Atmospheric oxidation, sources, budget of nitrous acid in an emerging city of Central China: Based on seasonal perspectives

Abstract

Nitrous acid (HONO) significantly affects atmospheric oxidation ability by generating hydroxyl radicals (·OH). A year-long observational study (2017–2018) was conducted in a central plain city with ammonia-rich conditions and high PM_2.5 pollution. The study monitored HONO concentrations across different seasons to assess the seasonal impacts of PM_2.5 and NH₃ on HONO formation via heterogeneous conversion. The HONO levels were highest in autumn and lowest in winter, with consistent diurnal variations peaking at 07:00 and reaching a nadir at 16:00. Homogeneous reactions contributed 22.6 % to 96.8 % to nocturnal HONO accumulation, with the highest average HONO/NO₂ ratio observed during summer nights (9.7 %) compared to other seasons (5.0 %–6.2 %). Heterogeneous conversion at night was regulated by relative humidity (RH). The presence of abundant NH₃ and PM_2.5 pollution appeared to enhance the conversion efficiency of NO₂. The net primary production rate of ·OH generated by diurnal HONO (P_·OH(HONO)) and O₃ (P_·OH(O₃)) was calculated for each season. The rate of P_·OH(HONO) / (P_·OH(HONO) + P_·OH(O₃)) varied across the seasons from 10.9 to 92.0 % (from highest to lowest as winter > spring > autumn > summer). This study enhances the understanding of the contribution of HONO to atmospheric oxidation across different seasons, particularly in ammonia-rich and polluted environments, and underscores the need for targeted pollution control strategies that consider these interactions.