Glass Transition Temperatures of Individual Submicrometer Atmospheric Particles: Direct Measurement via Heated Atomic Force Microscopy Probe.

The phase (solid, semisolid, or liquid) of atmospheric aerosols is central to their ability to take up water or undergo heterogeneous reactions. In recent years, the unexpected prevalence of viscous organic particles has been shown through field measurements and global atmospheric modeling. The aerosol phase has been predicted using glass transition temperatures ( T g ), which were estimated based on molecular weight, oxygen:carbon ratio, and chemical formulae of organic species present in atmospheric particles via studies of bulk materials. However, at the most important sizes for cloud nucleation (∼50-500 nm), particles are complex mixtures of numerous organic species, inorganic salts, and water with substantial particle-to-particle variability. To date, direct measurements of T g have not been feasible for individual atmospheric particles. Herein, nanothermal analysis (NanoTA), which uses a resistively heated atomic force microscopy (AFM) probe, is combined with AFM photothermal infrared (AFM-PTIR) spectroscopy to determine the T g and composition of individual particles down to 76 nm in diameter at ambient temperature and pressure. Laboratory-generated proxies for organic aerosol (sucrose, ouabain, raffinose, and maltoheptaose) had similar T g values to bulk T g values measured with differential scanning calorimetry (DSC) and the T g predictions used in atmospheric models. Laboratory-generated phase-separated particles and ambient particles were analyzed with NanoTA + AFM-PTIR showing intraparticle variation in composition and T g . These results demonstrate the potential for NanoTA + AFM-PTIR to increase our understanding of viscosity within submicrometer atmospheric particles with complex phases, morphologies, and compositions, which will enable improved modeling of aerosol impacts on clouds and climate.