NH₃-driven HONO production as a potential unknown source during snow cover and melt

Snow cover has the characteristic of high reflectivity, and the snow particles inside it are prone to form a quasi-liquid layer (QLL) on their surface at temperatures below 0 °C, both of which promote the photolysis and hydrolysis reactions of nitrous acid (HONO). In this study, the CAMx model was updated by incorporating 11 heterogeneous chemical reactions of HONO, including HONO depletion reactions, and was applied to conduct numerical simulations for March 2024 in Northeast China. The results showed that the average HONO flux during the snowmelt period (9.06 × 10¹⁴ molecules m^-2 s^-1) exceeded that of the snow accumulation period (7.74 × 10¹⁴ molecules m^-2 s^-1), while the flux during the snow-free period was significantly lower (3.12 × 10¹⁴ molecules m^-2 s^-1), indicating that HONO flux peaks during the snowmelt period. Moreover, this study summarized that HONO flux from mid-latitude snow cover was found to be two orders of magnitude higher than that in polar regions, which indicates that snow cover in mid-latitude regions might be an important potential source of atmospheric HONO. Enhancement factor functions were established based on ammonia (NH₃) concentrations, relative humidity (RH), and unknown HONO sources, which reveals substantial contributions to unknown HONO during the snow-covered (8.25 %) and snowmelt (19.78 %) periods. Compared to HO₂ and RO₂, the enhancement factor contributes the most to OH, with a maximum contribution of 24.76 %, while its maximum contributions to HO₂ and RO₂ are 12.14 % and 9.73 %, respectively. These findings elucidate HONO formation mechanisms over seasonal snow and quantify cryosphere-atmosphere flux exchange.