Search for Cleaner Gas Is Balancing Act
All of the efforts to regulate the formulation of gasoline are designed to reduce two key pollutants that are precursors to smog: nitrogen oxides and hydrocarbons, which react with sunlight to create ozone. And that calls for a delicate balancing act.
“In trying to find the ideal recipe for gasoline, it’s important to understand that basic combustion poses a dilemma for air quality,” Bill Sessa, a spokesman for the California Air Resources Board, said.
“Most measures that improve the combustion of fuel promote the formation of nitrogen oxides, and most that promote reduction of nitrogen oxides decrease combustion efficiency, leading to greater hydrocarbon emissions,” he said. “In many respects, emission control systems balance those things to come up with simultaneous reduction of all of the emissions.”
In attempting to devise a gasoline formula, regulators have focused on a number of key elements of gasoline:
* Volatility, or the tendency of gasoline to evaporate. The higher a gasoline’s volatility, the easier it burns, but the more likely it is to generate hydrocarbon emissions through evaporation. Lowering volatility reduces such emissions.
* Aromatics, high-octane components of gasoline. When burned, they give off nitrogen oxide and benzene, a highly toxic and carcinogenic chemical. Lowering aromatics lowers the peak combustion temperature of gasoline, resulting in fewer nitrogen oxide emissions.
* Olefins, another high-octane hydrocarbon component of gasoline. They are highly reactive with sunlight. Lowering olefins lowers the portion of the hydrocarbon mix that is the most likely to produce ozone.
* Oxygen, in the form of oxygenates such as methyl tertiary butyl ether (MTBE), ethanol or other chemicals. Increasing the level of oxygen in gasoline boosts octane and encourages more efficient burning, thereby lowering hydrocarbon emissions and providing an alternative to aromatics. But some oxygenates may increase levels of nitrogen oxide emissions, and data is incomplete.
