This paper describes the chemical, aerosol, and meteorological measurements taken at Mount Bachelor Observatory (MBO), a new mountaintop site in central Oregon, USA (44.0°N, 121.7°W, 2763 m above mean sea level). During the initial campaign (28 March to 19 May 2004) we evaluated the utility of this location as a site to observe the global atmosphere, especially the free troposphere (FT). We observed some boundary layer (BL)/upslope flow during the daytime, which produced a 37–62% higher average water vapor mixing ratio (WV) compared to radiosonde data taken from similar altitudes in western Oregon. However, recently subsided air masses with low WV contained significantly elevated concentrations of carbon monoxide (CO), total gaseous mercury (TGM), and ozone (O3), (25 ppbv, 0.16 ng/m3, and 13.9 ppbv, respectively), compared to periods with high‐WV measurements. These enhancements represent a significant influence from Asian long‐range transport (ALRT). One specific time period (“event”) on 25 April produced some of the largest CO, TGM, O3, and particulate scattering (σsp) measurements ever seen along the West Coast due to ALRT. Enhancement ratios between these species are consistent with the major source being Asian industrial emissions. In particular, the ΔTGM/ΔCO enhancement ratio was 0.0045–0.0048 ng/m3/ppbv for all ALRT events, similar to the value previously obtained from pollution plumes directly downwind of east Asia. A smaller pollution event of North American origin produced a ΔTGM/ΔCO value of only 0.0013 ng/m3/ppbv, suggesting that the ΔTGM/ΔCO value is an effective tracer of ALRT. Finally, thousands of kinematic back trajectories were calculated for each event to evaluate sources and transport processes.

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In this study we have evaluated the role of wildfires on concentrations of fine particle (d < 2.5 microm) organic carbon (OC) and particulate mass (PM2.5) in the Western United States for the period 1988-2004. To do this, we examined the relationship between mean summer PM2.5 and OC concentrations at 39 IMPROVE sites with a database of fires developed from federal fire reports. The gridded database of area burned was used to generate a database of biomass fuel burned using ecosystem-specific fuel loads. The OC, PM2.5, and fire data were evaluated for five regions: Northern Rocky Mountains (Region 1), Central Rocky Mountains (Region 2), Southwest (Region 3), California (Region 4), and Pacific Northwest (Region 5). In Regions 1, 2, and 5, we found good correlations of seasonal mean PM2.5 concentrations among the sites within each region. This indicates that a common influence was important in determining the PM concentration at all sites across each region. In Regions 1 and 2, we found a significant correlation between PM2.5 and both the area burned and biomassfuel burned in each region. This relationship is statistically significant using either the area burned or fuel burned, but the correlations are stronger using the biomass fuel burned. In all five regions we found a statistically significant relationship between biomass burned and organic carbon. Using these relationships, we can estimate the amount of PM2.5 due to fires in each region during summer. For the Regions 1 through 5, the average summer-long enhancement of PM2.5 due to fires is 1.84, 1.09, 0.61, 0.81, and 1.21 microg/m3, respectively, and approximately twice these values during large fire years.