Quantifying HONO Production from Nitrate Photolysis in a Polluted Atmosphere.

The photolysis of particulate nitrate (pNO 3 - ) has been suggested to be an important source of nitrous acid (HONO) in the troposphere. However, determining the photolysis rate constant of pNO 3 - ( j pNO 3 - ) suffers from high uncertainty. Prior laboratory measurements of j pNO 3 - 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 pNO 3 - for HONO production ( j pNO 3 -  → HONO ) using aerosol filters with identical chemical compositions but different aerosol loadings. We applied the method to quantify j pNO 3 -  → HONO over the North China Plain (NCP) during the winter haze period. After correcting for the shadow effect, the normalized average j pNO 3 -  → HONO at 5 °C increased from 5.89 × 10 -6 s -1 to 1.72 × 10 -5 s -1 . The j pNO 3 -  → HONO decreased with increasing pH and nitrate proportions in PM 2.5 and had no correlation with nitrate concentrations. A parametrization for j pNO 3 -  → HONO was developed for model simulation of HONO production in NCP and similar environments.