Perturbation to the Atmospheric Radiation Field from Carbonaceous Aerosols

Abstract

Carbonaceous aerosols from combustion processes enter the atmosphere in the form of submicron particles. Subsequent processes involving coagulation and wet and dry removal give rise to a microparticle size spectrum whose shape continually evolves as the aerosol-laden air mass ages and disperses. After a transport time of ~10–20 days, an asymptotic size distribution function seems to be approached, usually consisting of a single particle size mode with about 80% of the particle mass contained between particle radii limits of 0.06 to 0.3 µm; particles smaller than this range have been removed by coagulation or molecular diffusion processes while those larger have been removed by sedimentation, impaction and nucleation. It is found, empirically, that the carbonaceous aerosol at distant “background” locations, like the Arctic, are mixed with a sulfate aerosol which derives mainly from the nucleation of natural and anthropogenic trace sulfur-bearing gases. In the Arctic, the carbonaceous aerosols constitute 10–30% of the aerosol mass, while in the southern polar regions the percentage is much smaller, presumably due to the remoteness of anthropogenic and natural combustion sources.

The quantity of particles in the air column above the northern polar regions and their size is such that, apparently, significant interactions can occur between the particles and visible band radiative fluxes passing through the atmosphere. This raises the possibility that interactions with the radiation field may influence terrestrial climate by introducing heating into the earth-atmosphere system. It also introduces an opportunity to employ passive ground-based measurements of certain atmospheric optical parameters to deduce characteristics of the aerosols. The theory of determining aerosol parameters is described briefly with the aid of a two-stream approximation to the equation of radiative transfer and its use is illustrated for optical data taken near Fairbanks during an episode of Arctic-derived haze. It has been deduced that the albedoof single scattering for Arctic haze is lower than had been expected—about 0.6 to 0.8—and that the optical thickness of the arctic aerosol during the spring months can be as large as 0.25 (at 500 nm wavelength). The cause of the arctic haze phenomenon seems to be associated with anthropogenic emissions at the mid-latitudes.