Latitudinal variations of trace gas concentrations in the free troposphere measured by solar absorption spectroscopy during a ship cruise

The latitudinal variations of atmospheric trace gas column abundances have been measured during a ship cruise between 57 degrees N and 45 degrees S on the central Atlantic. The measurements were performed in October 1996 using high-resolution solar absorption spectroscopy in the infrared. The analysis method employed permits the retrieval of the total column densities of 20 different trace gases and for a few compounds the vertical mixing ratio profiles. For CH4 an interhemispheric difference of 3% was observed. The total columns of the shorter-lived trace gases CO and C2H6, analyzed between 57 degrees N and 45 degrees S, reveal a slight maximum in the tropics and a substantial increase north of 45 degrees N. The total columns of C2H2 and HCN, detectable between 30 degrees N and 30 degrees S, reveal a maximum in the tropics of the Southern Hemisphere. For CH2O, studied between 57 degrees N and 45 degrees S, a well-pronounced maximum is observed in the tropics. The profile retrieval gives high mixing ratios for CO, C2H6 and O-3 north of 40 degrees N in the lower troposphere. In the tropics high concentrations are found for all three compounds in the entire troposphere, even above 12 km. The measurements have been used to estimate averaged mixing ratios of the trace gases for the free troposphere between 0 and 12 km. In the tropics the data give high values: for example, more than 200 pptv for HCN, 750 pptv for CH2O, 100 ppbv for CO and 100 pptv for C2H2 These values are comparable to or higher than what has been observed at midlatitudes, indicating the importance of biomass burning emissions on the tropospheric composition.