Regional Differences in Ozone
Region 8: Montana, North Dakota, South Dakota, Wyoming, Utah, and Colorado
Local Sources of VOCs
The largest human-generated source of VOCs in Region 8 is solvent use, at 31 percent, higher than the nation as a whole. Highway vehicles contribute just over one-fourth at 27 percent, lower than the 29 percent produced nationally. Off-highway vehicle use, the third largest source, is 18 percent, the same as the national rate. In 1999, Region 8 produced 778,485 tons of VOCs, the second lowest of all the regions.
Local Sources of NOx
The largest sources of NOx emissions in Region 8 were electric utilities and off-highway vehicles, which each generated one fourth of the total of 1.5 million tons in 1999. Nearly another fourth came from highway vehicles (23%). While the electric utility and off-highway contributions are higher than the nation as a whole, the highway vehicle contribution is lower by ten percent (23% v 33%). Industrial fuel combustion in the region makes up a much larger proportion, at 20 percent, than it does nationwide (12%). Industrial sources are less of a factor than in the nation as a whole (2% v 4%).
Trends
Monitored ozone levels dropped by 9 percent between 1982 and 2001, a trend slightly behind the nation as a whole, which dropped 11 percent during the same period. Furthermore, the long-term decline could have been greater had not the region’s ozone levels dropped by only 1 percent in the 1990s.
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