α-Keto acids are important, atmospherically relevant species, and their photochemistry has been considered in the formation and processing of aerosols. Despite their atmospheric relevance, the photochemistry of these species has primarily been studied under extremely low pH conditions. Using a variety of analytical techniques, we characterize the extent of hydration and deprotonation for solutions of two α-keto acids, pyruvic acid and 2-oxooctanoic acid, as a function of pH. We find that changes in the initial solution composition govern the accessibility of different photochemical pathways, resulting in slowed photolysis under high pH conditions and a shift in photoproducts that can be predicted mechanistically.

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.

We have monitored the speeds of evaporating helium atoms dissolved in liquid octane, isooctane, 1-methylnaphthalene, dodecane, squalane, ethylene glycol, and two jet fuels. In all cases, the average kinetic energies of the evaporating He atoms exceed the Maxwellian value of 2RT. The energies roughly track solvent surface tensions; this correlation may reflect the tighter packing and attractions of interfacial solvent molecules that restrict the gaps through which He atoms escape. Mixtures of dodecane, squalane, and 1-methylnaphthalene generate He evaporation energies that lie between the pure liquid values. We find, however, that He atoms evaporate from pure 1-methylnaphthalene with kinetic energies lower than expected based on its high surface tension, perhaps because the sideways packing of the aromatic rings provides more direct channels for the escaping He atoms. Additionally, He evaporates from two complex fuel mixtures, Jet A and JP-8, with nearly identical energies, implying that the extra additives in JP-8 do not segregate to the surface in ways that alter the dynamics of evaporation. ■ INTRODUCTIONHelium atom scattering is an enormously powerful tool to elucidate the surface structure and surface vibrations of crystalline solids, organic monolayers, and even polymer films. 1−5 The diffraction features that make this technique so useful are unfortunately lost for room-temperature liquids because of long-range disorder and extensive thermal motions that promote energy transfer between the He and surface atoms. However, it may still be possible to glean insights into interfacial motions and packing by replacing He atom scattering with He atom evaporation from liquids. 6 We continue our studies here by measuring the energy distributions of He atoms evaporating from a wide range of linear and branched hydrocarbon liquids, including mixtures, jet fuels, and ethylene glycol (as a model for a deicing additive). The experiments are performed by dissolving He atoms into each liquid, injecting the liquids into a vacuum as microjets, 7,8 and monitoring the velocities of the evaporating He atoms.The evaporation of He atoms dissolved in hydrocarbons, alcohols, and salty water appears to be special: these small and weakly attractive atoms evaporate at speeds that are faster than predicted by a Maxwell−Boltzmann (MB) distribution. This behavior stands in contrast to larger and more polarizable species such as Ar, N 2 , O 2 , H 2 O, CO 2 , HCl, and HNO 3 , which evaporate with flux-weighted average energies of 2RT liq consistent with a MB energy distribution for a liquid at temperature T liq . 6,9 We recently found that the He speed distributions depend on the nature of the solvent molecules, with average kinetic energies ranging from 1.14 × 2RT liq for He evaporation from dodecane to 1.7 × 2RT liq from a 7.5 M LiBr/ H 2 O solution. 6 Detailed balancing of the incoming and outgoing fluxes implies that He atoms must preferentially dissolve at higher kinetic energies as well. 10 In this timereversed picture, the attractive...