Trees are valued in California's urban areas for the many benefits they provide, including modification of the local environment. Besides serving as a source of oxygen and water vapor, trees give off other chemical compounds into the atmosphere, including volatile organic compounds (VOC) that can react in the atmosphere and contribute to the formation of ozone, a principal air quality problem in California. In this publication we will briefly describe how ozone near the Earth's surface is

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... and how trees can affect the formation of ozone, and we will suggest ways to take this aspect of tree biology into consideration when selecting trees for planting in large numbers in urban areas.