Technical Note: Improved total atmospheric water vapour amount determination from near-infrared filter measurements with sun photometers

[摘要] In this work we explore the effect of the contribution of the solar spectrum to the recorded signal inwavelengths outside the typical 940-nm filter's bandwidth. We employ gaussian-shaped filters aswell as actual filter transmission curves, mainly AERONET data,to study the implicationsimposed by the non-zero out-of-bandcontribution to the coefficients used to derive precipitable water from the measured water vapour band transmittance. Published parameterized transmittance functions are applied to the datato determine the filter coefficients.We also introduce an improved, three-parameter, fitting function that can describe the theoretical dataaccurately, with significantly less residual effects than with the existing functions.The moderate-resolution SMARTS radiative transfer code is used to predict theincident spectrum outside the filter bandpass for different atmospheres, solar geometries and aerosoloptical depths. The high-resolution LBLRTM radiative transfer code is used to calculate the watervapour transmittance in the 940-nm band. The absolute level of the out-of-band transmittance hasbeen chosen to range from 10⁻⁶ to 10⁻⁴, and typical response curves of commerciallyavailable 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 thederived columnar water vapour is mainly underestimated by a few percents. The actual errordepends on the specific out-of-band transmittance, optical air mass of observation and water vapouramount. Further investigationswill use experimental data from field campaigns to validate these findings.

[学科分类] 大气科学