Fog event is possibly a source rather than a sink of atmospheric nitrate aerosols: Insights from isotopic measurements in Nanjing, China

Abstract

Nitrate is the major component of fine particle matter in recent years, and fog deposition is an important sink form of atmospheric nitrate aerosol. However, evidence shows that high relative humidity, and stable atmosphere condition may lead to the rapid increase of secondary aerosols, including nitrate. Therefore, whether fog event is a sink or a source of nitrate aerosol is still an open question. Simultaneous observations of concentration and isotopes of nitrate both in PM2.5 and fog water during fog event were conducted during a heavy fog event happened in winter of 2018 in Nanjing, a megacity sited in the Yangzi River Delta, China. Based on nine 12-h time resolution fog water samples and eleven 24-h time resolution PM2.5 samples, it was found that NO3− concentration in PM2.5 increased during the fog event (from 10.4 ± 8.6 μg m−3 to 21.2 ± 11.0 μg m−3). Compared with Δ17O–NO3- in PM2.5 before (28.4 ± 3.7‰) and after the fog event (28.6 ± 1.3‰), Δ17O–NO3- in PM2.5 during the fog event was higher (31.6 ± 1.2‰) and Δ17O–NO3- in fog water was lower (24.3 ± 1.2‰). The increased NO3− in PM2.5 during the fog event had a different formation pathway compared with NO3− in PM2.5 when there is no fog, and NO3− in fog water had another formation pathway as well. Explosive growth of NO3− in PM2.5 may come from active NO3 + HC reaction, along with happen of fog event through the reaction of NO2 with ·OH or H2O and hydrolysis of N2O5. As the fog event finished, the fog water possibly mainly settled NO3− newly generated during the fog event. It seems the fog event may be a source rather than a sink of atmospheric nitrate aerosols in this case.