High accuracy transition intensities for ozone

Ozone is present in low concentrations throughout the Earth's atmosphere. In the troposphere ozone is a pollutant which largely results from human activity. However, ozone is harmful to humans, animals and plants at even trace concentrations. Conversely stratospheric ozone, the ``ozone layer'', provides an extremely important shield of solar ultraviolet radiation. Human activity has resulted in a significant reduction in stratospheric ozone and this loss has lead to increased holes at the poles.

Studies of atmospheric ozone concentrations rely heavily on the use of spectroscopic remote sensing from a mixture of ground-based, airborne and satellite instruments. These instruments observe the characteristic absorption features of ozone either in the infrared or the ultraviolet. Retrievals based on these observations require accurate laboratory data to make them useful. In particular the many studies of atmospheric currently being conducted require intensity / cross section data for both ultraviolet (UV) and infrared (IR) which is accurate to 1% or better.

Unfortunately, as has been extensively documented in the scientific literature, the situation with the laboratory intensity determinations is far from satisfactory. Firstly, there are many measurements showing systematic differences between atmospheric studies performed at infrared and ultraviolet wavelengths at the 4 to 5 % level. Secondly, while laboratory measurements of the ultraviolet cross sections show a measure of agreement, those for the infrared do not. A recent (2012) analysis concluded that for the key 10 micron region agreement between measurements was only at best 4% with intensity discrepancies much higher than this. There are other discrepancies within the infrared region. There is an urgent need for a solution to this problem for missions such as TES+OMI on Aura satellite mission (NASA), IASI+GOME-2 on Metop satellite (ESA) AIRS on the Auqa satellite (NASA).

The proposal will use high accuracy, first principles quantum mechanical methods to compute the transition intensities for both the IR and UV portions of the spectrum. For the IR region, methods of computing high accuracy dipole moment surfaces already used successful for water and CO2, will be employed. These will be combined with measured transition frequencies to complete line lists with intensities accurate to about 0.5%.

New methodologies will be developed to transfer the experience gained computing IR vibration-rotation intensities (which require electronically diagonal dipole moments) to electronic transitions in the UV. Initial work will focus on the Huggins band and will also require further development of the methods used for treating nuclear motion.

These calculations will provide complete independent assessment of the absolute line intensities / cross sections removed from experimental issues such as the ozone concentration. Results will be made widely available via the web, databases and submitted for inclusion in standard compilations used for atmospheric studies such as HITRAN. HITRAN will be a project partner on the proposal and undertake independent evaluation of the results.

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 (including ones relating to atmospheric physics) as the basis of popular talks which I give to schools and other
non-specialist groups.