Water activity and equilibrium freezing temperatures of aqueous NH 4 HSO 4 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 (NH 4 ) 2 SO 4 ‐H 2 SO 4 ‐H 2 O phase diagram. In the completed laboratory work, the NH 4 HSO 4 ‐H 2 O phase diagram is determined over the domain relevant to the troposphere. The H 2 O vapor pressures of aqueous NH 4 HSO 4 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% NH 4 HSO 4 (aq), the first solid to form from supersaturated NH 4 HSO 4 (aq) is (NH 4 ) 3 H(SO 4 ) 2 (S), not NH 4 HSO 4 (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].