The small organic acid, pyruvic acid (PA), and its conjugate base, the pyruvate ion (PA -), are common chemical constituents of aerosols, fogs, clouds as well as seawater. PA is ubiquitous in the atmosphere and emitted from both biogenic and anthropogenic sources and has been detected as a major component of secondary organic aerosols (SOA) at many geographic locations. These include the urban atmospheres of Japan, China, and Mongolia 9 but PA has also been detected in remote aerosols at mountaintops in China, tropic Indian aerosols, marine aerosols, arctic aerosols, and even in biomass burning aerosols in Brazil. The particle-phase concentration of PA has been observed to possess a large spatiotemporal variability with average particle-phase concentrations measured as high as 49.7 ng m -3 in SOA of the urban atmosphere of Tokyo, Japan and as low as 0.03 ng m -3 in marine aerosols over the Southern Ocean. Furthermore, particle-phase concentrations of PA are observed to fluctuate significantly depending on season. In the aqueous phase, PA has been subjected to many photochemical investigations and has been reported to produce acetoin, lactic acid, and acetic acid and possibly also contribute to SOA mass through an oligomerization process, which goes via the parapyruvic acid dimer. Furthermore, PA has been found to exist as a mixture of the keto form and its hydrate, the geminal diol 2,2-dihydroxypropanoic acid (termed diol in this study). Hence, PA interconverts between the diol, the conjugate base of the diol, and the conjugate base of PA, PA -, in a cyclic manner. The ratio of abundances of the keto form and the diol has previously been reported to be 65% diol and 35% keto in aqueous solution, at 298 K and neutral pH. Furthermore, the pK a of PA in its keto form in aqueous solution and at 298 K is determined to be 2.18 while for the diol, it is determined to be 3.6. In perspective, aerosols are usually acidic with a median pH of 2.5, which implies that PA in its keto form is the most favored in aerosols at normal ambient conditions. In addition to the pure aqueousphase studies of PA, there has also been conducted several photochemical experiments of PA in e.g. multiphase environments and at air-water interfaces in order to probe the dynamic equilibrium between PA in the aqueous phase and PA in the gas-phase. PA has also been studied extensively in the gas-phase. The gas-phase mixing ratio of PA has mainly been measured in the range 10-100 pptv but has been detected as high as 400 pptv in the Amazonas region of Brazil and in the southern US. The atmospheric sources of PA in the troposphere are manifold and the routes to its formation goes mainly through gas-phase photolysis of small organic precursors. This includes the photo-oxidation of isoprene via the ozonolysis of methyl vinyl ketone, the photolysis of methylglyoxal, the reactions of peroxy radicals formed in the oxidation of propane, acetone, and hydroxyacetone, and in the photo-oxidation of aromatic compounds in the presence of NO x . The atmospheric sinks of PA, on the other hand, are generally four-fold: i) By rapid gas-phase photolysis induced by actinic radiation; ii) by slow gas-phase reaction with OH radicals; 2 by wet and dry deposition, and iv) by partitioning into the aerosol particle-phase. The atmospheric fate of PA via photolysis proceeds primarily via an exothermic decarboxylation, which involves a five-membered transition state that decomposes into CO 2 and methylhydroxycarbene with the latter rapidly rearranging to acetaldehyde. The gas-phase lifetime of PA with respect to the photolysis channel is a few hours according to previous experimental studies. Conversely, the gas-phase lifetime of PA against the OH channel is significantly longer at about three months. The atmospheric fate of PA via the dry and wet deposition mechanisms as well as by the partitioning into the aerosol-phase are particularly pronounced at high relative humidity due to the high solubility of PA in water and can thus contribute to the formation of SOA. While the gas-phase and aqueous phase chemistry of PA has been extensively studied, the transition between the two has not yet been investigated in detail. The uptake of PA onto aerosol particles must occur via the interfacial surface layer. As a first approximation the local environment of the interaction between the gas phase PA and the surface can be viewed as a small cluster of molecules consisting of water and ions. Despite previous studies on the PA monohydrate cluster, the diol in the liquid phase and the PA clusters with one to four water molecules, there has, to the best of our knowledge, not been reported any studies on how PA or similar carboxylic acids simultaneously interact with water molecules and ions.