Nitrogen dry deposition at an AmeriFlux site in a hardwood forest in the MidWest

This paper presents observations of ammonia, nitric acid and size segregate d chemically speciated aerosols above the canopy of a secondary successiona l deciduous forest in Indiana during spring. Average nitric acid concentrat ions range between 0.1-1 mu g m(-3) and vertical gradients from 1 to 9 ng m (-3)/m. The ammonia probability distribution indicates an average concentra tion of 0.8 mu g m(-3), although concentrations in excess of 2 mu g m(-3) a re observed. Aerosol measurements indicate ammonium is largely confined to the accumulation mode (0.1 < D-p < 1.0 mu m) with a mean concentration of 1 mu g m(-3). Nitrate exhibits a bi-modal distribution (with mass in both th e accumulation and coarse (D-p > 1.0 mu m) modes), and is present at mean c oncentrations of 1.2 mu g m(-3). Average dry deposition fluxes calculated f rom these measurements are: Nitric acid = 0.38 mg-N m(-2) dy(-1), ammonia = 1.07 mg-N m(-2) dy(-1), aerosol-nitrate = 0.27 mg-N m(-2) dy(-1), aerosol- ammonium = 0.25 mg-N m(-2) dy(-1). The total dry deposition nitrogen flux o f over 1.9 mg-N m(-2) dy(-1) is approximately equal to the flux in precipit ation as measured at NADP sites in the region.