Nitrate debuts as a dominant contributor to particulate pollution in Beijing: Roles of enhanced atmospheric oxidizing capacity and decreased sulfur dioxide emission

Abstract Implementation of strict emission mitigation measures since 2013 has significantly changed air pollutants in the Beijing-Tianjin-Hebei region (BTH), China. Observations show that ozone (O3) concentrations have increased by 62.40% (27.84%) and SO2 concentrations have decreased by 56.42% (35.07%) during particulate pollution episodes in Beijing (BTH) in the autumn from 2013 to 2015. The measured nitrate concentration in Beijing has increased markedly, which to a large degree offsets the sulfate decrease caused by SO2 emission mitigation. Using the WRF-Chem model, we demonstrate that the enhanced nitrate formation is primarily attributed to increasing atmospheric oxidizing capacity (AOC) and decreasing sulfate competition for base ions. A 9.41–46.24% (7.58–40.97%) decrease in OH radical (O3) concentrations in October 2015 reduces nitrate and fine particulate matters (PM2.5) concentrations by 2.51–18.18% and 3.15–18.90% in Beijing, respectively. Based on the scenario in October 2015, if the SO2 emission increases by 20.00–100.00%, the PM2.5 concentration increases by 3.02–11.21%, but the nitrate level decreases by 2.48–21.87% simultaneously. Our results suggest that the nitrate aerosol has become a dominant contributor to particulate pollution in Beijing and that decreasing AOC is critical to mitigate nitrate and PM2.5 concentrations.