Infrared Observations of the Response of NaCl, MgCl2, NH4HSO4, and NH4NO3Aerosols to Changes in Relative Humidity from 298 to 238 K

Abstract

Infrared extinction spectroscopy has been used to monitor liquid-to-solid and solid-to-liquid phase transitions of a variety of model tropospheric aerosols over a temperature range from 298 to 238 K. Uptake and loss of water were induced by controlling the relative humidity (RH) as submicron-sized aerosols passed through a low-temperature, atmospheric pressure flow tube on a time scale of tens of seconds. Observation of the absorption features of condensed-phase liquid water showed that NaCl aerosols deliquesced at a RH within ±2% of the thermodynamic value from 298 to 253 K, whereas efflorescence did not occur until 40 ± 5% RH. No hydrate formation was observed. By contrast, aerosols composed of species with considerably lower deliquescence points, including MgCl2, NH4HSO4, and NH4NO3, exhibited a strong inhibition to efflorescence down to 2% RH. As aerosols, the only one of these three which began to effloresce was MgCl2, and only at relative humidities below 2%. MgCl2·6H2O is the likely product. NH4HSO4 aerosols were induced to effloresce when sprayed onto an infrared window and subjected to relative humidities of less than 1% for periods of hours. These findings strongly suggest that, in the absence of heterogeneous nuclei, a wide variety of inorganic aerosols will exist as liquid solutions in the atmosphere regardless of relative humidity and temperature conditions.