Vertical profiles of the H2 mixing ratio in the troposphere were obtained by collecting air samples during aircraft flights at several locations over North America. Three sets of profiles at two locations have been measured for periods of at least 1 year, two over Scottsbluff, Nebraska, and one over the Pacific, 200 km offshore from Santa Barbara, California. The individual profiles indicate relatively large variations with time and space. However, no significant systematic seasonal variation of the tropospheric H2 mixing ratio could be established. The annually averaged profiles show that H2 is rather well mixed vertically and horizontally within the troposphere. The average H2 mixing ratio for the region of 26°–42°N was found to be 0.503 ppmv (parts per million by volume). The only systematic gradient in the troposphere was observed near the surface over the Pacific. It is ascribed to local sources. In the stratosphere the H2 mixing ratio increases to a slight maximum of about 0.59 ppmv around 27-km altitude and decreases above to a value of 0.40 ppmv at 50 km.
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