Atmospheric aerosol over Alaska - 1. Spatial and seasonal variability

Aerosol chemical composition data for seven National Park Service locations in Alaska for the period from 1986 to 1995 were analyzed. Data for elements, black carbon, and total mass were obtained for samples of atmospheric particles less than 2.5 mu m in diameter. Assuming that all elemental aerosol S was from SO4=, concentrations of excess (nonmarine) sulfate (XSO4=) were calculated by subtraction of the sea salt SO4= mass fraction to Na and observed Na concentrations. Mass concentration of XSO4= ranged from 0.01 to 3.9 mu g m(-3), and mass concentration of black carbon (BC) ranged from 0.01 to 5 mu g m(-3). Seasonal variations with maxima during the winter/spring season and minima in the summer for most of the sites in Alaska were observed for the XSO4= but not for the BC and the fine mass concentration. The seasonal variations of XSO4= concentration were always higher in northwestern Alaska. BC concentration in central Alaska peaked during the summertime because of the high BC emission from forest fires. The Pb concentration in central Alaska shows a negative trend over the timescale of measurements. It is concluded that all of the sites experience aerosol pollution from long distance, regional, and local sources. Seasonal variations of the XSO4= concentration are related to the seasonal variations in long-range transport of midlatitudinal S emissions, to the varying rate of photochemical SO4= fine aerosol production and to fine aerosol removal processes in the troposphere. The spatial distribution of XSO4= shows a strong northwest to southeast negative gradient that can be associated with the long-range transport of anthropogenic aerosol from pollution sources located to the north or northwest of Alaska.