Mass spectrometry of individual particles between 50 and 750 nm in diameter at the Baltimore supersite

The performance of the real-time single-particle mass spectrometer RSMS III is evaluated for ambient fine and ultrafine particle number concentration measurements. The RSMS III couples aerodynamic size selection with laser ablation time-of-flight mass spectrometry for single-particle analysis. It was deployed at the Baltimore particulate matter Supersite for semi-continuous operation over an 8-month period. The sampling protocol adopted for this study permitted the analysis of on average 2000 particles per day. The number of particles analyzed is a tradeoff between generating a statistically significant data set and maintaining instrument operation over a long period of time. The optimum particle size range of analysis was found to be ca. 50-770 nm in diameter, although particles as small as 45 nm and as large as 1250 nm were also analyzed. While nitrate, sulfate, and carbon (elemental and organic) were found to dominate the ambient aerosol, over 10% of the detected particles contained transition and/or heavy metals. The (size-dependent) detection efficiency, defined as the fraction of particles entering the inlet that are analyzed, was determined by comparison with scanning mobility particle sizing data. Using the experimentally determined detection efficiencies, particle number concentrations of specific chemical components were estimated. While the sampling protocol allowed the particle concentrations of major chemical components to be followed as a function of both time and particle size, minor components required averaging over time and/or size to achieve adequate precision.