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

Abstract. The production mechanism of atmospheric nitrous acid (HONO), an important precursor of hydroxyl radical (OH), was still controversial. Few studies have explored the effects of particulate nitrate photolysis on HONO sources in different environment conditions across China. Here, the photolysis rate constants of particulate nitrate for HONO production (J_HONO) were determined through photochemical reaction system with PM_2.5 samples collected from five representative sites in China. To eliminate the “shadowing effect” — potential light extinction within aerosol layers at heavy PM_2.5 loadings on the filters, the relationship between light screening coefficient and EC, the dominant light-absorbing component in PM_2.5, was established (R²=0.73). The corrected J_HONO values varied with sampling period and location over a wide range, distributing from 1.6×10⁻⁶ s⁻¹ to 1.96×10⁻⁴ s⁻¹, with a mean (± 1 SD) of (1.71±2.36)×10⁻⁵ s⁻¹. Chemical compositions, specifically nitrate loading and organic component, affected the production of HONO through particulate nitrate photolysis: high J_HONO values were generally associated with the PM_2.5 samples with high OC/NO₃⁻ ratio (R²=0.86). We suggested that the parameterization equation between J_HONO and OC/NO₃⁻ established in this work can be used to estimate J_HONO in different aerosol chemical conditions, thus reducing the uncertainty in exploring HONO daytime sources. This study confirms that the photolysis of particulate nitrate can be a potential HONO daytime source in rural or southern urban sites, which were characterized by high proportion of organic matter in PM_2.5, while the contribution of this process to HONO daytime formation was still limited.