Where Are All The Cool Parking Lots?

"The 1970s energy crisis spurred implementation of parking lot shading ordinances in cities such as Sacramento, Davis, and Los Angeles, California. These ordinances required that 50% of the total paved area be shaded within 15 years of the issuance of development permits.

Tree Lists contained the 15-year crown diameter and crown projection area (i.e., area under a tree’s dripline) of species recommended for planting, data used by landscape architects to calculate paved area that would be shaded under each tree. The Sacramento ordinance, adopted in 1983, ensures a distribution of shade throughout the lot and had not been evaluated or amended since its inception.

"Parking lots occupy about 10% of the land in our cities and can be characterized as miniature heat islands and sources of motor vehicle pollutants. By cooling these heat islands, trees reduce air temperatures and vehicle hydrocarbon emissions. "Oasis effects" of 5 C to 7 C (9-12.6 F) have been measured due to direct shading of the ground surface and transpiration of water through leaves. By lowering parking lot temperatures, trees can reduce evaporative emissions of hydrocarbons (HC) from parked cars. These evaporative HC emissions account for 16% of total motor vehicle HC emissions in Sacramento, and motor vehicles are responsible for 49% of countywide HC emissions. During summer, HCs and NOx form ozone through photochemical reactions. Currently, 159 million people live in areas where ozone concentrations violate federal air quality standards. Although the shading and cooling provided by parking lot trees was originally valued for energy conservation, air quality benefits may be important because increased air temperatures increase the rate of evaporative HCs released from leaky fuel tanks, worn hoses, and saturated carbon canisters."

See also: 4 page color factsheet

This paper was prepared as a synopsis for a June 2, 2004 EPA Air Quality conference call featuring Dr. McPherson.

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