Study of atmospheric aerosols in the Chicago region by Total Reflection X-ray Fluorescence Spectrometry

Atmospheric aerosols were collected by filtration at the Lake Shore Campus of Loyola University Chicago during summer/fall 2000. Sampling times varied from 15 minutes to 2 hours depending on the meteorological situation and particle concentration. Longer times were chosen for lower particle concentration and shorter ones for higher particle freights. The filter samples were digested and subsequently analyzed by total reflection X-ray fluorescence (TXRF). TXRF is an energy dispersive X-ray technique and utilizes the total reflection of an X-ray beam in highly polished (mirrorlike) material. For analysis the sample is placed onto a quartz glass carrier and exposed to the X-ray beam under an extremely small angle. The beam penetrates through the sample into the carrier and is totally reflected in the carrier material. As a result the sample is irradiated two times - ones by the original beam and ones by the totally reflected beam. The fluorescence radiation of the sample is then recorded by a Si-pin electrode detector. The instrument in use is portable and therefore ideally suited for field measurements. It is the only one of this type existing in the United States. TXRF is extremely sensitive with detection limits ranging from 200ng/g for light elements like sulfur to 200pg/g for heavier elements like strontium or lead. Due to the unique set-up only tiny sample amounts of a few nanogram are required for reliable analysis thus making TXRF an excellent tool for characterization of atmospheric particles, occurring naturally only in small amounts. The low sample amounts required make it further possible to reduce collection times for aerosols dramatically and monitor air masses passing through the sampling region by consecutively taking samples. In particular near Lake Michigan air masses are arriving from different locations and change often rapidly. Short term sampling enables us to monitor this changes by recording the variations in composition and elemental concentration of the aerosol. It is known that certain elements are indicative for different sources: Vanadium, nickel and lead for instance are released from anthropogenic sources, whereas calcium, potassium and iron are characteristic for soil dust. Together with the back trajectories calculated for the time of collection source identification and important insight about air mass movement on a regional scale can be obtained. Ultimately it is intended to use several trace metals as tracers for air masses and perform source apportionment studies. The results obtained from this measurements are extremely useful as input for model studies on the local and regional scale. Further future studies will include field measurements at different locations as well as Lagrangian experiments.