Glyoxal in Aqueous Ammonium Sulfate Solutions: Products, Kinetics and Hydration Effects

Abstract

Reactions and interactions between glyoxal and salts in aqueous solution were studied. Glyoxal was found to react with ammonium to form imidazole, imidazole-2-carboxaldehyde, formic acid, N-glyoxal substituted imidazole, and minor products at very low concentrations. Overall reaction orders and rates for each major product were measured. Sulfate ions have a strong and specific interaction with glyoxal in aqueous solution, which shifts the hydration equilibria of glyoxal from the unhydrated carbonyl form to the hydrated form. This ion-specific effect contributes to the observed enhancement of the effective Henry's law coefficient for glyoxal in sulfate-containing solutions. The results of UV-vis absorption and NMR spectroscopy studies of solutions of glyoxal with ammonium, methylamine, and dimethylamine salts reveal that light absorbing compounds require the formation of nitrogen containing molecules. These findings have implications on the role of glyoxal in the atmosphere, both in models of the contribution of glyoxal to form secondary organic aerosol (SOA), the role of nitrogen containing species for aerosol optical properties and in predictions of the behavior of other carbonyls or dicarbonyls in the atmosphere.

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

“…Formic acid, especially under acidic conditions, is a sufficiently strong reducing agent to complete the reductive amination step as shown in Scheme 1. Formic acid was not 30 detected in this study in either positive or negative ion mode (as m/z 47 or 45, respectively) but it has been reported as a side product of glyoxal/AS and methyglyoxal/AS in several other studies (Galloway et al, 2009;De Haan et al, 2009;Sareen et al, 2010;Yu et al, 2011). The evidence outlined here lends confidence to our assignment of m/z 109 as 2,5-DMP.…”

“…In our study, absorbance has been normalized by TOC concentration for each sample to provide a bulk solution phase mass absorption coefficient, 25 which accounts for the loss of any organic carbon to the gas phase. The results in Figure 1a are consistent with Kampf et al (2012) and Yu et al (2011) and suggest that over a pH range between 2 and 9, the role of nitrogen as a nucleophile is more important for chromophore formation than acid-catalyzed aldol condensation. The observed changes in absorptivity (rather than absorbance) imply that solution pH can promote one product (like methylglyoxal self-reaction products) at the expense of another (N-containing products) or that unreacted methylglyoxal remains in solution after one week.…”

“…In a related study, Kampf et al (2012) observed a novel, highly absorptive chromophore in the glyoxal/ammonium sulfate system, the imidazole bicycle, whose production 5 rate declined as the pH of the reaction dropped. Similarly, Yu et al (2011) observed an exponential dependence on pH for the rate of formation of imidazole and related products and concluded that available ammonia, dictated by solution pH, drives this relationship. In the context of food chemistry, it is thought that neutral or slightly alkaline conditions favor melanoidins generally (Kwak et al, 2005) and in particular dimethylpyrazine (Koehler and Odell, 1970).…”

“…The mechanistic details of aqBrC formation, including the roles of pH and evaporation, are still incomplete, yet some trends in aqBrC absorptivity 30 have been identified. In most cases, it was concluded that chromophores form fastest in basic solution, because nitrogen (in ammonia, methylamine, or amino acids) must initiate a nucleophilic attack at the carbonyl carbon atom (Yu et al, 2011;Kampf et al, 2012). However, several studies have shown that aldol condensation of dicarbonyls can be acid catalyzed (Noziere et al, 2009) and that aldol condensation products may contribute to visible light absorption (Sareen et al, 2010).…”

Atmospheric water-soluble organic nitrogen (WSON) in the eastern Mediterranean: origin and ramifications regarding marine productivity

“…Atmospheric organic nitrogen can also be formed through chemical reactions. For example, reactions between volatile organic compounds, NO x and ammonium sulfate aerosols may lead to the formation of nitrogen-containing compounds (Surratt et al, 2008;Galloway et al, 2009;De Haan et al, 2011;Yu et al, 2011). Furthermore, atmospheric organic nitrogen plays an essential role in many global processes which may impact the chemistry of the atmosphere as well as climate and biogeochemical cycles.…”