Phosgene is most famous for being part of the chemical arsenal used in the trenches during World War I - generals preferred it over chlorine because soldiers coughed less and therefore inhaled more. It was still stockpiled in military arsenals after the Second World War but its presence in the atmosphere today is due to man-made chlorinated hydrocarbons used in the chemical industry.

Phosgene still plays a major role in the preparation of pharmaceuticals, herbicides, insecticides, synthetic foams, resins and polymers.

Professor Peter Bernath of the Department of Chemistry at the University of York and a research team have carried out the first study of the global distribution of the gas. Between February 2004 and May 2006, they used the Canadian Atmospheric Chemistry Experiment (ACE) satellite to measure the incidence of the gas.

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The scientists discovered that it was present in some quantity in all latitudes, though there is a main atmospheric concentration of the gas was above the Equator. They found that levels of phosgene in the atmosphere had reduced since previous studies in the 1980s and 1990s, though its continued presence is a contributor to ozone depletion.

Professor Bernath said: ”There is a small, but not negligible, concentration of phosgene in the troposphere. Chlorinated hydrocarbons don't occur in nature but as chlorinated solvents they are used by industry. They are short-lived and they decay rapidly, but they decay into phosgene.

“It's very toxic and pretty nasty stuff - its reputation is well deserved. Considering the health hazards associated with phosgene, the chemical industry is trying to find substitutes to eliminate its use. But the use of chlorinated hydrocarbons is being reduced because of the legal restrictions of the Montreal Protocol, so phosgene is also decreasing.”

Higher up in the atmosphere phosgene can be slowly oxidized by ultraviolet rays, and so it continues to play a role in the depletion of the ozone layer.

The research was financed by the Canada Space Agency (CSA) and the Natural Sciences and Engineering Research Council of Canada.

Research team: Dejian Fu, Chris Boone, Sean McLeod and Ray Nassar, the Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada; Peter Bernath, Department of Chemistry, University of York, Heslington, York, UK; Kaley Walker, Department of Physics, University of Toronto, Toronto, Canada; Gloria Manney, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.

Article: Global phosgene observations from the Atmospheric Chemistry Experiment (ACE) mission, September 2007 Geophysics Research Letters.