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)(2)SO4-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 degrees 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 degrees C and above 42 wt% NH4HSO4(aq), the first solid to form from supersaturated NH4HSO4(aq is (NH4)(3)H(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].