Deliquescence, efflorescence, and supercooling of ammonium sulfate aerosols at low temperature: Implications for cirrus cloud formation and aerosol phase in the atmosphere

Motivated by the roles which tropospheric aerosols play in cloud formation processes and heterogeneous chemistry, low-temperature phase transitions of submicron-sized (NH4)(2)SO4 aerosols have been directly observed using Fou rier transform infrared extinction spectroscopy. In particular, deliquescen ce relative humidities have been measured down to 254 K and efflorescence p oints to 238 K. In agreement with previous studies, it was observed that so lid particles deliquesced at the thermodynamic value, whereas efflorescence of aqueous aerosols did not occur until a substantially lower relative hum idity was reached. By observing the formation of spectral features due to i ce, the same apparatus was used to monitor freezing of supercooled, micron- sized aerosol particles over a composition range from 0 to 49 weight percen t (wt%) (NH4)(2)SO4 The freezing of pure water aerosols occurred at 39 deg of supercooling, i.e., at a temperature of 234 K, in good agreement with th e literature value. Ammonium sulfate aerosols exhibited significantly less than 39 deg of supercooling when referenced to the equilibrium freezing lin e. These results indicate that ammonium sulfate aerosols will promote cirru s cloud formation in the upper troposphere at higher temperatures and lower relative humidities than previously believed.