Atmospheric science. Getting to the critical nucleus of aerosol formation.

A better understanding of how aerosols form in the atmosphere could greatly improve climate models. A tmospheric aerosols—microscopic particles suspended in Earth's atmo-sphere—are a major environmental problem. They degrade visibility, negatively affect human health, and directly and indirectly influence climate by absorbing and refl ecting solar radiation and modifying cloud formation. Researchers do not, however , fully understand at the molecular level how aerosols form, creating one of the largest sources of uncertainty in atmospheric models and climate predictions (1). Recent fi nd-ings suggest a path to a better understanding of aerosol formation (2– 4). Aerosols can be directly emitted into the atmosphere—for example by plants, combustion , or sea spray—or form through a chemical process known as nucleation, in which gaseous molecules bond. Nucleation produces a large fraction of atmospheric aerosols, and investigators have frequently observed nucleation in various environments, including urban, forested, and marine areas (5). New particle formation is commonly considered to be a two-step process: First, nucleation forms a " critical nucleus, " which then grows to a detectable size (6). Classical nucleation theory reveals that when the critical nucleus forms, the free energy of the nucleating system reaches a maximum— " the nucleation barrier " —beyond which aerosol growth becomes spontaneous. The rate at which nucleation occurs is related to the chemical makeup of the critical nucleus and the gaseous concentrations of the nucleating species (7). That rate is an important variable in simulations of aerosol formation in atmospheric models (8). Previous studies, however, have not been able to directly measure nucleation rates whose by-product after reaction is water. The products of the catalytic reaction described by Uyanik et al. create the framework for a number of natural products with varied biological effects. The benzofuran products can form the starting point for the synthesis of more complex pharmaceutical candidates. For example, tremetone has both antifungal and insecticidal properties and is derived from a plant extract. A similar plant isolate, rotenone, is a potent anti-leukemic drug candidate as well (14). In 2005, the Merck Company reported the synthesis of a drug candidate with this same backbone that modulates the levels of serum triglyc-erides and high-density lipoprotein in the blood (15). A synthesis of this target with this new method could be accomplished in fewer steps than the 2005 method, produce less waste, and reduce cost. The future of hypervalent iodine is likely to be as varied …