Regional budgets for nitrogen oxides from continental sources: Variations of rates for oxidation and deposition with season and distance from source regions

Measurements of nitrogen deposition and concentrations of NO, NO 2 , NO y (total oxidized N), and O 3 have been made at Harvard Forest in central Massachusetts since 1990 to define the atmospheric budget for reactive N near a major source region. Total (wet plus dry) reactive N deposition for the period 1990–1996 averaged 47 mmol m −2 yr −1 (126 μmol m −2 d −1 , 6.4 kg N ha −1 yr −1 ), with 34% contributed by dry deposition. Atmospheric input adds about 12% to the N made available annually by mineralization in the forest soil. The corresponding deposition rate at a distant site, Schefferville, Quebec, was 20 mmol m −2 d −1 during summer 1990. Both heterogeneous and homogeneous reactions efficiently convert NO x to HNO 3 in the boundary layer. HNO 3 is subsequently removed rapidly by either dry deposition or precipitation. The characteristic ( e ‐folding) time for NO x oxidation ranges from 0.30 days in summer, when OH radical is abundant, to ∼1.5 days in the winter, when heterogeneous reactions are dominant and O 3 concentrations are lowest. The characteristic time for removal of NO x oxidation products (defined as NO y minus NO x ) from the boundary layer by wet and dry deposition is ∼1 day, except in winter when it decreases to 0.6 day. Biogenic hydrocarbons contribute to N deposition through formation of organic nitrates but are also precursors of reservoir species, such as peroxyacetylnitrate, that may be exported from the region. A simple model assuming pseudo first‐order rates for oxidation of NO x , followed by deposition, predicts that 45% of NO x in the northeastern U.S. boundary layer is removed in 1 day during summer and 27% is removed in winter. It takes 3.5 and 5 days for 95% removal in summer and winter, respectively.