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Computer Modeling of Los Angeles Airshed Tests Clean Air Strategies

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By Allan Chen,

March 25, 1997

BERKELEY, CA -- Computer models of the Los Angeles airshed indicate that innovative methods of reducing air pollution such as increasing urban vegetation and making roofs and other urban surfaces more reflective to sunlight can improve air quality, says Haider Taha, a scientist at the Ernest Orlando Lawrence Berkeley National Laboratory.

Taha, who is in the Environmental Energy Technologies Division, has been running computer simulations of the air quality in Los Angeles to test the effectiveness of these air pollution reduction strategies. His studies will help the South Coast Air Quality Management District (SCAQMD) decide if it is worth boosting efforts at urban revegetation and re-roofing buildings with materials that efficiently reflect solar radiation.

"Past research has demonstrated that increasing urban vegetation and coating roofs with paint or roofing material that is highly reflective to sunlight can reduce the temperature of the urban heat island," says Taha. "We think that these methods can also reduce the formation of photochemical smog."

The heat island effect is an elevation of temperature of urban areas relative to the surrounding countryside caused by the removal of vegetation, and the use of materials such as asphalt that absorb the sun's heat. The temperature elevation in Los Angeles can be five to ten degrees (Fahrenheit) on hot summer days.

Berkeley Lab researchers have shown that re-roofing with "high-albedo" (solar-reflective) materials, and planting vegetation can reduce the magnitude of the heat island effect, and as a result, air conditioning bills and energy use.

High-albedo materials reflect more of the sun's heat than low-albedo materials. Thus buildings don't absorb as much heat and require less air conditioning. With sufficient revegetation and use of reflective materials in roofs and roads, an urban area can be a few degrees cooler overall. Berkeley Lab researchers have found a number of materials of different colors that efficiently reflect the sun's heat.

"The heat island effect also causes an increased probability that photochemical smog will form because smog is more likely to form as the temperature increases," says Taha. "Reducing the temperature of the urban heat island could reduce the formation of photochemical smog."

Taha used a computer model to simulate the meteorology and its sensitivity to changes in the surface albedo of the South Coast Air Basin, and a second model to simulate the impacts of changes in meteorology and emissions on ozone air quality (smog).

Dividing the L.A. basin into cells, Taha assumed that the maximum albedo of any cell on land is 0.30 (reflects 30 percent of the sun's radiation), given limitations on how much land area might be a candidate for increased albedo. Today, the albedo of most of L.A.'s land areas is between 0.14 and 0.17. Then he ran simulations using temperature and smog data recorded from a major smog episode on August 26-28, 1987.

The result is a map of ozone concentration in the L.A. basin. Ozone formation is an indicator of photochemical smog. "Increasing the albedo in possible areas of the basin reduced the peak ozone concentration of this smog episode at 3 p.m. (usually the peak hour for the area's smog) by up to 7 percent. This represents a reduction from 220 to 205 parts per billion ozone," says Taha.

Reducing the smog in the basin, of course, results in less human exposure to smog. Taha calculated the "population-weighted" reduction in exposure to smog relative to the federally prescribed level (National Ambient Air Quality Standard), and the state level (California Ambient Air Quality Standard). The increased albedo strategy led to a reduction in population-weighted exposure above the NAAQS threshold of 16 percent during the peak afternoon hours and 10 percent during the day. Under the more stringent CAAQS standard, the reduction during peak hours was 12 percent.

Locally, there are variations in the magnitude of the ozone reduction. Some areas see larger reductions than others, in part because of the influence of wind currents and other weather-related factors.

Simulations testing the effect of increasing the vegetation in the basin produced encouraging results as well. "The net basin-wide effect of increased tree-planting is reduced ozone concentrations and exposure to ozone if the additional urban trees planted are low emitters of biogenic hydrocarbons," says Taha. (Some trees are known natural emitters of hydrocarbons and can contribute to photochemical production of smog.)

Taha's results suggest that revegetation and solar-reflective materials could be a useful part of SCAQMD's anti-air pollution strategy, which also includes a variety of other measures. The agency is considering various ways to encourage revegetation and cool materials such as free market incentives and building code modifications.

"Nonetheless, there are always uncertainties in the model," says Taha, "these results are specific to this site, to the emissions data that we think reflects the actual emissions in the Los Angeles basin, and to the meteorological conditions prevailing during the episode. "

Taha plans to continue refining these models and working with SCAQMD and other agencies. This work is published in the journal Atmospheric Environment.

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

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