Atmospheric NOx oxidation as major sources for nitrous acid (HONO)

Abstract

Nitrous acid (HONO) is the major precursor of hydroxyl (OH) radicals to initiate tropospheric chemistry leading to formation of secondary pollutants. The sources of atmospheric HONO, however, are not fully understood. Here we show two additional HONO sources that stem from atmospheric oxidation of nitrogen oxide (NOx = NO + NO2). Nitric acid (HNO3) formed from photooxidation of NO2 can be converted into HONO with a yield of ~53%, and dark NO oxidation by NO3 radicals in the presence of H2O produces HONO with a yield of 2%. The diurnal variations of HONO levels from field observations in the urban (Beijing) and rural (Wangdu) areas of the North China Plain can be well reproduced by the WRF-Chem model when the two new HONO sources are taken into account. The findings imply that atmospheric NOx oxidation pathways are the major sources for HONO, which can significantly accelerate ozone formation in polluted regions as well.