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Investigation of atmospheric phase transitions in low temperature, mixed aerosols Bodsworth, Aidan Neville Fredrick

Abstract

Atmospheric aerosols are very small liquid or solid droplets which are suspended in a gas. They are present in our atmosphere in great numbers (10³ particles per cm³ in unpolluted rural environments). They represent a substantial source of uncertainty in climate models and influence human health, atmospheric reactivity and visibility. Field studies have shown that mixed organic-inorganic aerosols are abundant in the atmosphere. Mixed aerosols undergo phase transitions between aqueous and crystalline phases, called efflorescence and deliquescence. By examining laboratory prepared mixed droplets in a temperature and humidity controlled flow cell, these phase transitions were observed optically. We studied the efflorescence properties of mixed citric acid-ammonium sulfate particles as a function of temperature to better understand the efflorescence properties of mixed organic-inorganic particles in the middle and upper troposphere. Data at 293 K illustrates that the addition of citric acid decreases the efflorescence relative humidity (ERH) of ammonium sulfate, which is consistent with the trends observed with other systems containing highly oxygenated organic compounds. At low temperatures the trend is qualitatively the same, but efflorescence can be inhibited by smaller concentrations of citric acid. For example at temperatures < 250 K an organic/(organic+sulfate) mass ratio of only 0.333 is needed to inhibit efflorescence of ammonium sulfate. In the upper troposphere the organic/(organic+sulfate) mass ratio can often be larger than this value. We also studied phase transitions in ammonium sulfate-3-hydroxy-4-methoxy-mandelic acid (HMMA) systems. These systems showed similar trends in deliquescence and efflorescence to the citric acid case, but also underwent phase separations at an RH of approximately 77%. Again, low temperatures inhibited phase transitions. It is believed that the inhibition of efflorescence and phase separation stems from either an increase in viscosity or the formation of an organic glass, this is investigated using classical nucleation theory and DSC measurements. These studies imply that particles in the upper troposphere may be less likely to form or remain in the crystalline state than previously thought and that solid ammonium sulfate may be less likely to participate in heterogeneous ice nucleation in the upper troposphere. Additional studies are required on other model organic systems.

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