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

Results from previous laboratory experiments indicate that both the protonated water dimer H3O+(H2O), and molecular cluster-ions, X+(H2O)n absorb infra-red (IR) radiation in the water vapour continuum region between 4 and 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 clear-sky conditions or periods of enhanced 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 545 m has been used to detect IR absorption by corona-generated positive atmospheric cluster ions. The column concentration of ions in the laboratory spectroscopy experiment was estimated to be ∼1013m-2; the column concentration of protonated atmospheric ions estimated using a simple model is ∼1014m-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 ambient carbon dioxide, the strongest absorption region is at 9.5-8.8μm (1050-1140cm-1) with a fractional change in transmissivity of 0.03±0.015, and the absorption at 12.5-12.1μm (800-825cm-1) is 0.015±0.008.