Combining remote sensing and in situ aerosol measurements for the determination of aerosol optical properties and radiative effects

The largest uncertainty in the estimates of the effects of atmospheric aerosols on climate stems from the uncertainties in the determination of their microphysical properties, including the aerosol complex index of refraction which in turn determines the optical properties of the aerosols.;In this thesis, methodologies to estimate the aerosol complex index of refraction from a combination of aerosol in situ size distribution and remote sensing measurements during NASA's Pacific Exploratory Mission West-B (PEM West-B) and the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) are developed. In particular, the remote sensing of aerosols with airborne lidar is utilized to derive vertical profiles of aerosol backscatter.;For the PEM West-B data analysis, a modified Klett inversion algorithm was adopted to utilize the aerosol in situ size distribution data to provide the height dependent lidar ratio and the aerosol backscatter at the aircraft altitude. In all three PEM West-B cases studied, the aerosol measurements could be explained using a two-layer aerosol model with distinct aerosol refractive indices as indicated by a best-fit backscatter refractive index estimation method. The real parts of the aerosol refractive indices retrieved are in between 1.42 and 1.60, while the imaginary part ranges from 10--6 to 0.163.;For the TARFOX data analysis, the incorporation of aerosol optical depth measurements obtained using an airborne sunphotometer system yields an additional constraint on the estimate of the complex aerosol index of refraction. The aerosol refractive indices thus retrieved are generally smaller than the values estimated for the PEM study, with values ranging from 1.33 to 1.45 for the real part and 0.001 to 0.008 for the imaginary part, respectively.;The methodology devised in this study provides, for the first time, a complete set of vertically resolved aerosol size distribution and refractive index data, yielding the vertical distribution of aerosol optical properties required for the determination of aerosol induced radiative flux changes. Calculations with the 18 channel broadband Fu-Liou radiative transfer model indicate instantaneous shortwave aerosol radiative forcings at the top of the atmosphere between --0.5 Wm--2 and +4.5 Wm--2 for the PEM case studies and values of the order of --36 Wm--2 for the TARFOX case studies, respectively.;The vertical structure of the aerosol induced flux changes is important to climate studies, since it affects local heating rates and thereby convective processes, the formation and lifetime of clouds, and the distribution of chemical constituents in the troposphere. To the author's knowledge, the calculations carried out in this work represent the first observationally-based estimate of the vertical structure of the aerosol radiative forcing. Therefore, these results are quite significant for the design of future field campaigns aimed at the determination of aerosol climatic effects.