Mobile and Highly Time-Resolved Measurements of Metals and Trace Elements using a new Microwave-Induced Plasma Time-of-Flight Mass Spectrometer

Metals and trace elements in atmospheric particles are among the main drivers of aerosol-induced adverse health outcome. Because their emissions can be highly localized, mobile and/or highly time-resolved measurements are an important strategy for identifying point sources, characterizing spatial variability, and assessing specific micro-environments. However, existing metal/trace element measurement techniques based on x-ray fluorescence lack the requisite time resolution for such applications, whereas inductively coupled plasma (ICP)-based mass spectrometers are logistically impractical for field measurements due to their reliance on an Ar-based plasma.

We present initial field deployments of a recently developed microwave-induced plasma time-of-flight mass spectrometer (mipTOF), which enables, for the first time, highly time resolved (~1 Hz), quantitative measurements of metals and trace elements. In contrast to conventional ICP-based systems, the mipTOF uses a microwave-assisted (MICAP) plasma source sustained with N2 that maintains performance and stability during direct sampling of ambient air. During two proof-of-concept field campaigns, the mipTOF was (1) deployed aboard a mobile sampling platform for 7 measurement drives in central Switzerland, and (2) deployed for ~1 month at a stationary site in Zurich, during which time its performance was cross-compared with the commonly used Xact ambient metals monitor.

During the mobile campaign, the mipTOF successfully resolved localized point sources such as vehicle tunnels, gravel pit emissions, and lead plumes from an airfield hosting small private aircraft. In addition, transient events with timescales <1 min were clearly resolved, including the passage of passenger vehicles, trains, and trams. Comparisons between mipTOF and Xact showed mostly good agreement within the PM2.5, with additional complexity in the coarse mode due to sampling line losses and/or instrument limitations. In all, the mipTOF represents an important step forward for understanding localized pollution dynamics and improving metal/trace element-based source apportionment.