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 NO y and O 3 eddy flux and concentration measurements and NO x 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 NO y flux from atmosphere to canopy were 3.4 and 3.2 μmole m −2 hr −1 at Harvard Forest in summer and winter, respectively, and 0.5 μmole m −2 hr −1 at Schefferville during summer. Nighttime values were 1.3, 2.0, and 0.15 μmole m −2 hr −1 , respectively. For 1990–1994, the net annual dry deposition of nitrogen oxides was 17.9 mmole m −2 yr −1 (2.49 kgN ha −1 y −1 ). Oxidized species such as HNO 3 dominated N deposition, with minor contributions from direct deposition of NO 2 . Emissions of NO from the forest soil were negligible compared to deposition. Comparison of NO y 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 NO 2 to HNO 3 in winter, leading to rates for dry deposition of NO y similar to fluxes in summer despite 10‐fold decrease in OH concentrations. In summer, formation of HNO 3 by heterogeneous reactions (mainly at night) could provide 25–45% of the NO 2 oxidation.