Products and Kinetics of the Liquid-Phase Reaction of Glyoxal Catalyzed by Ammonium Ions (NH₄⁺)

Abstract

Glyoxal, a common atmospheric gas, has been reported to be depleted in some regions of the atmosphere. The corresponding sink could be accounted for by reactions in or at the surface of atmospheric particles, but these reactions were not identified. Recently, we showed that inorganic ammonium ions, NH(4)(+), are efficient catalysts for reactions of carbonyl compounds, including glyoxal, in the liquid phase. To determine whether ammonium-catalyzed reactions can contribute to depletion of glyoxal in the atmosphere, the reactivity of this compound in aqueous solutions containing ammonium salts (ammonium sulfate, chloride, fluoride, and phosphate) at 298 K has been studied. The products identified by LC-HRMS and UV absorption revealed a mechanism involving two distinct pathways: a Bronsted acid pathway and an iminium pathway. The kinetics of the iminium pathway was studied by monitoring formation of a specific product. This pathway was second order in glyoxal in most of the solutions studied and should therefore be second order in most ammonium-containing aerosols in the atmosphere. The corresponding rate constant, k(II) (M(-1) s(-1)), increased strongly with ammonium ion activity, a(NH(4)(+)), and pH: k(II) (M(-1) s(-1)) = (2 +/- 1) x 10(-10) exp((1.5 +/- 0.8)aNH(4)(+)) exp((2.5 +/- 0.2)pH). This iminium pathway is a lower limit for the ammonium-catalyzed consumption of glyoxal, but the contribution of the acid pathway is expected to be small in tropospheric aerosols. With these results the reactive uptake of glyoxal on ammonium-containing aerosols was estimated and shown to be a possible explanation for depletion of this compound in Mexico City.

Glyoxal secondary organic aerosol chemistry: effects of dilute nitrate and ammonium and support for organic radical–radical oligomer formation

“…oxalate, oligomers, organosulfate and imidazole). [6,8,[32][33][34] Because these compounds are found predominantly in the condensed phase in the atmosphere, aqueous reaction of GLY contributes to SOA aq . [32,33] Organic nitrogen and sulfur compounds have been observed to form in highly concentrated aqueous solutions relevant to wet aerosols.…”

“…[32,33] Organic nitrogen and sulfur compounds have been observed to form in highly concentrated aqueous solutions relevant to wet aerosols. [6,[34][35][36][37] Evidence for the photochemical formation of organosulfates from GLY and other aldehydes is provided through studies of bulk sulfate solutions and through smog chamber experiments with ammonium sulfate seed particles and UV irradiation. [6,35,36] Aerosol mimic solutions containing GLY and inorganic ammonium salts have yielded products with carbon-nitrogen bonds in the absence of major atmospheric oxidants (i.e.…”

“…[37] In addition, imidazole formation has been detected in (dark) concentrated aldehydeammonium sulfate solutions and chamber experiments. [6,34] Inorganic and organic concentrations are orders of magnitude smaller in clouds and fogs. Although experiments have been performed to examine the effect of dilute (cloud-relevant) concentrations of acidic sulfate on GLY chemistry, [33] to our knowledge the degree to which GLY þ OH chemistry is altered by the presence of cloud-relevant concentrations of nitrate or ammonium has not been examined.…”

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

“…There may exist branching points in these reactions, favoring one type of chromophore over another according to the acidity. Noziere et al (2009) observed that the iminium ion pathway, incorporating N, is faster at higher pH while the traditional aldol condensation is favored at lower pH. This suggests that the incorporation of N-containing products has a pH dependence as well.…”

“…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). This leaves the relationship between pH and absorptivity in amine-aldehyde reactions unclear.…”