* Corresponding author. Tel.: #1-970-491-3700; fax: #1- 970-491-8598. E-mail address: ames@cira.colostate.edu (R.B. Ames). Atmospheric Environment 35 (2001) 905 } 916 Comparison of sulfate and nitrate particle mass concentrations measured by IMPROVE and the CDN Rodger B. Ames*, William C. Malm Cooperative Institute for Research in the Atmosphere (CIRA), Colorado State University, Fort Collins, CO 80523, USA Air Resources Division, National Park Service, CIRA, Colorado State University, Fort Collins, CO 80523, USA Received 1 November 1999; received in revised form 3 July 2000 Abstract 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 di!erences in the respective network's sampling protocols. A number of sampling protocol di!erences exist between the two networks that may lead to sampling bias, particularly for particle NO  . Observed di!erences between particle SO  mass concentrations reported by the two monitoring networks are generally small, yet statistically signi"cant at many comparison sites. Di!erences between particle NO  mass concentra- tions are substantial, statistically signi"cant at nearly all comparison sites, and the bias magnitude varies by geographic region. Di!erences in particle NO  , 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.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Aerosol monitoring; CASTNet; Measurement bias; Dry deposition; Visibility 1. Introduction The Interagency Monitoring of PROtected Visual En- vironments (IMPROVE) Network (Malm et al., 1994) began operation in 1988 following a cooperative agree- ment between the United States Environmental Pro- tection Agency (EPA) and federal land management agencies to establish a comprehensive visibility monitor- ing network. Many IMPROVE monitoring sites are located where aerosol samplers from the Stacked Filter Unit (SFU) Network (Flocchini et al., 1981) had been in operation previous to IMPROVE. The Clean Air Status and Trends Network (CAST- Net), or CASTNet Deposition Network (CDN) was established as a result of the 1990 Amendments to the Clean Air Act with the goal to determine the e!ect of emissions reductions mandated by the Act on air quality and deposition. Monitoring sites of the National Dry Deposition Network (NDDN), (CASTNet Deposition Summary Report, 1998), itself established by the EPA in 1986, became a part of the CDN in 1992. Beginning in 1994 monitoring at 17 primarily western United States national parks began under the CDN, National Park Service (NPS), and EPA. IMPROVE is primarily a visibility monitoring net- work, while the CDN design is to monitor dry deposition #ux. Each network has sampling protocols distinct from the other, although they have in common measurement of airborne species of interest to both visibility and 1352-2310/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 3 6 9 - 1