The seasonality of CO abundance in the Southern Hemisphere

CO mixing ratios in air have been measured continuously at Cape Point (34°21°S; 18°29’E) between 1978 and 1981. The results show a seasonal variation of the CO mixing ratios with minimum values of 53 p.p.b.v. during January/February and maximum values of 87 p.p.b.v. during September/October. Short-term variations of CO mixing ratios in clean, undisturbed air were lacking, indicating that CO is well mixed in the Southern Hemisphere at latitudes of 20-40° S and that the observed seasonal variation is not due to temporal changes of local and regional source strengths. The seasonality of CO is explained by the seasonal variation of OH and by the north-south shift of the intertropical convergence zone. The agreement of CO mixing ratios measured at Cape Point and over the Southern Atlantic in 1971/1972 indicates that the southern hemispheric CO mixing ratios cannot have changed by more than 5-10% during the last decade.