Highly time-resolved ambient trace element concentrations in Paris during the MEGAPOLI summer campaign

Atmospheric aerosols are of interest for their influence on human health and climate forcing.Understanding of their chemical composition is thus a key factor to better understand thehealth effects and to provide guidance for the development of mitigation and/or reductionstrategies, especially for densely populated areas, such as megacities. The MEGAPOLIproject took place in Paris, France in summer 2009 and in winter 2010. Among a large suiteof instruments to collect and analyze the chemical composition of aerosols were two rotatingdrum impactors (RDI) capable of sampling particulate matter (PM) in three size ranges with2-hour time resolution. The RDIs were placed at city background and suburban sites. Eachcampaign lasted four weeks.The samples were analyzed with synchrotron radiation-induced X-ray fluorescencespectrometry (SR-XRF) at beamlines at the Swiss Light Source (SLS) at PSI, Villigen,Switzerland, and at DESY/HASYLAB in Hamburg, Germany. The photon flux ofsynchrotron facilities yields good signal-to-noise levels of the aerosol spectra in reasonabletime (typically 30 s per sample). We measured up to two dozen elements in parallel (from Nato Pb). Subsequently, the resulting element time series are analyzed with statistical methods toattribute possible sources.The MEGAPOLI summer campaign showed generally low PM concentrations, as the weatherwas rather unsettled with frequent rain. The maritime origin of air masses becomes manifestin Na, Mg and Cl time series, which are similar for both sites. Transition metals show higherconcentrations at urban background than at suburban sites, as do typical traffic-relatedelements (Cu, Ba, Sb). Most mass of these elements occurs in the largest size fraction (PM10-2.5), which hints towards road dust resuspension. In this way trace elements help to refine theapportionment of PM sources