Paul Crutzen is best known for his research on ozone depletion. In 1970 Prof. Paul Crutzen pointed out that emissions of nitrous oxide (N2O), a stable, long-lived gas produced by soil bacteria, from the Earth's surface could affect the amount of nitric oxide (NO) in the stratosphere. Crutzen showed that nitrous oxide lives long enough to reach the stratosphere, where it is converted into NO. Crutzen then noted that increasing use of fertilizers might have led to an increase in nitrous oxide emissions over the natural background, which would in turn result in an increase in the amount of NO in the stratosphere. Thus human activity could have an impact on the stratospheric ozone layer. In the following year, Crutzen and (independently) Harold Johnston suggested that NO emissions from supersonic aircraft, which fly in the lower stratosphere, could also deplete the ozone layer. He lists his main research interests as “Stratospheric and tropospheric chemistry, and their role in the biogeochemical cycles and climate”
In 1995, he received the Nobel Prize in Chemistry, together with Mario. Molina and F. S. Rowland, U.S.A.