Water activity and equilibrium freezing temperatures of aqueous NH4HSO4 solutions from −30 to 25°C

One mechanism of cirrus cloud formation is homogeneous ice nucleation within ammoniated sulfuric acid particles. An important constraint on ice formation is the (NH4)2SO4‐H2SO4‐H2O phase diagram. In the completed laboratory work, the NH4HSO4‐H2O phase diagram is determined over the domain relevant to the troposphere. The H2O vapor pressures of aqueous NH4HSO4 solutions are measured from −30 to 25°C, and the compositions of the aqueous phase in equilibrium with a solid phase (i.e., ice or letovicite) are determined at several temperatures. Below 25°C and above 42 wt% NH4HSO4(aq), the first solid to form from supersaturated NH4HSO4(aq) is (NH4)3H(SO4)2(S), not NH4HSO4(s). Supercooling of nearly 20 K is required to initiate letovicite formation for 100 mL samples. The results of this study are in good agreement with the calculations of Clegg et al. [1998].