Assessment of the consistency of near‐infrared water vapor line intensities using high‐spectral‐resolution ground‐based Fourier transform measurements of solar radiation

Abstract

[1] Calculations of the absorption of solar radiation by atmospheric gases, and water vapor in particular, are dependent on the quality of databases of spectral line parameters. There has been increasing scrutiny of databases such as HITRAN in recent years, but this has mostly been performed on a band-by-band basis. We report nine high-spectralresolution (0.03 cm À1 ) measurements of the solar radiation reaching the surface in southern England over the wave number range 2000 to 12,500 cm À1 (0.8 to 5 mm) that allow a unique assessment of the consistency of the spectral line databases over this entire spectral region. The data are assessed in terms of the modeled water vapor column that is required to bring calculations and observations into agreement; for an entirely consistent database, this water vapor column should be constant with frequency. For the HITRAN01 database, the spread in water vapor column is about 11%, with distinct shifts between different spectral regions. The HITRAN04 database is in significantly better agreement (about 5% spread) in the completely updated 3000 to 8000 cm À1 spectral region, but inconsistencies between individual spectral regions remain: for example, in the 8000 to 9500 cm À1 spectral region, the results indicate an 18% (±1%) underestimate in line intensities with respect to the 3000 to 8000 cm À1 region. These measurements also indicate the impact of isotopic fractionation of water vapor in the 2500 to 2900 cm À1 range, where HDO lines dominate over the lines of the most abundant isotope of H 2 O.Citation: Casanova, S. E. B., K. P. Shine, T. Gardiner, M. Coleman, and H. Pegrum (2006), Assessment of the consistency of nearinfrared water vapor line intensities using high-spectral-resolution ground-based Fourier transform measurements of solar radiation,

Intensity measurements of H216O lines in the spectral region 8000–9350cm−1

“…According to several articles [2,[17][18][19], this spectral region is incomplete and presents systematic errors on the line intensities. Even if a correction of the intensities was applied in 2000 in the HITRAN database, there are certainly other sources of error.…”

“…This decreases the intensities in regards of the lower energy level value of the transition (up to 19%). Some different studies [2,[19][20][21] performed after the correction made by Giver et al [14] proved that a problem still persists with the intensity values in this spectral region. For example in the study of the Venus deep atmosphere, Bailey [2] shows that for this spectral window, the data available in HITRAN2004 [16] are incomplete and also show a large systematic error on the line intensities.…”

“…For example in the study of the Venus deep atmosphere, Bailey [2] shows that for this spectral window, the data available in HITRAN2004 [16] are incomplete and also show a large systematic error on the line intensities. By measuring the absorption of solar radiation reaching the earth's surface, Casanova et al [19] find that the line intensities are underestimated by 18% in the range 8000-9500 cm À1 . Also Belmiloud et al [20] reported that the integrated intensities measured are higher than those derived from the HITRAN96 [11] database, while Smith et al [21] found the opposite.…”

Sensitivity of model-based quantitative FTIR to instrumental and spectroscopic database error sources

“…However, uncertainties associated with the measurements are not always reported and so the uncertainty information available is variable. Various authors have critiqued the accuracy of the parameter information in the database comparing real measurements to forward models, and also examining the internal consistency and the affects of the changes made as a result of each update [32][33][34][35].…”

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

“…These instruments have been used for many years for ground-based remote sensing of the atmosphere using the Sun as a source [3][4][5], and the National Physical Laboratory (NPL) has been involved in various aspects of the validation of these measurements [6][7][8]. To date, these measurements have not included an absolute radiative calibration of the FTIR signal that is required to study broadband continuum effects.…”