State of the art of IBA analysis of high time resolution aerosol samples

1. INFN-Florence and Department of Physics and Astronomy, University of Florence, via G. Sansone 1, 50019-Sesto Fiorentino, Italy
2. Department of Physics and Astronomy, University of Florence, via G. Sansone 1, 50019-Sesto Fiorentino, Italy

Most environmental studies on particulate matter (PM) are based on 24-h data. However many PM emissions as well as atmospheric transport and dilution processes may change within a few hours. As a consequence, the measurement of the PM composition with higher time resolution gives a better insight on aerosol emission, transport and dilution processes, as well as a better quantification of the human exposure. Furthermore, receptor models, which are used for the aerosol source apportionment, need series of samples containing material from the same set of sources in different proportions: increasing the time resolution provides samples that have greater between-sample variability and this strongly enhance the capabilities of these techniques. 

PM samplings with 1 h time resolution can be performed by the Streaker sampler (PIXE Int. Corp.). With this device the aerosol fine (<2.5 μm) and coarse (2.5–10 μm) fractions are simultaneously collected on two rotating substrata: this produces a circular continuous deposition of PM (“streak”) on both stages. The analysis of these aerosol streaks with a properly collimated ion beam, which scans the deposit in steps corresponding to 1 h of aerosol sampling, provides the aerosol composition with hourly time resolution. It is worth noting that IBA techniques are unrivalled in the analysis of this kind of samples, which cannot be analysed chemical methods.

At the 3 MV Tandetron accelerator of the LABEC laboratory aerosol Streaker samples have been analysed by Particle Induced X-ray Emission (PIXE) since a long time, and the external beam set-up dedicated to these analysis has been continuously improved to increase the measurement sensitivity and throughput (hourly time resolution studies produce a high number of very low mass aerosol samples: 168 for stage for weak). The adopted strategy is the use of a quite high proton current (100-500 nA) and a two-detector system, optimized for low-Z and medium-high-Z elements, to balance for the different cross sections. Si(Li) detectors have been replaced by SDDs, as they provide better resolution with modest cooling and can cope with higher counting rates, and, recently the SDD used for medium-high-Z elements has been duplicated to double the statistics.

The optimised setup, together with a convenient choice of beam energy and suitable collecting substrata, allows obtaining good statistics in less then 1 min per 1-hour sample. Particle Induced Gamma-ray Emission (PIGE) is also routinely used, simultaneously with PIXE, to correct the underestimation of PIXE in quantifying the concentration of the lightest detectable elements, like Na or Al, due to X-ray absorption inside aerosol particles, while the use of elastic scattering for H, C, O and N detection in these samples is still under development.  A detailed description of the experimental set-up and of the adopted measurement conditions will be given, together with some examples of application in recent PM field campaigns.