The USEPA has proposed to regulate PM(coarse) (particulate matter 2.5 to 10 microm in diameter). Exceedance of the proposed National Ambient Air Quality Standard (NAAQS) for PM(coarse) is expected within the Columbia Plateau of the Pacific Northwest United States based on the high frequency of dust storms and the large contribution of crustal material to fugitive dust in the region. The objective of this study was to explore the implication of the proposed NAAQS for PM(coarse) on air quality. Concurrent observations of both PM10 (particulate matter </=10 microm in diameter) and PM(fine) (particulate matter </=2.5 microm in diameter) were made at Kennewick, WA from 1999 through 2005. Daily PM(coarse) concentration was determined as the difference between PM10 and PM(fine) concentrations. The number of exceedances of the proposed USEPA NAAQS for PM(coarse) was determined for various levels of the standard (the proposed daily level is 70 microg m(-3)). Over the 7 yr of this study, the PM10 standard was exceeded on 16 d with PM(fine) constituting 4 to 7% of PM10. The proposed PM(coarse) standard would have been exceeded on 35 d and represents a 120% increase in the number of exceedances over the current PM10 standard. Changing the level of the proposed PM(coarse) standard to that of the current PM10 standard (150 microg m(-3)) would result in a 20% decrease in the number of exceedances of the PM standard. The results of this study suggest that the proposed NAAQS for PM(fine) and PM(coarse) will be exceeded more frequently than the current PM(fine) and PM10 standard in a region subject to seasonal dust storms.