Photochemical source of nitrous acid in the marine boundary layer at Tudor Hill Marine Atmospheric Observatory in Bermuda

Gaseous nitrous acid (HONO) plays an important role in the oxidation capacity of the troposphere, as its rapid photolysis produces hydroxyl radical, the main oxidizing species. In the marine boundary layer (MBL), previous field measurements suggested the existence of an active daytime HONO source that sustains the observed HONO mixing ratio (up to several tens of pptv) against its fast photochemical loss. In the MBL, gaseous nitric acid (HNO₃) may be effectively scavenged by sea-salt aerosols to form particulate nitrate (pNO₃). Laboratory experiments showed that the photolysis of pNO₃, which proceeds at a much faster rate than gaseous HNO₃, could be a significant source of HONO. Here we present temporal distributions of measured HONO, HNO₃ and pNO₃, along with several chemical and meteorological parameters during the spring and the late summer of 2019 at Tudor Hill Marine Atmospheric Observatory in Bermuda. In clean marine air, the range of HONO mixing ratio is from the detection limit of 0.5 pptv to 20 pptv. HONO mixing ratios exhibit distinct diurnal cycles that were highest during noontime and lowest during nighttime, confirming the importance of photochemical processes as HONO source. The mixing ratio of total inorganic nitrate (the sum of HNO₃ and pNO₃) range from a detection limit of 15 pptv to 300 pptv, with pNO₃ the dominant species. The pNO₃ photolysis rate constants leading to HONO formation are determined in the laboratory using aerosol samples collected during the field campaigns. Results from both field observations and laboratory photochemical experiments are used to assess if pNO₃ photolysis is an important or even a major daytime HONO source in the MBL, as previously proposed. Implications regarding reactive nitrogen chemistry and atmospheric oxidation capacity in the MBL will be discussed.