Using X-ray Techniques to Monitor Air Pollution

Air quality is a global concern, but even more so for urban areas and poor countries. The pollution in the air goes directly into our lungs, which makes monitoring the particulates and various levels of pollution a major concern. Johan Boman of the Department of Chemistry & Molecular Biology at University of Gothenburg has been studying the air quality in several different regions using energy dispersive X-ray fluorescence (EDXRF) and total reflection X-ray fluorescence (TXRF). Here, he discusses the challenges faced in this research and plans for future studies. 

What are the most important questions or problems that atomic spectroscopy needs to solve in the next 5–10 years? What role does your research play in tackling these areas?

Boman: In the field of X-ray fluorescence there is an ongoing development of devices as well as complete systems for elemental analysis. What is lacking is a closer cooperation between the developers and researchers working in the air quality field. Very much more information can be gained from collected aerosol particles if the analytic systems could be adapted for analysis of filters with particulate matter.

Your research focuses on energy dispersive X-ray fluorescence and total reflection X-ray fluorescence. How do these two techniques differ and what kind of analyses do you usually conduct with each one?

Boman: Both techniques are excellent for analyzing the elemental content of aerosol particulate samples. The EDXRF setup I use is based on what we call three-axial geometry. Instead of letting the X-rays from the X-ray tube excite the elements in the sample directly, they are first hitting a secondary target. It’s the characteristic radiation from the secondary target that excites the elements in the sample. In this way we lose most of the intensity from the X-ray tube, but by utilizing three orthogonal directions for the different beam paths we gain in signal-to-background ratio. So despite the lower intensity, the detection limits are lower compared to direct-excitation EDXRF. It is a laboratory-built spectrometer that can be optimized for different kinds of samples. At the moment it is optimized for analysis of aerosol particles collected on filters. With this setup there is no need for any sample preparation. The filters are removed from the sample collector and then inserted into the spectrometer for analysis. TXRF is a special version of EDXRF. The difference is the incident angle. In EDXRF it’s 45°, whereas it’s below 0.1° in TXRF. Using this small angle, below what we call the critical angle, together with a very well-polished sample carrier, we obtain total reflection of the incoming X-rays. This leads to even lower detection limits compared to those achieved by our EDXRF system. The detection limits are lowered further by the very tight geometry of the TXRF spectrometer. With TXRF it is possible to analyze the elemental content of very minute samples. The drawback of TXRF is that there is a step of sample preparation needed to apply the sample onto the sample carrier.

In a talk given at this year’s Denver X-ray conference (and a paper in press [1]), you discussed research on air pollution in Kingston, Jamaica, using a mini-vol sampler to collect samples and then analyzing them using particle-induced X-ray emission (PIXE). What was the biggest challenge in this research? Will your results impact future air pollution studies?

Boman: The greatest challenge in the research in Jamaica is to get a representative set of samples. So far only two measurement series have been made to collect PM2.5 samples with research in the focus. The National Environmental and Planning Agency collects a few PM2.5 samples in Jamaica yearly for monitoring purposes. During the evaluation of the analyzed samples another challenge is to understand the local sources that might influence the elemental composition of the particulate samples. We are also trying to estimate the health impact of the air pollution. The knowledge obtained in Jamaica can be a basis for understanding the air quality situation in other urban areas in low- and middle-income countries.

Several of your recent papers (1–3) investigate the inorganic composition of atmospheric aerosol particles (PM2.5). Why are PM2.5 particles an important sample for testing air quality?

Boman: PM2.5, particles with an aerodynamic diameter of 2.5 µm or smaller, have the ability to penetrate deep into our lungs and thus have the possibility to cause relatively large damage to the human health. PM2.5 particles have been studied for a long time, but it is still not clear why they influence our health. Obtaining information about the inorganic content of the particles can add to the knowledge of the relationship between the particle concentrations and the human health.

Source apportionment based on the inorganic content of PM2.5 is an important part in selecting abatement techniques to limit the emissions of particulate pollutants and their precursors.

Your research in Nairobi, Kenya, was conducted during a 2-year period. Did that extended research time present any additional challenges for your results? How did you decide on Nairobi as the location? Do you have any other long-term research projects going on in other cities?

Boman: The long-term measurements are needed to better understand the air quality situation in Nairobi, and elsewhere. The weather system is complex and influences the air quality in an area like Nairobi, and to get the best possible understanding of the air quality in Nairobi in this case long-term measurements are needed. Short-term measurements are a little like measuring ambient temperature and precipitation for a week at a selected site and based on these limited measurements predicting the weather at the site for the rest of the year. Long-term measurements are also the only reliable way to determine trends and changes in the air quality.

Africa is a continent undergoing rapid changes and thus huge challenges for a sustainable development. Based on earlier research collaboration and exchange of staff, Nairobi was selected to better understand the air quality, and life quality, in an urban environment where these changes are very obvious at the same time as there is a lack of knowledge about the current air quality. Through our collaboration, we built a local capacity to deal with these challenges in Nairobi.

So far Nairobi is the only city where I have been able to do this long-term study. Shorter measurement campaigns have been performed by our research group and colleagues in locations such as Ouagadougou, Burkina Faso; Cairo, Egypt; Taif, Saudi Arabia; and Kingston, Jamaica. Hopefully some of these studies will be prolonged. Currently, Jamaica looks the most promising.

Have you seen any air pollution policies go into effect yet as a result of your studies?

Boman: We cannot say that we have had that influence yet, but by widening our research team and involve stakeholders like UNEP, the World Bank, and UN-HABITAT we will have a better chance to influence policies in the future. Our ambition is to present a better understanding of the actually air quality in Nairobi. This is needed to establish relevant policies.

What are the next steps in your research?

Boman: Currently, I’m working with several other researchers in a wider approach to address the air and life quality development in Nairobi. If it evolves as anticipated this will be a long-term study with researchers from many different research fields and from many parts of the world. It will later be applied in other parts of Africa south of the Sahara.

China and India are other countries facing severe air quality problems and we are extending our research in these countries as well.

Finally, back to spectroscopy, I’m chairing the upcoming conference on X-ray spectroscopy, EXRS2016, in Gothenburg, Sweden in June 2016. We expect participation of 300 delegates from all over the world and plan an exhibition with 20 companies. You are very welcome to attend!

For more information on Professor Boman's work, please visit his research page at cmb.gu.se. 

References

(1) J. Boman and S.M. Gaita, Nuclear Instruments and Methods in Physics Research B in press (2015). (http://www.gu.se/english/research/publication?publicationId=222103)

(2) A. Shaltout, J. Boman, Z.F. Shehadeh, D.-a.R. Al-Malawi, O.M. Hemeda, and M.M. Morsy, Journal of Aerosol Science79, 97–108 (2015). (http://www.gu.se/english/research/publication?publicationId=205855)

(3) S.M. Gaita, J. Boman, M.J. Gatari, J.B.C. Pettersson, and S. Janhäll, Atmospheric Chemistry And Physics14, 9977–9991 (2014). (http://www.gu.se/english/research/publication?publicationId=203127)