The long life decay products 210Pb, 210Bi and 210Po of 222Rn are adsorbed in atmospheric particulate matters, thus making it possible to estimate the average residence time by measuring the specific activity or activity ratio of radon daughters. However, the existing research results are rather bad in consistency and the contradictory phenomena cannot be reasonably explained, which makes it inapplicable to atmospheric environmental monitoring. For this reason, starting with the basic attributes of lower atmosphere, based on the decay characteristics, transfer rates and other related information of radioactive nuclides in atmospheric particulates, a mathematical model is built to reflect the relationship among the particle numbers of the absorbed, decayed and transferred. Then, a new method is presented to estimate the average residence time of atmospheric aerosol. The method can solve the confusion that have appeared in the current research, reveal that the atmosphere has limited self-purification function, and provide the condition for nuclides in the same decaying chain to decay to the activity balance and the way to reduce the average residence time. All these conclusions will provide a scientific basis for further study and exploitation in atmospheric environment monitoring.