Comparison of sulfate and nitrate particle mass concentrations measured by IMPROVE and the CDN

The 1977 and 1990 Amendments to the Clean Air Act call for visibility and atmospheric deposition monitoring throughout the United States. We compare sulfate and nitrate particle mass concentrations measured by two regional air quality networks, the Interagency Monitoring of PROtected Visual Environments (IMPROVE) network and the Clean Air Status and Trends Network (CASTNet), or CASTNet Deposition Network (CDN). The intent of this comparison is to quantify bias that may be introduced from differences in the respective network's sampling protocols. A number of sampling protocol differences exist between the two networks that may lead to sampling bias, particularly for particle NO₃^?. Observed differences between particle SO₄^2? mass concentrations reported by the two monitoring networks are generally small, yet statistically significant at many comparison sites. Differences between particle NO₃^? mass concentrations are substantial, statistically significant at nearly all comparison sites, and the bias magnitude varies by geographic region. Differences in particle NO₃^?, based on data from monitoring sites selected for this comparison, are 40% in the west, 56% in the interior desert/mountain region, and ?9% in the east, expressed as the IMPROVE mean subtracted from the CDN mean, as a percent of the IMPROVE mean. Comparisons are made using data from 23 locations where monitoring sites from IMPROVE and CDN are within approximately 50 km.