DETERMINING MARINE AEROSOL SCATTERING CHARACTERISTICS AT AMBIENT HUMIDITY FROM SIZE-RESOLVED CHEMICAL-COMPOSITION

Although the variation of aerosol optical properties with humidity dep ends critically on particle size and chemical composition, existing mo dels usually employ empirical growth factors for water uptake. For som e size ranges and many types of particles these empirical factors are not well known. We describe here a model that applies a thermodynamic analysis to arbitrary aerosol chemical size distributions to compute w ater uptake, refractive index, number distribution, and optical extinc tion. By starting with the fundamental chemistry of the aerosols the m odel is able to compute the derivative of extinction with sulfate, sul fate plus ammonium, and other scenarios that may be derived from globa l change and pollution control strategies. It can also supply informat ion on tile variation of aerosol size and extinction with humidity and with details of the aerosol size distribution. We demonstrate this mo del using five detailed chemical size distributions measured with a ca scade impactor in the marine boundary layer. The results are in good a greement with published extinction coefficients and with nephelometer data collected in concert with one of our samples. Calculated iron-sea -salt sulfate mass-scattering efficiencies of our samples ranged from 0.6 to 2.6 g m(-2) for dry particles and 5.3 to 13 g m(-2) at 80% rela tive humidity.