Purves emphasized that cutting the human-caused sources may have been worthwhile even if ozone levels did not decrease. "Even keeping the air quality the same might have been an achievement because if we hadn't done anything it might have worsened," said Purves.
The study did not measure actual ozone levels. Instead it focused on VOCs, a crucial part of the chemical reaction that produces ozone. The other critical ingredient is a class of gasses known as NOx (various combinations of nitrogen and oxygen), which are almost entirely man-made. The ozone-producing reaction happens most readily in hot weather, which is also when trees produce the most VOCs.
Further studies at Princeton and the federal Geophysical Fluid Dynamics Lab at Princeton are using sophisticated computer models to estimate the changes in ozone caused by the changes in tree-produced VOCs. Purves noted that interactions between VOCs, NOx and ozone are complex -- some may actually lower pollution -- so it would be premature to base environmental policy on studies of VOCs alone.
Purves, a postdoctoral fellow, wrote the article in collaboration with Stephen Pacala, professor of ecology and evolutionary biology at Princeton, as well as John Casperson of the University of Toronto, Paul Moorcroft of Harvard University and George Hurtt of the University of New Hampshire. The article is scheduled to be published later this fall in the journal Global Change Biology.
The scientists conducted the study by analyzing data collected by the U.S. Forest Service, which measured and cataloged 2.7 million trees on 250,000 plots of land across the country. They calculated the VOC emissions for each tree and each plot and used their findings to map VOC
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Contact: Steven Schultz
sschultz@princeton.edu
609-258-5729
Princeton University
28-Sep-2004