Aerosols

Dr Hugh Reiley of the Department of Chemistry and Dr Andrew Hodgson of the Surface Science Research Centre have been successful in securing a major grant from the National Environment Research Council to study reactions which are believed to be crucial to the depletion of ozone in the stratosphere. The £120k award will allow the continuation of Dr Reiley's NERC funded research into the chemical reactions which occur on the surfaces of model Polar Stratospheric Clouds and fund the start of a new project which aims to study the rates of reaction of atmospheric trace gases with liquid aerosol droplets.

The ozone layer is to be found in a narrow vertical 5km region about 20km above the Earth's surface. It is vital to our continued existence on the planet since it absorbs harmful solar ultraviolet radiation (UV-B) and is largely responsible for the thermal structure of the stratosphere which ensures the integrity of our immediate atmosphere, the troposphere. The problems of ozone destruction in the Antarctic are well-known to us all in the shape of the infamous 'ozone hole', which re-occurs every springtime above the South Pole. The 'hole' is actually a temporary, if drastic, depletion of the equilibrium concentration of ozone present in this region of the stratosphere. The chemistry which gives rise to this temporary loss is due to the heterogeneous processing of chlorine and nitrogen oxides on the surfaces of water and nitric acid/water ice particles over the Antarctic winter (which is dark) to produce reactive but latent compounds. When the sunshine returns in the spring, these chlorine compounds are broken down by the light to produce chlorine atoms when then destroy the ambient ozone. As the stratosphere warms up the PSCs melt and the ozone levels rise due to the influx of stratospheric air from mid-latitudes. The overall result over a period of years has been a gradual decline in the average concentration of ozone.

Although many of us are aware of the ozone hole over the Antarctic, its effects seem remote: the only animal-life affected are penguins and a few polar bears! However, when we ourselves become affected by reductions in levels of stratospheric ozone we (and our politicians) start to pay a little more attention. We are all more aware of the increased incidence of melanomas (skin cancer) in Oceania and of the increased skin protection required if we ourselves venture into the sun, but how many of us realise that annual reductions in ozone also occur over the Arctic? Or read the article in The Independent earlier this year which reported the record lowest ozone concentration above the UK since records began - a reduction of almost 50% since the 1980s.

It has been recently realised that heterogeneous reactions are in fact more general than originally conceived: any aqueous acid aerosol can act as an agent for the catalytic production of compounds which are potentially destructive for the ozone layer. This means that ozone destruction takes place on the surfaces of sulphuric and nitric acid based aerosols which are abundant in the troposphere. Dr Reiley and Dr Hodgson's new experiment is designed to investigate the rates and mechanisms of these potentially damaging reactions.