Exploring HONO production from particulate nitrate photolysis in representative regions of China: characteristics, influencing factors, and environmental implications

Abstract. The production mechanism of atmospheric nitrous acid (HONO), an important precursor of the hydroxyl radical (OH), remains controversial. Few studies have explored the effects of particulate nitrate photolysis on HONO sources under different environment conditions across China. In this work, the photolysis rate constant of particulate nitrate for HONO production (JNO3--HONO) was determined through a photochemical reaction system with PM2.5 samples collected from five representative sites in China. We developed a method to correct and quantify the “shadowing effect” – potential light extinction within aerosol layers under heavy PM2.5 loading conditions on the filters – for JNO3--HONO measurements, showing that elemental carbon (EC), the dominant light-absorbing component in PM2.5, plays a dominant role in it. The corrected JNO3--HONO values varied with the sampling period and location over a wide range, from 1.6 × 10−6 to 1.96 × 10−4 s−1, with a mean (±1 SD) of 1.71 (± 2.36) × 10−5 s−1. Chemical compositions, specifically those concerning nitrate loading and organic components, affected the production of HONO through particulate nitrate photolysis: high JNO3--HONO values were generally associated with PM2.5 samples with a high organic carbon (OC) / NO3- ratio (R2 = 0.86). We suggest that the parameterization equation between JNO3--HONO and the OC / NO3- ratio established in this work can be used to estimate JNO3--HONO under different aerosol chemical conditions, thus reducing the uncertainty in exploring daytime HONO sources. This study confirms that the photolysis of particulate nitrate can be a potential daytime HONO source in rural or southern urban sites, which are characterized by PM2.5 containing high proportions of organic matter.