Seasonal Oxidant Observations in the Tropical North Atlantic Atmosphere

The tropical marine boundary layer acts as an engine room for the self cleansing of the Earths atmosphere, - a result of high solar radiation and an abundance of water vapour and ozone. It is in these regions that the effects of certain greenhouse gases may be acutely influenced, since the oxidation chemistry occurring is potent in controlling the lifetime and hence future concentrations of gases such as methane and ozone. Atmospheric observations in tropical regions are however sparse and for the dominant atmospheric oxidant OH almost non-existent. A measurement of OH alone however is of only limited use, - the key to improving and testing our understanding is to subsequently compare those concentrations measured locally with concentrations calculated from first principles when all other trace species which form and react with OH are accounted for. This requires a formidable array of analytical instrumentation to measure species such as ozone, nitrogen oxides, organic and halogen compounds and aerosols. To attempt to understand OH over a seasonal cycle, which is of by far the most value to global and climate models, is feasible only where long time series observations of other gases can also be made. Within this proposal we identify that to obtain year-round measurements of OH, and its partner species HO2, in conjunction with a highly instrumented atmospheric observatory located within the tropical marine boundary layer would be a unique and world leading experiment. Our experiment places a laser induced fluorescence instrument capable of high sensitivity HO/HO2 measurements at the SOLAS Cape Verde atmospheric Observatory. To support nearly one year of radical observations starting in 2008, we propose that the detailed trace gas time series at the Observatory be extended by a further 9 months to end of Apr 2009. Numerous links and users of the potential dataset can be identified already in the global atmospheric modelling communities.