Remote measurement of tropospheric gases under cloud with the AERI Instrument

The remote measurement of tropospheric gas concentrations can be achieved by utilizing thermal emission under cloud. The cloud deck acts as a cold optically thick blackbody at the cloud base temperature. The emission features from the atmospheric gases below the cloud are superimposed on the cloud blackbody emission. In the thermal spectral region, there are bands and lines from ozone, carbon dioxide, water vapor, methane, nitrous oxide, CFCs, nitric acid and many other gases. The measurements made under cloudy conditions enable the radiative flux from tropospheric O3 to be determined since the downward emission from the stratospheric ozone is blocked by the cloud layer. The measurement of the surface forcing irradiance flux from gases requires a well calibrated high resolution measurement of spectral radiance in the thermal infrared region from 700 to 2500 wave numbers (cm-1). Such measurements are made routinely with the AERI instruments at the DOE ARM sites. Well calibrated infrared spectral measurements of the downward infrared thermal radiation have been routinely made by the robotic AERI instruments at the three main ARM sites for over 11 years with a 15-year record at the ARM South Great Plains site. The AERI instrument is a Fourier Transform Spectrometer which has internal blackbody calibration sources. It is fully automated for operation over the long term. Zenith spectra of the radiance from clouds above the sites will be shown. The features from carbon dioxide, water vapor, ozone and several greenhouse gases are evident. Although the technique was designed for radiative forcing measurements of greenhouse gases, the conversion to mixing ratios is simple. Of particular interest is the measurement of ozone below cloud since the combination of the total column and the cloud base height yields a mean mixing ratio below the cloud base. Although the technique was designed for radiative forcing measurements of greenhouse gases, the conversion of this quantity into mixing ratios is simple. Examples of measurements of the ozone mixing ratio in the lower troposphere will be shown. A time series of the tropospheric ozone mixing ratio measurements will be shown. Sample measurements of CFC11, CFC12 and nitric acid mixing ratios will be shown. Potential applications to air quality field projects will be discussed.