Trace Element Analysis

Trace metals comprise only a few percent of total aerosol mass, yet they are important due to their potential toxicity and as tracers for source apportionment. Their generally low atmospheric abundance in the order of nanograms per cubic meter require highly sensitive techniques for their analysis when sampled with a high time resolution (1 hour). Sampling is done with rotating drum impactors, and elemental analysis is done with synchrotron radiation induced X-ray fluorescence spectrometry (SR-XRF).

Rotating Drum Impactor Sampling

Rotating drum impactors (RDI) of the Lundgren type have been used since the 1960s. Air is sucked through nozzles specifically designed such that a select size range of particles is impacted on a mylar or polypropylene tape, while the remaining particles are carried by the airflow to the next impaction stage. In this way, a series of particle size ranges can be collected. Our RDIs segregate particles into the three size ranges 10 - 2.5 micrometers, 2.5 - 1.0 micrometer, and 1.0 - 0.1 micrometer, representing the relevant size ranges for regulatory purposes and for the study of health effects. During sampling the impactor drums are kept fixed, then they are rotated to the next position for the next sample. This generates a barcode-like pattern of deposited material. The polypropylene tapes are greased with Apiezon-L to increase the collection efficiency for particles. Sampling times can be set in the order of 1 hour, which increases the time resolution substantially when compared to conventional 24-h filter sampling. Diurnal variations in trace element concentrations become measurable.

Synchrotron Radiation-induced X-ray Fluorescence Spectrometry

X-rays are used for the excitation of elements in ambient samples and their detection via the element-characteristic fluorescence lines. After excitation of the sample elements, the emitted fluorescent radiation is measured with an energy-dispersive detector, and a spectrum of a wide range of elements present in the sample is obtained. Since the amount of sample material is limited, the high sensitivity and low detection limits achieved with the XRF method at synchrotron facilities is crucial for the detection of trace elements. In addition to the high sensitivity, the fact that no further sample preparation is needed is an important aspect of synchrotron radiation induced X-ray fluorescence spectrometry (SR-XRF). Our technique allows for the analysis of several thousands of individual samples in reasonable time, given the generally restricted amount of beamtime available at the synchrotron facilities. Measurements have been performed so far at the Swiss Light Source (SLS), at HASYLAB (Hamburg, Germany), and at the Advanced Light Source (ALS, Berkeley, USA).

Projects and field campaigns

• NF China-XRF (2016-2019) with measurements in Beijing (2017), New Delhi (2019), Krakow (2018, 2019)
• ClearfLo - Clean Air for London - United Kingdom (2012)
• XRF-PMF (2011-2013)
• RDI-SR-XRF (2008-2011)
• CalNex-LA, Pasadena, California, USA (2010)
• MEGAPOLI, Paris, France (2009, 2010)
• EUCAARI, Zürich, Switzerland, 2008-2009
• DAURE, Barcelona, Spain (2008)
• EMEP/EUCAARI, Payerne, Switzerland, (2008)
• FAME'08, Finokalia, Crete, Greece (2008)
• APART, Zürich and Reiden, Switzerland (2007-2008)

Theses

Analysis of major and trace elements in ambient aerosols and their sources in European and Asian cities.
Rai, P., 2020.
Diss. ETH No. 26608 Thesis, ETH Zürich, Villigen, Switzerland.
DOI:

Highly time- and size-resolved ambient trace element analysis and source apportionment.
Visser, S., 2015.
Diss. ETH No. 22520 Thesis, ETH Zürich, Villigen, Switzerland.
DOI: 10.3929/ethz-a-001987748

Determination of trace elements in ambient aerosols with synchrotron induced X-ray fluorescence spectrometry and subsequent source apportionment.
Richard, A. C. J., 2011.
Diss. ETH No. 19667 Thesis, ETH, Zürich, Switzerland, 160 pp.
DOI: 10.3929/ethz-a-006550233
 

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