Abstract
Abstract. Absorption of solar radiation by water dimer molecules in the Earth's atmosphere has the potential to act as a positive feedback effect for climate change. There seems little doubt from the results of previous laboratory and theoretical studies that significant concentrations of the water dimer should be present in the atmosphere, yet attempts to detect water dimer absorption signatures in atmospheric field studies have so far yielded inconclusive results. Here we report spectral measurements in the near-infrared around 750 nm in the expected region of the | 0〈f | 4〉b|0 〉 overtone of the water dimer's hydrogen-bonded OH stretching vibration. The results were obtained using broadband cavity ringdown spectroscopy (BBCRDS), a methodology that allows absorption measurements to be made under controlled laboratory conditions but over absorption path lengths representative of atmospheric conditions. In order to account correctly and completely for the overlapping absorption of monomer molecules in the same spectral region, we have also constructed a new list of spectral data (UCL08) for the water monomer in the 750–20 000 cm−1 (13 μm–500 nm) range. Our results show that the additional lines included in the UCL08 spectral database provide an improved representation of the measured water monomer absorption in the 750 nm region. No absorption features other than those attributable to the water monomer were detected in BBCRDS experiments performed on water vapour samples containing dimer concentrations up to an order of magnitude greater than expected in the ambient atmosphere. The absence of detectable water dimer features leads us to conclude that, in the absence of significant errors in calculated dimer oscillator strengths or monomer/dimer equilibrium constants, the widths of any water dimer absorption features present around 750 nm are of the order of 100 cm−1 HWHM, and certainly greater than the 25–30 cm−1 HWHM reported in the literature for lower energy water dimer transitions up to 8000 cm−1.
Highlights
Absorption of solar radiation by water dimers in the Earth’s atmosphere has potentially important consequences for the planet’s radiative balance and for amplifying the feedback effects of climate change (e.g. Chylek and Geldart, 1997)
Our results show that the additional lines included in the UCL08 spectral database provide an improved representation of the measured water monomer absorption in the 750 nm region
In order to test the efficacy of our new UCL08 line list relative to HITRAN08 in the region expected for the dimer’s 0 f | 4 b |0 overtone, broadband cavity ringdown spectroscopy (BBCRDS) spectra of water vapour were recorded in synthetic air at atmospheric pressure for a variety of water monomer concentrations and temperatures
Summary
Absorption of solar radiation by water dimers in the Earth’s atmosphere has potentially important consequences for the planet’s radiative balance and for amplifying the feedback effects of climate change (e.g. Chylek and Geldart, 1997). There have been several attempts to detect signs of water dimer absorption directly in atmospheric field studies, with authors variously reporting a dimer absorption band around 750 nm (Pfeilsticker et al, 2003), a potential weak but inconclusive dimer signal in the same region (Sierk et al, 2004) or no observable dimer absorptions at all (Daniel et al, 1999; Hill and Jones, 2000) This disparity in field results has prompted further laboratory investigations into the absorption properties of the water dimer and/or water continuum. Shillings et al.: An upper limit for water dimer absorption in the 750 nm spectral region under atmospherically relevant conditions These studies have tended to concentrated on infrared frequencies up to 8000 cm−1, including the fundamental, first overtone and first combination band vibrations of the water monomer. A laboratory study by Kassi et al (2005) was unable to reproduce an absorption feature seen near 750 nm in field observations by the Pfeilsticker et al (2003) and initially attributed to the water dimer ( this claim to have observed the water dimer has since been revoked, Lotter, 2006)
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