Absorption of infra-red radiation by atmospheric molecular cluster-ions

Abstract

Results from previous laboratory experiments indicate that both the protonated water dimer H 3 O + (H 2 O), and molecular cluster-ions, X + (H 2 O) n absorb infra-red (IR) radiation in the water vapour continuum region between 4-14µm (2500-714cm -1 ).Protonated water clusters are a common species of atmospheric molecular cluster-ion, produced by cosmic rays throughout the troposphere and stratosphere. Under clearsky conditions or periods of increased atmospheric ionisation, such as solar proton events, the IR absorption by atmospheric ions may affect climate through the radiative balance. Fourier Transform Infrared Spectrometry in a long path cell, of path length 545m has been used to detect IR absorption by corona-generated positive molecular cluster-ions. The column concentration of ions in the laboratory spectroscopy experiment was estimated to be ~10 13 m -2 ; the column concentration of protonated * Corresponding author: email k.l.aplin@rl.ac.uk 2 atmospheric ions estimated using a simple model is ~10 14 m -2 . Two regions of absorption, at 12.3 and 9.1µm are associated with enhanced ion concentrations. After filtering of the measured spectra to compensate for spurious signals from neutral water vapour and residual carbon dioxide, the strongest absorption region is at 9.5 to 8.8 µm (1050 cm -1 to 1140 cm -1 ) with a fractional change in transmissivity of 0.03 ± 0.015, and the absorption at 12.5 to 12.1 µm (800 cm -1 to 825 cm -1 ) is 0.015 ± 0.008.

Terahertz vibrational properties of water nanoclusters relevant to biology

“…Ambient water vapour containing 10 17 [18]. This greatly enhances the possible contribution of water clusters to radiation absorption and emission and has been argued to explain the strong far-infrared and submillimeter absorption of radiation in Earth's atmosphere [3,4].…”

“…Scientific interest in water nanoclusters has been motivated by their possible roles in biology [1,2], atmospheric science [3,4], and astrophysics [5], as well as by their relevance to the structure and properties of liquid water [6]. In a recent letter by this author and collaborators [7], it has been suggested that because of their large electric dipole moments, protonated water clusters such as (H 2 O) 21 H + shown in Fig.…”

Perspective: Water cluster mediated atmospheric chemistry

“…5-7, 10, 22, 43, 44 Water complexes can be diffuse absorbers of solar radiation, contributing to atmospheric heating, an effect resulting from changes occurring upon complex formation in the IR and near IR spectra of constituent monomers. 7,8,[10][11][12]45 Intermolecular interactions responsible for complex formation cause shifts and increased linewidth in ro-vibrational spectra of water complexes compared with the constituent monomers, thereby enhancing their "greenhouse" gas potential. 8,11 Only water complexes with abundant atmospheric gases have atmospheric concentrations large enough to affect radiative transfer.…”

Atmospheric ionisation in Snowdonia

“…Laboratory experiments showed that atmospheric cluster-ions can absorb radiation in the 9-12 µm range of the infra-red (IR) spectrum [10]. However, it has not been possible to detect any relationship between atmospheric ionisation and the downwelling broadband IR radiation in clear sky at the Snowdon Summit site, due to atmospheric temperature effects modulating both the muon data and downwelling IR.…”