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, NOy (total oxidized N), and O-3 have been made at Harvard Forest in centr al Massachusetts since 1990 to define the atmospheric budget for react ive N near a major source region. Total (wet plus dry) reactive N depo sition for the period 1990-1996 averaged 47 mmol m(-2) yr(-1) (126 mu mol 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 a nnually by mineralization in the forest soil. The corresponding deposi tion rate at a distant site, Schefferville, Quebec, was 20 mmol m(-2) d(-1) during summer 1990. Both heterogeneous and homogeneous reactions efficiently convert NOx to HNO3 in the boundary layer. HNO3 is subseq uently removed rapidly by either dry deposition or precipitation. The characteristic (e-folding) time for NOx oxidation ranges from 0.30 day s in summer, when OH radical is abundant, to similar to 1.5 days in th e winter, when heterogeneous reactions are dominant and O-3 concentrat ions are lowest. The characteristic time for removal of NOx oxidation products (defined as NOy minus NOx) from the boundary layer by wet and dry deposition is similar to 1 day, except in winter when it decrease s to 0.6 day. Biogenic hydrocarbons contribute to N deposition through formation of organic nitrates but are also precursors of reservoir sp ecies, such as peroxyacetylnitrate, that may be exported from the regi on. A simple model assuming pseudo first-order rates for oxidation of NOx, followed by deposition, predicts that 45% of NOx in the northeast ern U.S. boundary layer is removed in 1 day during summer and 27% is r emoved in winter. It takes 3.5 and 5 days for 95% removal in summer an d winter, respectively.