Single Particle Studies in an Optical Levitator: Influence of Organics on Contact Efflorescence
Abstract
The phase state of atmospheric aerosols is a key factor in determining their impacts on climate and chemistry. Soluble inorganics in the atmosphere can be found either as crystalline or aqueous particles depending on the relative humidity (RH) and RH history of the air parcel. However, salt aerosols are often mixed with organic compounds, which can shift the humidity conditions necessary for efflorescence and deliquescence to occur. The diverse range of organics in the atmosphere further complicates their impact on the phase of mixed aerosols. Here we use raffinose and polyethylene glycol (PEG-400) as two model organic systems. At room temperature, raffinose is known to form a glass while PEG-400 phase separates from ammonium sulfate ((NH4)2SO4) below 90% RH, forming an organic coating around an aqueous core. In the present study we optically levitate single particles of (NH4)2SO4 mixed with equal parts of either raffinose or PEG-400 to investigate particle efflorescence behavior. We observe that the glassy raffinose inhibits homogeneous crystal formation of (NH4)2SO4, even when dried to as low as 1% RH. The PEG-400 coating, unlike raffinose, does not change the efflorescence RH of (NH4)2SO4 when compared to the homogeneous efflorescence RH of 35% RH. We also investigated the effect of the organics on contact efflorescence of (NH4)2SO4 by colliding the mixed aerosol with a crystal of pure (NH4)2SO4. We observe that contacting pure (NH4)2SO4 with a crystal of itself completely shuts down hysteresis, allowing for crystallization to occur at all humidities below the deliquescence RH. When (NH4)2SO4 is mixed with organics, we find that both raffinose and phase-separated PEG-400 still allow for contact nucleation at high RH. At lower humidities, the PEG-400 coating again did not interfere with efflorescence of the pure (NH4)2SO4 core. However, mixture with raffinose delay (but not prevent) contact efflorescence, presumably due to slower water diffusion in the semisolid particle. Models have shown that collisions between aerosols in the atmosphere occur regularly. Thus, when two aerosol plumes mix, contact efflorescence is possible and can result in more particles being crystalline than previously believed.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A53M2947U
- Keywords:
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- 0305 Aerosols and particles;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0317 Chemical kinetic and photochemical properties;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0320 Cloud physics and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES