A Lagrangian air quality model is developed which represents the airborne particle complex as a source-oriented external mixture. In a source-oriented external mixture, particles of the same size can evolve to display different chemical compositions that depend on the chemical and hygroscopic properties of the primary seed particles initially emitted from different sources. In contrast, previous models initialize the airborne particles as an internal mixture in which all particles of the same size are assumed to have the same chemical composition. Test cases show that representation of the aerosol as an internal mixture can distort the predicted particle composition and concentration in the HNO3/NH3/HCl/H2SO4/aerosol Cl-/SO4=/NO3-/NH4+/Na+ system when Na+ and SO4= exist in separate particles, as may occur when sea spray coexists with long-distance transport of anthropogenic sulfates. Tests also indicate that the external mixture model can predict the evolution of a nearly monodisperse aerosol into a bimodally distributed aerosol as relative humidity increases, qualitatively matching observations. The source-oriented external mixture model is applied to predict the size and composition distribution of airborne particles observed at Claremont, California, on August 28, 1987. Calculations produce an aerosol mass distribution that is distinctly bimodal in the size range from 0.1 μm to 1.0 μm particle diameter, matching field observations. External mixture calculations also predict specific differences in composition between particles of the same diameter. The external mixture model is expected to have applications including exploration of the cause of the particle-to-particle differences seen by time-of-flight mass spectrometers that measure single particle size and composition in the atmosphere.¿ 1997 American Geophysical Union