May 11--Sulfur dioxide pollution in the Yellowstone Valley fell to a new low last year, continuing a steady decline since 2004.

The valley's seven major industrial sources pumped 8,118 tons of sulfur dioxide into the air in 2008, a 10 percent reduction from the year before.

And for the first time, the ConocoPhillips refinery became the valley's smallest source-emitting only 108 tons of sulfur dioxide -- while processing a record amount of crude oil.

ConocoPhillips edged out The Western Sugar Co., whose beet plant historically has been the smallest source. The sugar plant, which operates seasonally and burns coal, emitted 138 tons of sulfur dioxide last year. Jim Hughes, an environmental specialist with the Montana Department of Environmental Quality, attributed the decline to a combination of plant operations and pollution controls.

The latest figures reflect the valley's progress from the pollutant's peak in 1993 at 35,581 tons. In 1993, the Environmental Protection Agency ordered Montana to revise its pollution control plan for the Billings-Laurel area, saying it failed to protect public health. Sulfur dioxide emissions have declined 77 percent since then.

In recent years, EPA reached consent decrees with the three valley refineries to settle allegations that refineries nationwide were violating clean-air laws by expanding operation without proper permitting. The decrees imposed pollution-cutting requirements on the refineries.

The valley's air-quality monitors also continue to show low readings for the long-term annual concentration, Hughes said. "Occasionally we do see one to three hours of elevated S02 readings from time to time, but they're infrequent," he said.

The DEQ received seven complaints last year about the industries, including a recurring complaint about flaring at the CHS refinery in Laurel, Hughes said.

Sulfur dioxide is produced by burning fossil fuels such as coal and fuel oil. The gas is colorless and has a pungent, burning odor. Sulfur dioxide makes breathing difficult for asthmatics and can worsen respiratory disease and cause other health problems. It also is a component of acid rain.

Six of the valley's sources cut their emissions last year; only one, Montana Sulphur & Chemical Co., increased emissions.

ConocoPhillips reduced sulfur dioxide release 6 percent from 2007 while processing a record 21.6 million barrels of oil.

Steve Steach, refinery manager, attributed the decline to burning more natural gas in the boiler house and to less flaring from more reliable operations.