Inconsistent capacity of potential HONO sources to enhance secondary pollutants: Evidence from WRF-Chem modeling.

Nitrous acid (HONO) is a crucial source of OH radicals in the troposphere, significantly enhancing secondary pollutants like secondary organic aerosols (SOA) and peroxyacetyl nitrates (PAN). While prior research has examined HONO sources and their total impacts on secondary pollution, the specific enhancement capacity of each individual HONO source remains underexplored. This study uses observational data from 2015 to 2018 for HONO, SOA, and PAN across six sites in China, combined with WRF-Chem model adding six potential HONO sources to evaluate their capacity: traffic emissions (E_traffic), soil emissions (E_soil), indoor-outdoor exchange (E_indoor), nitrate photolysis (P_nit), and NO heterogeneous reactions on aerosol and ground surfaces (Het_a, Het_g). The simulated HONO contributions near the ground in urban Beijing were: 12 % from NO + OH (default source), 10 %-20 % from E_traffic, 1 %-12 % from P_nit, 2 %-10 % from Het_a, and 50 %-70 % from Het_g. For SOA and PAN, we calculated incremental contributions enhanced by each HONO source and derived enhancement ratios (ERs) normalized against HONO's contribution: ∼7 for P_nit, ∼2 for Het_a, ∼0.9 for Het_g, ∼0.8 for E_soil, ∼0.3 for E_traffic, and ∼0.1 for E_indoor. HONO sources' capacity to enhance secondary pollutants varies, being larger for aerosol-related sources. Vertical analysis on HONO concentration, spatial distribution, RO radical cycling rates, and OH enhancements revealed that aerosol-related HONO sources, especially P_nit, contribute more to secondary pollution. Future research should focus more on assessing real-world impacts of HONO sources, besides identifying their budgets. Additionally, uptake coefficient (γ) and nitrate photolysis frequency (J) critically affect HONO and secondary pollutant formation, necessitating further investigations.