A new microphysical model of aerosol size spectra accounting for hygroscopic growth is used to calculate the aerosol scattering and absorption coefficients of solar and longwave radiation. Dependence of the extinction coefficient is explored theoretically, starting from the Junge <1963> power spectra of the dry aerosol and the Kohler equation <Khvorostyanov and Curry, this issue>. It is shown that the general dependence of the aerosol scattering and absorption coefficients on λ and H can be presented in the form &sgr;~λ-&ggr;(1-H)R. This model explains both the empirical angstrom wavelength law and Kasten <1969> Hanel <1976> humidity law and identifies a functional relationship between them. The empirical coefficients of these laws are related to the aerosol microstructure and to each other by a very simple relationship. An explanation is offered for the spectral behavior of the scattering coefficient and its variation with relative humidity for various types of atmospheric hazes. It is shown that at very high humidities a dense haze with visibility less than 1 km may form under subsaturated conditions and without activated droplets. Application of this model to the longwave spectrum allows us to explain quantitatively the negative temperature dependence of atmospheric absorption in the atmospheric window without depending on the presence of water vapor dimers. Recommendations for aerosol optical and microphysical measurements are given. This simple model can be used as a tool in remote sensing for evaluation of aerosol scattering and absorption optical thicknesses and for parameterization of aerosol optical properties in general circulation and cloud models. ¿ 1999 American Geophysical Union