Nitrous acid (HONO) is a key precursor of OH (hydroxyl radicals) and plays an important role in regulating air quality. However, the mechanisms driving its formation remain incompletely understood. To investigate these pathways, continuous observations were conducted during wintertime in urban Shanghai to measure HONO, related pollutants, meteorology, and photolysis frequencies. Budget analysis revealed an additional nighttime HONO source beyond direct emissions and homogeneous reactions, pointing to a significant contribution from heterogeneous processes on both ground and aerosol surfaces. To quantify this contribution, an RF (Random Forest) model was developed, and SHAP (Shapley Additive Explanations) was applied to assess feature importance and pollutant response variations. The results indicated that NO 2 , PM 2.5 , and NH 3 were the key chemical factors of the nighttime HONO source linked to heterogeneous pathways, representing the dominant chemical regulatory factors independent of meteorological variability, with a sensitivity analysis showing that PM 2.5 exerted the strongest influence, followed by NH 3 and NO 2 . Crucially, simultaneous reductions of PM 2.5 and NH 3 were found to produce the largest reduction in predicted nighttime heterogeneous HONO formation, suggesting the potential importance of their coordinated control during wintertime to limit nocturnal HONO production in urban Shanghai and enhance seasonal air quality management strategies.
Zhao et al. (Fri,) studied this question.