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Abstract

The formation, growth, and composition of mixed cluster ions HSO₄^-−A_aW_w and H+A_aW_w (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 A_aW_w 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.