He formerly led the Langley Aerosol Research Group, which conducts research to characterize the sources and fundamental properties of atmospheric aerosols; investigate links between aerosols and cloud microphysical properties; provides validation data for remote sensors and transport/transformation models; and contributes to assessments of human impacts upon the environment (i.e., aviation impacts on local air quality and climate). He has flown instruments aboard nine different aircraft including the NASA DC-8 (31 field missions) and P-3B (6) and has participated in over 40, NASA-sponsored airborne field experiments. He has also been very active in supporting NASA ARMD initiatives to determine the impacts of aviation on the atmosphere. This work includes being project/platform scientist on studies to characterize aircraft emissions during flight (SNIF, SUCCESS, ACCESS-1 and -2, ND-MAX) and ground-based operations (EXCAVATE, APEX-3, PW308, AAFEX-I and II, ECLIF-1); several of these studies have specifically investigated the effects of alternative fuels on engine emission performance.