An investigation into the chemistry of HONO in the marine boundary layer at Tudor Hill Marine Atmospheric Observatory in Bermuda

Abstract

Abstract. Here we present measurement results of temporal distributions of nitrous acid (HONO) along with several chemical and meteorological parameters during the spring and the late summer of 2019 at Tudor Hill Marine Atmospheric Observatory in Bermuda. Large temporal variations in HONO concentration were controlled by several factors including local pollutant emissions, air mass interaction with the island, and long-range atmospheric transport of HONO precursors. In polluted plumes emitted from local traffic, power plant, and cruise ship emissions, HONO and nitrogen oxides (NOx) existed at substantial levels (up to 278 pptv and 48 ppbv, respectively), and NOx-related reactions played dominant roles in daytime formation of HONO. The lowest concentration of HONO was observed in marine air, with median concentrations at ∼ 3 pptv around solar noon and < 1 pptv during the nighttime. Considerably higher levels of HONO were observed during the day in the low-NOx island-influenced air ([NO2] < 1 ppbv), with a median HONO concentration of ∼ 17 pptv. HONO mixing ratios exhibited distinct diurnal cycles that peaked around solar noon and were lowest before sunrise, indicating the importance of photochemical processes for HONO formation. In clean marine air, NOx-related reactions contribute to ∼ 21 % of the daytime HONO source, and the photolysis of particulate nitrate (pNO3) can account for the missing source assuming a moderate enhancement factor of 29 relative to gaseous nitric acid photolysis. In low-NOx island-influenced air, the contribution from both NOx-related reactions and pNO3 photolysis accounts for only ∼ 48 % of the daytime HONO production, and the photochemical processes on surfaces of the island, such as the photolysis of nitric acid on the forest canopy, might contribute significantly to the daytime HONO production. The concentrations of HONO, NOx, and pNO3 were lower when the site was dominated by the aged marine air in the summer and were higher when the site was dominated by North American air in the spring, reflecting the effects of long-range transport on the reactive nitrogen chemistry in background marine environments.