter haze periods and often been underestimated by models due to some missing formation mechanisms. Here we investigated the heterogeneous reaction dynamics of SO 2 oxidation by the abundantly co-existing O 3 and NO 2 in the urban atmosphere of China by using a laboratory smog chamber simulation technique. Our results showed that with an increase of NH 3 concentrations from 0.05 ppm to 1.5 ppm, SO 2 oxidation by O 3 can be greatly promoted and lead to an exponential increase of diameter growth factor (GF) of particles in the chamber from 1.29 to 1.98 for NaCl seeds and from 1.20 to 1.60 for (NH 4 ) 2 SO 4 seeds, along with an increasing uptake coefficient (γ) of SO 2 from 4.47 × 10 -5 to 1.52 × 10 -4 on NaCl seeds and from 2.32 × 10 -5 to 5.74 × 10 -5 on (NH 4 ) 2 SO 4 seeds, respectively. The heterogeneous production of sulfate from oxidation of SO 2 under NH 3 -rich conditions by O 3 and NO 2 mixture in the chamber was 2.0-3.5 times the sum of sulfate from SO 2 oxidations by O 3 and NO 2 , suggesting a strongly synergetic effect of the mixed oxidants on the heterogeneous oxidation of SO 2 , which can cause rapid formation of (NH 4 ) 2 SO 4 and NH 4 NO 3 and is responsible for the explosive growth of PM 2.5 in the winter haze period of China. Our chamber results further showed that such synergetic process is only efficient under NH 3 -rich conditions, clearly indicating that the combined controls on O 3 , NOx and NH 3 are necessary for further mitigating the PM 2.5 pollution in China., Competing Interests: Declaration of competing interest We declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Ammonia (NH 3 ), as a dominant alkaline gas in the atmosphere, plays a vital role in Chinese urban haze formation process, but its source in urban areas of China is controversial. To identify the sources of urban NH 3 in the semi-arid region of East Asia, real-time measurements of NH 3 and NH 4 + of PM 2.5 in the urban atmosphere of Xi'an, inland China during the winter and summer of 2017 were performed and their stable nitrogen isotope composition were analyzed. NH 3 was 38.0 ± 9.4 μg/m 3 in the summer, which is 1.5 times higher than that in the winter. Concentration of NH 3 in both seasons well correlated with that of PAHs in PM 2.5 and the mass ratio of (BbF + BeP + IP + BghiP) to the total PAHs, suggesting that fossil fuel combustion is an important source of NH 3 in Xi'an. Moreover, diurnal variation pattern of NH 3 was consistent with that of CO in the summer, peaking in the morning and evening rush hours, respectively, further indicating an importance of the contribution of traffic emissions to NH 3 in the city. Based on the source apportionment by using isotope mixing model, we found that 66.4% and 62.5% of NH 3 in the urban atmosphere were contributed by non-agricultural sources in the summer and winter, respectively. Our work revealed that non-agricultural sources dominate the atmospheric NH 3 of Xi'an, where haze pollution is still severe, and suggested that emission controls of non-agricultural NH 3 could be an effective way to mitigate the air pollution problem in the semi-arid region of East Asia., Competing Interests: Declaration of competing interest We declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)