Technical Note: Improved total atmospheric water vapour amount determination from near-infrared filter measurements with sun photometers F. Mavromatakis1, C. A. Gueymard2, and Y. Franghiadakis1 1Department of Scienses, Technological Education Institute of Crete, Estavromenos, P.O. Box 1939, 710 04 Heraklion, Crete, Greece 2Solar Consulting Services, P.O. Box 392, Colebrook, NH 03576, USA
Abstract. 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 employ gaussian-shaped filters as
well as actual filter transmission curves, mainly AERONET data, 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. Published parameterized transmittance functions are applied to the data
to determine the filter coefficients.
We also introduce an improved, three-parameter, fitting function that can describe the theoretical data
accurately, with significantly less residual effects than with the existing functions.
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 mainly underestimated by a few percents. The actual error
depends on the specific out-of-band transmittance, optical air mass of observation and water vapour
amount. Further investigations
will use experimental data from field campaigns to validate these findings.
Citation: Mavromatakis, F., Gueymard, C. A., and Franghiadakis, Y.: Technical Note: Improved total atmospheric water vapour amount determination from near-infrared filter measurements with sun photometers, Atmos. Chem. Phys., 7, 4613-4623, doi:10.5194/acp-7-4613-2007, 2007.