National Science Foundation Final Project Report Part I - Project Identification

The 11-level GFDL global chemical transport model has been used to assess the impact of stratospheric NOx production on tropospheric reactive nitrogen (N0 y) concentrations. A temporally varying source function was constructed using specified two-dimensional, monthly-average 03 , N 20, temperature and surface pressure data generated by the GFDL 'SKYHI' model. The calculated yearly NO y production rate is 0.64 tg N (0.64 x 1012 g N). A wet removal scheme, which distinguishes between stable and convective rain based on the bulk Richardson number, is introduced. Simulations have been performed with a simplified chemical mechanism which fractionates NO y into soluble and insoluble species. The role of PAN in determining the impact of stratospheric injection on the tropospheric NOy budget is studied by comparing results of simulations with and without PAN chemistry. We conclude that: (1) The stratospheric source is too small to account for background surface NOy concentrations observed in the remote (i.e., regions a few thousand kilometers from continental source regions) troposphere. Surface NO y mixing ratios seldom exceed 10 pptv in the model Northern Hemisphere, and are always below 20 pptv. Together, fossil-fuel combustion emissions and stratospheric injection account for less than 10% of observed surface nitrate concentrations in the remote tropical Pacific; (2) The impact of the stratospheric source is comparable to that of the fossil-fuel combustion source in terms of NOy mixing ratios in the Northern Hemisphere at the 500 rnb model level, and is more important in the midand high latitudes of the Southern Hemisphere. At the 315 mb model level, the stratospheric source contribution to NO y levels is more important than that of the fossil fuel source at all latitudes, except in the tropics. However, substantial contributio0s from other NO y sources are needed to explain observations in the remote midand upper troposphere; and (3) Inclusion of PAN chemistry has the effect of increasing model-calculated surface NO y mixing ratios in the Northern Hemisphere midand high latitudes by factors of 1.5-3 during winter/spring, and by factors of 2-4 during summer/fall. Surface NOy mixing ratios in the Southern Hemisphere show a smaller