Seasonal variation of fine particulate composition in the centre of a UK city

Abstract

In order to better project the impacts of air quality control measures, it is necessary to study the behaviour of particulates beyond mass-based metrics. To study the long term and seasonal trends of particulate chemical composition in Manchester, UK, seven Aerosol Mass Spectrometer (AMS) datasets obtained from ambient sampling in central Manchester over a seven year period have been combined and compared. The size-resolved submicron nitrate, sulphate, ammonium and organic matter data were examined to assess the relative importance of season, local meteorology and synoptic transport in determining ambient concentrations. Systematic trends were observed that were not previously possible through the analysis of individual datasets in isolation. The results obtained here indicate that prevailing westerly winds tend to bring relatively consistent low levels of pollutants, whereas Easterly and Northerly winds bring much more variable pollutant concentrations, whose chemical compositions are strongly dependent on season. Specifically, continental air in the summer brings with it high levels of sulphates whereas in winter similar synoptic conditions and cold ground temperatures tend to induce strong inversions, trapping local pollution and leading to elevated organic matter. Local temperature was also found to be important, with the very low temperatures in winter and very high temperatures in summer giving the greatest concentrations of all components except ammonium nitrate in summer, which began to evaporate at the higher temperatures. These results will assist in evaluating the chemical nature of air pollution episodes and also provide a phenomenological basis for testing chemical transport and exposure models. While the results focus on Manchester, similar phenomena could be expected of a number of UK cities.