Do atmospheric aerosols form glasses?

Abstract

Abstract. A new process is presented by which water-soluble organics might influence ice nucleation, ice growth, chemical reactions and water uptake of aerosols in the upper troposphere: the formation of glassy aerosol particles. Glasses are disordered amorphous (non-crystalline) solids that form when a liquid is cooled without crystallization until the viscosity increases exponentially and molecular diffusion practically ceases. The glass transition temperatures, Tg, homogeneous ice nucleation temperatures, Thom, and ice melting temperatures, Tm, of various aqueous inorganic, organic and multi-component solutions are investigated with a differential scanning calorimeter. The investigated solutes are: various polyols, glucose, raffinose, levoglucosan, an aromatic compound, sulfuric acid, ammonium bisulphate and mixtures of dicarboxylic acids (M5), of dicarboxylic acids and ammonium sulphate (M5AS), of two polyols, of glucose and ammonium nitrate, and of raffinose and M5AS. The results indicate that aqueous solutions of the investigated inorganic solutes show Tg-values that are too low to be of atmospheric importance. In contrast, aqueous organic and multi-component solutions readily form glasses at low but atmospherically relevant temperatures (≤230 K). To apply the laboratory data to the atmospheric situation, the measured phase transition temperatures were transformed from a concentration to a water activity scale by extrapolating water activities determined between 252 K and 313 K to lower temperatures. The obtained state diagrams reveal that the higher the molar mass of the aqueous organic or multi-component solutes, the higher Tg of their respective solutions at a given water activity. To a lesser extent, Tg also depends on the hydrophilicity of the organic solutes. Therefore, aerosol particles containing larger and more hydrophobic organic molecules (≳150 g mol-1) are more likely to form glasses at intermediate to high relative humidities in the upper troposphere. Our results suggest that the water uptake of aerosols, heterogeneous chemical reactions in aerosol particles, as well as ice nucleation and ice crystal growth can be significantly impeded or even completely inhibited in organic-enriched aerosols at upper tropospheric temperatures with implications for cirrus cloud formation and upper tropospheric relative humidity.

Multiphase OH oxidation kinetics of organic aerosol: The role of particle phase state and relative humidity

“…One possible mechanism for the enhancement in γ with increasing RH is that the particle's liquid water content controls particle phase state and the rate of OH loss. Previous studies indicate that LEV constitutes a highly viscous (glassy) solid or semisolid amorphous phase state under dry conditions [ Zobrist et al , ; Mikhailov et al , ]. However, LEV aerosol particles undergo a humidity‐induced phase transformation to less viscous, i.e., semisolid or liquid‐like, particles at RH = 30–60% [ Mikhailov et al , ].…”

“…This suggests that OH can react not only on the surface of amorphous LEV particles but also within the particle bulk at higher RH. LEV represents just one of many types of amorphous organic material, which also include carbohydrates, proteins, carboxylic acids, SOA, and humic‐like substances [ Shiraiwa et al , ; Mikhailov et al , ; Zobrist et al , ; Murray , ; Wang et al , ]. Thus, consideration of OH reactivity with other amorphous organic substances will further improve our understanding of its influence on chemical aging and thus atmospheric chemistry and climate.…”

“…In general, RH is higher in the lower section of the atmospheric column near the surface; thus, it is expected that OH oxidation of amorphous organic aerosol is enhanced near the ground. At higher altitudes, e.g., above the boundary layer, where RH is relatively low and ambient T can be near the glass transition point of amorphous organic aerosol [ Zobrist et al , ; Koop et al , ; Wang et al , ], particle oxidation is expected to be less efficient as a result of reduced translational motions of reactant species yielding longer particle lifetimes. OH uptake by aqueous LEV solutions is likely representative of cloud‐processing conditions when LEV particles activate and become highly diluted water droplets.…”

Cirrus cloud formation and ice supersaturated regions in a global climate model

“…These large ice crystals are suggested to have grown on particles that acted as ice nuclei at low supersaturations with respect to ice (Jensen et al 2008). Very recently, Zobrist et al (2008) based on laboratory experiments showed that mixed organic/inorganic aerosols may form glasses at temperatures prevalent in the upper troposphere, and that this may greatly reduce their uptake of water and hence their homogeneous ice formation ability.…”

A rapid scan vacuum FTIR method for determining diffusion coefficients in viscous and glassy aerosol particles

“…20,21 The organic-rich phase of atmospheric aerosols can be classified as a liquid ( < 10 2 Pa·s; where is the dynamic viscosity), a semi-solid (10 2 Pa·s < <10 12 Pa·s) or a solid ( > 10 12 Pa·s), depending on ambient conditions and chemical composition. [22][23][24][25] Indeed, Virtanen et al showed that secondary organic aerosol (SOA) could exist in a glassy phase, while other studies suggest that aerosol particles could be characterised as gels or rubbers. [26][27][28][29] Based on the relationship between viscosity and diffusion constant suggested by the Stokes-Einstein equation, it might be expected that diffusion constants could be characterised by a similarly wide range, depending on the viscosity and phase of the aerosol.…”