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programs and identification ofthe entities utilizing the various strategies are detailed in Section 3.0. The second phase of the region's air quality improvement program includes a regional commitment to continued air quality monitoring, photochemical modeling, and technical analyses to facilitate design and implementation of additional strategies for future years. Since 1996, ambient air monitoring has been conducted and emission inventories have been compiled for the region. Photochemical modeling of a 1995 episode has also been performed. Modeling and analyses ofa 1999 high ozone episode are underway, with completed studies expected in early 2002. Results ofthis work are expected to enhance the region's ability to select the most effective emission reduction strategies. MOBILES has been used for this work; analysts will use MOBILE61 to evaluate mobile source emission reduction measures. Using the results of this additional technical work, interlocal agreements may be developed among the participants to carry out future programs and measures, or the signatories may choose to amend the existing 03 Flex Agreement. 1.4 Geographic Coverage of the 03 Flex Plan The proposed 03 Flex Plan applies to the five counties included in the Austin/San Antonio Metropolitan Statistical Area (A/SM MSA). These counties are Bastrop, Caldwell, Hays, Travis, and Williamson counties (Figure 1.1). The A/SM MSA is defined by the U.S. Office of Management and Budget (based on data generated by the U.S. Census Office) and is representative ofthe historical basis used by the EPA for defining nonattainment area boundaries. For Central Texas, using the defined MSA is a reasonable and suitable approach to setting the area's 03 Flex Plan boundaries. The predominant sources of anthropogenic VOC and NOx in the region are on-road, non- road, and area. The impacts of, and increases in, emissions from these sources are primarily related to the urban character of the region (e.g., population densities, urban/suburban growth, commuting patterns, etc.). I MOBILE6 is the most recent version (released January 2002) of EPA's emission factor model for on-road mobile sources. Input to the model includes fuel, temperature, speed, driving behavior, and emission reduction control strategies. From this input, the model predicts emission factors expressed as a rate per unit of activity, typically rate per vehicle mile traveled. 4