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 production1–7. The observed concentrations of nitrous acid are thought to be sustained by heterogeneous reactions involving precursors such as nitrogen oxides1–3,8–12 and nitric acid5,6,8,13. 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. To quantitatively assess the source strength of nitrous acid (HONO) at the forest canopy surface in a rural environment, we conducted measurements of ambient HONO concentration and HONO flux on the 32-m PROPHET tower (∼11m above the forest canopy level) at the University of Michigan Biological Station from 15 July to 10 August, 2008. A detailed site description can be found elsewhere14. Other parameters were measured at the site, including NOx(=NO+NO2), OH, and Eppley ultraviolet intensity. Sixteen sets of leaf samples were collected at the top of the forest canopy at different times of the day from 25 July to 5 August, 2008, tomeasure the leaf surface nitrate loadings. Positive upward HONO fluxes were observed most of the time during the measurement period, with an average of +0.37 × 10−6 molm−2 h−1, consistent with our understanding that the canopy surface is mainly a HONO source at this site8,15. The highest upward HONO fluxes, +4.7×10−6 molm−2 h−1, occurred around solar noon during the day after a high-NOx and highNOy episode, suggesting that HONO was produced from the precursors deposited onto the forest canopy surface. Significant downward fluxes, −1.8×10−6 molm−2 h−1, occurred occasionally only during the events of rain, fog or dew, because of enhanced HONO deposition to the wet canopy surface8. The observed