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^- (j_pNO) suffers from high uncertainty. Prior laboratory measurements of j_pNO 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 (j_pNO) using aerosol filters with identical chemical compositions but different aerosol loadings. We applied the method to quantify j_pNO over the North China Plain (NCP) during the winter haze period. After correcting for the shadow effect, the normalized average j_pNO at 5 °C increased from 5.89 × 10^-6 s^-1 to 1.72 × 10^-5 s^-1. The j_pNO decreased with increasing pH and nitrate proportions in PM_2.5 and had no correlation with nitrate concentrations. A parametrization for j_pNO was developed for model simulation of HONO production in NCP and similar environments.