Cold air seals in pollution from valley | TahoeDailyTribune.com

Cold air seals in pollution from valley

Gregory Crofton, Tahoe Daily Tribune

It’s a fact that summer weather patterns can push smoke from distant forest fires or smog from the Sacramento Valley into the Lake Tahoe Basin.

It’s also true that colder temperatures in the winter act as a lid, trapping pollution produced by cars motoring down Highway 50.

But does any of that dirty air end up depositing chemicals in the lake? If so, does it have an impact on Tahoe’s clarity?

The California Air Resources Control Board, the Tahoe Regional Planning Agency and variety of agencies from Nevada and California want to find out. They have undertaken a $2 million study to determine what chemicals end up in the lake and whether or not they affect its clarity. Preliminary results of the study are expected next year.

Based on prior studies, air quality experts say pollution from outside the basin gets transported inside almost exclusively in summer. They don’t know yet how much comes in, but they estimate it could account for up to 50 percent of the haze visible over the lake in the summer.

If the CARB study does determine that polluted air from Sacramento has a direct impact on the clarity of Tahoe, would it be the job of the Tahoe Regional Planning Agency to curb that pollution?

The answer is no, according to the TRPA.

“Our responsibility is to do what we can to coordinate with other agencies,” said Jennifer Quashnick, TRPA’s air quality program manager. “We don’t have authority to tell Sacramento what to do.”

Other than working with agencies to improve air quality, the TRPA can also comment on projects that affect the basin. The agency has provided comments on development planned for Martis Valley, an area north of Tahoe just outside the basin and the jurisdiction of the TRPA. The proposed development is expected to impact the traffic flow and the number of vehicle miles traveled in the basin, Quashnick said.

El Dorado County says it too relies in part on the work of other air quality agencies to address pollution that blows east into its back yard.

“The region is moving toward achieving compliance,” said Marcela McTaggart, air pollution control officer for El Dorado County. “It is working together to minimize emissions from mobile sources. Most of it is transported in from the Sacramento region and the Bay Area.”

McTaggart cited a new law that limits bus idling in Sacramento as an example of what is being done to reduce smog in the foothills of El Dorado County.

The research being conducted at the California Air Resources Control Board, called the Lake Tahoe Atmospheric Deposition Study, involves air samples collected west of the basin, on the lake and from the air above it. Results will be part of a pollution-loading amount to be established by the Lahontan Regional Water Quality Control Board to determine how much nitrogen, phosphorous and road dust the lake can absorb without continuing to lose clarity.

The U.S. Forest Service and the TRPA plan to incorporate those numbers, called the “total maximum daily load,” as part of their regional plans that are due to be updated by 2007.

CARB is not the only group studying the air at Tahoe. UC Davis just delivered a report that analyzes the size of particles that polluted air brings with it as it moves over and into the basin.

“My personal opinion based on data is that lake clarity, with the exception of major forest fires, is not being affected by pollutants transported in from outside the basin,” said Tom Cahill, a professor of atmospheric science and physics at UC Davis. “Lake clarity is a local problem.”

Cahill said the cold temperature of the lake forms a layer of air, or an inversion, over the lake every night. In the summer it can act as a shield protecting the lake and allowing pollutants to blow in and blow out of the basin without coming in contact with its water.

In winter, inversions can have the opposite effect and hold pollutants against the lake surface. Still, the dirty air doesn’t have much of an impact on the lake because the concentration of pollutants is low and comprised of fine particles that don’t tend to settle on Tahoe, said Cahill, who has been studying the lake since 1972.


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