The direct radiative effect of smoke aerosols on atmospheric absorption of visible sunlight

Abstract

Smoke aerosol has a significant effect on the atmospheric radiation budget due to its strong absorbing properties. Observational studies of the smoke radiative effect (SRE) usually suffer from a shortage of in-situ measurements of aerosol optical properties. This study introduces a new approach to determine SRE, i.e. the amount of solar energy absorbed in the atmosphere, under any sky conditions, using satellite and surface measurements. The method requires no observation of aerosol optical properties. It is based on a satellite inversion algorithm that retrieves surface net solar radiation in the visible spectrum (400–700 nm). The algorithm was first validated under a variety of sky conditions ranging from clear, to smoky, and to cloudy skies. It is found that the accuracy of retrieval is affected only by absorbing aerosols such as smoke. Without correction for this effect, the difference between observed and estimated fluxes is a good estimate of SRE. Following this approach, instantaneous, daily and monthly mean SRE were computed, over a study site in the remote boreal forest region of western Canada, where fire activities dominate the variation of aerosol loading during the summer season. The monthly and daytime mean SRE reaches a maximum value of 26.0 W m^-2 during a period of active burning in July 1994, in comparison to a total deduction of solar radiation budget by both clouds and smoke of 76.7 W m^-2 at the surface within the specified visible solar spectral spectrum.