Soot Affects Snow, Skies in California

American Geophysical Union, Fall Meeting 2007

Air pollution falls to earth in Sierra Nevada snowfall and blankets the skies over California's valleys. Researchers at Scripps Institution of Oceanography at UC San Diego have made direct measurements of black carbon amounts in snow and rain in Northern California and found that the pollution is probably enhancing snowmelt.

Scripps graduate student Odelle Hadley and atmospheric science researchers Craig Corrigan and V. Ramanathan conducted an analysis of snow collected from the Sierra Nevada and rain collected from a coastal area near the California-Oregon border. They found that trace amounts of soot and other forms of black carbon were present in the snow. While direct measurements of the albedo, or brightness, of the snow are difficult to make, computer models suggest that the observed levels of black carbon would have a growing impact as the snowpack ages. Similarly significant levels of black carbon were present in the coastal rain.

"What we are finding is that almost all of this black carbon is getting entrained in the snow," said Ramanathan. "It's good that the air is cleaning itself but it's also dumping the black carbon all into the snowpack."

The finding implies that pollution is causing the dirty snow to melt faster, contributing to a well-documented trend of earlier river runoff in the Sierra Nevada and other Western mountain ranges. Disruptions of runoff timing can have serious consequences for municipal and agricultural users of water in the West.

"The bottom line is there is measurable black carbon that we can observe," said Hadley. "How that will change as snow ages, we don't know."

Though Ramanathan's team has done extensive work to track the transport of aerosols such as soot from Asia to the U.S. West Coast, Ramanathan and Hadley said it is too soon to distinguish whether the black carbon measured came from local or distant sources.

"We were there too brief a time to make any generalizations," said Ramanathan in reference to a six-week survey of snowfall at the Donner Pass and a more remote location in Lassen National Park in Northern California.

The researchers believe that they have enough evidence to suggest that the levels of black carbon they found at the two Sierra Nevada locations is typical of snow throughout the mountain range.

Ramanathan's team is funded by the California Energy Commission to continue the snow and rain sampling this winter over a longer time frame than in the initial field study. To witness the same phenomena from a different vantage point, they are embarking on an aerial survey of Southern California skies using a fleet of unmanned aircraft.

The scientists also hope to begin an aerial survey in February that will characterize the black carbon found in the skies over Southern California. The planned California AUAV (autonomous unmanned aerial vehicle) Pollution Profiling project (CAPPS) will rely on unmanned aircraft to create detailed meteorological profiles of aerosols and clouds. The methodology will be very similar to that used in Ramanathan's landmark 2006 study called the Maldives AUAV Campaign, which was the first project to create dimensional profiles of the atmosphere through the use of aircraft flying in formation.

Flights will originate at the Dryden Flight Research Center at Edwards Air Force Base north of Lancaster, Calif.