Seminar

Abstract

The Earth Observing System (EOS) Era is drawing to a close, having produced almost 25 years of global aerosol data from instruments such as the Multi-angle Imaging SpectroRadiometer (MISR) and the MODerate resolution Imaging Spectroradiometer (MODIS). As MISR represented an entirely new instrument concept when it began taking data in late February 2000, for much of this time, work by the instrument team focused on establishing the radiometric and geometric calibration, developing and subsequently refining the Level 2 aerosol retrieval algorithms, and validating the Level 2 aerosol products. Although we were always learning something about Earth from MISR beginning with the first images, for about 15 years after launch we were learning more about MISR than about Earth. That balance has subsequently reversed. We now have the tools, the experience, and the understanding to extract the scientific value of the MISR and associated aerosol data records in service of learning about Earth – the evolution of wildfire smoke, desert dust, and volcano plumes, air quality and particulate pollution mapping, globally. We have demonstrated all these capabilities in published studies, and validated with ground-based-network observations and in situ data from field campaigns wherever possible.

But beyond offering MISR and MODIS data products, our efforts can contribute to advancing climate analysis and prediction, a primary goal for fielding these instruments in the first place. But doing so will require greater synthesis of satellite and suborbital measurements with models. Each of these elements has a role to play in addressing societally important questions, from constraining aerosol forcing in climate models to issuing smoke advisories for communities downwind of major wildfires. We have demonstrated the satellite capabilities to constrain the 3-D distribution of aerosol amount, and to generate qualitative aerosol-type maps, frequently, over large areas. But associating aerosol type with detailed particle properties needed for many applications requires in situ measurements. Such measurements provide the most direct way to quantify particle single-scattering albedo and size distribution, offer by far the best constraints on particle hygroscopicity and CCN size spectra, and produce the only available constraints on mass extinction efficiency and actual particle composition. Models are needed to target aircraft measurements and to identify likely sources and aging histories of suborbital samples and satellite aerosol-type maps. They are also required to fill gaps in available measurements, to calculate aerosol forcing and its impacts, and to make climate and air quality predictions. But without adequate observational constraints, even the model uncertainties are typically not well characterized. As such, each of these three elements has unique and essential contributions to make to the other two: a three-way street. We have over the years attempted to effect some of the required connections between measurements and models. So, this presentation will review the aerosol-related capabilities we have demonstrated with MISR and MODIS, will discuss how the results fit into the larger context of constraining aerosol-climate forcing, and conclude by pointing to next steps required to meet the promise of the three-way street.

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Ralph Kahn received his PhD in applied physics from Harvard University in 1980. He joined the Laboratory for Atmospheric & Space Physics (LASP) at the University of Colorado in January 2024, after 38 years as a research scientist and senior research scientist at The Jet Propulsion Laboratory and NASA Goddard. Kahn specializes in the remote sensing of airborne particles. He is Aerosol Scientist for the NASA Earth Observing System's Multi-angle Imaging SpectroRadiometer (MISR) instrument, using MISR's unique observations, combined with other data and numerical models, to study wildfire smoke, desert dust, volcano and air pollution particles and their impacts on climate and air quality. Kahn has lectured on Climate Change and atmospheric physics at Caltech, UCLA, and other venues, was Adjunct Professor at the University of Maryland (2016-2023), and is editor and founder of PUMAS, the on-line journal of science and math examples for pre-college education. Kahn has authored over 250 publications, chaired COSPAR Commission A through mid-2024, and is co-chair of AeroSat, the international association of the satellite aerosol remote-sensing community. He has received the NASA Exceptional Service and Outstanding Leadership Medals, NASA's Nordberg Award for Earth Science, the AGU's Kaufman award, the Simpson award from the AMS, and is Fellow of the American Geophysical Union.