[1] Eddy covariance flux measurements of NO, NO 2 , and O 3 were obtained above the mixed deciduous canopy at Harvard Forest in central Massachusetts from April to November 2000. Net deposition of NO x was observed throughout the measurement period, with average velocity of $0.2 cm s À1 . At night, NO 2 is deposited at a rate that depends nonlinearly on NO 2 concentration and cannot be explained by N 2 O 5 hydrolysis, suggesting HONO formation by heterogeneous disproportionation of NO 2 . During the day, photochemically driven NO x fluxes conform to the predicted behavior based on gradients of light and eddy diffusivity through the canopy, with residual net flux attributable to both stomatal and nonstomatal processes. The results were consistent with a compensation point for NO 2 near 1.5 nmol mol À1 . These results were confirmed by independent evidence from NO, NO 2 , and O 3 profiles acquired at the site over several years. If the rate of NO x deposition observed at this site in April through November continues during the winter, it would have a larger potential impact on tropospheric chemistry because mixing depths are shallow and chemical NO x oxidation is slow during winter; the impact at night is important for the same reasons. The results contradict widely used parameterizations of NO 2 deposition that both overestimate stomatal uptake and do not allow for surface uptake when stomates are closed.

No abstract

No abstract