Behaviour of aromatic hydrocarbons in urban atmospheres
Quasi-continuous measurement of benzene, toluene, ethylbenzene and isomers of xylene in northern England has been performed using an automated thermal desorption - gas chromatography - flame ionisation detector (TD-GC-FID) system. The TD-GC-FID system was mounted in a mobile laboratory, together with a TEOM (Tapered Element Oscillating Microbalance) system for particulate monitoring, and a weather station. Both systems have been applied to urban and suburban sites in order to examine the behaviour of BTEX aromatics at these localities, under a range of boundary layer environments. Measurement of BTEX aromatics and particulate matter has also been performed within a bus depot, a site which was effectively isolated from the free boundary layer overnight, to contrast the behaviour of BTEX in sunlit and non-sunlit atmospheres. Time series analysis of the concentration ratios of individual BTEX aromatics has been performed and diurnal seasonality has been demonstrated at a number of sites. Significant differential removal of individual BTEX aromatics has been found to occur overnight. Relative removal rates have been measured for this period, and values indicate that accepted tropospheric transformation mechanisms of the BTEX aromatics are not responsible. The available experimental and theoretical evidence collected in this research serves to support the supposition that a physical process, which is related to atmospheric temperature, volatility of individual BTEX aromatics, and pressures of individual BTEX aromatics, governs nocturnal differentiation of the concentration of BTEX aromatics. A postulated effect, which may contribute to nocturnal differential, is preferential absorption of particular BTEX aromatics to a liquid organic carbon phase, which has been shown to be associated with diesel carbonaceous combustion aerosol. The latter supposition has been partially corroborated by the findings of a separate study exploring the composition of carbonaceous combustion aerosol by GCMS, XRF, DTA, DSC, and FTIR.