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Study finds new clues to the fate of smog in Sierra Nevada forests

– Most scientists believe that when smog ozone is taken up by the forests of the Sierra Nevada, it is mostly absorbed by trees and plants. But new research at the University of California, Berkeley, suggests a large proportion of the ozone is actually transformed by chemical reactions in the air with compounds emitted by the forest.

The new findings, published online in April in Geophysical Research Letters, indicate that much of the ozone entering pine forests in the Sierra Nevada could be reacting with natural hydrocarbons emitted by plants. One outcome of this reaction is the formation or growth of aerosols.

"We care about aerosols in the atmosphere because they can affect human health and visibility," said Allen Goldstein, associate professor of biogeochemistry at UC Berkeley's College of Natural Resources and principal investigator of the study. Aerosols also potentially impact climate by increasing the formation of clouds and scattering sunlight. They are generally believed to have a cooling effect on the environment.

The study has implications for managing both air quality and air pollution impacts on forests. Previous studies have shown that air pollution from manmade sources such as car exhaust and power plants travels up from the Central Valley to the Sierra Nevada Mountains. The ozone formed from those emissions is considered damaging to forest health - it leads to discoloration and loss of needles from pine trees, inhibits growth and potentially increases susceptibility to diseases.

"In order to relate the dose of ozone to the damage, we need to do a better job of quantifying uptake by the trees," said Meredith Kurpius, lead author of the study and a former UC Berkeley graduate student in ecosystem science.

"The forest acts as a type of 'sink,' a place where a lot of the ozone generated as pollution ends up being taken up by the trees, deposited on surfaces or transformed in chemical reactions in the air," said Kurpius, now a postdoctoral research associate at Oregon State University's Department of Oceanic and Atmospheric Science. "Our findings suggest that, in the summer, half of the ozone is lost by another mechanism that had never been quantified before."

In addition to forming aerosols, the reaction of ozone with hydrocarbons emitted by plants creates hydroxyl radicals, important elements in atmospheric chemistry near the Earth's surface.

"Traditionally, people thought that most of the ozone was going into the plants and trees and causing damage, but the amount of ozone lost through other processes was never measured," said Goldstein. "We set out to determine what controls the amount of ozone taken up by the forest, and what we found was very surprising."

Over the course of a year, the UC Berkeley researchers determined the amount of ozone lost in a ponderosa pine plantation in the Sierra Nevada Mountains, about 50 miles from Sacramento. They estimated that 45 to 55 percent of the ozone was lost in the summer through gas phase chemical reactions within the forest canopy.

As the temperature cooled going from fall into winter, the amount of hydrocarbons released by the trees decreased, leading to a subsequent decrease in the amount of ozone lost through gas phase chemistry. During the cooler months, more ozone was absorbed by the trees and plants, according to the study.

Kurpius and Goldstein suggest that the main hydrocarbons reacting with ozone are a group called terpenes. These highly reactive chemicals involved in the atmospheric reactions with ozone are so unstable that they are difficult to measure, but Goldstein said understanding such natural processes is necessary in the development of scientifically based air quality management policy.

"In order to develop legislation to control the amount of aerosols in the atmosphere, we need to better understand how much of the aerosol results from natural processes and how much is coming from manmade sources," he said.

The research was supported by the Environmental Protection Agency, the National Science Foundation and the California Air Resources Board.