Photoinduced Oligomerization of Aqueous Pyruvic Acid

Abstract

The 320 nm-band photodecarboxylation of aqueous pyruvic acid (PA), a representative of the alpha-oxocarboxylic acids widely found in the atmospheric aerosol, yields 2,3-dimethyltartaric (A) and 2-(3-oxobutan-2-yloxy)-2-hydroxypropanoic (B) acids, rather than 3-hydroxy-2-oxobutanone as previously reported. A and B are identified by liquid chromatography with UV and ESI-MS detection, complemented by collisionally induced dissociation and 2H and 13C isotope labeling experiments. The multifunctional ether B gives rise to characteristic delta approximately 80 ppm 13C NMR resonances. Product quantum yields are proportional to [PA](a + [PA])(-1) in the range [PA] = 5-100 mM. CO2(g) release rates are halved, while A and B are suppressed by the addition of >1.5 mM TEMPO. A and B are only partially quenched in air-saturated solutions. These observations are shown to be consistent with an oligomerization process initiated by a bimolecular reaction between 3PA and PA producing ketyl, CH3C(OH)C(O)OH, and acetyl, CH3C(O)*, radicals, rather than by the unimolecular decomposition of 3PA into 1-hydroxyethylidene, 3HO(CH3)C: (+CO2), or [CH(3)C(O)* + *C(O)OH] pairs. A arises from the dimerization of ketyl radicals, while B ensues the facile decarboxylation of the C8beta-ketoacid formed by association of acetyl radicals with the ketyl radical adduct of PA. Since the radical precursors to A and B are scavenged by O2 with a low probability per encounter (k(sc) approximately 1 x 10(6) M(-1) s(-1)), PA is able to accrete into multifunctional polar species in aerated aqueous media under solar illumination.

Photochemistry of aqueous pyruvic acid

“…Organic polymers have been identified as important contributors to SOAs. These polymeric species do not result from gas phase reactions (11) but rather appear to be products of organic reactions in condensed (aqueous) phase, where high-molecular-weight compounds, polymers, and oligomers can be generated (1,5,(12)(13)(14)(15). Recent results have mainly addressed radical reactions in aqueous environments (5,7,16), with only a few using direct photolysis (13,17).…”

“…These polymeric species do not result from gas phase reactions (11) but rather appear to be products of organic reactions in condensed (aqueous) phase, where high-molecular-weight compounds, polymers, and oligomers can be generated (1,5,(12)(13)(14)(15). Recent results have mainly addressed radical reactions in aqueous environments (5,7,16), with only a few using direct photolysis (13,17). The need for understanding organic reaction mechanisms in aqueous solutions representative of aqueous media in the atmosphere (aerosols, cloud droplets, fog, and rain) has been recognized (1,2,7,18).…”

Reply to Eugene et al.: Photochemistry of aqueous pyruvic acid

“…When examined in its entirety, the evidence for the production of acetoin, lactic, and acetic acids in PA photolysis is irrefutable, and the mechanism presented is sound, being grounded in theoretical calculations as well as supported by the older literature (5,6), and providing an alternative explanation of Guzman et al's prior published results (3,4).…”

Aerosol formation and heterogeneous chemistry in the atmosphere

“…Very recently, some laboratory studies have shown that oligomers are produced in the liquid phase after aqueous phase hydration and/or photooxidation of some of the oxidation products of isoprene, i.e. glyoxal (Hasting et al 2005;Loeffler et al 2006;Carlton et al 2007), methylglyoxal (Loeffler et al 2006;Altieri et al 2008), pyruvic acid (Guzmán et al 2006;Altieri et al 2006;Carlton et al 2006), methacrolein El Haddad et al 2009), and glycolaldehyde (Perri et al 2009). The chemical pathways leading to the observed oligomers have been investigated by very few authors.…”

“…Alternatively, there is evidence that radical processes may also contribute to the formation of oligomers in the aqueous phase. Kinetic studies of the formation of oligomers lead some authors to propose combination reactions of organic radicals during the aqueous phase direct photolysis of pyruvic acid (Guzmán et al 2006) (Figure 16) or during OH-initiated oxidation of methacrolein and methylvynilketone Liu et al 2010). For all aqueous phase studies, the formed oligomers have shown molecular masses up to 1400 Da (see the mass spectrum in Figure 15), and are thus non volatile material, which is likely a source of SOA after cloud's processing.…”