Gas-particle partitioning process contributes more to nitrate dominated air pollution than oxidation process in northern China

Nitrate has been recognized as a key aerosol component in regional haze formation in China. However, reducing nitrate aerosol concentration remains a major challenge. Generally, the formation of particulate nitrate (NO3-) is mainly affected by two processes: oxidation (to generate gaseous HNO3 or particulate NO3-) and gas-particle partitioning (HNO3-NO3- partition). Here, we proposed a new method to explore the contributions of above two processes (COxiobs (%) and CG/Pobs (%)) to nitrate formation based on field observation, and combined theoretical calculation and modeling to verify it. Quantitative results showed that gas-particle partitioning process (average CG/Pobs (%) was 64.90%) always contributed more than oxidation process (average COxiobs (%) was 35.10%) for particulate nitrate formation under different pollution scenarios in the ambient environment. We argued that this phenomenon was mainly caused by high aerosol pH (>4.5). Nevertheless, as pollution level rose, the COxiobs (%) will also increase (contributing to 32%, 38%, 40% and 41% under clean, light, medium and heavy pollution levels) which may be attributed to the increased HNO3 production rate and relatively enhanced heterogeneous reaction pathway. The results indicate future strategies for prevention and control of nitrate pollution should both consider reducing precursors emission and regulating aerosol acidity, in order to increase the effectiveness of reducing nitrate dominated pollution.