Ralph Kahn NASA Goddard Space Flight Center Winner of the 2019 Nordberg Award

 
"Smoke Gets in Your Optics - The Airborne Particles We See from Space"
  

Except in the context of major pollution events, the impact of airborne particles on planetary habitability was largely unappreciated until the dawn of the satellite age. Space-based imagery starting in the late 1960s provided evidence that aerosols can be transported thousands of kilometers, frequently in sufficient amounts to affect global climate as well as regional air quality, with the potential to fertilize remote ocean and land areas. For three decades, satellite remote-sensing first tracked bright aerosol plumes, and subsequently, derived qualitative column-amount, but only over dark ocean surfaces. Advances in imager spectral and spatial resolution, radiometric calibration, as well as innovative instrument design ideas, suggested that global aerosol loading estimates could be placed on more solid, quantitative footing, and that a great deal of additional information about airborne particle microphysical properties could be retrieved from space. Both NASA and the European Space Agency developed instruments to realize the potential. This is where our story really begins...

On 24 February 2000 the MODerate resolution Imaging Spectroradiometer (MODIS) and the Multi-angle Imaging SpectroRadiometer (MISR), both aboard the NASA Earth Observing System's Terra satellite, each acquired their first images of Earth from space, marking the beginning of the EOS era. Both instruments continue to acquire data nominally after more than 22 years in orbit. MODIS, a broad-swath, single-angle imager with 36 spectral channels, represented the next generation of multi-spectral instruments, embodying all the refinements in resolution and calibration developed over the previous two decades. The MISR design was entirely new - nine separate cameras pointing toward Earth at different angles, each acquiring data in four spectral bands. The cameras are configured so that, over a period of seven minutes, as Terra carries the instrument roughly from pole to pole, MISR sweeps out 36 strips of near-coincident imagery. However, unlike MODIS, which samples the entire Earth surface once every day or two, MISR's narrower swath samples the whole planet about once per week.

It required almost two decades to gain sufficient understanding of the MISR data, and based upon that understanding, to develop the analysis tools, needed to squeeze the observations for their information content about airborne particles. Since MISR first light, we have been learning about Earth, but only recently we are learning more about Earth than about MISR. From MISR hyper-stereo observations we derive geometrically the heights and associated motion vectors for wildfire, volcano, and dust plumes, wherever contrast features are visible in the multi-angle views. From reflectance measured at slant paths through the atmosphere varying systematically up to a factor three for the steepest-viewing MISR cameras, aerosol amount can be retrieved with substantial confidence, even over relatively bright land surfaces. Taken together, the multi-angle, multi-spectral data also contain qualitative information about particle size, shape, and light-absorption properties, provided the column-amount is sufficiently high.

With many contributions from students, post-docs, and colleagues, we are gleaning from the 22-year MISR data record lessons about how wildfire smoke particles evolve downwind, what volcanos, even in remote areas, emit over time, and the way desert dust is mobilized and transported across oceans. Most importantly, these results can be used to constrain and to refine climate and air quality models, improving forecasting and supporting the diagnosis of major events such as the Australia wildfires in 2019-2020 and the eruption of Hunga Tonga early this year. The EOS era is drawing to a close, but we, and especially our younger colleagues, are preparing to field the next generation of aerosol-monitoring instruments. New ideas are emerging for using smallsat and geostationary observing platforms, as well as for obtaining systematic aircraft in situ measurements of aerosol properties unobtainable from space. And so, the story continues...

About the Speaker

Dr. Kahn received his PhD in applied physics from Harvard University in 1980. He spent 21 years as a Research Scientist and Senior Research Scientist at the Jet Propulsion Laboratory, where he studied climate change on Earth and Mars, and also led the Earth & Planetary Atmospheres Research Element. Kahn is Aerosol Scientist for the NASA Earth Observing System's Multi-angle Imaging SpectroRadiometer (MISR) instrument. He focuses on using MISR's unique observations, combined with other data and numerical models, to learn about wildfire smoke, desert dust, volcano and air pollution particles, and to apply the results to regional and global climate-change and air quality questions. Kahn has lectured on Climate Change and atmospheric physics at Caltech, UCLA, and many other venues, is Adjunct Professor at the University of Maryland, and is editor and founder of PUMAS, the on-line journal of science and math examples for pre-college education. He has authored over 200 publications in refereed scientific journals and book chapters, has received the NASA Exceptional Service and Outstanding Leadership Medals, and is a Fellow of the American Geophysical Union.

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