Atmospheric aerosol over Alaska - 2. Elemental composition and sources

The fine particle (< 2.5 mu m) composition data from seven National Park Service locations in Alaska for the period from 1986 to 1995 was performed using a new type of factor analysis, positive matrix factorization (PMF). This method uses the estimates of the error in the data to provide optimum data point scaling and permits a better treatment of missing and below detection limit values. Eight source components were obtained for data sets from the Northwest Alaska Areas and the Bering Land Bridge sites. Five to seven components were obtained for the other Alaskan sites. The solutions were normalized by using aerosol fine mass concentration data. Squared correlation coefficients between the reconstructed mass obtained from aerosol composition data for the sites and the measured mass were in the range of 0.74-0.95. Two factors identified as soils were obtained for all of the sites. Concentrations for these factors for most of the sites have maxima in the summer and minima in the winter. A sea-salt component was found at five locations. A factor with the highest concentrations of black carbon (BC), H+, and K identified as forest fire smoke was obtained for all data sets except at Katmai. Factors with high concentrations of S, BC-Na-S, and Zn-Cu were obtained at all sites. At three sites, the solutions also contained a factor with high Pb and Br values. The factors with the high S, Pb, and BC-Na-S values at most sites show an annual cycle with maxima during the winter-spring season and minima in the summer. The seasonal variations and elemental compositions of these factors suggest anthropogenic origins with the spatial pattern suggesting that the sources are distant from the receptor sites. The seasonal maxima/minima ratios of these factors were higher for more northerly locations. Four main sources contribute to the observed concentrations at these locations: long-range transported anthropogenic aerosol (Arctic haze aerosol), sea-salt aerosol, local soil dust, and aerosol with high BC concentrations from regional forest fires or local wood smoke. A northwest to southeast negative gradient suggesting long-range transport of air masses from regions north or northwest of Alaska dominated the spatial distribution of the high S factor concentrations.