Showing papers by "James Walega published in 1995"

Regional ozone and urban plumes in the southeastern United States: Birmingham, A case study

Abstract: Aircraft measurements of ozone and the oxides of nitrogen have characterized the horizontal and vertical extent of the urban plume downwind of Birmingham, Alabama. Derived NOx emission rate estimates of 0.6×1025 molecules s−1, with an uncertainty of a factor of 2, for this metropolitan area are in reasonable accord with the 1985 National Acid Precipitation Assessment Program inventory, which gives 1.2×1025 molecules s−1 for daytime emissions. These estimates are from two flights in 1992 when the urban plume was well separated from the plumes from two power plants northwest of the city. During three flights in 1990 the plumes of the Birmingham metropolitan area and the two power plants were combined; good agreement was found between the estimated fluxes (2.0 to 5.5×1025 molecules s−1) and the emission inventory (3.7×1025 molecules s−1) for the combined sources. The enhancement of O3 in the urban plume indicates photochemical formation and shows that during the summertime, approximately seven O3 molecules can be formed per NOx molecule added by the urban and power plant emissions. This production efficiency applies both to the isolated urban plume and to the combined urban-power plant plumes and is similar to that derived for rural areas from surface studies. Comparison of the results from several flights indicates the contribution of the regional ozone levels to the O3 concentrations observed within the urban plumes. The aircraft measurements, in combination with surface measurements of ozone, show that the interaction of ozone concentrations entering the urban area, the photochemical formation of ozone during the oxidation of the urban emissions, and the wind speed and direction determine the location and the magnitude of the peak ozone concentrations in the vicinity of this metropolitan area.

O3, NO y , and NO x /NO y in the upper troposphere of the equatorial Pacific

Abstract: 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.