Integrating the updated HONO formation mechanism to better understand urban O₃ formation chemistry

As a vital precursor of hydroxyl radicals (OH), atmospheric nitrous acid (HONO) plays a significant role in tropospheric chemistry and the production of secondary pollutants. However, knowledge of its sources remains insufficient. To comprehensively investigate the HONO chemistry in polluted cities and alleviate the O₃ pollution, based on a comprehensive HONO-related field campaign in Zibo City, on the North China Plain, the parameterized formulas of additional HONO sources were validated in a box model (based on the default MCMv3.3.1) and the resulting chemical impact of HONO was determined. The results showed that the updated mechanism well reproduced the diurnal profile of observed HONO, and budget analysis revealed the predominant role of the NO₂ photo-enhanced heterogeneous reaction on the ground surface in HONO formation, with contributions of about 70% for both high-O₃ episodes and clean days. Underestimations of 32.1% net O_x production rate and 28.5% HO_x concentrations were simulated by the default mechanism when HONO was not constrained, while the adoption of the updated mechanism decreased these underestimations to 5.3% and 5.4%, respectively. Additionally, sensitivity tests of NO_x emission changes showed that the urban O₃ photochemical regime was more inclined to NO_x-limited after considering variations in HONO concentrations caused by changes in NO_x concentrations. These results can contribute to implementing more precise O₃ pollution control in urban areas, especially in areas lacking HONO observations.