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, NO₂, NO_y (total oxidized N), and O₃have 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 HNO₃ in the boundary layer. HNO₃ 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 O₃ 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.