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particulate matter formation processes in Southeast Texas Progress Summary/Accomplishments: Measurements for the Houston fine particulate matter supersi e were completed in late 2001. Continuing work is focussing on data analysis and this progress report describes analysis of data collected in the Washburn tunnel during the August – September 2000 intensive sampling period. The goal of the sampling conducted in the Washburn Tunnel was to determine fuel-based emission factors for the Houston on-road fleet; this was accomplished and, in this report, the fuel based emission factors for Houston are compared to emission factors determined in other tunnel studies. 3Overview Measurements collected in a Houston tunnel were used to develop CO, NOx, and NMHC (non-methane hydrocarbon) emission factors for on-road gasoline vehicles. Samples collected in the tunnel were also used to develop chemical fingerprints of fine particulate matter emitted by on-road diesel vehicles