Images reveal that atmospheric particles can undergo liquid–liquid phase separations

Abstract

A large fraction of submicron atmospheric aerosol particles contains both organic material and inorganic salts. As the relative humidity cycles in the atmosphere and the water content of the particles correspondingly changes, these mixed particles can undergo a range of phase transitions, possibly including liquid-liquid phase separation. If liquid-liquid phase separation occurs, the gasparticle partitioning of atmospheric semivolatile organic compounds, the scattering and absorption of solar radiation, and the reactive uptake of gas species on atmospheric particles may be affected, with important implications for climate predictions. The actual occurrence of liquid-liquid phase separation within individual atmospheric particles has been considered uncertain, in large part because of the absence of observations for realworld samples. Here, using optical and fluorescence microscopy, we present images that show the coexistence of two noncrystalline phases for real-world samples collected on multiple days in Atlanta, GA as well as for laboratory-generated samples under simulated atmospheric conditions. These results reveal that atmospheric particles can undergo liquid-liquid phase separations. To explore the implications of these findings, we carried out simulations of the Atlanta urban environment and found that liquid-liquid phase separation can result in increased concentrations of gas-phase NO 3 and N 2 O 5 due to decreased particle uptake of N 2 O 5 .chemistry | physical state | secondary organic aerosol | ambient aerosol | atmospheric chemistry I n the atmosphere, single particles containing both organic species and inorganic salts are abundant (1, 2). The number of different types of inorganic salts is relatively small, with ammonium sulfate considered to be one of the most important in particles less than 1 μm in diameter (3, 4). In contrast, the number of organic species in a single atmospheric particle is on the order of thousands, with only ∼10% of these species identified at the molecular level (5). As the relative humidity (RH) cycles in the atmosphere through high and low values, the water content of the particles increases and decreases as a hygroscopic response. In response to variable water content, the mixed particles can undergo a range of phase transitions including crystallization (i.e., efflorescence), dissolution (i.e., deliquescence), and liquidliquid phase separation (Fig. 1) (6-12).Results of laboratory measurements or calculations for particles or solutions containing ammonium sulfate mixed with one or a few specific organic molecules suggest, after extrapolation to atmospheric conditions, that liquid-liquid phase separations can occur in atmospheric particles (6, 10, 13-16). Atmospheric particles, however, are far more complex than the simple proxies used in these studies. A few studies have inferred that liquidliquid phase separation occurs in particles containing ammonium sulfate and secondary organic material (SOM) generated from the dark ozonolysis of α-pinene (17-20). In the present...

Hydrolysis of Organonitrate Functional Groups in Aerosol Particles

“…This phase separation has been observed for similar laboratory SOA samples and for ambient samples (You et al 2012). The separation of the organic and aqueous phase would confine the interaction of water and ON molecules to the interface of the two phases, significantly lowering the rate of hydrolysis reactions.…”

Modeling kinetic partitioning of secondary organic aerosol and size distribution dynamics: representing effects of volatility, phase state, and particle-phase reaction

“…While the focus of the present work is on kinetic partitioning of organic gases to particulate organic phase, the new framework can be readily adapted to kinetically partition water soluble organic gases into the particulate aqueous phase if that is the only liquid phase in the particle. However, additional research is needed to extend the present framework to mixed inorganic-organic particles that experience liquidliquid phase separation (i.e., coexistence of separate aqueous and organic phases; You et al, 2012). Table 1.…”

“…However, additional work is needed to extend the present framework to mixed inorganic-organic particles in which water and organics may form separate liquid phases (You et al, 2012).…”

Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosol

“…Organic aerosol (OA) often dominates over sulfate (Zhang et al, 2007), in particular in the Southeast in summer where there is a large OA source from biogenic hydrocarbons Marais et al, 2016). Mixing of organic and sulfate aerosol may slow down mass transfer due to phase separation, in which the organic aerosol fraction coats the predominantly aqueous inorganic core, as has been observed in many laboratory studies of organic ammonium sulfate particles (Anttila et al, 2007;Ciobanu et al, 2009;Bertram et al, 2011;Koop et al, 2011;You et al, 2013) as well as in the field in the Southeast (You et al, 2012). …”