Observations of extinction in the atmospheric continuum at 19 frequencies between 1202 and 479 cm−1 have been made by two methods: (I) solar absorption, between 1202 and 790 cm−1; (II) 200 and 400 m horizontal paths using a Nernst source, between 716 and 479 cm−1. Method I was made absolute by reference to a blackbody; Method II provided extinction coefficients relative to that for 1119 cm−1, given by Method I. In Method I, the water vapor in the path was deduced from the area of the line at 871 cm−1, and some measure of the particulate matter in the path was obtained from the extinction in the visible. In Method II, the horizontal visibility was used for the latter purpose, and observations with a cell of pure CO2 were made in order to determine the limits of CO2 absorption.
The following conclusions are drawn from the observations: (1) The results strongly support Elsasser’s suggestion that the continuum is mainly due to the accumulated wings of distant lines in the water-vapor bands at 1595 and 200 cm−1. The observed absorption coefficient increases from 0.06 g−1 cm2 at 1100 cm−1 to about 4.1 g−1 cm2 at 480 cm−1, though nearby lines account for half of the latter. (2) None of the line shapes examined gives a satisfactory representation of the continuum over the whole range studied, though Lorentz shape, in both approximate and exact form, gives qualitative agreement with observation. (3) In the interpretation of the observations reported here, CO2 absorption may be ignored outside the frequency range 790 to 560 cm−1. It is much more confined than would be the case if the CO2 line wings had Lorentz structure. (4) The extinction due to particulate matter is small compared with that due to water vapor throughout the range observed, except under very hazy dry conditions. (5) Evidence is presented for a considerable self-broadening effect (about 30 times air broadening), and if correct, this will be of the utmost importance in the analysis of remote terrestrial radiation measurements which involve correction for the water-vapor continuum. This must, however, be confirmed under controlled conditions.
© 1963 Optical Society of AmericaFull Article | PDF Article
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