Asian aerosols in North America: Extracting the chemical composition and mass concentration of the Asian continental aerosol plume from long-term aerosol records in the western United States

[1] Empirical assessment of the frequency and intensity of dust transport from Asia to North America has found that the dust regularly impacts elevated sites in the western United States, revealing a pattern of consistent, frequent transport above the marine boundary layer. Using the dust as a marker for Asian transport, a subset of Asian-influenced samples was identified within a decade of routine aerosol samples from two sites in the western cordillera of North America: Crater Lake, Oregon, and Mount Lassen, California. This subset was used to guide a statistical analysis to isolate Asian aerosol against the "background'' of local contaminants. The analysis was then generalized to all samples during the transport season ( March - October) for 1989 - 1999. A mixture of dust and combustion products dominates the Asian aerosol with typical concentration around 5 mug/m(3) and mass median diameter between 2 and 3 mum. Major fine particle (< 2.5 mu m diameter) constituent fractions are similar to 30% mineral, 28% organic compounds, 4% elemental carbon, 10% sulfate, < 5% nitrate, and < 1% sea salt. A second, possibly Asian, component of aged biomass smoke and sea salt is also present, with typical concentration ( when present) around 1.5 mu g/m(3). Averaged over the transport season the dusty Asian aerosol and the smoky aerosol comprise 60 and 6%, respectively, of total particle mass (< 10 mm diameter) and 72 and 13% of fine particle mass at these sites. These data indicate that the Asian continental plume is a significant contributor to aerosol loading at remote high-altitude sites across western North America. This implies a significant influence for Asian emissions on free troposphere aerosols over North America and suggests that they need to be explicitly accounted for in aerosol analyses ranging from climate studies to aerosol regulatory programs.