Secondary organic material formed by methylglyoxal in aqueous aerosol mimics

Abstract

Abstract. We show that methylglyoxal forms light-absorbing secondary organic material in aqueous ammonium sulfate and ammonium nitrate solutions mimicking tropospheric aerosol particles. The kinetics were characterized using UV-Vis spectrophotometry. The results suggest that the bimolecular reaction of methylglyoxal with an ammonium or hydronium ion is the rate-limiting step for the formation of light-absorbing species, with kNH4+II=5×10−6 M−1 min−1 and kH3O+II≤10−3 M−1 min−1. Evidence of aldol condensation products and oligomeric species up to 759 amu was found using chemical ionization mass spectrometry with a volatilization flow tube inlet (Aerosol-CIMS). Tentative identifications of carbon-nitrogen species and a sulfur-containing compound were also made using Aerosol-CIMS. Aqueous solutions of methylglyoxal, with and without inorganic salts, exhibit significant surface tension depression. These observations add to the growing body of evidence that dicarbonyl compounds may form secondary organic material in the aerosol aqueous phase, and that secondary organic aerosol formation via heterogeneous processes may affect seed aerosol properties.

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.…”

“…while the structure provided in their Supporting Information corresponds to a different formula, C 6 H 11 O 4 N, matching the structure initially reported in Sareen et al (2010). No structure was provided matching the observed ion C 9 H 11 N 3 , however.…”

“…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.…”

“…This leaves the relationship between pH and absorptivity in amine-aldehyde reactions unclear. For example, Sareen et al (2010) showed that the peak absorbance (located at 282 nm) in the products of methylglyoxal/AS was linearly inversely related to the pH of samples after 24 hours, and concluded that the formation of light-absorbing products is enhanced by the presence of ammonium and hydronium ions through the acid-catalyzed aldol condensation mechanism. 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.…”

“…Reaction products of the methylglyoxal/AS system listed in Table 2 are very typical Maillard reaction products, observed in both cloud water simulations Lin et al, 2015;Sareen et al, 2010;Hawkins et al, 2016) and the chemical characterization of baked bread, cooked meat, aged cheese and other reactions between carbonyl containing compounds and 30 amines or amino acids (Koehler and Odell, 1970;Adams et al, 2008;Van Lancker et al, 2010;Divine et al, 2012). It is therefore surprising that pyrazine-based products, which are also well-known to form in Maillard reactions in food, have not been identified in the context of atmospheric reactions.…”