Quantifying HONO Production from Nitrate Photolysis in a Polluted Atmosphere
Yifan Jiang, Men Xia, Likun Xue, Xinfeng Wang, Xuelian Zhong, Yongchun Liu, Markku Kulmala, Tong Ma, Jiaqi Wang, Yurun Wang, Jian Gao, Tao Wang
Abstract
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.