Retrieving the vertical structure of the effective aerosol complex index of refraction from a combination of aerosol in situ and remote sensing measurements during TARFOX

The largest uncertainty in estimates of the effects of atmospheric aerosols on climate stems from uncertainties in the determination of their microphy sical properties, including the aerosol complex index of refraction, which in turn determines their optical properties. A novel technique is used to e stimate the aerosol complex index of refraction in distinct vertical layers from a combination of aerosol in situ size distribution and remote sensing measurements during the Tropospheric Aerosol Radiative Forcing Observation al Experiment (TARFOX). In particular, aerosol backscatter measurements usi ng the NASA Langley LASE (Lidar Atmospheric Sensing Experiment) instrument and in situ aerosol size distribution data are utilized to derive vertical profiles of the "effective" aerosol complex index of refraction at 815 nm ( i.e., the refractive index that would provide the same backscatter signal i n a forward calculation on the basis of the measured in situ particle size distributions for homogeneous, spherical aerosols). A sensitivity study sho ws that this method yields small errors in the retrieved aerosol refractive indices, provided the errors in the lidar-derived aerosol backscatter are less than 30% and random in nature. Absolute errors in the estimated aeroso l refractive indices are generally less than 0.04 for the real part and can be as much as 0.042 for the imaginary part in the case of a 30% error in t he lidar-derived aerosol backscatter. The measurements of aerosol optical d epth from the NASA Ames Airborne Tracking Sunphotometer (AATS-6) are succes sfully incorporated into the new technique and help constrain the retrieved aerosol refractive indices. An application of the technique to two TARFOX case studies yields the occurrence of vertical layers of distinct aerosol r efractive indices. Values of the estimated complex aerosol refractive index range from 1.33 to 1.45 for the real part and 0.001 to 0.008 for the imagi nary part. The methodology devised in this study provides, for the first ti me, a complete set of vertically resolved aerosol size distribution and ref ractive index data, yielding the vertical distribution of aerosol optical p roperties required for the determination of aerosol-induced radiative flux changes.