Is Air Pollution Reducing California's Water Supply?

New project seeks to verify role smog may play in stifling precipitation

The Sierra Nevada snowpack that supplies more than 30 million Californians with their water has shown a steady decline over the past century, possibly in part because of the state's air pollution.

But as drought preparations ramp up across the state, researchers are attempting to verify this hypothesis by analyzing the particles in air pollution and precipitation that descend on the Sierra mountain range. A team including scientists at Scripps Institution of Oceanography, UC San Diego took an unprecedented look in late February and early March at a snowstorm and the chemistry that influenced it in the first of several field studies that will take place where Interstate 80 runs through the Sierras.

"The exciting thing for us is to be able to make coupled aerosol chemistry and state-of-the-art meteorological measurements which should allow us to be better understand the types of air pollution that are impacting cloud and precipitation processes," said Scripps atmospheric chemistry Professor Kim Prather, who also holds an appointment at UCSD’s Department of Chemistry and Biochemistry. Prather is a co-investigator of the CalWater project funded by the California Energy Commission.

Particulates in air pollution affect precipitation by changing the number and composition of cloud condensation and ice nuclei—the tiny particles of dust, sea salt, organic materials produced in combustion and human-produced pollutants such as diesel soot around which cloud droplets form. The moist droplets need to grow large enough to fall from the sky as rain or snow, so changes in the quantity of seed particles in the air can influence how much rain or snow a cloud is capable of producing.

Making analysis of these influences possible is an instrument co-developed by Prather known as an aerosol time-of-flight mass spectrometer (ATOFMS). Continuously pulling in air samples, the instrument can chemically characterize the individual particles present that form the cloud seeds. The fingerprints of each individual particle measured by ATOFMS reveal to scientists where particles likely originated and how long they have been airborne. In addition, UCSD graduate students Jessie Creamean and Andrew Ault collected rainwater and snow samples. The ATOFMS was used to analyze the chemistry of these samples to gain insights into which particles actually seeded the snow and rain.

The team, led by NOAA researcher Christopher Williams, established an Aerosol-Met observatory at Sugar Pine Reservoir in the foothills east of Sacramento. The observatory included the mass spectrometer and profilers that measured the vertical structure of clouds and the locations within them where snow melted into rain.

CalWater will run a more detailed analysis of precipitation through an entire winter starting in November 2010 when in-flight measurements of the chemistry of cloud seeds by an A-ATOFMS (aircraft-ATOFMS) will supplement ground-based measurements.

The February-March run just completed showed what could turn out to be a typical pattern: A snow event recorded by the instruments revealed that streams of pollution transported from Asia mixed with a "river" of moisture that was pumped northward from tropical latitudes to influence snowfall in the central part of the range.

—Robert Monroe

April 2009