Analysis of aerosol particles collected in Ljubljana, Slovenia, using Particle Induced X-ray Emission with a focused proton beam and a Helium Ion Microscope

The correlative approach of real-time aerosol measurements with offline filter analysis and ParticleInduced X-ray Emission (PIXE) can significantly enhance the scope of aerosol studies. Aerosol particles have diverse physical and chemical properties, thus having a direct impact on air quality, cloud nucleation the planetary radiation balance, public health, etc. Essential information on the chemical composition of aerosol particles can be deduced from the elemental concentrations measured simultaneously for many elements by non-destructive and undemanding PIXE (Lucarelli, 2018). Furthermore, PIXE measurements can be performed with a focused beam allowing the analysis of individual particles (Biancato, 2006). Determining elemental concentrations is also important input information for aerosol source apportionment models and consequently abatement measures in order to improve air quality (Artaxo, 1999). Complementary information of aerosol particles structure down to nm scale, can be obtained with a Helium Ion Microscope (HIM), which, to our knowledge, has never been used before for aerosol characterization. The detection of secondary electrons and backscattered ions enables sub nm lateral resolution and large depth-of-field, also on insulating samples. In addition, a concurrent secondary ion mass spectrometry (ToF-SIMS) integrated in the HIM can provide insights of the composition of elements and molecules with imaging capabilities of sub 8 nm (Klingner, 2019).
In this study we coupled real-time measurements of optical properties of aerosols with an Aethalometer (Drinovec, 2015) and their carbon content with a Total Carbon Analyzer (Rigler, 2019) with PIXE and HIM analysis. Ambient aerosols were collected on quartz filters and quartz filters with Teflon coating during different pollution events (traffic or biomass burning dominated, Saharan dust dominated, etc.) at an urban background sampling site in Ljubljana, Slovenia (46o04’17’’N, 14o30’06’’E). The PM2.5 inlet was used for sampling carbonaceous aerosol while a virtual impactor was used for concentrating coarse particles during Saharan dust events.
PIXE measurements have been performed on these collected samples and compared to optical properties and source apportionment obtained by the online instruments. The PIXE measurements were performed across several hundred-micrometer regions and on individual points and results are presented including a description of the procedures for quantification. Complementary high-resolution imaging and sputtered ion analysis on single black carbon and Saharian dust aerosol particles was done on the HIM to study their structure and coating composition. The combination of all methods yields a comprehensive view of the aerosol particles.