Gaseous biogenic volatile organic compounds (BVOCs) are immediately oxidized by gaseous oxidants to form BVOC-acids that rapidly condense onto aqueous aerosol phase and thus contribute to the growth of atmospheric particles. Because BVOC-acids are highly hydrophobic and hence surface-active in nature, it seems critical to study the oxidation by gaseous hydroxyl radical (center dot OH(g)) at the air-water interface. Here we report on the fast (<= 10 mu s) oxidation of aqueous cis-pinonic acid (C10H16O3, CPA, cis-pinonate anion's m/z = 183), a representative BVOC-acid, by center dot OH(g) at the air-water interface for the first time. We find that cis-pinonate anion is more enriched at the air-water interface by similar to 4 and similar to 14 times than n-octanoate anion at 10 and 100 mu M, respectively, as revealed by an interface-specific mass spectrometry of the equimolar mixture of microjets. Exposure of aqueous CPA microjets to center dot OH(g) pulses from the 266 nm laser photolysis of O-3(g)/O-2(g)/H2O(g)/N-2(g) mixtures yields pinonic peroxyl radicals (m/z = 214) that lead to the functionalization products carbonyls (m/z = 197), alcohols (m/z = 199), and pinonic hydroperoxides (m/z = 215) in addition to smaller-mass products including carbonyls (m/z = 155 and 157). We confirmed the formation of the corresponding alcohols, aldehydes, and hydroperoxides in experiments performed in D2O solvent. The analysis of total mass balance implies a significant amount (>70%) of products would be emitted into the gas-phase during the heterogeneous center dot OH-oxidations. Our results suggest center dot OH-oxidations of amphiphilic BVOC-acids at the air-water interface may play a far more significant role in photochemical aging process of aqueous aerosols than previously assumed.