"Aerosol" means the suspended liquid or solid particles in air (except cloud droplets). "Ambient aerosol" is aerosol that has taken up ambient water through hygroscopic growth. The extent of hygroscopic growth depends on the relative humidity and the composition of the aerosol. The "Angstrom exponent" appears in the formula relating aerosol optical thickness to the wavelength of incident radiation: T(lambda) = T(lambda0) * [lambda/lambda0] ** (-1 * alpha) where alpha is the Angstrom exponent, lambda is the wavelength of incident radiation, lambda0 is a reference wavelength, T(lambda) and T(lambda0) are the values of aerosol optical thickness at wavelengths lambda and lambda0, respectively.