The population balance equations for simultaneous coagulation and deposition of free molecule aerosols in turbulent pipe flow have been cast in dimensionless form and integrated to obtain a simplified two-equation model. The relative importance of coagulation and deposition is shown to depend upon a dimensionless parameter A, which is a ratio of a characteristic deposition time to a characteristic coagulation time. Solutions to these equations are presented graphically and range from the coagulation-dominated to the deposition-dominated regime. This analysis accounts for the enhancement in coagulation rates due to London-van der Waals attractive forces; this effect is often important for free molecule aerosols. In a separate paper (Brockmann et al., 1982) experimental data are shown to be in good agreement with theoretical predictions.