Abstract
An understanding of the distribution, variation, and photochemical interactions of reactive species in the global troposphere is fundamental to questions about the budgets and trends of tropospheric ozone and other trace gases. These are among the major objectives of the IGAC-GLOCHEM Activity. Recently, several field campaigns have measured reactive trace gases and radical species in the remote troposphere to test models of transport and chemical processes. The experimental programs described here are STRATOZ III, TROPOZ II, and MLOPEX II. Examples from each of these programs will highlight some of the results.
Global-scale distributions of trace gases were measured during June 1984 (STRATOZ III) and January 1991 (TROPOZ II). The measurements cover latitudes from 60°N to 60°S and altitudes from 0 to 12 km. A number of different chemical species were measured during the flights, but here we focus on measurement of nitrogen oxides, NOX, in the troposphere. The oxides of nitrogen to a large extent control hydroxyl radical concentration and ozone production in the troposphere. Thus, accurate knowledge of the global NOX distribution is required for the understanding of tropospheric chemistry.
The northern hemispheric summertime vertical profiles are “C-shaped,” with high NO mixing ratios at 8 to 10 km altitude and a strong vertical gradient. In contrast, high concentrations were observed in the northern hemisphere during winter at all altitudes nearly without a vertical gradient.
A two-dimensional, zonal model with limited chemistry of NO, NO2 and HNO3, applied to the latitudinal band of 40 to 50°N, reproduces the measured vertical profiles reasonably well. Even the formation of a maximum of NO mixing ratio at 8 to 10 km altitude over Europe and North America during summertime can be explained by fast vertical transport of surface-emitted NO, and the NO emissions by aircraft in the upper troposphere.
The Mauna Loa Observatory Photochemical Experiment (MLOPEX II) was conducted to measure simultaneously a large number of related chemical species to characterize the photochemical state of the remote free troposphere and to test our current understanding of photochemical interactions and processes. The measurements at this location were carried out in 1991 and 1992 during four seasonal intensive experiments, each lasting 5 to 6 weeks. Aircraft studies during one intensive were made to relate measurements at the ground site to trace gas concentrations on a large, regional scale. In addition to trace gas composition, free radical species (HO2, RO2, HO) and photolysis rates (for NO2, O3) were measured.
Because the experiment was only recently completed, data have only begun to be integrated and analyzed. This paper presents examples of the different measurements obtained during the experiment, and emphasizes the variations and relationships observed over diurnal, synoptic, and seasonal time scales.