Characteristics and sources of wintertime nitrous acid (HONO): Insights from year-to-year variations and the impact of COVID-19.

As a key precursor of hydroxyl radicals (•OH), nitrous acid (HONO) substantially influences atmospheric oxidation and air quality. Although the role of HONO in atmospheric chemistry is well recognized, systematic analyses that simultaneously consider interannual variability and large-scale disruptions such as the COVID-19 pandemic remain limited. This study addresses this gap by examining HONO variations across three distinct periods: Period I (9-31 January 2019), Period II (9-26 January 2020), and Period III (27 January-29 February 2020, during the COVID-19 lockdown) in a megacity in central China. Results revealed a 46 % reduction in measured HONO concentrations during Period III compared to Period II, primarily due to decreased vehicle emissions. During the lockdown, heterogeneous conversion played a more critical role in nighttime HONO formation, exhibiting strong sensitivity to relative humidity (RH) and temperature (T). HONO photolysis remained the dominant pathway for daytime HONO loss (74-82 %) across the three periods. Although daytime direct HONO emissions dropped by 50 %, HONO photolysis accounted for nearly half of daytime •OH production (47 %) during the lockdown. The findings underscore the importance of direct emission control strategies, as well as coordinated regulation on NO2 and PM2.5. While targeting HONO and its precursors is effective for mitigating atmospheric oxidation capacity (AOC) under polluted conditions, photochemical factors such as ozone photolysis and solar radiation intensity become the dominant daytime regulators under cleaner atmospheric scenarios.