Source variability of WSOC in atmospheric precipitation

A wide range of organic (carbon containing) compounds produced both naturally by biological and chemical processes but also by mans activities (fires, vehicle and industrial emissions etc.) readily dissolve in water. Together these compounds are termed 'water soluble organic carbon', or WSOC for short. In some senses WSOC can be considered as a somewhat vague and all-encompassing term because it is a complex mixture of organic molecules which vary in their contributions to the total and are extremely difficult to isolate and analyse individually. Nevertheless, as a so-called 'bulk' component, WSOC is an important constituent of atmospheric water and is reactive. Its constituent molecules undergo sunlight-induced (photochemical) reactions that result in a range of reactive products which influence atmospheric chemical processes, influencing the climate on both local and global scales. WSOC can be an important source of cloud condensation nuclei (CCN) which are required for cloud formation, it contributes to rainwater acidity, and it can be a nutrient source to lakes, rivers and the ocean. Recent trends of increasing rainfall in the UK and worldwide mean that the need for detailed study of WSOC will become ever more important. In order to predict its impact on the environment, its various individual sources need to be better identified and characterised in terms of their composition and variabilty in time and space. Detailed chemical analysis of WSOC is costly, not readily accesible and in any case does not provide a high level of identification; rather only 'average compositions'. Recently a large fraction of WSOC was found to be fluorescent, which means that it emits electromagnetic radiation, especially visible light, if stimulated by the absorption of incident radiation from some source. The wavelength spectrum emitted can be analysed and used as a 'fingerprint' to characterise WSOC (and other forms of dissolved organic matter) of different 'types' (so-called 'flourophores') that enable a means to deduce information about different WSOC sources contributing to a sample and provide a measure of their relative importance (from their fluorescence intensities). The advantages of fluorescence are its relative simplicity and low cost. The idea of this work is to collect rainwater samples from specific rainfall 'events' and analyse these by fluorecence to identify the fluorophores present and their intensities. Using these measurements along with meteorological data from an adjacent meteorological mast and software that uses these data to predict areas from which air masses have originated, it will be possible to determine the regions in which the various source 'types' identified originated. This will be an important first step towards a better understanding of the origin and subsequent fate of WSOC in the environment. The results from this short-duration study will be used to help formulate a more detailed programme of WSOC research in the near future.