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Seasonal Variation in In-Use Emissions
We used multiple years of data from several states to examine average daily emissions from initial IM240 tests of passenger cars. We compared seasonal variation in average daily emissions with maximum daily temperature and changes in fuel composition (winter oxygenates in Arizona and Colorado, reformulated gasoline (RFG) in Wisconsin, late introduction of RFG in Arizona). There is a large seasonal variation in average emissions and I/M failure rates; The figure below demonstrates how the trend in average daily IM240 CO emissions (circles) mirrors the trend in maximum daily temperature (gray lines) in Phoenix. However, these variations are not consistent across I/M programs. These results are summarized in an October 28, 1999 memo to EPA. We also compared emissions by second from Arizona vehicles tested in the winter with those from vehicles tested in the summer. Seasonal trends suggest an emissions impact due to changes in fuel composition, but more research is necessary.
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