• visibility;
  • soil

[1] We estimated the contributions of windblown dust from nearby area sources to dust concentrations at Class I areas in the western United States including Alaska and Hawaii. The approach utilized multivariate linear regression of dust concentrations against categorized wind conditions (wind direction and speed) for all 2001–2003 data for 70 Interagency Monitoring of Protected Visual Environments sites. Statistically significant associations between dust concentrations and at least one of the wind variables were found at 41 sites with correlation coefficients as high as 0.97. At some sites, primarily in New Mexico and Texas, windblown dust from nearby sources accounted for up to 3 μg m−3 over the 2001–2003 period. In addition, the impact of local windblown dust sources during the 20% worst visibility days when dust was the major component of visibility reduction (worst dust days) was examined. A total of 608 worst dust days were identified for 2001–2003, mostly at Class I areas in southwestern states during spring and summer with 24-h average dust concentrations as high as 153 μg m−3. Windblown dust from local sources was present with statistical confidence on many of the worst dust days at sites in New Mexico, Utah, Colorado, southern Texas, and Death Valley in California. A smaller percentage of worst dust days were associated with local windblown dust in Arizona and other sites in southern California, suggesting either nonwindblown or distant sources of dust. The methods discussed can serve as a useful, semiquantitative tool for identifying sites where local wind conditions affect dust concentrations.