Abstract. Chemical ionization (CI) mass spectrometers are used to study atmospheric nucleation by detecting clusters produced by reactions of sulfuric acid and various basic gases. These instruments typically use nitrate to deprotonate and thus chemically ionize the clusters. In this study, we compare cluster concentrations measured using either nitrate or acetate. Clusters were formed in a flow reactor from vapors of sulfuric acid and dimethylamine, ethylene diamine, tetramethylethylene diamine, or butanediamine (also known as putrescine). These comparisons show that nitrate is unable to chemically ionize clusters with high base content. In addition, we vary the ion–molecule reaction time to probe ion processes which include proton-transfer, ion–molecule clustering, and decomposition of ions. Ion decomposition upon deprotonation by acetate/nitrate was observed. More studies are needed to quantify to what extent ion decomposition affects observed cluster content and concentrations, especially those chemically ionized with acetate since it deprotonates more types of clusters than nitrate.

Model calculations of the neutral and ion cluster formation pathways are also presented to better identify the cluster types that are not efficiently deprotonated by nitrate. Comparison of model and measured clusters indicate that sulfuric acid dimers with two diamines and sulfuric acid trimers with two or more base molecules are not efficiently chemical ionized by nitrate. We conclude that acetate CI provides better information on cluster abundancies and their base content than nitrate CI.