IMPLICATIONS OF LARGE-SCALE SHIFTS IN TROPOSPHERIC NOX LEVELS IN THE REMOTE TROPICAL PACIFIC

A major observation recorded during NASA's western Pacific Exploratory Mission (PEM-West B) was the large shift in tropical NO levels as a f unction of geographical location. High-altitude NO levels exceeding 10 0 pptv were observed during portions of tropical flights 5-8, while va lues almost never exceeded 20 pptv during tropical flights 9 and 10. T he geographical regions encompassing these two flight groupings are he re labeled ''high'' and ''low'' NOx regimes. A comparison of these two regimes, based on back trajectories and chemical tracers, suggests th at air parcels in both were strongly influenced by deep convection. Th e low NOx regime appears to have been predominantly impacted by marine convection, whereas the high NOx regime shows evidence of having been more influenced by deep convection over a continental land mass. DMSP satellite observations point strongly toward lightning as the major s ource of NO, in the latter regime. Photochemical ozone formation in th e high NOx regime exceeded that for low NOx by factors of 2 to 6, wher eas O-3 destruction in the low NOx regime exceeded that for high NOx b y factors of up to 3. Taking the tropopause height to be 17 km, estima tes of the net photochemical effect on the O-3 column revealed that th e high NO, regime led to a small net production. By contrast, the low NO, regime was shown to destroy O-3 at the rate of 3.4% per day. One p roposed mechanism for off-setting this projected large deficit would i nvolve the transport of O-3 rich midlatitude air into the tropics. Alt ernatively, it is suggested that O-3 within the tropics may be overall near self-sustaining with respect to photochemical activity. This sce nario would require that some tropical regions, unsampled at the time of PEM-B, display significant net column O-3 production, leading to an overall balanced budget for the ''greater'' tropical Pacific basin. D etails concerning the chemical nature of such regimes are discussed.