From Evans et al 2006. Methodology
P1.7 Measurements of the Radiative Surface Forcing of Climate (2006 - Annual2006_18climatevari)
2. METHODOLOGY The measurements of the downward atmospheric thermal emission were collected using a Magna 550 FTIR spectrometer or a high resolution Bomem DA8 system; the instruments were capable of resolutions of 0.25 cm^-1 and 0.02 cm^-1, respectively. Both instruments incorporated a liquid-nitrogen cooled, narrow-band, mercury cadmium telluride (MCT) detector with a 1 mm^2 element. The downward zenith sky radiation from the clear sky was collected by positioning a gold-coated mirror at the emission port along the optical axis of the instrument. A stored-phase correction was applied to the measured interferogram before conversion was made to the spectral domain in order to account for phase changes that were present at 750 and 2000 cm^-1. The thermal emission background of the instrument was characterized by measuring a negligible source of thermal radiation which consisted of a blackened dewar containing liquid nitrogen. The background measurement was taken immediately prior to and after the measurement of the sky radiation to ensure that the spectrometer was thermally stabilized. The calibration of the atmospheric measurements was performed by placing an ambient blackbody source beneath the gold mirror, filling the field-of-view of the spectrometer. The temperature of the blackbody was monitored by a chromelalumel thermocouple. The atmospheric emission measurements required 15-30 minutes of observing time. This resulted in a typical root-mean-square noise value of about 5.0×10^-9 W/(cm^2 sr cm^-1) in the midinfrared region. The greenhouse radiation from tropospheric ozone was measured by a technique in which the base of cold clouds was used as a target. The thermal emission from the warm atmosphere below the cloud was measured against the low background emission from the cold cloud base (Puckrin et al., 1996). The cloud also screened out the emission from the stratospheric ozone above it, effectively restricting the sampling area to the lower troposphere.
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