Field Measurements of Water Continuum and Water Dimer Absorption by Active Long Path Differential Optical Absorption Spectroscopy (DOAS)
530 | 530 Physics
arxiv: Physics::Atmospheric and Oceanic Physics
Water vapor plays an important role in Earth's radiative budget since water molecules strongly absorb the incoming solar shortwave and the outgoing thermal infrared radiation. Superimposed on the water monomer absorption, a water continuum absorption has long been recognized, but its true nature still remains controversial. On the one hand, this absorption is explained by a deformation of the line shape of the water monomer absorption lines as a consequence of a molecular collision. One the other hand, water dimers possibly contribute to water continuum absorption. Field measurements addressing water continuum and water dimer absorption in the visible and near-infrared spectral region were carried out by means of active Long Path DOAS. The spectral regions of the 4n, 4n+d, and 5n water polyads were selected for the measurements. In these regions three water dimer absorption bands are predicted to exist almost free of interference by strong water monomer absorption. Within the scope of this thesis it is shown that the quality of existing spectral line databases is insuffcient to provide accurate water monomer references in order to confidently detect superimposed water dimer absorption. Therefore, only upper limits of water dimer absorption are obtained. An upper limit of Kp(301K)=0.055/atm is inferred for the water dimer equilibrium constant. The water dimer band broadening is at least 40/cm (FWHM). Water continuum absorption is detected in all three water bands. The measured water continuum absorption and the predictions by the semi-empirical CKD_2.4.1 and MT_CKD_1.0 water continuum models are of same order of magnitude. In contrast, the Ma and Tipping far wing line shape theory underestimates water continuum absorption by one order of magnitude in the 4n and 5n water bands, and by two orders of magnitude in the 4n+d water band. Based on the present state of knowledge about the spectroscopic and thermochemical properties of the water dimer, their contribution to the absorption the visible and near-infrared spectral region is minor.