The evolution of positive air ions is mathematically simulated considering 150 trace gases and more than a thousand ion-molecule reactions. The main attention is paid to the evolution of near-ground air ions in the age interval from 10 ms to 3 s where the knowledge about ion evolution is most limited. Recently, detailed experimental data about the time variation of the air ion mobility spectrum became available, and the results of mathematical simulation can be compared with the observed transformations. Unfortunately, the ion mobility measurements have not been accompanied by data about concentrations of the trace gases;;therefore the mobilities can be interpreted only ambiguously. Nevertheless, the main features of the measured evolution shape of ions can be explained by the results of simulation, and the best agreement is achieved by the assumption of an enhanced concentration of either acetone or pyridine. In this case, the younger ions should be mainly ions H3O+(H2O)(n), while the more aged ions should be either acetone or pyridine family clusters. (C) 1999 Elsevier Science Ltd. All rights reserved.