Ice nucleation in NH4HSO4, NH4NO3, and H2SO4 aqueous particles:: Implications for cirrus cloud formation

The temperature at which ice nucleation occurs in aqueous NH4HSO4, NH4NO3, and H2SO4 particles has been studied as a function of solute concentration. The onset of freezing was determined using FTIR extinction spectroscopy coupled to a low-temperature flow tube system. The greatest supercooling with respect to equilibrium freezing was observed for H2SO4/H2O. Considerably less supercooling was observed for NH4NO3/H2O, and less yet for the NH4HSO4/H2O system. All three aerosol types exhibit a lower freezing temperature, as a function of solute concentration, than (NH4)(2)SO4 particles previously studied by this approach. These results suggest that air masses containing ammoniated sulfate particles will promote cirrus cloud formation at higher temperatures and lower relative humidities than will those containing pure H2SO4 particles.