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Published January 29, 2019 | Published
Journal Article Open

Detecting Intermediates and Products of Fast Heterogeneous Reactions on Liquid Surfaces via Online Mass Spectrometry


One of the research priorities in atmospheric chemistry is to advance our understanding of heterogeneous reactions and their effect on the composition of the troposphere. Chemistry on aqueous surfaces is particularly important because of their ubiquity and expanse. They range from the surfaces of oceans (360 million km2), cloud and aerosol drops (estimated at ~10 trillion km2) to the fluid lining the human lung (~150 m2). Typically, ambient air contains reactive gases that may affect human health, influence climate and participate in biogeochemical cycles. Despite their importance, atmospheric reactions between gases and solutes on aqueous surfaces are not well understood and, as a result, generally overlooked. New, surface-specific techniques are required that detect and identify the intermediates and products of such reactions as they happen on liquids. This is a tall order because genuine interfacial reactions are faster than mass diffusion into bulk liquids, and may produce novel species in low concentrations. Herein, we review evidence that validates online pneumatic ionization mass spectrometry of liquid microjets exposed to reactive gases as a technique that meets such requirements. Next, we call attention to results obtained by this approach on reactions of gas-phase ozone, nitrogen dioxide and hydroxyl radicals with various solutes on aqueous surfaces. The overarching conclusion is that the outermost layers of aqueous solutions are unique media, where most equilibria shift and reactions usually proceed along new pathways, and generally faster than in bulk water. That the rates and mechanisms of reactions at air-aqueous interfaces may be different from those in bulk water opens new conceptual frameworks and lines of research, and adds a missing dimension to atmospheric chemistry.

Additional Information

© 2019 The Author(s). This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0). Received: 8 January 2019 / Revised: 20 January 2019 / Accepted: 22 January 2019 / Published: 26 January 2019. (This article belongs to the Special Issue Physical Chemistry of the Air-Water Interface) A.J.C. acknowledges support from the National Science Foundation USA, grant AGS-1744353. SE acknowledges support from the JSPS KAKENHI, grant numbers 15H05328 and 15K12188. Author Contributions: A.J.C. and S.E. designed experiments. S.E. performed experiments. A.J.C. and S.E. analyzed results and wrote the paper. The authors declare no conflict of interest.

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