Photometer-based aerosol phase functions for simulating the internal radiation field in smoky atmospheres

Over the last two decades and due to rapid economic growth, South-East Asia experiences mild to severe smoke episodes generated from annual biomass burning, usually occurring during the dry season period (from June to October). In Singapore, in-situ aerosol measurements are now performed by our AERONET sun photometer site since 2006. Such measurements readily provide optical characterization of atmospheric aerosols, such as optical depth and Angstrom number, as well as other physical properties such as refractive index, single scattering albedo, particle size distribution and scattering phase function when available. However, realistic aerosol phase functions can be quite complex and sometimes prohibitive to use in full radiative transfer calculations. This is due to the fact that many radiative transfer codes use Legendre polynomials and/or generalized spherical functions to represent the aerosol phase function. In such a cases, the number of expansion terms, for realistic aerosol particles, could be very large. In this work, a double Henyey-Greenstein function approximation is adopted to generate aerosol phase functions from level 1.5 AERONET measurements during smoke/smoke-free periods.