PhD Defense: Reed Espinosa
TITLE: Comprehensive airborne in situ characterization of atmospheric aerosols: from angular light scattering to particle microphysics
ABSTRACT: A comprehensive understanding of atmospheric aerosols is necessary both to understand Earth's climate as well as produce skillful air quality forecasts. In order to advance our understanding of aerosols, the Laboratory for Aerosols, Clouds and Optics (LACO) has recently developed the Imaging polar Nephelometer (I-Neph) instrument concept for the in situ measurement of optical scattering properties. Imaging Nephelometers provide measurements of absolute phase function and polarized phase function over a wide angular range, typically 3 to 177 degrees, with an angular resolution smaller than one degree. The first of these instruments, the Polarized Imaging Nephelometer (PI-Neph), has taken part in five airborne field experiments and is the only modern aerosol polar nephelometer to have flown aboard an aircraft. A method for the retrieval of aerosol optical and microphysical properties from I-Neph measurements is presented and the results are compared with existing measurement techniques. A synopsis is then presented of aerosol scattering measurements made by the PI-Neph during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEA4CRS) and the Deep Convection Clouds and Chemistry (DC3) field campaigns. To better summarize these extensive datasets a novel aerosol classification scheme is developed to identify the different aerosol types measured during the deployments. This scheme makes use of ancillary data that includes gas tracers, chemical composition, aerodynamic particle size and geographic location, all independent of PI-Neph measurements. Principal component analysis (PCA) is then used to reduce the dimensionality of the multi-angle PI-Neph scattering data and the results are examined as a function aerosol type. Strong clustering is observed in the PCA score space, corresponding to the ancillary classification results, suggesting a robust link between the angular scattering measurements and the aerosol type or composition. Retrievals of the DC3 scattering data suggest the presence of a significant amount of mineral dust aerosol in the inflow of storms sampled during this campaign, with the quantity of dust depending strongly on the geographic location. The retrieved size distributions of all fine mode dominated aerosols were found to be remarkably similar among a range of aerosol types, including biomass burning, biogenic emissions as well as urban and industrial emissions. There were however consistent differences between the angular scattering patterns of biomass burning samples and the other fine mode cases, especially in the polarization signature of the scattered light. The GRASP retrieval attributed these differences almost entirely to a higher real refractive index in the case of the biomass burning samples. The work concludes with recommendations for improvements to the I-Neph design and data processing algorithm as well as suggestions for future analysis efforts involving I-Neph data sets.