His principal interest is in the oxidation chemistry of Earth's atmosphere — specifically the oxidation of organic compounds and the associate radical processes in the atmosphere. Two closely connected areas are ozonolysis chemistry and the chemistry controlling organic-aerosol levels and properties in the atmosphere. Aerosols — fine particulate matter, or PM — are of interest for two major reasons: particles play a central role in climate, and they kill people. The leading uncertainty on the forcing side of climate science is the degree to which cloud properties have changed between 1850 and now due to changes in the number concentrations of fine, water-soluble particles that act as cloud-condensation nuclei (CCN). Also, approximately 50,000 people die prematurely each year in the U.S. alone from inhalation of elevated levels of fine PM. More than half of the fine PM mass is composed of a very complex mixture of highly oxidized organic compounds. They are water soluble and have unknown health effects but appear to correlate positively with observed health endpoints.