Measurement report: Elevated atmospheric ammonia may promote particle pH and HONO formation – insights from the COVID-19 pandemic

Abstract. HONO plays a crucial role as a precursor to OH radicals in the tropospheric atmosphere. The incongruity between HONO concentration and NOx emissions during the COVID-19 pandemic remains puzzling. Here, we show evidence from field observations of 10 sites in China where 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 conversion of NH4+ to NH3. Sensitivity analysis indicated that the decrease in anion concentrations (especially sulfate and nitrate) and the increase in cation concentrations during the COVID-19 pandemic led to an increase in particle pH. In other words, changes in the excess ammonia drove changes in particle pH that may consequently have impacted the rate of HONO formation. The calculation of reaction rates indicates that during the epidemic, the increase in pH may promote the generation of HONO by facilitating redox reactions, which highlights the importance of coordinating the control of SO2, NOx, and NH3 emissions.