"It's a bit of detective work," says Greg Evans, a professor in the Department of Chemical Engineering and Applied Chemistry. "We happened to know when that forest fire was happening in Quebec and we realized that this mixture of different particles that we found in downtown Toronto is a signature for a forest fire." With the dust particles from the Sahara, the researchers recognized sand-like particles and were ultimately able to track their trajectory from the desert, across the Atlantic Ocean to Mexico, then north through the United States to Toronto.
The researchers used a device known as a laser ablation mass spectrometer (LAMS), which pulls in air from College Street and accelerates the pollutants to close to the speed of a bullet. As a particle passes by two lasers, sensors calculate its exact speed and tell the LAMS when to fire a third, high-powered laser that vaporizes a portion of the particle, sending fragments hurtling along a "flight tube". Lighter molecules take less time to travel down the flight tube, giving the researchers the particle's chemical signature. Evans says that once they build up a library of particles, this research could make it possible to identify pollutants without any knowledge of their origin.