Iron-driven secondary brown carbon formation reactions from water-soluble organics in cloud droplets and aerosols create insoluble and soluble products of emerging atmospheric importance. This work shows, for the first time, results on dark iron-catalyzed polymerization of catechol forming insoluble black polycatechol particles and colored water-soluble oligomers under conditions characteristic of viscous multicomponent aerosol systems with relatively high ionic strength (I = 1–12 m) and acidic pH (∼2). These systems contain ammonium sulfate (AS)/nitrate (AN) and C3–C5 dicarboxylic acids, namely, malonic, malic, succinic, and glutaric acids. Using dynamic light scattering (DLS) and ultra high pressure liquid chromatography-mass spectrometry (UHPLC-MS), we show results on the rate of particle growth/agglomeration and identity of soluble oligomeric reaction products. We found that increasing I above 1 m and adding diacids with oxygen-to-carbon molar ratio (O:C > 1) significantly reduced the rate of polycatechol formation/aggregation by a factor of 1.3 ± 0.4 in AS solution in the first 60 min of reaction time. Using AN, rates were too slow to be quantified using DLS, but particles formed after 24 h reaction time. These results were explained by the relative concentration and affinity of ligands to Fe(III). We also report detectable amounts of soluble and colored oligomers in reactions with a slow rate of polycatechol formation, including organonitrogen compounds. These results highlight that brown carbon formation from iron chemistry is efficient under a wide range of aerosol physical states and chemical composition.

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Published in Environmental Science & Technology, v. 55, issue 1.

Copyright © 2020 American Chemical Society

This is an open access article published under a Creative Commons Non-Commercial No-Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes.

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H.A.A. acknowledges the partial funding from Laurier, NSERC, Canadian Foundation for Innovation, and the Fulbright Canada Research Chair in Atmospheric Chemistry, Air Quality and Climate Change program at the University of California Irvine. M.I.G. thanks the U.S.A. National Science Foundation for research funding under award 1903744.

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.0c05678. It is also available for download as the additional file listed at the end of this record.

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Detailed materials and methods, additional figures and tables, equilibrium constants for different reactions, and list of abbreviations

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