SOAPEX-2 is primarily an experiment to study atmospheric cleansing by free radicals in extremely clean and slightly perturbed tropospheric air and focuses on a field campaign carried out at Cape Grim, Tasmania in January-February1999. Oxidation of almost all trace gases released into the atmosphere is initiated by hydroxyl (OH) radicals, produced mainly from the action of near-UV light on ozone in the presence of water vapour. Increasing evidence suggests that the oxidative capacity of the troposphere has been perturbed in recent years due to the emission of gases such as methane, carbon monoxide, non-methane hydrocarbons and nitrogen oxides from man-made sources. These perturbations may be causing changes in the natural atmospheric composition, for instance increasing tropospheric levels of the greenhouse gas ozone, which has important consequences for climate and human health. It is also possible that the rates of oxidation of gases such as methane, and production of sulphate aerosols from the oxidation of sulphur dioxide, have been modified. Taken together a change in the oxidative capacity of the atmosphere has many consequences for the long-term stability of the Earth's climate.

SOAPEX-2 builds upon the success of the original SOAPEX-I experiment conducted at Cape Grim in January/February 1995 which resulted in the publication of several papers to the literature on the relationship between concentrations of peroxy radicals and uv light levels in different NOx concentration regimes, and the consequences for ozone production and loss in the marine boundary layer. SOAPEX-2 is a more complete experiment with the addition of atmospheric measurements of key new species including hydroxyl, hydroperoxyl, halogen oxide and nitrate radicals, non methane hydrocarbons, speciated aldehydes, PAN and halocarbons. SOAPEX-2 involves four groups of tropospheric scientists from the UK and Australia, namely the Universities of East Anglia, Leeds and Leicester along with CSIRO (Commonwealth Scientific Research Organisation), Melbourne.

The clean air photochemistry experiment is an essential prerequisite for experiments carried out in more polluted atmospheres. The data obtained is allowing rigorous testing of basic mechanisms which describe the behaviour of free radical concentrations at differing light levels, water vapour and nitrogen oxide concentrations, etc. The measurements performed in this project are expected to yield valuable information on chemical changes that are affecting the oxidative capacity of the global troposphere and, therefore, the rate at which the global atmosphere can cleanse itself of pollutants. The measurements are also highly relevant to the situation in more polluted atmospheres, where increased levels of confidence in our understanding of atmospheric chemistry is an essential prerequisite to any legislation designed to reduce regional and global pollution.

The specific objectives of SOAPEX-2 are:

The main feature of the work programme was a ground-based measurement campaign at the Cape Grim Baseline Atmospheric Pollution Station (CGBAPS) in January/February 1999. The instruments deployed during the field campaign and the partners responsible for each measurement (defined below) are summarised in Tables 1 and 2.