Research Highlight: Modeling Atmospheric Transformation of PAHsBy Bevin P. Engleward
MIT SRP Program Director
Jul 08, 2021
As part of the MIT SRP, the Kroll Lab at MIT is working to better understand what happens to PAHs during their chemical transformations in the atmosphere. This is important because PAHs are known to have negative impacts on human health, and so better understanding of their evolution in the atmosphere will inform how long they pose a threat after emission. Importantly, some reactions in the atmosphere produce reaction products that can be equally or more harmful than the parent PAH,1 yet these reaction products are not currently monitored. By studying PAH evolution over time, harmful byproducts of atmospheric reactions can be identified and suggested for routine monitoring.
Studying the evolution of PAHs in the atmosphere first requires an understanding of PAH emission. PAHs can be emitted in both the gas and particle phase depending on the size of the compound. PAHs in both phases are important, but larger PAH molecules mainly reside in the particle phase and are typically more harmful to human health than gas phase PAHs.2 For this reason, the work of the MIT SRP is mainly focused on particle phase PAHs. Once in the atmosphere, there are three predominant processes that affect their evolution, illustrated in Figure 2: 1) reaction with UV light produced by the sun, 2) reactions between particle phase PAHs and gas phase oxidants (i.e., heterogeneous reactions), and 3) the condensation of gas phase reactions. The relative importance of these pathways is determined by how quickly the process degrades the parent PAH and the types of reaction products that are formed. Fast degradation pathways of the parent PAH will decrease the health risk if the products formed from reaction are innocuous, but if the products formed are more harmful than the parent compound the reaction pathway becomes detrimental.
In order to determine which process is most impactful on human health, we have developed an experimental setup that can generate particle phase PAHs and simulate the three reaction pathways. Researchers measure PAH evolution using an Aerosol Mass Spectrometer (AMS) and Proton Transfer Reaction Mass Spectrometer (PTR) that can measure the concentration of individual PAHs and reaction products in the particle phase over time. To date this setup has been used to study heterogeneous reactions between particles and gas phase oxidants. Particle phase perylene, a representative PAH, has been created and reacted with two common atmospheric oxidants, hydroxyl radical (OH˙) and ozone (O3). Figure 3 shows the degradation of perylene by hydroxyl radical and ozone over time, and Figure 4 shows the reaction products products that are formed during perylene degradation by hydroxyl radical over time.
The reactions can be compared using the two factors mentioned previously, decay of parent PAH and formation of products. Results from the AMS show that the reaction of perylene with O3 in the atmosphere is initially faster than OH˙, but levels off after around one day of exposure while the reaction with OH˙ keeps going (Figure 3). This could mean that PAHs emitted in areas where heterogeneous reaction with O3 is the only process, only around 30% of the PAH will go away and the rest will remain a risk for human exposure. In terms of reaction products, similar products are formed in both reactions, but the products are formed more quickly from reaction with OH˙. This could mean that for PAHs that spend less than a day in the atmosphere, the products from reaction with OH˙ are of most concern to human health.
Future work on this project will include extending the experimental setup to other atmospheric processes and testing additional PAHs. This will begin to inform what processes are most important as well as identify potentially harmful byproducts. Beyond this, we aim to identify which chemical species of PAHs and degradation products have impacts on health. Collaboration with Projects 3, 4, and 5 of the MIT SRP will help identify the health risks of PAHs and their byproducts so that harmful ones can be suggested for routine monitoring.
 Finlayson-Pitts, B. J., & Pitts Jr, J. N. (1999). Chemistry of the upper and lower atmosphere: theory, experiments, and applications. Elsevier.
 Calvert, J. G., Atkinson, R., Becker, K. H., Kamens, R. M., Seinfeld, J. H., Wallington, T. H., & Yarwood, G. R. E. G. (2002). The mechanisms of atmospheric oxidation of the aromatic hydrocarbons. Oxford University Press.