The regional air quality, atmospheric chemistry and climate change are largely influenced by the chemical composition of ambient aerosols and more importantly, by their residence time on a spatial and temporal scale. The environmental radionuclide Pb-210 (t(1/2)= 22.3 years), injected into the atmosphere by in situ decay of its parent nuclide Rn-222 (t(1/2)= 3.8 days) from ground sources, is highly particle reactive and thus serves as an ideal tracer to assess the residence time of anthropogenic and natural aerosols from ground-based sources. We report on the temporal variability in residence time of ambient aerosols studied from an urban site (Ahmedabad) and a high altitude site (Mt Abu) in western India. The residence time of aerosols, ranging from similar to 2 to 8 days, is predominantly controlled by regional meteorology and high dust abundance (shorter removal time) in semiarid regions. These observations raise the issue of uncertainty in tracing the source region of atmospheric pollutants based on air-mass back trajectory analyses without knowing their actual residence time for a given time-period over a study region.