Combined retrievals of boreal forest fire aerosol properties with a polarimeter and lidar

Absorbing aerosols play an important, but uncer- tain, role in the global climate. Much of this uncertainty is due to a lack of adequate aerosol measurements. While great strides have been made in observational capability in the pre- vious years and decades, it has become increasingly apparent that this development must continue. Scanning polarimeters have been designed to help resolve this issue by making accu- rate, multi-spectral, multi-angle polarized observations. This work involves the use of the Research Scanning Polarimeter (RSP). The RSP was designed as the airborne prototype for the Aerosol Polarimetery Sensor (APS), which was due to be launched as part of the (ultimately failed) NASA Glory mission. Field observations with the RSP, however, have es- tablished that simultaneous retrievals of aerosol absorption and vertical distribution over bright land surfaces are quite uncertain. We test a merger of RSP and High Spectral Res- olution Lidar (HSRL) data with observations of boreal for- est fire smoke, collected during the Arctic Research of the Composition of the Troposphere from Aircraft and Satel- lites (ARCTAS). During ARCTAS, the RSP and HSRL in- struments were mounted on the same aircraft, and valida- tion data were provided by instruments on an aircraft fly- ing a coordinated flight pattern. We found that the lidar data did indeed improve aerosol retrievals using an optimal es- timation method, although not primarily because of the con- traints imposed on the aerosol vertical distribution. The more useful piece of information from the HSRL was the total column aerosol optical depth, which was used to select the initial value (optimization starting point) of the aerosol num- ber concentration. When ground based sun photometer net- work climatologies of number concentration were used as an initial value, we found that roughly half of the retrievals had unrealistic sizes and imaginary indices, even though the re- trieved spectral optical depths agreed within uncertainties to independent observations. The convergence to an unrealis- tic local minimum by the optimal estimator is related to the relatively low sensitivity to particles smaller than 0.1 (µm) at large optical thicknesses. Thus, optimization algorithms used for operational aerosol retrievals of the fine mode size distribution, when the total optical depth is large, will require initial values generated from table look-ups that exclude un- realistic size/complex index mixtures. External constraints from lidar on initial values used in the optimal estimation methods will also be valuable in reducing the likelihood of obtaining spurious retrievals.