Photographic aureole measurements and the validity of aerosol single scattering.

A refined photographic system for solar aureole measurements is described in which a neutral density filter occulting disk blocks the direct sunlight. Experimental data for solar aureoles are presented for several wavelengths from 309 nm to 640 nm and solar zenith angles of up to 88 degrees . The results are compared with a modified single scattering calculation that includes corrections for Rayleigh multiple scattering, attenuation by ozone, ground reflection, and spherical atmosphere geometry and, at 309 nm, are compared with a Monte Carlo multiple scattering calculation. Useful analytic functions for the aerosol size distribution and for the molecular, ozone, and aerosol height distributions are given. Our near sunset calculation represents an intermediate step between single scattering and full multiple scattering for a Rayleigh-ozone-aerosol atmosphere. We show that aerosol multiple scattering may not be ignored at large solar zenith angles and long wavelengths.