Absolute high spectral resolution measurements of surface solar radiation for detection of water vapour continuum absorption

Abstract

Solar-pointing Fourier transform infrared (FTIR) spectroscopy offers the capability to measure both the fine scale and broadband spectral structure of atmospheric transmission simultaneously across wide spectral regions. It is therefore suited to the study of both water vapour monomer and continuum absorption behaviours. However, in order to properly address this issue, it is necessary to radiatively calibrate the FTIR instrument response. A solar-pointing high-resolution FTIR spectrometer was deployed as part of the 'Continuum Absorption by Visible and Infrared radiation and its Atmospheric Relevance' (CAVIAR) consortium project. This paper describes the radiative calibration process using an ultra-high-temperature blackbody and the consideration of the related influence factors. The result is a radiatively calibrated measurement of the solar irradiation at the ground across the IR region from 2000 to 10 000 cm −1 with an uncertainty of between 3.3 and 5.9 per cent. This measurement is shown to be in good general agreement with a radiative-transfer model. The results from the CAVIAR field measurements are being used in ongoing studies of atmospheric absorbers, in particular the water vapour continuum.

Can Measurements of the Near‐Infrared Solar Spectral Irradiance be Reconciled? A New Ground‐Based Assessment Between 4,000 and 10,000 cm−1

“…In addition, the instrument relies on an external optical setup which is exposed to the elements (Gardiner et al, ). This results in a loss of mirror reflectivity over time, which must be corrected for, but this was not done in Menang et al ().…”

The effect of chosen extraterrestrial solar spectrum on clear-sky atmospheric absorption and heating rates in the near infrared

“…Menang et al [3] have recently produced a high-resolution ESS ("CAVIAR ESS") in the near-IR from 2000-10000 cm -1 (1-5µm). The CAVIAR ESS is based on a Langley analysis of ground-based sun-pointing measurements using a calibrated high-resolution (0.03 cm -1 ) Fourier Transform Spectrometer (see [4] for the instrument configuration) made by the National Physical Laboratory as part of the UK consortium "Continuum Absorption by Visible and Infrared radiation and its Atmospheric Relevance" (CAVIAR). The analysis has been shown to be capable of reproducing the fine resolution features of the solar spectrum seen in limited spectral regions in earlier measurements.…”

Water in the gas phase

“…Gardiner et al [6] demonstrate the necessary steps required for radiative calibration of a sun-pointing spectrometer, and show that the spectral irradiance reaching the surface can be measured to better than 6 per cent uncertainty. Airborne and satellite measurements analysed by Newman et al [7] indicate broad agreement with the laboratory measurements in Ptashnik et al [4], but they present evidence that there may be a hitherto undetected temperature dependence in the continuum of water-air mixtures at atmospheric temperatures.…”