Abstract
Direct measurements of the phase separation relative humidity (RH) and morphology of aerosol particles consisting of liquid organic and aqueous inorganic domains are presented. Single droplets of mixed phase composition are captured in a gradient force optical trap, and the evolving size, refractive index (RI), and morphology are characterized by cavity-enhanced Raman spectroscopy. Starting at a RH above the phase separation RH, the trapped particle is dried to lower RH and the transition to a phase-separated structure is inferred from distinct changes in the spectroscopic fingerprint. In particular, the phase separation RHs of droplets composed of aqueous solutions of polyethylene glycol (PEG-400)/ammonium sulfate and a mixture of C6-diacids/ammonium sulfate are probed, inferring the RH from the RI of the droplet immediately prior to phase separation. The observed phase separation RHs occur at RH marginally higher (at most 4%) than reported in previous measurements made from studies of particles deposited on hydrophobic surfaces by brightfield imaging. Clear evidence for the formation of phase-separated droplets of core-shell morphology is observed, although partially engulfed structures can also be inferred to form. Transitions between the different spectroscopic signatures of phase separation suggest that fluctuations in morphology can occur. For droplets that are repeatedly cycled through the phase separation RH, the water activity at phase separation is found to be remarkably reproducible (within ±0.0013) and is the same for the 1-phase to 2-phase transition and the 2-phase to 1-phase transition. By contrast, larger variation between the water activities at phase separation is observed for different droplets (typically ±0.02).
Highlights
Aerosol particles are an important atmospheric constituent, influencing atmospheric chemistry,[1,2] air quality, and global climate.[3−7] Their impact is highly dependent on their properties, including particle size, phase, and morphology.[3,8,9] the dependence of these properties on particle composition and environmental conditions, relative humidity (RH) and temperature, remains poorly understood, leading to uncertainties in the magnitudes of the impacts of aerosols on climate and chemistry
Data is presented showing how the relative humidity at which liquid−liquid phase separation occurs varies with composition for single levitated particles containing poly ethylene glycol (PEG)-400/ammonium sulfate/water and a mixture of C6-diacids/ammonium sulfate/ water
In the PEG/ammonium sulfate (AS) system, the observed separation RH (SRH) in the optical tweezers is in general agreement with the data of Ciobanu et al.[41] which indicated liquid−liquid phase separation in droplets of the same compositions deposited on hydrophobic substrates, indicating the substrate does not have an appreciable effect on the relative humidity at which separation occurs for this system
Summary
Aerosol particles are an important atmospheric constituent, influencing atmospheric chemistry,[1,2] air quality, and global climate.[3−7] Their impact is highly dependent on their properties, including particle size, phase, and morphology.[3,8,9] the dependence of these properties on particle composition and environmental conditions, relative humidity (RH) and temperature, remains poorly understood, leading to uncertainties in the magnitudes of the impacts of aerosols on climate and chemistry. Tropospheric aerosol is complex in composition containing organic matter (both primary and secondary with a large range of O:C ratios), inorganic salts (mainly ammonium, sulfate, and nitrate in the accumulation mode particle size range10), and water. These components can be internally mixed and, depending on the composition of the aerosol and the ambient conditions, may be one liquid phase, two liquid phases, an amorphous glassy state, a solid crystalline state, or a mixture of these states.[11]. There has been little consideration of the phase separation of a Received: February 18, 2015 Revised: April 8, 2015 Published: April 16, 2015
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