NASA Langley Research Center (LaRC) recently developed an airborne High Spectral Resolution Lidar (HSRL) to measure aerosol and cloud distributions and optical properties. The HSRL technique takes advantage of the spectral distribution of the lidar return signal to discriminate aerosol and molecular signals and thereby measure aerosol extinction and backscatter independently. The LaRC airborne HSRL measures aerosol backscatter and depolarization at 532 and 1064 nm and aerosol extinction at 532 nm. The HSRL had a very successful first deployment during the Megacity Aerosol Experiment in Mexico City (MAX-MEX)/Intercontinental Chemical Transport Experiment-B (INTEX B) joint experiment conducted in March 2006. The major objective of the MILAGRO mission was to investigate the evolution and transport of pollution from a tropical megacity – in this case Mexico City. During this mission, the HSRL collected approximately 50 hours of data during 14 science flights on the NASA King Air B200. Initial comparisons showed excellent agreement between the HSRL aerosol extinction profiles and the corresponding profiles derived from coincident airborne Sun photometer measurements of aerosol optical thickness and in situ measurements of aerosol scattering and absorption. During flights over Mexico City, HSRL measurements reveal significant variations in aerosol loading and optical properties. The HSRL measurements of aerosol “color” ratio, extinction/backscatter ratio (“lidar ratio”), and depolarization are used to infer variations in aerosol type over Mexico City. On one flight, aerosol optical thickness that was derived from the HSRL aerosol extinction profile, increased by nearly an order of magnitude over a distance of about 10 km. This increase was also well correlated with corresponding changes aerosol properties and trace gas amounts measured simultaneously by in situ instruments flying on another aircraft. HSRL measurements of aerosol backscatter color ratio, lidar ratio, and depolarization indicate that dust and pollution were dominant aerosol types over the city. The HSRL aerosol measurements are also used to help evaluate coincident aerosol measurements from both MODIS and MISR. The HSRL is also scheduled to participate in the Texas Air Quality Study / Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS/GoMACCS) field mission to occur over and around Houston during August-September 2006. This intensive field study will focus on providing a better understanding of the sources and atmospheric processes responsible for the formation and distribution of ozone and aerosols in the atmosphere and the influence that these species have on the radiative forcing of climate regionally and globally as well as their impact on human health and regional haze. HSRL measurements from both the MILGARO and TexAQS/GoMACCS field experiments will be discussed in this presentation.