MODELING THE AIRBORNE PARTICLE COMPLEX AS A SOURCE-ORIENTED EXTERNAL MIXTURE

A Lagrangian air quality model is developed which represents the airbo rne particle complex as a source-oriented external mixture. In a sourc e-oriented external mixture, particles of the same size can evolve to display different chemical compositions that depend on the chemical an d hygroscopic properties of the primary seed particles initially emitt ed from different sources. In contrast, previous models initialize the airborne particles as an internal mixture in which all particles of t he same size are assumed to have the same chemical composition. Test c ases show that representation of the aerosol as an internal mixture ca n 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 in dicate that the external mixture model can predict the evolution of a nearly monodisperse aerosol into a bimodally distributed aerosol as re lative humidity increases, qualitatively matching observations. The so urce-oriented external mixture model is applied to predict the size an d 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 mu m to 1.0 mu m particle diameter, matching field observations. Externa l mixture calculations also predict specific differences in compositio n between particles of the same diameter. The external mixture model i s expected to have applications including exploration of the cause of the particle-to-particle differences seen by time-of-flight mass spect rometers that measure single exist in separate particles.