Measurement Report: Elevated excess-NH₃ can promote the redox reaction to produce HONO: Insights from the COVID-19 pandemic

The incongruity between atmospheric oxidizing capacity and NOx emissions during the COVID-19 pandemic remains puzzling. Here, we show evidence from field observations of ten sites in China that there was a noticeable increase in NH3 concentrations during the COVID-19 pandemic. In addition to the meteorological conditions, the significant decrease in sulfate and nitrate concentrations enhanced the portioning of NH4+ to NH3 Such conditions enable enhanced particle pH values, which in turn accelerate the redox reactions between NO2 and SO2 to form HONO. This mechanism partly explains the enhanced atmospheric oxidizing capacity during the pandemic and highlights the importance of coordinating the control of SO2, NOx, and NH3 emissions.