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

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 (O-3) 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 (O-3) 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.

Automated summary

The implementation of strict emission mitigation measures in the Beijing-Tianjin-Hebei region since 2013 has led to significant changes in air pollutants. Nitrate aerosols have become the dominant contributor to particulate pollution in Beijing, emphasizing the importance of reducing atmospheric oxidizing capacity to mitigate nitrate and PM2.5 concentrations.