HONO chemistry affected by relative humidity and ammonia in the North China Plain during winter

Abstract

Nitrous acid (HONO) is a critical precursor of the hydroxyl radical (OH) and plays a pivotal role in atmospheric photochemistry. Although nitrogen dioxide (NO₂) heterogeneous reactions (HET) on ground and aerosol surfaces are widely recognized as major paths of HONO production, their influencing factors are not well characterized in air quality models, limiting the understanding of HONO formation and the quantification of their regional impact. In this study, a novel parameterization scheme for the NO₂ uptake coefficient, including the effects of solar radiation, relative humidity (RH) and ammonia (NH₃), was developed and coupled into the Weather Research and Forecasting model with Chemistry. Nine simulation scenarios were designed to assess the impacts of RH and NH₃ on HONO chemistry and O₃ levels in the North China Plain (NCP). The results showed that the RH-impacted HET contributed 10−25% of HONO, with a significant increase of more than 35% during the haze periods; whereas the NH₃-impacted HET contributed 15% of nighttime HONO and <5% of noontime HONO, playing a more significant role in rural areas. Vertically, the RH-impacted HET contribution to nighttime HONO concentrations remained 26−31% at an altitude of 700–900 m due to the higher RH levels (50−60%) during the haze periods; whereas the NH₃-impacted HET contribution was minor above 500 m owing to the fast-decreasing NH₃ concentrations with height. When RH exceeded the turning point (70%), nighttime HONO was suppressed by up to 1 ppb in eastern NCP. The combination of RH and NH₃ produced a ground daily maximum 8h averaged O₃ enhancement of 6–14 μg m⁻³ during the haze periods, exceeding the effect of solar radiation. These findings deepen our understanding of the role of RH and NH₃ in HONO chemistry and imply the importance of reasonably expressing HET in air quality models.