Investigation the effects of realistic dust-pollution mixed scenarios on the degree of linear polarization

Previous work has demonstrated that mineral dust will cool or warm the atmosphere depending on its microphysical properties (refractive indices index and size distributions) and its vertical location and layering with clouds and other tropospheric aerosols. We address the role of the multi-layered aerosol vertical distribution, the mixing state of dust and pollution (external vs. internal mixing), dust particle shapes, and the regional specifics of aerosol composition on the Degree of Linear Polarization (DOLP) through integrated analyses of NASA satellite aerosol products, and ground-based airborne lidar and sunphotometer observations. We will use several case studies from the MILAGRO (Megacity Initiative: Local and Global Research Observations) field campaign, which took place during March 2006 in Mexico City. During the campaign, biomass-burning particles were present in the atmosphere together with transported dust and local pollution. We have identified several days during MILAGRO when transported dust was mixed with Mexico City pollution aerosol, and when collocated measurements were available from the High Spectral Resolution Lidar (HSRL) flying on the NASA King Air aircraft, and MISR on the Terra satellite. Aerosol vertical profiles and properties were determined by the HSRL and the surface properties and aerosol loadings/spatial distribution were constrained with MISR satellite observations. We will show results of modeling studies that investigate the effects of realistic dust-pollution mixed scenarios (constrained by satellite and ground-based observations) on the degree of linear polarization at visible-NIR wavelengths. We will address aerosol effects upon the Stokes parameters (I, Q, and U) and Degree of Linear Polarization (DOLP) by employing a Successive Orders of Scattering (SOS) forward radiative transfer model. The SOS approach is highly flexible and allows for multiple atmospheric aerosol layers. Aerosol mixing treatments are currently done off-line. Each layer in the model atmosphere is calculated considering aerosol mixing state and molecular components. The optical properties used to describe the model atmosphere are scattering coefficient, extinction coefficient, and the fully polarized scattering matrix for each aerosol component. Optical properties are currently pre-calculated for non-spherical dust models at visible-NIR wavelengths with DDA and T-matrix techniques. Optical properties of pollution and smoke components are calculated through a choice of Mie theory depending upon the mixing state option and aerosol size distribution reported by field observations. Molecular scattering is handled explicitly within the SOS code. We will assess the information content of polarimetric observations for the realistic dust-pollution mixed scenarious by modeling the DOLP of dust-pollution mixtures for HSRL measured aerosol vertical profiles. The assessment is directly applicable to evaluation of polarimetric remote sensing capability to quantify microphysical and optical properties of dust and dust-pollution mixtures. The focus will be on determining the extent to which polarimetric data can aid in improving predictions of aerosol radiative forcing.