Two-Dimensional Distribution of Light Hydrocarbons: Results From the STRATOZ III Experiment

During the STRATOZ III flights in June 1984, about 120 whole-air samples were collected and analyzed later on in the laboratory for several atmospheric trace components, including light hydro-carbons. The STRATOZ III mission consisted of 21 flight segments and covered a latitude range from 70°N to 60°S and altitudes up to 12 km. The results of these measurements were used to construct latitude-altitude profiles in the form of isolines for ethane, ethene, acetylene, propane, propene, n-butane, isobutane, n-pentane, and isopentane. These results are compared with the latitudinal cross sections obtained during a previous, very similar flight mission (STRATOZ II) in May-June 1980. Also, the few published latitudinal or vertical profiles for these nonmethane hydrocarbons (NMHCs) are used for a comparison. The two-dimensional distributions for the longer-lived NMHCs, especially ethane and to some extent also propane and acetylene, are reasonably representative, even on a global scale. The high variability of the short-lived NMHCs, the C4 and C5 alkanes, and the light alkenes, prevents the determination of representative two-dimensional distributions for these species. The distributions of these short-lived compounds give at best an extremely rough idea on the distributions and should in general be considered as descriptions of a given momentary situation. These latitude-altitude profiles indicate the existence of fast mechanisms for vertical mixing in the troposphere. The observation of high mixing ratios of short-lived hydrocarbons in the middle and upper troposphere proves the existence of vertical mixing processes with a time scale comparable to, or even shorter than, the atmospheric lifetime of these reactive NMHCs. As a consequence, there exist several regions, even above the boundary layer, with NMHC mixing ratios high enough to make them important participants in the atmospheric photochemical reaction cycles.