Short wavelength absorption by water vapour

The two dominant radiative transport processes in our atmosphere are absorption of incoming sunlight, and the absorption of outgoing radiation in what is commonly called the greenhouse effect. Despite occurring at significantly different wavelengths, the rotation-vibration spectrum of water is both the dominant absorber of sunlight and the major greenhouse gas. Thus the rotation-vibration spectrum of water is, by some distance, the single most important spectrum for atmospheric processes. Accurate knowledge of water spectra is required for models of global radiative transport and the earth's energy budget and for more detailed studies such as retrievals of column densities and profiles of other species by remote sensing. The spectrum of water is of course very well studied but remains a challenge: it is very extended, complicated (with no regular structure at high resolution) and the intensities of individual atmospherically important transitions have a huge dynamic range. The demands of modern remote sensing satellites require water line intensities with high accuracy for both monitoring water columns and, because water absorption is so ubiquitous that its lines interferes other retrievals, for detection of a long list of trace species. Failure to model water absorptions accurately at best introduces a major source of error into retrievals and at worst can mean they fail altogether thus severely degrading the usefulness of remote observations. Many species, such as HONO, OClO, NO2, SO2, O3, BrO, HCHO, O4, IO and Glyoxal are monitored using their ultraviolet (UV) spectrum. It has become apparent from recent atmospheric studies that accurate representation of water absorption in the near UV is essential for their accurate retrieval. Retrieval of water columns is a major and important activity. Retrieval of water columns in the near UV has significant advantages since the Earth reflects sunlight in a much more uniform fashion at these wavelengths and the weaker absorption means that optical thickness effects which prevent the determination of reliable water columns in humid atmospheres are largely eliminated.

However, precise retrievals rely on the availability of accurate laboratory data which are largely lacking. Satellites flying or planned such as NASA's first Earth Venture Instrument Class mission TEMPO (Tropospheric Emissions: Monitoring Pollution) mission, ESA's Sentinal series and Korea's GEMS (geostationary environmental monetaring satellite) mission will analyse the chemical composition of air with high spatial resolution at near UV wavelengths. All these missions will require high quality laboratory data for water over an extended wavelength range stretching into the near-UV. At present these data are simply not available: there are no direct, high-resolution laboratory or atmospheric measurements of water vapour spectra in the region, and atmospheric database such as HITRAN, contain no relevant information on it.

The aim of this proposal is to provide comprehensive and accurate data on water absorption at short wavelengths. These data will be generated using techniques of first principle quantum mechanics that have been successfully applied to both absorption by water vapour at longer wavelengths and other key atmospheric species. Where possible the positions of absorption features will be adjusted using laboratory measurements. The resulting line lists will be made available to key groups involved monitoring the Earth's atmosphere in the near UV, placed in data depositories and made available to databases such as HITRAN.

There is considerable interest in accurate line lists from the non-academic community. Besides the needs of the atmospheric remote sensing community (discussed under academic beneficiaries), there are many others involved in environmental monitoring. For example we have a collaboration with Servomex plc. Servomex undertakes gas analysis for hydrocarbon processing, industrial gas production and respiratory medicine markets. They are very interested in using line lists generated by my group at UCL for work in optimising their products.

The line intensities (and associated line list) generated in this project will be distributed widely and vigorously to maximise
its potential impact. It will be made available via (atmospheric) databases HITRAN and GEISA; via BADC and via
web portals such as the Virtual Atomic and Molecular Data Centre (VAMDC), and my own exomol.com website.

My work on molecular spectroscopy regularly gets extensive national and international press coverage. I use topics from
this work as the basis of popular talks which I give to schools and other non-specialist groups: one of these talks is explicitly on water.