Formation and growth of sulfuric acid-water cluster ions: Experiments, modelling, and implications for ion-induced aerosol formation

The formation, growth, and composition of mixed cluster ions HSO4-−AaWw and H+AaWw (A=H2SO4, W=H2O) with a up to 6 were investigated in the laboratory using a flow reactor-mass spectrometer apparatus. In the flow reactor the buffer gas (N2) pressure and temperature were kept constant at 80 hPa and 295 K, respectively, while the partial pressures of the trace gases H2SO4 and H2O and thereby also the relative humidity were varied. It was found that for small a (<5) positive ions are markedly enriched in H2O and negative ions are enriched in H2SO4 molecules. For larger a the composition of positive and negative ions becomes similar and approaches the modelled composition of a liquid AaWw droplet (respectively, for a>8 and a>15). When increasing the partial pressure of gaseous H2SO4 in the flow reactor both positive and negative ions grow via A uptake. This growth is more pronounced for negative ions where it becomes very efficient for a>8 reflecting A attachment to become faster than A detachment. We also have made model calculations indicating that for a>10 negative ion growth via A attachment occurs on nearly every collision of an A-molecule with an ion