Ion-induced aerosol formation: new insights from laboratory measurements of mixed cluster ions HSO sub(4) super(-)(H sub(2)SO sub(4)) sub(a)(H sub(2)O) sub(w) and H super(+)(H sub(2)SO sub(4)) sub(a)(H sub(2)O) sub(w)

Here we report on laboratory investigations of mixed cluster ions composed of sulphuric acid and water molecules HSO sub(4) super(- )(H sub(2)SO sub(4)) sub(a)(H sub(2)O) sub(w) and H super(+)(H sub(2)SO sub(4)) sub(a)(H sub(2)O) sub(w). Such ions have potentially an important role in atmospheric H sub(2)SO sub(4)/H sub(2)O aerosol formation via ion- induced nucleation. The investigations were made using a flow reactor equipped with two ion mass spectrometers which had either a high mass resolution but a relatively small mass range (0-750 amu) or a low mass resolution but a large mass range (0-10000 amu). Our experiments led to the following major findings: (a) Both negative and positive cluster ions take up H sub(2)SO sub(4) molecules very efficiently. (b) For negative ions the sulphuric acid mole fraction is initially large, but as the ions grow, it rapidly approaches the value expected for neutral clusters. (c) For positive ions the sulphuric acid mole fraction is initially small, but as the ions grow, it rapidly increases and approaches the value expected for neutral clusters. (d) The measurements indicate that thermal detachment of sulphuric acid molecules from the cluster ions is important and strongly limits ion growth. The thermal detachment of sulphuric acid molecules is more efficient for positive ions compared to negative ions. Our experiments have interesting implications for ion-induced nucleation of H sub(2)SO sub(4)/H sub(2)O in the atmosphere which are addressed.