Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland .1. Measurements and mechanisms

We present 5 years of NOy and O-3 eddy flux and concentration measurements and NOx concentration measurements at Harvard Forest (1990-1994), a mixed deciduous forest in central Massachusetts, and 2 months of data for a spruce woodland near Schefferville, Quebec, during the NASA ABLE3B/Northern Wetlands Study (1990). Mean midday values of net dry NOy flux from atmosphere to canopy were 3.4 and 3.2 mu mole m(-2)hr(-1) at Schefferville during summer. Nighttime values were 1.3, 2.0 and 0.15 mu mole m(-2)yr(-1) (2.49 kgN ha(-1)y(-1)). Oxidized species such as HNO3 dominated N deposition, with minor contributions from direct deposition of NO2. Emissions of NO from the forest soil were negligible compared to deposition. Comparision of NOy deposition at Harvard Forest and Schefferville and analysis of the dependence on meteorological parameters show that anthropogenic sources dominate the nitrogen oxide inputs over much of North America. Heterogeneous reactions account for >90% of the conversion of NO2 to HNO3 in winter, leading to rates for dry deposition of NOy similar to fluxes in summer despite 10-fold decrease in OH concentrations. In summer, formation of HNO3 by heterogeneous reactions (mainly at night) could provide 25-45% of the NO2 oxidation.