In this study, all available PM2.5 sites in the United States with more than 5 years of Federal Reference Method data are studied. The critical design values (CDV) for each site are calculated. The CDV concept developed by Chu (Chu, S.-H., 2000. CDV estimation and its applications. Presented at the 93rd AWMA Annual Meeting, San Diego, CA) has proven to be useful in PM10 Limited Maintenance Plan applications (US EPA, 2001. PM10-NAAQS Implementation-Guidance Document-Limit Maintenance Plan Option for Moderate PM10 Nonattainment Areas. US EPA, Research Triangle Park.) because of its ability in predicting the likelihood of future violations of the National Ambient Air Quality Standards (NAAQS) based on the existing design values (DV) and their inter-annual variability. Thus, the potential for future violations of PM2.5 standards among these sites can be estimated and compared. The results suggest that given the current PM2.5 NAAQS, most of the high-risk areas of potential future violation of the annual standard are in the East and California. However, only California and a few isolated areas in the West are at risk of violation of the 24-h standard in the near future. The higher risk of violating the PM2.5 annual NAAQS in the East and California is largely due to the existing high level of average design value (ADV) concentrations. On the other hand, the higher risk of violation of the 24-h NAAQS in the West (essentially California) is largely attributable to inter-annual variability, particularly in natural emissions such as wildfires, since these events play a significant role in the quick rise of short-term PM2.5 levels in the West. The more frequent occurrence of wildfires in the West is known to be associated with its much drier climate and frequent droughts. On the other hand, higher SO2 emissions in a more humid East, particularly in the summer, lead to much higher sulfate concentrations and enhanced secondary organic aerosol production (Chu, S.-H., 2004. PM2.5 episodes as observed in the speciation trends network. Atmospheric Environment 38, 5237-5246). Thus, the much higher annual PM2.5 DVs in the East are largely due to the high level of sulfate and organic aerosol concentrations. These results may be used by the regulators to identify seriously polluted areas and prioritize their regional control strategies. Published by Elsevier Ltd.
