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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-63
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Feb 2018

Research article | 07 Feb 2018

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This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.

Evidence for pyrazine-based chromophores in cloudwater mimics containing methylglyoxal and ammonium sulfate

Lelia Nahid Hawkins1, Hannah Greer Welsh1, and Matthew Von Alexander2 Lelia Nahid Hawkins et al.
  • 1Dept of Chemistry, Harvey Mudd College, 301 Platt Blvd, Claremont, CA 91711
  • 2Dept of Chemistry, Pomona College, Claremont, CA 91711

Abstract. Simulating aqueous brown carbon (aqBrC) formation from small molecule amines and aldehydes in cloud water mimics provides insight into potential humic-like substance (HULIS) contributors and their effect on local and global aerosol radiative forcing. Previous work has shown that these (Maillard type) reactions generate products that are chemically, physi- cally, and optically similar to atmospheric HULIS in many significant ways, including in their complexity. Despite numerous characterization studies, attribution of the intense brown color of many aqBrC systems to specific compounds remains in- complete. In this work, we present evidence of novel pyrazine-based chromophores (PBC) in the product mixture of aqueous solutions containing methylglyoxal and ammonium sulfate. PBC observed here include 2,5-dimethyl pyrazine (DMP) and products of methylglyoxal addition to the pyrazine ring. This finding is significant as the literature of Maillard reactions in food chemistry tightly links the formation of pyrazine (and related compounds) to browning in foods. We investigated both the roles of cloud processing (by bulk evaporation) and pH on absorptivity and product distribution in microliter samples to understand the contribution of these PBC to aqBrC properties. In agreement with previous work, we observed elevated absorptivity across the entire UV/visible spectrum following simulated cloud processing as well as higher absorptivity in more basic samples. Absorptivity of the pH 2 sample, following evaporation, exceeded that of the unevaporated pH 9 sample, indicating that cloud processing can overcome the previously observed kinetic barrier imposed on aqBrC formation in acidic conditions. Further, the fraction of pyrazine compounds in the product mixture increased by up to a factor of four in response to drying with a maximum observed contribution of 16% at pH 5. Therefore, cloud processing under more acidic conditions may produce PBC at the expense of imine and imidazole-derived compounds. This finding has implications for further BrC reactivity and degradation pathways.

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Lelia Nahid Hawkins et al.
Lelia Nahid Hawkins et al.
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Short summary
Atmospheric reactions can change the color of particles; this has implications for global climate. We present evidence of pyrazine compounds in cloud water mimics. We measured changes in brownness and composition during evaporation and acidity changes to understand the importance of the new compounds because the reactions parallel browning in foods. Drying favors browning and pyrazine formation, while acidity favors only pyrazine formation. Even acidic cloud water, when dried, produce pyrazines.
Atmospheric reactions can change the color of particles; this has implications for global...
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