THE GLOBAL TROPOSPHERIC DISTRIBUTION OF NOX ESTIMATED BY A 3-DIMENSIONAL CHEMICAL TRACER MODEL

The global distribution of NOx in the troposphere is calculated using a simple three-dimensional chemical tracer model. This model includes a simplified chemistry scheme for the tracers NOx equivalent to NO + N O2 and HNO3, which are redistributed by advection, dry and wet convect ion, and large-scale diffusion. The sources of NOx considered are foss il fuel combustion, emissions from soil microbial activity, biomass bu rning, lightning discharges, emissions by aircraft, and downward trans port from the stratosphere. Dry and wet deposition act as final sinks. At northern middle and high latitudes the calculated tropospheric NOx content is dominated by the surface sources, fossil fuel combustion i n particular. In the tropical free troposphere, lightning discharges p rovide about 80% of the total NOx throughout the year. The zonally ave raged fractional contribution of aircraft emissions strongly depends o n the season. The largest contribution of this source, over 60%, occur s during January in the upper troposphere between 45 degrees N and 60 degrees N. The NO mixing ratios determined by the model show good over all agreement with vertical profiles measured during the Stratospheric Ozone Experiment (STRATOZ) III aircraft campaign.