SCISAT focuses its attention in the stratosphere, where the ozone layer is located. SCISAT is providing the most accurate measurements to date of chemicals that affect ozone, which blocks the sun's biologically damaging ultraviolet radiation and prevents most of it from reaching the Earth's surface.

It measures more than 30 different molecules, which is "more thorough than anything that's up there," said Peter Bernath, a University of Waterloo chemistry professor who heads the SCISAT science team. Ozone—comprised of three atoms of oxygen—is constantly being created and destroyed by natural chemical processes in the atmosphere. The amount of ozone present at any given time varies depending on the balance between the processes of creation and destruction.

Canada's SCISAT satellite

Industrial activities on Earth produce chemicals— notably chlorofluorocarbons (CFCs)—that affect this balance by destroying stratospheric ozone. They not only reduce the ozone shield around the globe, they actually eat large holes in the ozone layer over the Antarctic each year, and also cause significant losses over the Arctic. In the past two decades, average ozone levels over Canada have dropped about 6%, while severe declines of 20—40% have occurred over the Arctic in the spring.

The Atmospheric Chemistry Experiment (ACE) on SCISAT was launched into a 650-kilometre-high, high-inclination orbit that takes it over the polar regions of the Earth, as well as tropical and mid-latitude locations. It measures chemical molecules that influence the distribution of stratospheric ozone, particularly in the Arctic. These data are making an important contribution to international environmental policy making aimed at protecting the ozone layer, such as the Montreal Protocol that bans certain CFCs.

SCISAT observations are also helping scientists better understand the effects of atmospheric chemistry, clouds and small particles (such as aerosols) on Earth's climate.

ACE measures the absorption of solar light by the atmosphere at sunrise and sunset. Different atmospheric constituents absorb different wavelengths of light in characteristic ways—a signature by which they can be identified. This technique is what allows ACE to make extremely accurate measurements.

The trade-off for this precision is that SCISAT takes measurements in a limited number of locations. Other satellites do provide more global coverage, but their data are not as accurate. "That's why there's great value in combining the two," said Bernath.

With its instrument performing so well, SCISAT is now moving beyond its original mandate and providing excellent data related not only to ozone depletion, but also to climate change, and air quality and pollution. "There are many things we're doing now that we really didn't know we could do," Bernath said.

ACE has been used to measure the distribution of several molecules correlated with air pollution and biomass burning, providing data that complements the observations of the MOPITT satellite. "Together, they give you a picture of what's going on, and it's not a good picture. A lot of pollution that come from industrial activity travels all over the globe," Bernath noted.

ACE observations are also useful in climate studies. For example, they've shown that previously observed increases in the amount of water vapour being injected into the stratosphere have stopped. Water vapour is the most powerful natural greenhouse gas and plays a key role in the Earth's climate. "No one knew why it was increasing and we don't know why it stopped, so there are quite a few mysteries left," said Bernath.

Originally intended to last two years, the satellite's operations have already been extended to 2007, and Bernath hopes it will continue to function beyond that. "It's been a real success."

(Article by Lydia Dotto. Source: Space Science Research in Canada 2004-2005, Beijing, China, July 2006, The CSA's Report to the 36th COSPAR Meeting)

www.space.gc.ca/asc/eng/sciences/atmosphere.asp