Vertical Distribution of Sources and Atmospheric Impacts of HONO in the North China Plain

Nitrous acid (HONO) plays a significant role in tropospheric air quality as a crucial precursor of hydroxyl radicals (OH). While extensive research has focused on ground-level HONO, the sources and atmospheric implications of high-altitude HONO remain poorly understood. In this study, the Community Multiscale Air Quality (CMAQ) model was employed to explore the vertical distribution of HONO chemistry. The revised model, incorporating five additional HONO sources, effectively reproduced observed HONO concentrations and enhanced the model's performance in simulating ozone and secondary particulate matter. The analysis revealed that the predominant source of HONO near the surface was heterogeneous reactions of NO₂ on the ground surface, with sinks mainly consisting of vertical diffusion, dry deposition and photolysis. In the upper air layer (90-600 m), the primary sources of HONO included the homogeneous reaction of NO with OH, the heterogeneous reaction of NO₂ on the aerosol surface and vertical diffusion, while the main sinks were photolysis and horizontal advection. This study provides valuable insights into the vertical distribution characteristics of HONO chemistry and underscores its critical influence on tropospheric air quality.