An upper limit for water dimer absorption in the 750 nm spectral region and a revised water line list

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⟩[subscript f] | 4⟩ [subscript 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[superscript −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[superscript −1] HWHM, and certainly greater than the 25–30 cm[superscript −1] HWHM reported in the literature for lower energy water dimer transitions up to 8000 cm[superscript −1]

Measurements and modelling of molecular iodine emissions, transport and photodestruction in the coastal region around Roscoff

Iodine emissions from the dominant six macroalgal species in the coastal regions around Roscoff, France, have been modelled to support the Reactive Halogens in the Marine Boundary Layer Experiment (RHaMBLe) undertaken in September 2006. A two-dimensional model is used to explore the relationship between geographically resolved regional emissions (based on maps of seaweed beds in the area and seaweed I[subscript 2] emission rates previously measured in the laboratory) and in situ point and line measurements of I[subscript 2] performed respectively by a broadband cavity ringdown spectroscopy (BBCRDS) instrument sited on the shoreline and a long-path differential optical absorption spectroscopy (LP-DOAS) instrument sampling over an extended light path to an off-shore island. The modelled point and line I[subscript 2] concentrations compare quantitatively with BBCRDS and LP-DOAS measurements, and provide a link between emission fields and the different measurement geometries used to quantify atmospheric I[subscript 2] concentrations during RHaMBLe. Total I[subscript 2] emissions over the 100 km[superscript 2] region around Roscoff are calculated to be 1.7×10[superscript 19] molecules per second during the lowest tides. During the night, the model replicates I[subscript 2] concentrations up to 50 pptv measured along the LP-DOAS instrument's line of sight, and predicts spikes of several hundred pptv in certain conditions. Point I[subscript 2] concentrations up to 50 pptv are also calculated at the measurement site, in broad agreement with the BBCRDS observations. Daytime measured concentrations of I[subscript 2] at the site correlate with modelled production and transport processes. However substantial recycling of the photodissociated I[subscript 2] is required for the model to quantitatively match measured concentrations. This result corroborates previous modelling of iodine and NO[subscript x] chemistry in the semi-polluted marine boundary layer which proposed a mechanism for recycling I[subscript 2] via the formation, transport and subsequent reactions of the IONO[subscript 2] reservoir compound. The methodology presented in this paper provides a tool for linking spatially distinct measurements to inhomogeneous and temporally varying emission fields