Do atmospheric aerosols form glasses?

[摘要] A new process is presented by which water soluble organics might influence ice nucleation, icegrowth, chemical reactions and water uptake of aerosols in the upper troposphere: the formation ofglassy aerosol particles. Glasses are disordered amorphous (non-crystalline) solids that form whena liquid is cooled without crystallization until the viscosity increases exponentially andmolecular diffusion practically ceases. The glass transition temperatures, T_g,homogeneous ice nucleation temperatures, T_hom, and ice melting temperatures,T_m, of various aqueous inorganic, organic and multi-component solutions are investigatedwith a differential scanning calorimeter. The investigated solutes are: various polyols, glucose,raffinose, levoglucosan, an aromatic compound, sulfuric acid, ammonium bisulfate and mixtures ofdicarboxylic acids (M5), of dicarboxylic acids and ammonium sulfate (M5AS), of two polyols, ofglucose and ammonium nitrate, and of raffinose and M5AS. The results indicate that aqueoussolutions of the investigated inorganic solutes show T_g values that are too low to be ofatmospheric importance. In contrast, aqueous organic and multi-component solutions readily formglasses at low but atmospherically relevant temperatures (≤230 K). To apply thelaboratory data to the atmospheric situation, the measured phase transition temperatures weretransformed from a concentration to a water activity scale by extrapolating water activitiesdetermined between 252 K and 313 K to lower temperatures. The obtained state diagramsreveal that the higher the molar mass of the aqueous organic or multi-component solutes, the higherT_g of their respective solutions at a given water activity. To a lesser extent,T_g also depends on the hydrophilicity of the organic solutes. Therefore, aerosolparticles containing larger (≳150 g mol⁻¹) and more hydrophobic organicmolecules are more likely to form glasses at intermediate to high relative humidities in the uppertroposphere. Our results suggest that the water uptake of aerosols, heterogeneous chemicalreactions in aerosol particles, as well as ice nucleation and ice crystal growth can besignificantly impeded or even completely inhibited in organic-enriched aerosols at uppertropospheric temperatures with implications for cirrus cloud formation and upper troposphericrelative humidity.

[学科分类] 大气科学