Nitric acid photolysis on forest canopy surface as a source for tropospheric nitrous acid

Photolysis of nitrous acid generates hydroxyl radicals-a key atmospheric oxidant-in the lower atmosphere. Significant concentrations of nitrous acid have been reported in the rural atmospheric boundary layer during the day, where photolysis of nitrous acid accounts for up to 42% of sunlight-induced radical production(1-7). The observed concentrations of nitrous acid are thought to be sustained by heterogeneous reactions involving precursors such as nitrogen oxides(1-3,8-12) and nitric acid(5,6,8,13). Here, we present direct measurements of nitrous acid flux over a rural forest canopy in Michigan, together with surface nitrate loading at the top of the canopy. We report a significant upward flux of nitrous acid during the day, with a peak around noontime. Daytime nitrous acid flux was positively correlated with the product of leaf surface nitrate loading and the rate constant of nitrate photolysis. We suggest that the photolysis of nitric acid on forest canopies is a significant daytime source of nitrous acid to the lower atmosphere in rural environments, and could serve as an important pathway for the remobilization of deposited nitric acid.