Global and Regional Aerosol Mapping: Analysis of Aerosol Air Mass Types

Global Aerosol Optical Depth (AOD) products have received vastly more attention from satellite instrument teams than aerosol type and aerosol microphysical property data products, despite their importance for many climate and air-quality related applications.  This is largely because aerosol properties are much more difficult to retrieve from remote sensing than AOD.  For example, single-view instrument data contain little or no information about aerosol type under most circumstances.  Even multi-angle instruments, such as the NASA Earth Observing System’s Multi-angle Imaging SpectroRadiometer (MISR), contain only qualitative constraints on aerosol size, shape, and single-scattering albedo (SSA).  The quality of aerosol type retrievals is much more dependent on observing conditions than for AOD, and, as we found in the past few years, they require significantly greater absolute and especially channel-to-channel relative calibration accuracy. Further, except for spectral AOD from surface-based sun photometers such as AERONET, the primary source of particle property “ground truth” is from occasional field campaign golden days.  Surface-based sky scan results are remote-sensing retrievals, entailing many more assumptions than the direct-sun AOD measurements.

Recent work with MISR in this area is supported by the NASA Atmospheric Composition Modeling and Analysis Program (ACMAP). A detailed analysis of the MISR Standard aerosol-type product showed both the strengths and limitations of the retrieved aerosol type information, including the degree to which each of particle sphericity, size, and absorption are constrained under a range of observing conditions [Kahn and Gaitley, JGR 2015]. Using the MISR Research Aerosol Retrieval algorithm, we explored the possibilities for improving upon the standard product over ocean with better surface modeling, some adjustments to the radiance selection, enhanced cloud screening, adaptive solution-acceptance criteria, and an improved climatology of aerosol types and mixtures in the algorithm [Limbacher and Kahn, 2014].  This work also exposed the need for significant radiometric calibration refinement for aerosol type retrieval, and the desirability of further refinement of the ocean-surface representation.  So we had to focus on calibration before pursuing further applications with the data, and in addition, incorporated an advanced ocean surface model that includes a self-consistent, Chlorophyll-a concentration retrieval [Limbacher and Kahn, 2015; 2016].  In this presentation, we will summarize our recent work under the ACMAP program, including the status of our improved aerosol type retrieval algorithm, and progress on the application to aerosol spatial variability.

References

 Kahn, R. A., and B. J. Gaitley, 2015. An analysis of global aerosol type as retrieved by MISR. J. Geophys. Res. Atmos. 120, doi:10.1002/2015JD023322.

Limbacher, J.A., and R.A. Kahn, 2014.  MISR Research-Aerosol-Algorithm: Refinements For Dark Water Retrievals.  Atm. Meas. Tech. 7, 1-19, doi:10.5194/amt-7-1-2014.

Limbacher, J.A., and R.A. Kahn, 2015. MISR Empirical Stray Light Corrections in High-Contrast Scenes.  Atmos. Meas. Tech. 8, doi: 10.5194/amt-8-1-2015.

Limbacher, J.A., and R.A. Kahn, 2016.  MISR Empirical Calibration Corrections, with Application to Combined Aerosol and Ocean Surface Chlorophyll-a Retrievals. Atmos. Meas. Tech., in preparation.