The activities of 214Pb (half-life 26.8 min) and 214Bi (half-life 19.7 min), decay products of 222Rn, are nearly equal in 187 aerosol samples collected over 1 year at an altitude of 12 m in New Haven, Connecticut. These observations do not agree with the large variations in (214Bi)/(214Pb) reported by others for southern California. We derive a reference ratio as a technique for relating the measured activities of 214Bi and 214Pb accumulated on filters over sampling times that are longer than their half-lives to the (214Bi)/(214Pb) in the atmosphere and derive simple models relating the reference ratio to aerosol residence time. The observed (214Bi)/(214Pb) ratios imply a mean residence time of 16 hours (with the 2 standard error limits of +72 and -7 of this mean) in a steady state box model and mean age of 2.8 hours (with the 2 standard error limits of +0.9 and -0.4 of this mean) in an isolated parcel model. The variance of the data indicates that the conditions of sampling are closer to the assumptions of the box model than the isolated parcel model. ¿ 1999 American Geophysical Union