The activities of 210Pb 210Bi and 210Po were measured in a large number of rain and snow samples that fell at Fayetteville, Arkansas. It was found that in nearly 82% of the samples measured, the residence times calculated from the 210Bi/210Pb and 210Po/210Pb ratios were found to be either concordant or near concordant. The discordant residence times are shown to result from the mixing of two different air masses, which results in the failure of the assumption of steady state atmospheric condition. By assuming a residence time of 4 days for aerosols near the surface of the earth, an average residence of 35 days is calculated for tropospheric aerosols.