Proton availability at the air/aerosol/water interface

The acidity of the water surface of aquated aerosols is examd. in expts. in which the uptake of gaseous trimethylamine, TMA, as TMAH+ on the surface of aquated aerosols is monitored via online electrospray mass spectrometry as a function of bulk pH, pHb. TMAH+ signal intensities track pHb along a titrn. curve displaying an equivalence point at pHb = 3.2, instead of the pKa(TMA) = 10.0 value measured on aq. TMA-HCl microdroplets. Inert X+ (X = Li, Na) cations, however, catalyze TMA uptake at pHb 4. The direct kinetic hydrogen isotope effects (1 < TMAH+/TMAD+ < 3.5 on H2O/D2O microdroplets) assocd. with TMA uptake enhancements reveal that they are driven by the competitive protonation (vs. desorption) of interfacial TMA. These outcomes are consistent with thermochem. data on gas-phase clusters showing that TMA can be protonated by the stronger acid H2O only after extensive solvation, but readily so by H3O+ or small (H2O)n.4Li+ clusters at the interface. We infer that protons become available at the aerial interface of neat water about its isoelec. point pIw = 3.2 rather than below pHb 7, and over broader pHb ranges in the presence of other cations