Determining aerosol temperatures by oxygen K-edge X-ray thermometry.

Determining the temperature of rapidly evaporating aerosol particles remains experimentally challenging because conventional thermometry cannot readily probe isolated droplets undergoing nonequilibrium cooling. In this work, we combine near edge X-ray absorption fine structure (NEXAFS) with simulations to assess the structure and temperature of glycerol aerosols. By connecting the nuclear and electronic structure information from simulation with the observed NEXAFS, we find that quantum nuclear effects are crucial to the structure of glycerol aerosols and that the NEXAFS spectrum broadens with decreasing temperature due to a phenomenon that we designate as a purple shift - a combined red- and blue-shift in the NEXAFS excitation energies of different oxygen centers on individual glycerol molecules that grows with H-bond strength. We find signatures of enhanced H-bonding structure in the glycerol aerosol NEXAFS that is ascribable to glycerol at roughly 175 K, which is 15 K below the glass transition temperature. These results establish X-ray thermometry as a direct experimental probe of the thermal and structural evolution of rapidly cooling aerosols.