Optimized Simulation of HONO Sources and Its Impact on Nitrate Formation in Guangzhou

• • Direct vehicle emissions contributed 49.7% to HONO concentration during winter haze in Guangzhou, highlighting traffic as the dominant source; controlling vehicular HONO precursors could significantly reduce secondary nitrate formation. • • Heterogeneous photosensitized reaction of NO2 on aerosol surfaces accounted for 23.0% of HONO, indicating that aerosol surface chemistry is a major non-vehicular source; models must include this pathway to avoid underestimation. • • Incorporating seven additional HONO sources increased simulated daytime HONO from (0.3±0.1) to (6.5±2.3) μg·m−3, matching observations; this improvement elevated ·OH concentrations by 1.2-fold and nitrate production rate by 3.5-fold, demonstrating the sensitivity of oxidative capacity to HONO. • • The simulated-to-observed nitrate ratio improved from 21% to 81% after optimizing HONO sources, indicating that accurate HONO representation is critical for reproducing nitrate levels; sensitivity tests suggest nitrate photolysis enhancement factor of 100 yields best results.