A new technique for measuring air pollutants in the lower part of the troposphere has been developed. The technique is based on Fourier-transform infrared (FTIR) spectroscopy, which is used to measure the atmospheric thermal emission from gases beneath uniform cloud cover. The cloud acts as a cold background emission source against which the emission from gases in the warmer atmosphere beneath the cloud may be detected. The region of the infrared spectrum near 2400 cm-1, which is nearly void of significant atmospheric water vapour emission, is used to infer the cloud base temperature. The FASCOD3 atmospheric transmission code is used to simulate the background emission spectrum below the cloud, which is then subtracted from the measured spectrum to yield the thermal emission band of a particular gas. Based on the band intensity, the average concentration of the gas in the lower atmosphere may be determined. In order to have sufficient detection sensitivity, the cloud base must exceed an altitude of about 1 km. The gases that have been successfully measured with this technique include tropospheric ozone, carbon dioxide, carbon monoxide and nitrous oxide. By comparing the tropospheric ozone amounts to the surface amounts measured with an ozone analyser over the past two summers, it was discovered that the ozone residing in the lower troposphere sometimes has a concentration that is nearly twice the value recorded at the surface. This result has important implications concerning air pollution models, which normally incorporate ozone amounts from meteorological stations at the surface.