Physical properties of aerosol particles, such as liquid–liquid phase separation (LLPS), have the potential to impact the climate system. Model systems have been shown to have size-dependent LLPS in the submicron regime; however, these systems are an extreme simplification of ambient aerosol, which can include myriad organic compounds. We expand the studies of LLPS in particles consisting of ammonium sulfate and more complex organic mixtures from multiple organic compounds to α-pinene secondary organic matter (SOM). All systems display a size-dependent morphology, with small particles remaining homogeneous while large particles phase-separate. Surprisingly, three-phase particles were also observed in some of the systems in addition to a new phase state that we have termed channel morphology, which can arise upon efflorescence. The existence of size-dependent LLPS in complex organic mixtures and SOM provides evidence that this is a relevant phenomenon for ambient aerosol and should be considered when modeling atmospheric aerosol.

Development of methods to measure the phase transitions and physical properties of submicron atmospheric aerosol particles is needed to better model these systems. In this paper, we present a method to flash freeze submicron particles to measure phase transitions as a function of relative humidity (RH). Particles are equilibrated at a fixed RH, vitrified in a temperature-controlled flow tube, and imaged with cryogenic transmission electron microscopy (cryo-TEM). We demonstrate the use of the technique for measuring the efflorescence relative humidity (ERH) of potassium sulfate and potassium chloride aerosol as well as the separation RH (SRH) for a multicomponent organic/inorganic system that undergoes liquid−liquid phase separation (LLPS). The location of phase transitions can shift between the micrometer and nanometer size regimes, and particles in a given population may have a range of RH over which a phase transition occurs. This technique addresses these requirements by allowing for characterization of the phase transitions for individual particles in a population on the submicron scale.

Nanoscale materials, when compared to their bulk components, possess unique properties. In particular, shifts in phase transitions can occur for submicrometer particles. For instance, small particles do not undergo the process of liquid–liquid phase separation (LLPS). LLPS has applications in emulsions such as Janus particles, controllable morphology to create drug-rich phases during drug delivery, and is often observed in atmospheric aqueous aerosol particles. In atmospheric particles, LLPS is tracked as a function of particle water activity, which is equivalent to the relative humidity (RH) at equilibrium. We probed three organic/inorganic aerosol systems in the range of RH over which phase separation occurs (SRH). Our findings indicate that SRH for submicrometer aerosol particles is lower than for micrometer-sized droplets. These findings show that it may be necessary to update the representation of phase transitions in aerosol particles in climate models. The vast majority of organic/inorganic aerosol particles have submicrometer diameters, and a decrease in SRH for submicrometer particles indicates that the current estimation of phase-separated aerosols may be overestimated. Furthermore, understanding the properties of LLPS at the nanoscale can provide key parameters to describe these systems and may lead to better control of phase separation in submicrometer particles.

For over 25 years, transmission electron microscopy (TEM) has provided a method for the study of aerosol particles with sizes from below the optical diffraction limit to several microns, resolving the particles as well as smaller features. The wide use of this technique to study aerosol particles has contributed important insights about environmental aerosol particle samples and model atmospheric systems. TEM produces an image that is a 2D projection of aerosol particles that have been impacted onto grids and, through associated techniques and spectroscopies, can contribute additional information such as the determination of elemental composition, crystal structure, and 3D particle structures. Soot, mineral dust, and organic/inorganic particles have all been analyzed using TEM and spectroscopic techniques. TEM, however, has limitations that are important to understand when interpreting data including the ability of the electron beam to damage and thereby change the structure and shape of particles, especially in the case of particles composed of organic compounds and salts. In this paper, we concentrate on the breadth of studies that have used TEM as the primary analysis technique. Another focus is on common issues with TEM and cryogenic-TEM. Insights for new users on best practices for fragile particles, that is, particles that are easily susceptible to damage from the electron beam, with this technique are discussed. Tips for readers on interpreting and evaluating the quality and accuracy of TEM data in the literature are also provided and explained.

Atmospheric processing of mineral aerosol by trace gases results in the formation of surface-adsorbed products that have the capacity to alter the chemical and physical properties of these airborne particulates. To investigate one potential impact of aerosol processing by biogenic volatile organic compounds (BVOCs), we investigated the heterogeneous decomposition of ozone on pure and monoterpene-processed kaolinite. We used a laminar flow reactor to measure O 3 reactive uptake coefficients on kaolinitecoated tubes as a function of relative humidity, O 3 concentration, and pre-exposure to gaseous limonene and αpinene. At 26% RH, kaolinite has a near equivalent of a monolayer of adsorbed water, and the ozone steady-state uptake coefficient was γ av = 2.9 × 10 −9 assuming the BET surface area. Pre-exposing kaolinite to limonene and α-pinene increased O 3 uptake coefficients by nearly 2 orders of magnitude to 2.1 × 10 −7 and 2.5 × 10 −7 , respectively. At all humidities studied (10−50% RH), O 3 uptake was at least 1 order of magnitude higher for monoterpene-processed kaolinite compared to that of pure kaolinite. This dramatic increase in O 3 reactivity is attributed to surface-adsorbed organics, namely limonenediol and α-terpineol, which contain alkene functionalities susceptible to ozonolysis. Increasing relative humidity decreased O 3 uptake for monoterpene-processed kaolinite consistent with competitive adsorption of water resulting in lower organic surface concentrations. These results demonstrate the significant impact adsorbed organics can have on O 3 uptake coefficients on mineral aerosol, which should be accounted for in atmospheric modeling studies.