Sulfuric Acid Dimer Formation in Laminar Flow Tube Experiments

Atmospheric nucleation rates have been shown to be dependent on sulphuric acid (H2SO4) concentration (Weber et al. 1996, Sihto et al., 2006; Riipinen et al., 2007; Kuang et al., 2009). However, laboratory experiments on H2SO4-nucleation have failed to reproduce ambient observations (e.g. Wyslouzil et al., 1991; Viisanen et al, 1997; Ball et al, 1999; Young et al 2008). Very recently Sipilä et al. (2010) showed that the disagreement between ambient data and laboratory studies is most probably related to improper experimental design and that sulphuric acid nucleation in ultra clean laboratory conditions can indeed explain atmospheric nucleation rates. If H2SO4 is responsible for the nucleation the first step should be formation of (hydrated) H2SO4-dimer which can and have been measured by means of chemical ionization mass spectrometry (CI-MS, Eisele and Hanson, 2000; Hanson and Eisele, 2002; Hanson and Lovejoy, 2006; Zhao et al., 2010).