Aerosol Remote Sensing and Modeling

The Climate and Radiation Lab (CRL) has a very active group studying the climate and health impacts of airborne particles (“aerosols”). Aerosol particles reflect sunlight, which tends to cool surfaces locally. Some also absorb sunlight, warming and stabilizing the ambient atmosphere while still cooling the surface below, sometimes suppressing cloud formation, and even affecting large-scale atmospheric circulation. In addition, aerosols are essential participants in the formation of cloud droplets and ice crystals, functioning as the collectors of water vapor molecules during the initial stages of cloud development. Particle abundance and properties affect the brightness, thickness, and possibly lifetimes of clouds and ultimately, precipitation and the terrestrial water cycle. And in significant near-surface concentrations they are pollutants, reducing visibility and raising health risks for those exposed.

Airborne particles originate from a great variety of sources, such as wildfires, volcanoes, exposed soils and desert sands, breaking waves, natural biological activity, agricultural burning, cement production, and wood, dung, and fossil fuel combustion. The particles having the largest direct environmental impact are sub-visible, ranging in size from about a hundredth to a few tenths the diameter of a human hair (about 0.1 to 10 microns). They typically remain in the atmosphere from several days to a week or more, and some travel great distances before returning to the Earth’s surface via gravitational settling or washout by precipitation. As such, they can affect regions thousands of kilometers from their sources: Dust from the Sahara Desert, transported across the Atlantic Ocean, supplies iron to the underlying ocean surface waters, occasionally limits visibility in Florida and the Caribbean, and possibly fertilizes the Amazon basin. Pollution and dust from East Asia sometimes reaches North America, and smoke from summertime fires in Siberia, northern Canada, and Alaska darken snow surfaces in the Arctic. 

The global scope of aerosol environmental influences makes satellite remote sensing a key tool for the study of these particles. Desert dust storms, wildfire smoke and volcanic ash plumes, and urban pollution palls on hot, cloud-free summer days are among the most dramatic manifestations of aerosol particles visible in satellite imagery (

Developing an understanding of the mechanisms that produce, transport, and remove particles from the atmosphere, and actually quantifying aerosol impacts, requires carefully calibrated, global data sets. Satellite aerosol retrievals must be validated with more detailed measurements obtained by aircraft and ground-based instruments. The resulting combined data products must be subjected to intense numerical analysis, and integrated into chemical transport and climate models. The constrained models are then used to simulate the effects aerosols have on the planet’s energy balance, atmospheric circulation, and water cycle, to make predictions about impacts, based on scenarios representing different choices civilization might make in the future.

The satellite measurement aspects of this field represent the primary focus of the CRL’s aerosol scientists, and through collaborations within NASA, with other US agencies, and with international colleagues, we apply these data to constraining climate and air quality modeling efforts. Our group includes the core aerosol science team for the NASA Earth Observing System’s MODerate resolution Imaging Spectroradiometer (MODIS) instruments, and the aerosol scientist for the Multi-angle Imaging SpectroRadiometer (MISR).The MODIS Dark Target, Deep Blue, and MAIAC aerosol algorithms are developed and maintained here, along with the MISR Research Aerosol Retrieval algorithm. We also contribute to the Total Ozone Mapping Spectrometer (TOMS) the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Suomi National Polar-orbiting Partnership’s Visible Infrared Imaging Radiometer Suite (SNPP-VIIRS) aerosol retrieval algorithms.

We perform validation studies on all these satellite aerosol products using ground-based remote-sensing aerosol measurements, such as those provided by the global Aerosol Robotic Network (AERONET) of Sun- and sky-scanning photometers and the Micro-Pulse Lidar Network (MPLNet). And through the Goddard Interactive Online Visualization ANd aNalysis Infrastructure (GIOVANNI), we have participated in the development of web-based tools to collocate multiple satellite and AERONET products and to analyze them statistically.

To retrieve aerosol properties in greater detail, and to better understand the processes by which they interact with the environment, we participate in field measurement campaigns and analyze the resulting data. During field operations, we contribute satellite-aircraft coordination and flight-planning expertise aimed at maximizing science return, some near-real-time satellite aerosol products, and on-the-spot analysis. In addition, we have developed and maintain a state-of-the-art, ground-based mobile facility for measuring the physical and chemical properties of aerosol and clouds, along with the ambient radiation fields (SMART-COMMIT-ACHIEVE), and the Cloud Absorption Radiometer (CAR) deployed in an aircraft nosecone, that can obtain radiance measurements over the entire sphere in 14 spectral bands.

The aerosol applications which we lead, and to which we contribute, range from the fundamental radiative transfer used in satellite aerosol retrieval algorithms, to detailed studies of wildfire smoke and volcanic ash plumes, aerosol pollution events and long-term exposure, as well as large-scale aerosol transports, global energy balance assessments, and climate change studies. Research outcomes of the group have contributed significantly to several national and international assessments conducted by the Intergovernmental Panel on Climate Change (IPCC) the U.S. Climate Change Science Program (CCSP), and the United Nations’ Task Force on Hemispheric Transport of Air Pollution (HTAP), among others. As part of our participation in the wider aerosol and climate change communities, we host the AeroCenter seminars, which are webcast regularly so that interested parties can attend remotely.

Contact:  Ralph Kahn