Articles | Volume 22, issue 20
https://doi.org/10.5194/acp-22-13783-2022
https://doi.org/10.5194/acp-22-13783-2022
ACP Letters
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01 Nov 2022
ACP Letters | Highlight paper |  | 01 Nov 2022

Not all types of secondary organic aerosol mix: two phases observed when mixing different secondary organic aerosol types

Fabian Mahrt, Long Peng, Julia Zaks, Yuanzhou Huang, Paul E. Ohno, Natalie R. Smith, Florence K. A. Gregson, Yiming Qin, Celia L. Faiola, Scot T. Martin, Sergey A. Nizkorodov, Markus Ammann, and Allan K. Bertram

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Cited articles

Aiken, A. C., DeCarlo, P. F., and Jimenez, J. L.: Elemental Analysis of Organic Species with Electron Ionization High-Resolution Mass Spectrometry, Anal. Chem., 79, 8350–8358, https://doi.org/10.1021/ac071150w, 2007. 
Allan, J. D., Jimenez, J. L., Williams, P. I., Alfarra, M. R., Bower, K. N., Jayne, J. T., Coe, H., and Worsnop, D. R.: Quantitative sampling using an Aerodyne aerosol mass spectrometer 1. Techniques of data interpretation and error analysis, J. Geophys. Res.-Atmos., 108, 4090, https://doi.org/10.1029/2002JD002358, 2003. 
Anttila, T., Kiendler-Scharr, A., Mentel, T. F., and Tillmann, R.: Size dependent partitioning of organic material: evidence for the formation of organic coatings on aqueous aerosols, J. Atmos. Chem., 57, 215–237, https://doi.org/10.1007/s10874-007-9067-9, 2007. 
Baltensperger, U., Dommen, J., Alfarra, R., Duplissy, J., Gaeggeler, K., Metzger, A., Facchini, M. C., Decesari, S., Finessi, E., Reinnig, C., Schott, M., Warnke, J., Hoffmann, T., Klatzer, B., Puxbaum, H., Geiser, M., Savi, M., Lang, D., Kalberer, M., and Geiser, T.: Combined determination of the chemical composition and of health effects of secondary organic aerosols: The POLYSOA project, J. Aerosol Med. Pulm. D., 21, 145–154, https://doi.org/10.1089/jamp.2007.0655, 2008. 
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Executive editor
Organic aerosol remain one of the more complex and hard to predict when studying atmospheric aerosols and their influences on air quality, meteorology and climate. Among its many complexities is the phase and viscosity of the organic matter, which dictates how it interacts with other particulate components and the gas phase, in turn affecting growth rates and cloud activation. There have been a number of previous works studying phase separation, where the organic matter becomes immiscible with an aqueous component (containing inorganic salts), but this new letter presents compelling visual evidence that different organic phases are also capable of separation. Different secondary organic aerosol (SOA) mixtures were created and some mixtures exhibited separation, with a factor being the oxygen-to-carbon ratio of the material, likely a surrogate for polarity. If this behaviour is found to be important in atmospheric aerosols this represents a new direction in how these may need to be represented in models.
Short summary
The number of condensed phases in mixtures of different secondary organic aerosol (SOA) types determines their impact on air quality and climate. Here we observe the number of phases in individual particles that contain mixtures of two different types of SOA. We find that SOA mixtures can form one- or two-phase particles, depending on the difference in the average oxygen-to-carbon (O / C) ratios of the two SOA types that are internally mixed within individual particles.
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