Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 7 3 2007 10.5194/acpd-7-6113-2007 http://www.atmos-chem-phys-discuss.net/7/6113/2007/ http://www.atmos-chem-phys-discuss.net/7/6113/2007/acpd-7-6113-2007.html http://www.atmos-chem-phys-discuss.net/7/6113/2007/acpd-7-6113-2007.pdf 6113 6141 2007-05-08 Improved total atmospheric water vapour amount determination from near-infrared filter measurements with sun photometers F. Mavromatakis fotis@stef.teiher.gr C. A. Gueymard Y. Franghiadakis Department of Scienses, Technological Education Institute of Crete, Estavromenos, P.O. Box 1939, 71004 Heraklion, Crete, Greece Solar Consulting Services, P.O. Box 392, Colebrook, NH 03576, USA In this work we explore the effect of the contribution of the solar spectrum to the recorded signal in wavelengths outside the typical 940-nm filter's bandwidth. We use gaussian-shaped filters as well as actual filter transmission curves to study the implications imposed by the non-zero out-of-band contribution to the coefficients used to derive precipitable water from the measured water vapour band transmittance. The moderate-resolution SMARTS radiative transfer code is used to predict the incident spectrum outside the filter bandpass for different atmospheres, solar geometries and aerosol optical depths. The high-resolution LBLRTM radiative transfer code is used to calculate the water vapour transmittance in the 940 nm band. The absolute level of the out-of-band transmittance has been chosen to range from 10^−6 to 10^−4, and typical response curves of commercially available silicon photodiodes are included into the calculations. It is shown that if the out-of-band transmittance effect is neglected, as is generally the case, then the derived columnar water vapour is systematically underestimated by a few percents. 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