Interfacial regions are unique chemical reaction environments that can promote chemistry not found elsewhere. The air−water interface is ubiquitous in the natural environment in the form of ocean surfaces and aqueous atmospheric aerosols. Here we investigate the chemistry and photochemistry of pyruvic acid (PA), a common environmental species, at the air−water interface and compare it to its aqueous bulk chemistry using two different experimental setups: (1) a Langmuir−Blodgett trough, which models natural water surfaces and provides a direct comparison between the two reaction environments, and (2) an atmospheric simulation chamber (CESAM) to monitor the chemical processing of nebulized aqueous PA droplets. The results show that surface chemistry leads to substantial oligomer formation. The sequence begins with the condensation of lactic acid (LA), formed at the surface, with itself and with pyruvic acid, and LA + LA − H 2 O and LA + PA − H 2 O are prominent among the products in addition to a series of higher-molecular-weight oligomers of mixed units of PA and LA. In addition, we see zymonic acid at the surface. Actinic radiation enhances the production of the oligomers and produces additional surface-active molecules known from the established aqueous photochemical mechanisms. The presence and formation of complex organic molecules at the air−water interface from a simple precursor like PA in the natural environment is relevant to contemporary atmospheric science and is important in the context of prebiotic chemistry, where abiotic production of complex molecules is necessary for abiogenesis.

The asymmetric water–air interface provides a dynamic aqueous environment with properties that are often very different than bulk aqueous or gaseous phases and promotes reactions that are thermodynamically, kinetically, or otherwise unfavorable in bulk water. Prebiotic chemistry faces a key challenge: water is necessary for life yet reduces the efficiency of many biomolecular synthesis reactions. This perspective considers water–air interfaces as auspicious reaction environments for abiotic synthesis. We discuss recent evidence that (1) water–air interfaces promote condensation reactions including peptide synthesis, phosphorylation, and oligomerization; (2) photochemistry at water–air interfaces may have been a significant source of prebiotic molecular complexity, given the lack of oxygen and increased availability of near-ultraviolet radiation on early Earth; and (3) water–air interfaces can promote spontaneous reduction and oxidation reactions, potentially providing protometabolic pathways. Life likely began within a relatively short time frame, and water–air interfaces offer promising environments for simultaneous and efficient biomolecule production.

Lactic acid, a small α‐hydroxy acid, is a multifunctional molecule that is prevalent on modern Earth and found in abiotic environments. Recently, there has been interest in the photochemistry of carboxylic acids in general. Specifically, the photochemistry of lactic acid, due to its prevalence and functionality, has garnered attention by the biomass valorization and abiotic chemistry communities. However, aqueous lactic acid photochemistry studies are limited, and gas‐phase lactic acid photolysis had not been performed. This work combines theory and experiments to explore the gas‐phase and aqueous phase photochemistry of lactic acid following excitation of S0 to S1 at 220–250 nm. We find that lactic acid primarily photodecomposes via decarboxylation in both phases. In the gas phase, secondary chemistry leads to mainly CO2 and CO, while in the aqueous phase, subsequent radical chemistry leads to a variety of products with one to four carbons. Isotopic substitutions, including the use of 13C tagged lactic acid and using D2O as a solvent, are used to infer mechanistic pathways for the major photolysis products. Computation shows that individual conformers may contribute to the overall photochemistry to a different degree than their relative abundance would suggest. The identified products and proposed mechanisms shown here serve to illustrate the photochemistry of lactic acid in the presence of high‐energy ultraviolet radiation. This knowledge may aid process and catalyst design for biomass valorization/organic synthesis and may provide insight into abiotic chemistry in environments exposed to high energy ultraviolet radiation.