The impact of sea spray aerosol on photochemical ozone formation over eastern China: heterogeneous reaction of chlorine particles and radiative effect

Abstract

Eastern China has suffered from severe photochemical O3 (ozone) pollution in recent years. In this coastal region, the atmospheric environment can be influenced by sea spray aerosols (SSAs) from marine emissions. However, the extent and mechanisms by which SSA affects O3 formation remain incompletely understood. Here, using the WRF-CMAQ model, this study investigates the comprehensive effect of SSA on radical chemistry and O3 formation in the lower troposphere across four seasons. SSA (over 50 % of which is particulate chlorine) can reach further inland through an atmospheric “bridge” aloft, interacting with the nitrogen-containing gases from continental anthropogenic emissions to reduce NOx levels and release Cl radicals. The NOx reduction increases O3 in volatile organic compound (VOC)-limited regions while decreasing it in NOx-limited zones. Elevated Cl radicals enhance VOC degradation and O3 formation during morning hours. Meanwhile, the scattering properties of SSA reduce daytime O3 formation by diminishing photolysis rates. Due to the contrasting effect of SSA via different mechanisms, the response of O3 varies seasonally and geographically. In winter, SSA increases O3 in eastern China due to the dominant effect of NOx reduction in VOC-limited regions. In spring and autumn, similar effects occur in the North China Plain, whereas southern China sees a decrease due to NOx reduction in the NOx-limited region and reduced photolysis rates. In summer, O3 increases are observed only around Bohai, with reductions elsewhere driven by NOx reductions in NOx-limited regions and decreased photolysis. This study highlights the important, varying, but previously unreported role of SSAs in shaping tropospheric photochemistry over eastern China.