Fluxes of nitrogen oxides over a temperate deciduous forest

Abstract

Eddy covariance flux measurements of NO, NO2, and O3 were obtained above the mixed deciduous canopy at Harvard Forest in central Massachusetts from April to November 2000. Net deposition of NOx was observed throughout the measurement period, with average velocity of ∼0.2 cm s−1. At night, NO2 is deposited at a rate that depends nonlinearly on NO2 concentration and cannot be explained by N2O5 hydrolysis, suggesting HONO formation by heterogeneous disproportionation of NO2. During the day, photochemically driven NOx 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 NO2 near 1.5 nmol mol−1. These results were confirmed by independent evidence from NO, NO2, and O3 profiles acquired at the site over several years. If the rate of NOx 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 NOx oxidation is slow during winter; the impact at night is important for the same reasons. The results contradict widely used parameterizations of NO2 deposition that both overestimate stomatal uptake and do not allow for surface uptake when stomates are closed.