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We describe here the instrumental setup and first experiments with the mobile single particle analysis and sizing system (SPASS) for the on-line characterization of single atmospheric aerosol particles. Aerosols are introduced into the SPASS via a differentially pumped particle inlet system using an aerodynamic lens that forms a narrow particle beam. The particles are sized with a two-laser velocimeter and subsequently desorbed and ionized with a highpower pulsed Nd:YAG laser operating at 266 nm. Positive and negative ions formed are simultaneously detected in a bipolar time-offlight mass spectrometer. Thus, the size and chemical composition of single aerosol particles can be characterized simultaneously in real time. The SPASS system has been installed inside a truck, creating a mobile unit. The performance of the SPASS in terms of mass resolution and sizing capabilities of the laser velocimeter has been evaluated. Positive and negative mass spectra from different types of particles have been obtained to identify "typical" peak patterns. The relative detection sensitivity depending on particle size and chemical composition was studied. Significant differences in detection sensitivities for different compounds were observed, demonstrating that the results obtained from ambient single particle measurements are strongly biased and dominated by "easy-to-detect" particles. The instrument performance is illustrated with results from a 24 h measurement period during winter in Milan, Italy. The period encompasses two meteorologically different episodes, a period of stagnant conditions, where regional background pollutants contribute significantly and the aerosol is dominated by ammonium nitrate and sulfate, and a North-Foehn event, where accumulation mode particles are scavenged and the urban aerosol population is dominated by organic matter due to local emissions. INTRODUCTIONThe importance of aerosols in chemical and physical processes involved in, for example, air pollution (human health, acid deposition, and global warming), combustion (soot and soot precursors), materials synthesis and processing (nanoparticles and coatings), and clean-room technology has increased dramatically in the past years. Their physical and chemical characterization and the study of their composition and reaction mechanisms represent an area in which basic information is still needed (Andreae and Crutzen 1997). It has become important not only to perform bulk analysis of aerosol particles but also to be able to analyze single aerosol particles, on-line in real time (Peter 1996). This capability will allow us in particular to investigate the mixing state of the particles. In an internally mixed aerosol, all particles have roughly the same chemical composition, which can be a mixture of inorganic and organic components. Externally mixed particles show chemically different subpopulations, e.g., with a fraction of the particles dominated by organic compounds and another fraction dominated by inorganic compounds. The mixing state is of relevan...

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