FACTORS INFLUENCING THE UPPER FREE TROPOSPHERIC DISTRIBUTION OF REACTIVE NITROGEN OVER THE SOUTH-ATLANTIC DURING THE TRACE-A EXPERIMENT

This paper evaluates the potential sources of the enhanced levels of N O that were observed throughout the upper troposphere over the equator ial and tropical South Atlantic. During September/October 1992 NOx (NO + NO2) mixing ratios in the 8-to 13-km region averaged 150 parts per trillion by volume (pptv) and were greatly affected by what appeared a s spatially large ''plumes'' (100 to 1000 km) with NO enhancements of over 800 parts per trillion by volume. Other trace gases were also enh anced within these plumes (e.g., CO, CO2, CH4, CH3Cl, C2H2, C2H6, C3H8 , and PAN). However, for these tracers of surface emissions, inconsist ent patterns of enhancement were found with respect to one another and to NO. We analyzed these plumes for indications of coherent relations hips between the enhanced levels of NO and the enhanced levels of biog enic and combustion-related tracers. This analysis indicated that the tracer relationships were primarily produced by their common injection via deep convection into the upper troposphere. A corollary analysis using a combustion tracer reference frame in combination with meteorol ogical analysis indicates a longer than expected lifetime of NO, in th e upper troposphere. This analysis also suggests that an efficient mec hanism may exist in the upper troposphere for recycling HNO3 back into NOx with a rate comparable to that predicted for the HNO3 formation. During the Transport and Atmospheric Chemistry Near the Equator Atlant ic study period this in-situ formation of NOx is estimated to provide the equivalent of approximately 0.7 TgN/yr of NOx within the South Atl antic basin's upper troposphere. This magnitude of local in situ sourc e is estimated to be comparable to the combined inputs from lightning and biomass burning, which are both injected via deep convection. Our analysis also suggests that lightning can contribute as much as half o f the external input of NOx into this region of the upper troposphere with biomass burning possibly representing the remainder.