400 600 800 1000 1200 1400 1600 Wavenumber (cm1)

FIGURE 14-4 Infrared emission from earth measured from the Nimbus 4 satellite (a) over the Niger Valley, North Africa (14.8°N, 4.7°W) at 12:00 GMT; (b) over Greenland (72.9°LN, 41.1°W) at 12:18 GMT, and (c) over Antarctica (74.6°S, 44.4°E) at 11:32 GMT. Emissions from blackbodies at various temperatures are shown by the dotted lines for comparison (adapted from Hanel et al, 1972).

acteristic of the particular temperature of the atmosphere where the emitting molecule is located. Contributions from C02 (600-750 cm"1) and 03 (1000-1070 cm"1) as well as from H20 [rotation bands below ~625 cm"1 (e.g., see Clough et al., f992) and vibration-rotation bands in the region from ~ 1200 to 2000 cm"1] are evident. There are also smaller contributions from other greenhouse gases such as CH4 and N20 (vide infra). Figures 14.4b and 14.4c show that the surface temperatures of Greenland and Antarctica are much colder, ~240 and 200 K, respectively, at the times these spectra were recorded. The emissions from other greenhouse gases such as CFC-11 and CFC-12 have also been observed from the earth's surface in downward radiation (e.g., see Walden et al., f 998).

The fact that both absorption and reemission contribute to the spectral features assigned to C02 and 03 is illustrated by the spectra in Fig. f4.4 as well. Over North Africa (Fig. f4.4a), the air temperature is less than that at the surface, so that the emission intensity around f040 cm"1 due to atmospheric 03 is less than its absorption, leading to a distinct "negative" band superimposed on the continuous emission curve. The

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