Laboratory measurement on dicarboxylic acid assisted nucleation by sulfuric acid/water system

Binary nucleation system consisted of sulfuric acid and water was shown to fail in explaining the field observed nucleation rate long time ago. Many improved nucleation mechanisms, such as ternary nucleation by sulfuric acid/water/amines, ion-induced nucleation, and organic acid assisted nucleation, have been proposed to reconcile the controversy. In this study, the effect of succinic acid (an example of dicarboxylic acid) on sulfuric/water nucleation rate was determined in the laboratory by flow tube technique. The gaseous concentrations of sulfuric acid and succinic acid were measured simultaneously by the ion drift-chemical ionization mass spectrometry (ID-CIMS) in the negative ion proton transfer (NI-PT) mode. Meanwhile, the particle concentration was measured using the particle size magnifier (PSM, home-build) and ultra fine condensation particle counter (UCPC, commercially available) coupled system. PSM was introduced to lower the size detection limit of UCPC from about 3 nm down to about 1.5 nm, which is usually considered to be the size of critical nuclei. The results show that 5 ppb of succinic acid can enhance the sulfuric acid/water nucleation rate by 102 orders of magnitude. The slope of lnJ versus the ln[SA] and ln[SUA] are around 4 and 2, respectively, indicating that the critical nuclei contains 4 molecules of sulfuric acids and 2 molecules of succinic acids. In addition, theoretical calculations also show that succinic acid can have strong hydrogen bonding with sulfuric acid, which is consistent with the laboratory obtained results.