The Northwest Regional Modeling Center (NWRMC) demonstration project was undertaken to determine the feasibility of using the Community Multiscale Air Quality (CMAQ) modeling system to investigate regional haze in the Pacific Northwest. The project was also aimed at establishing the NWRMC as a virtual modeling center accessible to all Northwest air quality stakeholders. This was a collaborative effort between the Environmental Protection Agency (EPA) Region 10, Washington State University (WSU), Washington State Department of Ecology, Idaho Department of Environmental Quality (ID DEQ), and Oregon Department of Environmental Quality (OR DEQ) to address regional haze and visibility problems in the Pacific Northwest. Canada also played a role in the project because of the common border and problems often experienced with data inconsistencies between the countries. The approach of the NWRMC involved two phases. In the first phase, results from CMAQ were compared to results from CALGRID, as discussed in O?Neill and Lamb (2002), to simulate ozone formation and transport along the I-5 corridor of western Washington and Oregon. The second phase of the project involved expanding to a regional scale domain to investigate aerosol formation and transport. The period of July 1-16, 1996 was selected as the study focus period.