Abstract. Sea-salt aerosol (SSA) particles are ubiquitous in the marine boundary layer and over coastal areas. Therefore SSA have ability to directly and indirectly affect the Earth's radiation balance. The influence SSA have on climate is related to their water uptake and ice nucleation characteristics. In this study, optical microscopy coupled with Raman spectroscopy was used to detect the formation of a crystalline NaCl hydrate that could form under atmospheric conditions. NaCl (s) particles (∼1 to 10 µm in diameter) deliquesced at 75.7 ± 2.5 % RH which agrees well with values previously established in the literature. NaCl (aq) particles effloresced to a mixture of hydrated and non-hydrated particles at temperatures between 236 and 252 K. The aqueous particles effloresced into the non-hydrated form at temperatures warmer than 252 K. At temperatures colder than 236 K all particles effloresced into the hydrated form. The deliquescence relative humidities (DRH) of hydrated NaCl (s) particles ranged from 76.6 to 93.2 % RH. Based on the measured DRH and efflorescence relative humidities (ERH), we estimate crystalline NaCl particles could be in the hydrated form 40-80 % of the time in the troposphere. Additionally, the ice nucleating abilities of NaCl (s) and hydrated NaCl (s) were determined at temperatures ranging from 221 to 238 K. Here, depositional ice nucleation is defined as the onset of ice nucleation and represents the conditions at which the first particle on the substrate nucleated ice. Thus the values reported here represent the lower limit of depositional ice nucleation. NaCl (s) particles depositionally nucleated ice at an average S ice value of 1.11 ± 0.07. Hydrated NaCl (s) particles depositionally nucleated ice at an average S ice value of 1.02 ± 0.04. When a mixture of hydrated and anhydrous NaCl (s) particles was present in the same sample, ice preferentially nucleated on the hydrated particles 100 % of the time. While both types of particles are efficient ice nuclei, hydrated NaCl (s) particles are better ice nuclei than NaCl (s) particles.

The effects of aerosol particles on heterogeneous atmospheric chemistry and climate are determined in part by the internal arrangement of compounds within the particles. We have used cryo-transmission electron microscopy to investigate the phase separation behavior of model organic aerosol composed of ammonium sulfate internally mixed with succinic or pimelic acid. We have found that no particle with a diameter <170 nm for succinic acid and 270 nm for pimelic acid is phase separated. Larger particles adopt a phase separated, partially engulfed structure. We therefore demonstrate that phase separation of aerosol particles is dependent on particle size and discuss implications for aerosol-climate interactions.

Organic aerosol is ubiquitous in the atmosphere, and impacts climate through the scattering and absorption of light and through the formation of nuclei for cloud droplets. These aerosol particles, which are composed of organic compounds and salts, are of great recent interest due to the complex chemistry that occurs within the particles as well as at the air-aerosol interface. Historically, organic aerosol was thought to undergo two phase transitions as the relative humidity around the particles is varied: efflorescence (crystallization) and deliquescence (water uptake). Recently, however, it was proposed that organic aerosol can undergo a phase transition in which liquid-liquid phase separation results in the formation of a particle with two liquid phases. This phenomenon has been recognized in the biophysical chemistry community for over a century, but atmospheric systems differ in several key aspects. Over the past 15 years, characterisation of the systems that undergo phase separation, the mechanisms by which this phase transition occurs, and the resultant morphologies have been investigated, sometimes with lingering questions. In addition, theory has been developed to model liquid-liquid phase separation in bulk systems. This review will cover these studies, focusing on experimental results, as well as covering recent results on the inhibition of liquid-liquid phase separation in nanoscale particles and studies that address the implications of this phase transition on climate-related properties of aerosol particles.

Abstract. Amorphous (semi-)solid organic aerosol particles have the potential to serve as surfaces for heterogeneous ice nucleation in cirrus clouds. Raman spectroscopy and optical microscopy have been used in conjunction with a cold stage to examine water uptake and ice nucleation on individual amorphous (semi-)solid particles at atmospherically relevant temperatures (200–273 K). Three organic compounds considered proxies for atmospheric secondary organic aerosol (SOA) were used in this investigation: sucrose, citric acid and glucose. Internally mixed particles consisting of each organic and ammonium sulfate were also investigated. Results from water uptake experiments followed the shape of a humidity-induced glass transition (Tg(RH)) curve and were used to construct state diagrams for each organic and corresponding mixture. Experimentally derived Tg(RH) curves are in good agreement with theoretical predictions of Tg(RH) following the approach of Koop et al. (2011). A unique humidity-induced glass transition point on each state diagram, Tg'(RH), was used to quantify and compare results from this study to previous works. Values of Tg'(RH) determined for sucrose, glucose and citric acid glasses were 236, 230 and 220 K, respectively. Values of Tg'(RH) for internally mixed organic/sulfate particles were always significantly lower; 210, 207 and 215 K for sucrose/sulfate, glucose/sulfate and citric acid/sulfate, respectively. All investigated SOA proxies were observed to act as heterogeneous ice nuclei at tropospheric temperatures. Heterogeneous ice nucleation on pure organic particles occurred at Sice = 1.1–1.4 for temperatures below 235 K. Particles consisting of 1:1 organic-sulfate mixtures took up water over a greater range of conditions but were in some cases also observed to heterogeneously nucleate ice at temperatures below 202 K (Sice= 1.25–1.38). Polynomial curves were fitted to experimental water uptake data and then incorporated into the Community Aerosol Radiation Model for Atmospheres (CARMA) along with the predicted range of humidity-induced glass transition temperatures for atmospheric SOA from Koop et al. (2011). Model results suggest that organic and organic/sulfate aerosol could be glassy more than 60% of the time in the midlatitude upper troposphere and more than 40% of the time in the tropical tropopause region (TTL). At conditions favorable for ice formation (Sice > 1), particles in the TTL are expected to be glassy more than 50% of the time for temperatures below 200 K. Results from this study suggests that amorphous (semi-)solid organic particles are often present in the upper troposphere and that heterogeneous ice formation on this type of particle may play an important role in cirrus cloud formation.

The morphology of aerosol particles impacts their role in the climate system. In the submicron size regime, the morphology of particles that undergo liquid-liquid phase separation is dependent on their size, where for some systems small particles are homogeneous and large particles are phase-separated. We use cryogenic transmission electron microscopy to probe the morphology of model organic aerosol systems. We observe that the transition region (where both homogeneous and phase-separated morphologies are seen) spans 121 nm at the fastest drying rates with a midpoint diameter > 170 nm. By slowing the drying rate over several orders of magnitude, the transition region shifts to smaller diameters (midpoint < 40 nm) and the width narrows to 4 nm. Our results suggest that the size-dependent morphology originates from an underlying finite size effect, rather than solely kinetics, due to the presence of a size dependence even at the slowest drying rates.