Ethane and propane in the Southern marine troposphere

Nearly 500 measurements of the ethane and propane mixing ratios have been made in clean marine air at Baring Head (New Zealand) and Scott Base (Antarctica) between 1991 and 1996. The annual averages of the mixing ratios (285 and 40 ppt) are lower than previously reported for the Southern Hemisphere. A striking feature of the seasonal cycle is the abrupt drop in mixing ratios of both compounds around November and a corresponding increase in the ethane/propane ratio (from about 7 in winter to > 10 in summer), suggesting a sharp decrease in Southern Hemisphere sources (e.g. biomass burning or fossil gas emissions) of these compounds in the spring. From a simple budget estimate it is concluded that biomass burning is most likely the dominant source of ethane and propane in the Southern Hemisphere. The seasonal variability of the emissions which are required to balance the Southern Hemisphere propane budget agrees very well with the seasonality derived for ethane emissions in a previous study.