Diurnal sources of atmospheric HONO in Haikou resolved by chemical model and oxygen-isotope constraints

Gaseous nitrous acid (HONO) is one of the important sources of atmospheric hydroxyl radicals (·OH).It affects the oxidative capacity of the atmosphere and promotes the rapid accumulation of fine-particle matter, but its sources remain difficult to quantify.We conducted wintertime observations of hourly concentrations of atmospheric pollutants together with day- and night-time δ18O-HONO in Haikou, a tropical city under marine climatic influence. The wintertime sources of HONO were constrained independently using the RACM2+ chemical mechanism and δ18O-HONO measurements. Both the model simulations and oxygen isotope results indicate that secondary production pathways—including heterogeneous conversion of NO2, photolysis of particulate nitrate (pNO3−), and the gas-phase reaction between NO and ·OH—dominate the HONO budget in Haikou, contributing 63.21%—87.50% of the total HONO sources. A comparison of HONO source-apportionment results across different Chinese cities shows that secondary sources account for (70.8% ± 21.7%) of HONO on average, further highlighting the key role of secondary formation in atmospheric HONO production.