Hazy Sky Linked to Air Conditioning

And while it's mainly power plants that churn out sulfur dioxide gas that becomes microscopic particles of sulfate, Cahill ties the increased summer electricity demand to the rapid growth of whole-house air conditioning, at least in the area studied.

During particularly bad summer episodes, typically several weeks in July and August, the regional haze peaks along with higher electricity demands for air conditioning during especially hot, humid days -- the same weather conditions that maximize the conversion of sulfur dioxide gas into the tiny sulfate particles that muddy distant images.

Cahill presented his information at a conference last month for the Air & Waste Management Association, representing industry as well as federal and state regulatory agencies.

Almost all sulfate originates as sulfur dioxide emissions from coal-burning power plants and other industrial sources, Cahill says. After escaping from smokestacks, invisible sulfur dioxide molecules can transform into much larger, view-busting sulfate molecules, especially in humid regions. Sulfate also is a main ingredient in acid rain.

The new analysis confirms that the East has been steadily losing its struggle to maintain postcard-perfect views of favorite summer vacation spots, even though U.S. sulfur dioxide emissions have been decreasing slightly. From 1982-1992, summer sulfate hazes in Great Smoky Mountains National Park, Tenn., soared almost 40 percent. Air quality in Shenandoah National Park, Va., was even worse. Summer concentrations at the eastern sites peaked in 1990 and 1991 and have began to level off or drop.

Cahill credits this recent change toward improved views during the past three years to reduced sulfur dioxide emissions from several power plants -- although it's too early to tell if this is a reversal of the trend toward hazier skies.

Recent summer sulfate levels have held steady at Shenandoah and dropped by roughly 30 percent at Great Smoky Mountains. In 1994, the large TVA Cumberland power plant was efficiently "scrubbed," reducing annual sulfur dioxide emissions from roughly 200,000 tons a year to 20,000 tons a year. Because this plant used to spew forth 1 percent of the total U.S. sulfur dioxide emissions, Cahill speculates that this control effort already has reduced summer sulfates at Great Smoky Mountains and other sites in the Appalachian Mountains.

The new study also answers lingering questions about a key scientific assumption behind the 1990 Clean Air Act amendments -- that air quality will improve in proportion to reduced emissions. "There's no substitute for removing the problem at the source," Cahill says.

The air over public lands has special visibility protection. When it revised the Clean Air Act, Congress mandated sulfur-dioxide emission controls, believing the visibility problem, as well as the acid rain problem, would decline in proportion to the reduced emissions. The new UC Davis study suggests that it may, indeed, be that simple.

In previous studies in the western United States, the UC Davis Air Quality Group has found similar reductions in regional sulfate levels with reduced or eliminated sulfur dioxide emissions from nearby sources.