Enhanced sulfate formation by nitrogen dioxide: Implications from in situ observations at the SORPES station

Abstract

Investigating sulfate formation processes is important not only for air pollution control but also for understanding the climate system. Although the mechanisms of secondary sulfate production have been widely studied, in situ observational evidence implicating an important role of NO 2 in SO 2 oxidation in the real atmosphere has been rare. In this study, we report two unique cases, from an intensive campaign conducted at the Station for Observing Regional Processes of the Earth System (SORPES) in East China, showing distinctly different mechanisms of sulfate formation by NO 2 and related nitrogen chemistry. The first case occurred in an episode of mineral dust mixed with anthropogenic pollutants and especially high concentrations of NOx. It reveals that NO 2 played an important role, not only in surface catalytic reactions of SO 2 but also in dust‐induced photochemical heterogeneous reactions of NO 2, which produced additional sources of OH radicals to promote new particle formation and growth. The second case was caused by aqueous oxidation of S(IV) by NO 2 under foggy/cloudy conditions with high NH 3 concentration. As a by‐product, the formed nitrite enhanced HONO formation and further promoted the gas‐phase formation of sulfate in the downwind area. This study highlights the effect of NOx in enhancing the atmospheric oxidizing capacity and indicates a potentially very important impact of increasing NOx on particulate pollution formation and regional climate change in East Asia.