The post-Second World War period was boom time in Canada for the chemical manufacturing sector, with companies like Canadian Industries Ltd., or C-I-L, offering the latest in goods to an eager nation of consumers. Especially popular was the tubular, airtight can that, as this 1951 ad from Chemistry in Canada shows, promised unprecedented convenience, able to safely hold such household substances as insect repellents and pigmented paints. It was a marketing and packaging bonanza for C-I-L, which promoted its aerosol cans to manufacturers with a handy manufacturing guide, Package for Profit.

Key to the aerosol can’s functioning was the propellant Freon, the trade name for several halocarbon, or chlorofluorocarbon (CFC) organic compounds first marketed by DuPont. As the ad claims, Freon was a remarkable ingredient, non-flammable and non-explosive and safe to use on virtually any surface, including skin. It was used not only in aerosol cans but refrigerators and air conditioners.

Later, in the 1980s, the dark side of CFCs came to light. CFCs — persistent in the environment — eventually rise into the stratosphere. Here, ultraviolet light reacts with the CFC molecule and causes a release of chlorine. The chlorine atom then degrades the thin layer of ozone (O3), a thin belt of atmosphere from 15 to 30 kilometres above the planet that plays a critical role in stopping ultraviolet rays, which have enough energy to penetrate the skin and damage DNA, causing skin cancer and other health problems.

The Montreal Protocol on Substances that Deplete the Ozone Layer was signed in 1987 and came into force in 1989. It severely restricts the use of CFCs, which have now been largely replaced by combinations of other chemicals, including hydrofluorocarbons (HFCs). Unfortunately, these chemicals contribute to another problem — climate change. The ability of certain HFC molecules to trap heat in the Earth’s atmosphere, as measured by their global warming potential (GWP), can be thousands of times higher than carbon dioxide (CO2), the primary greenhouse gas, even though their concentration in the atmosphere is lower.

In addition to helping negotiate the Montreal Protocol, Canada has played a leadership role in monitoring the effects of the ban. Since 2003 the Atmospheric Chemistry Experiment (ACE) on board the Canadian Space Agency’s SCISAT satellite has provided detailed observation of various chlorine species as well as dozens of other molecules. Using a Fourier Transform Spectrometer (FTS) and a UV-visible-NIR spectrophotometer, ACE can measure how the ratios of these chemical species change with altitude.

A recent finding from ACE shows a decline in the overall amount of chlorine in the atmosphere. However, in the Northern Hemisphere, a temporary slowdown in atmospheric circulation has increased the concentration of some chlorine-containing molecules. It is still predicted that O3 levels will return to normal within a century or so, but it does mean that the legacy of Freon lives on.