Laboratory study of cluster ions formation in H2SO4–H2O system: Implications for threshold concentration of gaseous H2SO4 and ion-induced nucleation kinetics

Ion-induced binary H2SO4–H2O nucleation is an important mechanism of aerosol formation in the atmosphere. Ions are created in the atmosphere mainly by galactic cosmic rays. The importance of ion-induced nucleation is recognized in some of the observed nucleation events in the background atmosphere. However, the predictions of current ion–aerosol models are highly uncertain mostly due to the lack of detailed experimental information concerning the thermodynamics and kinetics of ion clustering reactions. Here we continue the report of results of our laboratory experiments on the formation and growth of positive and negative cluster ions in H2SO4–H2O vapours in the flow reactor started in Wilhelm et al. [2004. Ion-induced aerosol formation: new insights from laboratory measurements of mixed cluster ions HSO4−(H2SO4)a(H2O)w and H+ (H2SO4)a(H2O)w. Atmospheric Environment 38, 1735–1744] and Sorokin et al. [2006. Formation and growth of sulphuric acid–water cluster ions: experiments, modelling, and implications for ion-induced aerosol formation. Atmospheric Environment 40, 2030–2045]. The main attention is given to the definition of the concentration of gaseous sulphuric acid in experiment and also to some aspects of the kinetics of small cluster ions formation. The performed analysis has indicated a threshold concentration of gaseous sulphuric acid for binary homogeneous nucleation of at least about 1010 cm−3 at room temperature and low relative humidity.