Measurement Report: Elevated 1 excess-NH3 can promote the redox reaction to produce HONO: Insights from the COVID-19 pandemic.

The incongruity between atmospheric oxidizing capacity and NO_x 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, NO_x, and NH₃ emissions. [ABSTRACT FROM AUTHOR]