O-3, NOY, AND NOX NOY IN THE UPPER TROPOSPHERE OF THE EQUATORIAL PACIFIC/

Two of the DC-8 flights during the 1991-1992 second Airborne Arctic St ratospheric Expedition (AASE 2) were between California and Tahiti. Ex tremely abrupt changes in O-3 and NOy were observed on both flights as the aircraft crossed the subtropical jet. They indicate that the widt h of the transition from midlatitude to tropical air in the tropospher e can be as short as 1 km. The NOy/O-3 ratio was remarkably stable acr oss the transition. We discuss some of the dynamical features associat ed with the transitions and speculate on the reasons for their abruptn ess. They occurred south of the subtropical frontal zone and were acco mpanied by changes in humidity, NOx/NOy, and modest changes in CO, CH4 , and CO2. In addition, a chemical model constrained by measurements o f the long-lived species is used to simulate the variation of NOx/NOy along the two flight tracks. Although this model is quite successful a t simulating observed NOx/NOy in midlatitude air, it drastically overe stimates NOx/NOy in tropical air. The rate at which the model converts NOx to HNO3 via the NO2 + OK reaction is very slow in the upper tropi cal troposphere because the low O-3 concentrations and cold temperatur es force most of the NO, to be in the form of NO during the day. We ar gue that there is an important NO to NO2 pathway in this region not pr esently included in models, that much of the NOy is in a stable (possi bly aerosol) form that is not readily converted to NOx, or that there has been insufficient time since convection for NOx to be released fro m other more stable forms of NOy. It is important to resolve this disc repancy because present models which have the correct O-3 and NO, may overestimate O-3 production rates and OH concentrations in the upper t ropical troposphere.