Scientists have discovered giant "tongues" of ozone routinely swoop down from the upper atmosphere over Eastern Canada and can exacerbate smog problems on the ground.
In one remarkable instance, ozone levels in a 1,000-kilometre swath from Montreal west across the Great Lakes almost doubled after an "ozone intrusion" dropped out of the stratosphere, the team reports in the journal Nature today.
The heavenly intrusions can have "significant" impacts on the ground, the researchers report, and are much more common than expected.
The assumption is that when you get this big increase in pollution it"s all manmade," says team leader Wayne Hocking, an atmospheric physicist at the University of Western Ontario. "What we are finding now is that at least some portion of it can be a natural."
"Sometimes, these natural events make at least a comparable contribution to any man-made ozone effects," he says.
The researchers believe the intrusions are a common phenomenon across Canada and many other regions of the world.
The study was conducted in southern Quebec and Ontario using ground-based radar and more than 100 scientific balloons launched between April 2005 and August 2006.
"What surprised me was that we seemed to see these intrusion events whenever we looked," says co-author David Tarasick, a research scientist with Environment Canada. The team witnessed at least one intrusion during each of its five 10-to 14-day field campaigns, suggesting they may be occurring almost weekly.
Ozone is an environmental friend or foe, depending on where it is. In the stratosphere, which begins about 10 kilometres above the ground, it forms a protective layer that absorbs harmful ultraviolet solar rays. But at ground level, ozone is a key component in smog that harms human health, forests, and crops. Smog is normally associated with pollution from automobiles, power plants and factories, and Hocking and his colleagues say human activities are clearly a major contributor to the smog that often chokes the Vancouver region and the Windsor-to-Quebec City corridor.
But the scientists say the intrusions of ozone from the upper atmospheric can make significant contributions to smog and air quality.
Turbulence generated by winds racing through the upper atmosphere is believed to cause the ozone to drop down. "It"s like waves breaking on a beach," says Tarasick, explaining how giant airwaves can break about 10 kilometres above the ground in the zone that separates the upper and lower atmosphere. The scientists say turbulence generated as winds speed over the mountains in Western Canada also generates intrusions.
The distinct tongues of stratospheric ozone often dissipate before they hit the ground, but Tarasick says they still can change air quality at street level. He estimates about 20 per cent of ozone at ground level originates in the upper atmosphere.
Sometimes, the research indicates it can be much higher. On May 3, 2005, the team picked up a giant ozone intrusion high in the atmosphere over Montreal. Four days later the stratospheric ozone reached the ground and ozone readings in the city almost doubled, from 15 parts per billion to 30 ppb at night and from 30 ppb to 50 ppb during the day.
Ozone can inflame the lungs, causing coughing wheezing and chest tightness and aggravate existing heart and lung conditions. Health experts say adverse effects can be seen at levels as low as 10 ppb.
"Stratospheric ozone had a large impact on the surface ozone densities," the scientists report. They later tied the intrusions to a "noticeable increase in surface ozone" from Montreal back to the Great Lakes 1,000 kilometres to the west.
The study was undertaken to get a better read on the poorly understood ozone intrusions and to assess if "windprofiler" radar can predict when intrusions will occur. The radars, being installed in Ontario and Quebec as part of a $6-million project financed largely by Canadian Foundation for Innovation, measure wind up to 15 kilometres off the ground. The radars are expected to improve weather forecasts and enable pilots to manoeuvre in and around winds to improve fuel efficiency.
