Quantifying HONO Production from Nitrate Photolysis in a Polluted Atmosphere

The photolysis of particulate nitrate (pNO3–) has been suggested to be an important source of nitrous acid (HONO) in the troposphere. However, determining the photolysis rate constant of pNO3– (jpNO3–) suffers from high uncertainty. Prior laboratory measurements of jpNO3– using aerosol filters have been complicated by the "shadow effect"─a phenomenon of light extinction within aerosol layers that potentially skews these measurements. We developed a method to correct the shadow effect on the photolysis rate constant of pNO3– for HONO production (jpNO3– → HONO) using aerosol filters with identical chemical compositions but different aerosol loadings. We applied the method to quantify jpNO3– → HONO over the North China Plain (NCP) during the winter haze period. After correcting for the shadow effect, the normalized average jpNO3– → HONO at 5 °C increased from 5.89 × 10–6 s–1 to 1.72 × 10–5 s–1. The jpNO3– → HONO decreased with increasing pH and nitrate proportions in PM2.5 and had no correlation with nitrate concentrations. A parametrization for jpNO3– → HONO was developed for model simulation of HONO production in NCP and similar environments.