Trees And Air Pollution

Scientists from Australia's federal science agency, CSIRO have been commissioned by the state of New South Wales Environmental Protection Authority (EPA) to investigate emissions of organic compounds from Australian eucalypt trees and grasses that contribute to the formation of photochemical smog.

"It's not just cars and industry that cause air pollution," says Mr Ian Galbally, from CSIRO Atmospheric Research.

"Plants release highly reactive hydrocarbons that can add significantly to photochemical smog problems. That is, smog caused by the reaction of sunlight with chemical compounds like those from industry, car exhausts - and now, as we've discovered, plants," he says.

"The blue haze you often see over the Dandenong Ranges in Victoria and in the Blue Mountains near Sydney is caused in part by the gases released by vegetation. We found that grasses, particularly when cut, are potent emitters of reactive hydrocarbons."

"Plants release these compounds into the atmosphere in large quantities. These volatile compounds add to the photochemical smog in the same way as emissions from human sources - there is no discrimination," says Dr Peter Nelson, senior research scientist with CSIRO Energy Technology.

"We are measuring the emissions rates of hydrocarbons from three Australian eucalypt species, using large branches of mature trees," says Dr Nelson.

"We deliberately sought trees that hadn't been grown under controlled conditions, but were 'real world' specimens. Previous studies have concentrated on a small number of single leaves of cultivated plants and performed measurements under very controlled conditions of temperature and radiation," says Dr Nelson.

"One of the things we have found already is a close relationship between the amount of the sun's radiation, of the type that is important for photosynthesis and the plant's growth, and the level of hydrocarbons they emit. Emission rates are highest during the day and drop off towards evening.

"We can use this information to assist the EPA to more accurately estimate chemical emissions from Australian trees and grasses," he says.

Researchers measured the emissions by two enclosure methods: a flexible Teflon film bag and a Teflon film Chamber.

The enclosures were ventilated with ambient air to control temperatures to near ambient conditions. Concentrations of carbon dioxide, nitric oxide, nitrogen dioxide, ozone and non-methane volatile hydrocarbons were monitored continuously with gas analysers to determine emission rates.

The plant compounds being measured include volatiles like isoprene and monoterpenes. "Such compounds were always the major contributors to hydrocarbon fluxes," says Dr Nelson.

Gas samples were periodically collected for detailed, high resolution, gas chromatographic analysis of isoprene and other hydrocarbons. Other physical parameters measured included temperature, total light intensity and photosynthetically-active radiation (PAR), or the portion of the sun's radiation spectrum that is important to plant photosynthesis.

By measuring each of these parameters, the emission rate of these hydrocarbons can be linked to both plant temperature and light intensity.