Vertical distributions of molecular hydrogen off the Eastern and Gulf Coasts of the United States

The vertical distributions of molecular hydrogen (H 2 ) and carbon monoxide (CO) were determined during spiraling maneuvers from aircraft off the eastern coast of the United States and over the Gulf of Mexico. H 2 was always at its lowest levels in the boundary layer, averaging about 500 parts per billion by volume (ppbv). H 2 mixing ratios determined in the free troposphere were typically higher (600–700 ppbv) and often suggested a small progressive increase with altitude. Several large free‐tropospheric H 2 plumes (mixing ratios as high as 1–2 parts per million by volume) were implied by the data. These H 2 plumes were not always accompanied by corresponding increases in CO mixing ratios. This result is most difficult to explain when it is noted that the primary atmospheric sources for molecular H 2 are considered to be combustion and photochemistry, both of which should be strong CO sources also.