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 C 4 and C 5 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.