O₃, NO_<i>y</i>, and NO_<i>x</i>/NO_<i>y</i> in the upper troposphere of the equatorial Pacific

Two of the DC‐8 flights during the 1991–1992 second Airborne Arctic Stratospheric Expedition (AASE 2) were between California and Tahiti. Extremely abrupt changes in O 3 and NO y were observed on both flights as the aircraft crossed the subtropical jet. They indicate that the width of the transition from midlatitude to tropical air in the troposphere can be as short as 1 km. The NO y /O 3 ratio was remarkably stable across the transition. We discuss some of the dynamical features associated with the transitions and speculate on the reasons for their abruptness. They occurred south of the subtropical frontal zone and were accompanied by changes in humidity, NO x /NO y , and modest changes in CO, CH 4 , and CO 2 . In addition, a chemical model constrained by measurements of the long‐lived species is used to simulate the variation of NO x /NO y along the two flight tracks. Although this model is quite successful at simulating observed NO x /NO y in midlatitude air, it drastically overestimates NO x /NO y in tropical air. The rate at which the model converts NO x to HNO 3 via the NO 2 + OH reaction is very slow in the upper tropical troposphere because the low O 3 concentrations and cold temperatures force most of the NO x to be in the form of NO during the day. We argue that there is an important NO to NO 2 pathway in this region not presently included in models, that much of the NO y is in a stable (possibly aerosol) form that is not readily converted to NO x , or that there has been insufficient time since convection for NO x to be released from other more stable forms of NO y . It is important to resolve this discrepancy because present models which have the correct O 3 and NO y may overestimate O 3 production rates and OH concentrations in the upper tropical troposphere.