Variability in atmospheric CO2 concentration over periods of 1--5 days at Cape Matatula, American Samoa, was studied. The variability was found to be the result of the alternating influences of three air mass source regions. Partitioning of Samoa CO2 data according to these air mass source regions revealed annual cycles in the partitioned data sets corresponding to those of the tropical South Pacific, the mid-latitude southern hemisphere, and the tropical North Pacific regions. ¿ American Geophysical Union 1988