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

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, physically, 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 incomplete. In this work, we present evidence of novel pyrazine-based chromophores (PBCs) in the product mixture of aqueous solutions containing methylglyoxal and ammonium sulfate. PBCs 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 the roles of both cloud processing (by bulk evaporation) and pH in absorptivity and product distribution in microliter samples to understand the contribution of these PBCs 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 over a period of days, exceeded that of the unevaporated pH 9 sample. In addition, mixtures of ammonium sulfate and methylglyoxal at pH 5 that were dried in under 1 h and analyzed 24 h later were as absorptive as pH 9 samples allowed to react for 7 days, indicating that evaporation occurring during cloud processing may provide a reaction pathway favorable for carbonyl–ammonia chemistry even under acidic conditions of aerosol and cloud water. The fraction of pyrazine compounds in the product mixture increased by up to a factor of 4 in response to drying with a maximum observed contribution of 16 % at pH 5. Therefore, cloud processing under acidic conditions may produce PBCs at the expense of imine- and imidazole-derived compounds. This finding has implications for further BrC reactivity and degradation pathways.