Aqueous phase reactions between methylglyoxal and ammonium sulfate are known to produce a wide range of products, including nitrogen-containing and visible light-absorbing compounds, through Maillard-type reactions initiated by C-N bond formation. This type of reaction may partially explain the presence of nitrogen-containing organics in aerosol and cloud water as well as some fraction of humic-like substances (HULIS). Imidazoles and their oligomers appear prevalent in mimics characterized by electrospray ionization; however, masses indicative of pyrazine-based products have also been detected using chemical ionization (APCI) and photo ionization (APPI). Further, 2,5-dimethylpyrazine (DMP) has been identified, but not quantified, in this reaction using GC-MS. This is significant because of the connection established in previous work between pyrazine-based compounds and chromophore content. Here we present quantitative analyses of 2,5-dimethylpyrazine in cloud water mimics containing methylglyoxal and ammonium sulfate under a range of conditions. The cloud water pH was varied between pH 2 and pH 9 to determine how pH impacts the yield of DMP. The greatest yield was observed at pH 5, consistent with the acid-catalyzed mechanism proposed previously. Passive evaporation to dryness (over 1 week) significantly increased the yield of DMP, and sample absorptivity below 400 nm, beyond that observed during aqueous aging. Rapid evaporation under filtered nitrogen gas (1 hr) showed little initial production of DMP and chromophores; in fact, samples containing only methylglyoxal achieved greater absorptivity than those containing ammonium sulfate. However, within a few hours after drying was complete, both absorptivity and DMP yield increased rapidly, resulting in greater absorptivity in the samples containing ammonium sulfate than those with methylglyoxal alone. Samples continued to brown and generate DMP for more than 7 days. The results presented here indicate that chromophores and pyrazine-based products can develop from methylglyoxal and ammonium sulfate on realistic timescales for cloud processing and under atmospherically relevant conditions, such as mildly acidic cloud water, and are therefore a plausible source for nitrogen-containing aromatic compounds in the atmosphere.

Quantifying pyrazine-based products in cloud water mimics containing ammonium sulfate and methylglyoxal: Effects of evaporation and pH on product formation

2019 Fall National Meeting

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