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New particle formation driven by acid–base chemistry was initiated in the\udCLOUD chamber at CERN by introducing atmospherically relevant levels of gas-phase sulfuric acid and dimethylamine (DMA). Ammonia was also present in the\udchamber as a gas-phase contaminant from earlier experiments. The composition\udof particles with volume median diameters (VMDs) as small as 10 nm was\udmeasured by the Thermal Desorption Chemical Ionization Mass Spectrometer\ud(TDCIMS). Particulate ammonium-to-dimethylaminium ratios were higher than the\udgas-phase ammonia-to-DMA ratios, suggesting preferential uptake of ammonia\udover DMA for the collected 10–30 nm VMD particles. This behavior is not\udconsistent with present nanoparticle physicochemical models, which predict a\udhigher dimethylaminium fraction when NH<sub>3</sub> and DMA are present at similar\udgas-phase concentrations. Despite the presence in the gas phase of at least\ud100 times higher base concentrations than sulfuric acid, the recently formed\udparticles always had measured base : acid ratios lower than 1 : 1. The\udlowest base fractions were found in particles below 15 nm VMD, with a strong\udsize-dependent composition gradient. The reasons for the very acidic\udcomposition remain uncertain, but a plausible explanation is that the\udparticles did not reach thermodynamic equilibrium with respect to the bases\uddue to rapid heterogeneous conversion of SO<sub>2</sub> to sulfate. These results\udindicate that sulfuric acid does not require stabilization by ammonium or\uddimethylaminium as acid–base pairs in particles as small as 10 nm

Unexpectedly acidic nanoparticles formed in dimethylamine–ammonia–sulfuric-acid nucleation experiments at CLOUD