Nitrogen Dry Deposition At An Ameriflux Site In A Hardwood Forest In The Midwest

This paper presents observations of ammonia, nitric acid and size segregated chemically speciated aerosols above the canopy of a secondary successional deciduous forest in Indiana during spring. Average nitric acid concentrations range between 0.1–1 µg m⁻³ and vertical gradients from 1 to 9 ng m⁻³ / m. The ammonia probability distribution indicates an average concentration of 0.8 µg m⁻³, although concentrations in excess of 2 µg m⁻³ are observed. Aerosol measurements indicate ammonium is largely confined to the accumulation mode (0.1 < D_p < 1.0 µm) with a mean concentration of 1 µg m⁻³. Nitrate exhibits a bi-modal distribution (with mass in both the accumulation and coarse (D_p > 1.0 µm) modes), and is present at mean concentrations of 1.2 µg m⁻³. Average dry deposition fluxes calculated from 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 of over 1.9 mg-N m⁻² dy⁻¹ is approximately equal to the flux in precipitation as measured at NADP sites in the region.