Airshed Calculation of the Sensitivity of Pollutant Formation to Organic Compound Classes and Oxygenates Associated with Alternative Fuels

Abstract

In response to the desire to allow the use of cleaner-burning automotive fuels, and at the same time to provide an equitable regulatory framework for comparing specific fuel/vehicle combinations, recent action by the California Air Resources Board proposed a reactivity-weighting for calculating the allowable automobile exhaust mass emission rate of non-methane organic gases. The "ozone-forming potentials" of individual organic compounds, times the mass fraction of those components in the exhaust, are summed to find the net ozone forming potential for a fuel/vehicle combination. In this way, the ozone production potentials of various alternative fuels (and reformulated gasolines) can be directly compared. Currently, the ozone-forming potentials of individual organics are calculated using the results of an average of about 35 episodes simulated with a chemically detailed, 1-dimensional model over short (less than one day) modeling periods. At question is whether using a physically detailed, three-dimensional model and multi-day simulations of a single severe episode gives comparable calculated ozone formation sensitivity, and also what the impact of changing emissions would have on other pollutants such as NO₂, PAN and formaldehyde.

This study uses a 3-D Eulerian photochemical model and an advanced chemical reaction mechanism to evaluate the sensitivity of pollutant levels to changes in emissions. In particular, the ozone forming potentials of classes of organic compounds are calculated, with particular emphasis on oxygenated organics associated with alternative fuels. Methanol, ethanol, MTBE, alkane and toluene emissions were found to add about one-fifth the ozone (on a carbon mass basis) as alkenes, aldehydes, non-toluene aromatics and ethene. On a per-carbon basis, formaldehyde added about ten times as much ozone as the least reactive organics tested. The results of the trajectory modelbased study usually compare well with those found here. The pollution formation potentials can now be used in assessing the relative impact of various exhaust gas compositions.