Instrument characterization and first application of the single particle analysis and sizing system (SPASS) for atmospheric aerosols

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 high-power pulsed Nd:YAG laser operating at 266 nm. Positive and negative ions formed are simultaneously detected in a bipolar time-of-flight 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.