Autumn leaves bring higher pollution levels

New research has found that emissions of volatile organic compounds (VOCs) shoot up when leaves rapidly assume their autumn colours and begin to fall.

Scientists from the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, have discovered that concentrations of at least two VOCs, methanol and acetaldehyde, more than doubled after a week of colder weather between 21-28 in northern Michigan, which signalled the beginning of autumn colours and falling leaves in that region. To measure foliage emissions, scientists from NCAR are taking continuous readings above the forest canopy near the town of Pellston with instruments which track both updrafts and downdrafts, monitoring ions that collide and transfer protons to the VOCs.

The scientists picked Michigan in part because the area’s abrupt, heavy frosts are likely to result in high emissions. In contrast, when leaves lose their summer green and wither more gradually, as in the southeast, the result is likely to be lower emissions that linger over a longer period of time. VOCs from plants are harmless in the absence of human activity, with trees and other types of vegetation benefiting air quality by absorbing ozone and other types of pollutants. But in an atmosphere modified by nitrogen oxides from human activity they react with sunlight to form ground-level ozone, the major component of smog. This reaction also impacts the climate by slowing the rate at which greenhouse gases are oxidised in the atmosphere.

In the past, some of the US Government’s anti-smog efforts have focused on reducing VOCs produced by motor vehicles and other human sources but that may be of little value, says the NCAR, since the bulk of those compounds actually comes from vegetation. A more effective way to fight smog, scientists argue, would be to curtail human-generated nitrogen oxide emissions.

Funded by the National Science Foundation and the US Environmental Protection Agency, the study is intended to help guide anti-pollution efforts by quantifying the chemicals that are given off by plants, as well as by human activities. “We hope to develop scenarios in which we can have forests and people and cars and power plants and factories, all existing together, without creating toxic levels of

pollutants,” NCAR researcher Alex Guenther said.

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