Multiphase Photochemistry of Pyruvic Acid under Atmospheric Conditions

Abstract

Aerosol and molecular processing in the atmosphere occurs in a complex and variable environment consisting of multiple phases and interfacial regions. To explore the effects of such conditions on the reactivity of chemical systems, we employ an environmental simulation chamber to investigate the multiphase photolysis of pyruvic acid, which photoreacts in the troposphere in aqueous particles and in the gas phase. Upon irradiation of nebulized pyruvic acid, acetic acid and carbon dioxide are rapidly generated, which is consistent with previous literature on the bulk phase photolysis reactions. Additionally, we identify a new C product, zymonic acid, a species that has not previously been reported from pyruvic acid photolysis under any conditions. Its observation here, and corresponding spectroscopic signatures, indicates it could be formed by heterogeneous reactions at the droplet surface. Prior studies of the aqueous photolysis of pyruvic acid have shown that high-molecular-weight compounds are formed via radical reactions; however, they are inhibited by the presence of oxygen, leading to doubt as to whether the chemistry would occur in the atmosphere. Identification of dimethyltartaric acid from the photolysis of multiphase pyruvic acid in air confirms radical polymerization chemistry can compete with oxygen reactions to some extent under aerobic conditions. Evidence of additional polymerization within the particles during irradiation is suggested by the increasing viscosity and organic content of the particles. The implications of multiphase specific processes are then discussed within the broader scope of atmospheric science.

Mechanistic Description of Photochemical Oligomer Formation from Aqueous Pyruvic Acid

“…This raises the possibility that these species might also be synthesized photochemically and act as reactive oligomeric intermediate species whose further chemistry contributes to the observed photoproducts in the light-initiated chemistry of pyruvic acid, as is discussed in detail below. The recent observation of zymonic acid as a photoproduct generated from the irradiation of multiphase pyruvic acid under atmospherically relevant conditions in environmental simulation chamber studies 61 lends credence to the photochemical formation and subsequent photochemistry of parapyruvic acid that may take place in aqueous solution.…”

“…The aqueous chemistry of pyruvic acid is generally defined by the formation of oligomeric species, under both light and dark conditions. The aqueous phase photochemistry of pyruvic acid has been studied previously in the literature 41,[43][44][45]56,60,61 and is known to generate a surprisingly complex mixture of observed photoproducts, many of which have been assigned following the literature mechanism.…”

“…The new mecha-nistic pathway we suggest for these products is informed by a recent investigation of the dark oligomerization processes of pyruvic acid, 66 as well as recent results examining the multiphase photochemistry of pyruvic acid in an environmental simulation chamber. 61 Pre-Irradiation Solutions and Dark Processes. The generation of covalently bonded dimers of pyruvic acid in the dark has been known since the 19th century; 78 − 80 nevertheless, these dark processes have been largely ignored by the modern literature.…”

Photochemical Synthesis of Oligomeric Amphiphiles from Alkyl Oxoacids in Aqueous Environments

“…This is consistent with previous studies on pyruvic acid that show that oligomers are formed in the presence of oxygen, 28,69 including under conditions that closely replicate the modern atmos-phere. 68 Nevertheless, the ability to shift the reaction branching ratios by shifting environmental conditions has significant implications, especially for longer, lipid-like oxoacids.…”

“…28,57,58 In the aqueous phase, pyruvic acid readily forms oligomers, 28,63,64,67,79 including in atmospherically relevant, multiphase studies using an environmental simulation chamber. 68 These covalently bonded dimers and trimers 80 are generated in relatively high yields from the recombination of photochemically generated organic radicals and are the major observed photoproducts, especially under oxygen-limited reaction conditions. 28,63,64,67 …”