SENSITIVITY OF URBAN AIRSHED MODEL RESULTS FOR TEST FUELS TO UNCERTAINTIES IN LIGHT-DUTY VEHICLE AND BIOGENIC EMISSIONS AND ALTERNATIVE CHEMICAL MECHANISMS - AUTO OIL AIR-QUALITY IMPROVEMENT RESEARCH-PROGRAM

Three sources of uncertainty in the air quality modeling performed for the Auto/Oil Air Quality improvement Research Program, Phase I, were investigated to assess their impact on predicted ozone for test fuels in Los Angeles in year 2010. First, quadrupling the estimated total organic gas (TOG) and tripling the CO emissions from light-duly gasoline vehicles in the air quality model increases the predicted peak ozone, as expected. The percent increase in peak ozone for the test fuels, about 25% of total ozone, is essentially the same as the percent increase in TOG emissions, about 25% of the total emissions from all sources. However, there is no important effect on the ranking of the test fuels from lowest to highest in predicted ozone formation. Second, replacing the original biogenic emission inventory with an alternative inventory having substantially lower biogenic emissions reduces the predicted peak ozone. The percent decrease in peak ozone, about 6% of total ozone, is considerably less than the percent decrease in TOG emissions, about 34% of total emissions. Fuel rankings are unchanged except for a reversal of two test gasolines in the ranking based on peak ozone. However, this reversal is not found in fuel rankings based on other measures of ozone formation. Third, replacing the Carbon Bond Mechanism version IV (CBM-IV) in the air quality model with an alternative representation of atmospheric chemistry, the Statewide Air Pollution Research Center (SAPRC) mechanism, increases the peak ozone by about 9%. There are also important changes in fuel rankings. For one research test gasoline, the contribution of light-duty gasoline vehicles to ozone is similar with both chemical mechanisms, but for another test gasoline that gives the lowest ozone with the CBM-IV, the contribution of light-duty gasoline vehicles to ozone is substantially higher with the SAPRC mechanism. With the CBM-IV mechanism, the most promising of the test gasolines studied has lower predicted ozone than any of the cases representing use of methanol fuels in prototype, flexible/variable fuel vehicles. With the SAPRC mechanism, the most promising test gasoline studied has lower predicted ozone than one methanol case and higher ozone than the other methanol case. These changes in fuel rankings are probably due to known differences in the reactivity of toluene and formaldehyde in the two mechanisms.