Atmosphere continuum absorption investigation at MM waves

Investigation of atmospheric gases spectral properties is demanded to develop absorption models. Such models are very important for the global Earth's atmosphere monitoring. Most of atmospheric lines and continuum spectral parameters are obtained in laboratory experiments. Recovered atmospheric parameters accuracy of depends directly on accuracy of laboratory measurements. Parameterization of the continuum absorption, in particular, the water vapor continuum is one of the most difficult problems. The problem is related to yet unknown physical origin of the continuum, its weakness in comparison with absorption in regular discrete lines of water molecule, and general difficulties of the water-related measurements. The water-related continuum broadband measurement by the resonator spectrometer [1] revealed strong influence of water adsorbing on resonator elements on results of the continuum parameters measurements. Analysis [2] of the MM/SubMM continuum measurement methods used in the earlier studies leads to the conclusion that water adsorbing onto mirrors was a common factor influencing the water-related continuum parameters obtained in all well-known laboratory experiments. In all these cases gas cells with multiple reflections of radiation off inner mirrors were used as the most appropriate technique for weak continuum absorption investigation, although the problem of separation of water vapor absorption and absorption by water adsorbed on mirrors was not solved in the experiments. To overcome the problem it was proposed to use the cavity length variation method [1], which permits separation of these two types of absorptions and, as a result, the accuracy of continuum absorption measurements increases.