Remote Sensing Measurements of Tropospheric Ozone by Ground-Based Thermal Emission Spectroscopy

Remote sensing measurements of the troposphere were obtained by a new technique in which spectra of the downward thermal radiation from tropospheric ozone were measured with a Fourier transform infrared spectrometer. The measurements were obtained near Peterborough, Ontario, about 100 km northeast of Toronto, a large metropolitan center. This site is subjected frequently to air pollution on days with a southerly air flow; days with northerly flows are characterized by clean air. The measurements were made for both cloudy skies and hazy, polluted conditions. For the former, the thermal radiation from ozone was measured against the background blackbody emission from the cloud; the cloud effectively forming a barrier against the detection of ozone in the upper atmosphere. In the case of hazy, polluted conditions, thermal emission spectra of the atmosphere were measured at a large zenith angle to enhance the detection of tropospheric ozone near the surface. Simulations of the background atmospheric emission, carried out using FASCOD3, were performed to extract the ozone emission band, from which ozone amounts were determined. The amounts of ozone derived from the remote sensing techniques were found to agree well with the results reported from in situ measurements obtained at the 440-m level of the CN Tower in Toronto. Agreement with the results from a more northerly station, Dorset, was also good for clean atmospheric conditions; however, the measurements obtained for polluted conditions at Peterborough were significantly higher than those for Dorset. This indicated that the polluted air mass may have dissipated by the time it reached Dorset, or the polluted layer was sufficiently localized as not to be detected by the in situ sensor located on the ground. These measuring techniques for tropospheric ozone may be potentially valuable in light of the fact that few remote sensing techniques exist for air pollution analysis.