Reactive oxygen species play key roles in the nitrite formation by the inorganic nitrate photolysis in the presence of urban water-soluble organic carbon

Abstract

Inorganic nitrates were considered to be a potential source of atmospheric NO2−/HONO during the daytime. To better evaluate the contribution of nitrate photochemistry on NO2−/HONO formation, the photolysis of nitrates in the real atmospheric environment needs to be further explored. Here, the NO2− generation by the photolysis of inorganic nitrates in the presence of total water-soluble organic carbon (WSOC) was quantified. The physicochemical properties of WSOC were measured to understand the underlying mechanism for the photolysis of inorganic nitrates with WSOC. WSOC enhanced or suppressed the photochemical conversion of nitrates to NO2−, with the quantum yield of NO2− (ΦNO2−) varying from (0.07 ± 0.02)% to (3.11 ± 0.04)% that depended on the light absorption properties of WSOC. Reactive oxygen species (ROS) generated from WSOC, including O2−/HO2 and OH, played a dual role in the NO2− formation. Light-absorbing substances in WSOC, such as N-containing and carbonyl aromatics, produced O2−/HO2 that enhanced the secondary conversion of NO2 to NO2−. On the other hand, OH deriving from the WSOC photochemistry inhibited the nitrate photodegradation and the NO2− formation. HONO source strength by the aqueous photolysis of nitrates with WSOC was estimated to be lower than 100 ppt h−1, which may partly contribute to the atmospheric HONO source in some cases.