Articles | Volume 8, issue 6
Research article
25 Jun 2015
Research article |  | 25 Jun 2015

The charging of neutral dimethylamine and dimethylamine–sulfuric acid clusters using protonated acetone

K. Ruusuvuori, P. Hietala, O. Kupiainen-Määttä, T. Jokinen, H. Junninen, M. Sipilä, T. Kurtén, and H. Vehkamäki

Abstract. Sulfuric acid is generally considered one of the most important substances taking part in atmospheric particle formation. However, in typical atmospheric conditions in the lower troposphere, sulfuric acid and water alone are unable to form particles. It has been suggested that strong bases may stabilize sulfuric acid clusters so that particle formation may occur. More to the point, amines – strong organic bases – have become the subject of interest as possible cause for such stabilization. To probe whether amines play a role in atmospheric nucleation, we need to be able to measure accurately the gas-phase amine vapour concentration. Such measurements often include charging the neutral molecules and molecular clusters in the sample. Since amines are bases, the charging process should introduce a positive charge. This can be achieved by, for example, using chemical ionization with a positively charged reagent with a suitable proton affinity. In our study, we have used quantum chemical methods combined with a cluster dynamics code to study the use of acetone as a reagent ion in chemical ionization and compared the results with measurements performed with a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (CI-APi-TOF). The computational results indicate that protonated acetone is an effective reagent in chemical ionization. However, in the experiments the reagent ions were not depleted at the predicted dimethylamine concentrations, indicating that either the modelling scheme or the experimental results – or both – contain unidentified sources of error.

Short summary
Ionization reagents suitable for accurate measurements of the atmospheric gas-phase amine vapour concentrations are needed. Based on computational results, acetone is a viable option for use as an ionization reagent in CI-APi-TOF measurements on atmospheric dimethylamine. However, comparison between the computational and experimental results revealed notable discrepancies. Further study is still required before the acetone CI-APi-TOF can be considered a viable option in practice.