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.