BERKELEY, CA — Computer models of air quality provide local governments with the scientific information they use to regulate air pollution emissions -- but these models are not always as accurate as regulators would like.

Atmospheric aerosols can extinguish light through scattering and absorption, reducing the rate of smog-forming reactions in the lowest layers of the troposphere while enhancing their rates in the higher layers.

Researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and the University of California at Berkeley have been studying the photochemical characteristics of air pollution in southern California as part of an effort funded by the California Air Resources Board (CARB) to improve the reliability of air quality models. The Berkeley Lab team’s work has yielded new insights into how variability in the solar flux and the concentration of aerosols in the atmosphere affect the formation of smog.

Since the passage of the Clean Air Act in 1990, computer-based air quality models have been the basis of air quality regulation in the United States. Reducing uncertainties in model results has been a focus of much research. "One of the uncertainties," says Berkeley Lab’s Laurent Vuilleumier, "is how well the models represent the optical properties of the atmosphere and their effect on the photochemical reactions that form smog." CARB designated variability in sunlight and its effect on photochemistry as one of the areas needing improvement in current air quality models.

Vuilleumier, a scientist in the Lab’s Environmental Energy Technologies Division, UC Berkeley’s Rob Harley, EETD’s Nancy Brown, and colleagues have been using data from CARB’s 1997 Southern California Ozone Study to gain a better understanding of the relationship between the amount of light entering the atmosphere and the rates of photochemical reactions that form ozone, a significant component of smog that influences the concentrations of other air pollutants.

"Ozone concentration is extremely sensitive to reactions that are driven by sunlight," explains Vuilleumier. "These photolysis reactions initiate the decomposition of chemical species such as nitrogen dioxide and formaldehyde by sunlight. The photolysis rates are variable because the amount of light reaching the lower atmosphere -- called the solar actinic flux -- is variable. Aerosols, particles in the atmosphere, can extinguish light through scattering and absorption, reducing the rate of certain smog-forming reactions in the lowest layers of the troposphere, while sometimes enhancing their rates in the higher layers."

Vuilleumier, Harley, Brown and colleagues used the SCOS 97 measurements of solar ultraviolet irradiance, taken at two stations in Riverside and Mount Wilson, to compute the atmosphere’s total optical depth. As a measure of the transparency of the atmosphere to the penetration of sunlight, optical depth is very influential in determining solar flux. Using a mathematical method called principal component analysis, the researchers separated the factors affecting optical depth into components, and determined which components were most significant.

The largest component, which the researchers attribute to the concentration of aerosols in the atmosphere, accounted for 91 percent of the variability in the data. The second component, the concentration of ozone, accounted for another eight percent of the observed variability.

"These results tell us that air quality models need to be modified to better account for the effects of aerosol and ozone concentration on smog formation," says Vuilleumier. "As a result of this work, we have prepared a report to CARB reviewing the mathematical methods used in the models to represent atmospheric optical properties and their effect on photochemical reactions, with suggestions on how to improve them. There are many other variables that affect the accuracy of these models, such as meteorological factors, chemistry and emissions inventories, and we hope to continue our studies of some of these for CARB."

Air quality models typically used by CARB today include the Urban Airshed Model and the SARMAP (San Joaquin Valley Air Quality Study Regional Model Adaptation Project) Air Quality Model.

"Variability in Ultraviolet Total Optical Depth during the Southern California Ozone Study (SCOS97)," by Vuilleumier, Robert Harley, and Nancy Brown of Berkeley Lab, and James Slusser (Colorado State University), Donald Kolinski (University Corporation for Atmospheric Research) and David Bigelow (Colorado State) will be published in the February 2001 issue of Atmospheric Environment.

Berkeley Lab is a U.S. Department of Energy laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.