Implications of large scale shifts in tropospheric NO_<i>x</i> levels in the remote tropical Pacific

Abstract

A major observation recorded during NASA's western Pacific Exploratory Mission (PEM‐West B) was the large shift in tropical NO levels as a function of geographical location. High‐altitude NO levels exceeding 100 pptv were observed during portions of tropical flights 5–8, while values almost never exceeded 20 pptv during tropical flights 9 and 10. The geographical regions encompassing these two flight groupings are here labeled “high” and “low” NO x regimes. A comparison of these two regimes, based on back trajectories and chemical tracers, suggests that air parcels in both were strongly influenced by deep convection. The low NO x regime appears to have been predominantly impacted by marine convection, whereas the high NO x 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 source of NO x in the latter regime. Photochemical ozone formation in the high NO x regime exceeded that for low NO x by factors of 2 to 6, whereas O 3 destruction in the low NO x regime exceeded that for high NO x by factors of up to 3. Taking the tropopause height to be 17 km, estimates of the net photochemical effect on the O 3 column revealed that the high NO x regime led to a small net production. By contrast, the low NO x regime was shown to destroy O 3 at the rate of 3.4% per day. One proposed mechanism for off‐setting this projected large deficit would involve the transport of O 3 rich midlatitude air into the tropics. Alternatively, it is suggested that O 3 within the tropics may be overall near self‐sustaining with respect to photochemical activity. This scenario 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. Details concerning the chemical nature of such regimes are discussed.