Reduced atmospheric sulfate enhances fine particulate nitrate formation in eastern China

Nitrate (NO₃^-) is a major component of atmospheric fine particles. Recent studies in eastern China have shown the increasing trend of NO₃^- in contrast to the ongoing control of nitrogen oxide (NO_x). Here, we elucidate the effects of reduced sulfur dioxide (SO₂) on the enhancement of NO₃^- formation based on field measurements at the summit of Mt. Tai (1534 m a.s.l.) and present detailed modelling analyses. From 2007 to 2018, the measured springtime concentrations of various primary pollutants and fine sulfate (SO₄^2-) decreased sharply (-16.4 % to -89.7 %), whereas fine NO₃^- concentration increased by 22.8 %. The elevated NO₃^- levels cannot be explained by the changes in meteorological conditions or other related parameters but were primarily attributed to the considerable reduction in SO₄^2- concentrations (-73.4 %). Results from a multi-phase chemical box model indicated that the reduced SO₄^2- levels decreased the aerosol acidity and prompted the partitioning of HNO₃ into the aerosol phase. WRF-Chem model analyses suggest that such a negative effect is a regional phenomenon throughout the planetary boundary layer over eastern China in spring. This study provides new insights into the worsening situation of NO₃^- aerosol pollution and has important implications for controlling haze pollution in China.