Who would have thought that lightning-induced forest fires in Canada could affect air pollution thousands of miles away? But according to researchers at the National Oceanic and Atmospheric Administration, fires burning in northwestern Canada sent plumes of carbon monoxide to the eastern seaboard and southeastern U.S., adding to the mix of human-induced air pollution, during summer of 1995.

The study, published in the April 14 issue of Science, shows that these 1995 forest fires were, at times, the largest source of summertime carbon monoxide pollution more than 2000 miles away in the eastern U.S. This is the first time that scientists have been able to show that forest fires that often occur naturally in high-latitude northern regions can have an atmospheric impact as far away as the mid-latitudes of the U.S.

Data from a 1995 air quality field experiment centered around Nashville, Tenn., and covering several eastern states in the region, together with models of the winds affecting the region, were used in the study.

Lead author Gerhard Wotawa from the University of Agricultural Sciences in Vienna and a visiting scientist at NOAA's Aeronomy Laboratory, and coauthor Michael Trainer, also from NOAA's Aeronomy Laboratory in Boulder, Colo., traced the winds backwards from the eastern states region, site of the Southern Oxidants air pollution study. They found that high periods of carbon monoxide in the region occurred in air that had originated during specific forest fire episodes in Canada.

"For certain periods of time, the northern high-latitude fires were the biggest source of the carbon monoxide pollution in the eastern U.S. region we studied," said Wotawa. "The fires added to the manmade pollution that was produced more locally in the region." Carbon monoxide levels were about doubled when the plumes of the forest fires passed through the region. Sometimes, higher ozone concentrations also accompanied the plumes. The researchers used additional data to show that the influence reached air quality monitoring stations in Maryland and Virginia. The summer of 1995 was one of the hottest on record in the eastern U.S., and the forest fires added to air pollution levels that were already quite high.

Air pollution is caused by emissions from human activities such as the burning of fossil fuels for transportation and power generation. Natural factors such as forest fires can further affect air quality by emitting carbon monoxide and other gases that lead to the production of ozone. High amounts of ozone can be harmful to human health and vegetation.

In the U.S., the National Ambient Air Quality Standards have led to efforts to reduce human emissions that cause pollution. "Our results show that there are times when the impact of North American fires may have to be taken into account when we are considering the causes of pollution," said Trainer.

Forest fires burned over 17 million acres in Canada during 1995, an area that is half the size of the state of Florida. Forest fires have been increasing in remote regions of North America during the last decades due to prolonged drought periods or climate variability.

Extreme forest fire conditions in North America occurred two times in the 1980s and three times during the 1990s.