On the Aerosol Particle Size Distribution Spectrum in Alaskan Air Mass Systems: Arctic Haze and Non-Haze Episodes

Abstract

Aerosols in central Alaskan winter air mass system were classified according to size by diffusive separation and light-scattering spectrometry. Particles entering central Alaska from the Pacific Marine environment had number concentrations ranging from 300 to 2000 cm?3 (geometric mean 685 cm?3) and unimodal size spectra, with maximum in number concentration near 1 ? 10?6 cm radius. Air masses entering Alaska from the Eurasian Arctic possessed a factor of two smaller aerosol number concentrations than Pacific Marine systems (e.g., 150?700 cm?3; geometric mean 386 cm?3) but contained a factor of two greater particle volume loading within the fine particle radius range ?5 ? 10?7 < r < 1 ? 10?5 cm. The particles in Eurasian Arctic air masses were bimodally distributed, with maxima in the particle size spectra near r = 3 ? 10?7 and 5 ? 10?6 cm. Sulfur was the predominant element in all cases studied. A particle depleted region was present in the size spectra obtained for Eurasian Arctic air masses. The deficiency of particles in the 10?6 cm radius range is interpreted as being the result of thermal coagulation taking place between sulfur-rich nuclei (produced at a rate of 10?20 to 10?18 g cm?3 s?1 and in sizes r < 10?6 cm) and ?large? (r ? 10?5 cm) imported primary particles. The primary particles are in the removal-resistant Greenfield Gap (r ? 10?5 cm) and seem to originate in the central Eurasian region.