Laboratory studies of model tropospheric aerosol phase transitions

Deliquescence and efflorescence phase transitions of sub-micron aerosol are of atmospheric interest. Studies with two experimental systems: a static mode chamber-FTIR and an aerosol flow tube-FTIR have been performed.; Novel deliquescence experiments at sub-eutectic temperatures, i.e. under ice-supersaturated conditions showed that although both deliquescence and ice formation are possible, deliquescence occurred at the thermodynamically predicted relative humidity (RH) for both ammonium sulphate (AS) and malonic acid (MA) particles. This implies that supercooled solution droplets can form at high RH in the atmosphere.; A detailed study into malonic and oxalic acid aerosol phase transitions was performed. Both species sustain a substantial level of solute supersaturation before efflorescence. Deliquescence occurs at the thermodynamically predicted RH, i.e. RH ∼100% for oxalic acid and between 69% RH and 91% RH (293 K--252 K) for MA. Malonic and oxalic acid efflorescence occurred at RH = 6 +/- 3% and RH ≤ 5% respectively (293 K).; Deliquescence and efflorescence of mixed AS-MA aerosol were studied. When the fraction of MA (fMA) by dry mass was <0.1 and >0.9, deliquescence occurred at the AS DRH and efflorescence relative humidity (ERH) respectively. For 0.1 < fMA < 0.25 deliquescence occurred at the eutonic DRH and the ERH is lowered. Between 0.25 < fMA < 0.90, no distinct deliquescence or efflorescence was observed. It was observed that crystallization, of both MA and AS, is suppressed by the presence of the other component. At fMA = 0.9, MA efflorescence was lowered to below the detection limits. Overall, the second component increased the RH range over which particles are aqueous.; AS mixtures with the C4--C6 dicarboxylic acids, ammonium oxalate and other complex aerosol mixtures were studied. It was possible to distinguish between the transitions of the AS and the dicarboxylic acid component. Continuous water uptake, implying crystallization has not occurred, is observed if there is a significant fraction of a second component. A general conclusion of this work is that either by sub-eutectic deliquescence or the presence of multiple solutes, aerosols are predicted to have more aqueous character than predicted with simple phase transition models.