Atmospheric aerosol particles formed from volatile organic compounds most likely exist as glassy, amorphous solids, rather than as liquid droplets, according to an international group of researchers led by Annele Virtanen of Tampere University of Technology, in Finland, and Thomas Koop of Bielefeld University, in Germany (Nature 2010, 467, 824). Aerosols play a role in Earth’s climate by scattering radiation and acting as cloud-condensation nuclei. Determining their physical state is important for modeling how they incorporate water and other molecules from the air, as well as how quickly or slowly chemicals in the particles might react. Working with a plant-growth chamber and in a boreal forest in Finland, the researchers looked at whether particles stuck to, or bounced off of, plates in an electrical low-pressure impactor. They compared the results with those of known liquid, crystalline, and glassy particles. That analysis, combined with electron microscopy studies, indicates that the particles are most likely in a solid, glassy state, rather than in a liquid state. The researchers suggest that particles may grow through adsorption rather than absorption into an organic liquid and that reactions within particles may be limited by diffusion.