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NASA Satellite Measurements of Atmospheric Dust Particles Validated
Billions of dust particles take to the sky each year. These tiny airborne particles, called aerosols, affect climate by interacting with solar radiation and changing cloud formation properties. While this is natural and ongoing, evidence suggests that aerosols may play a dramatic role in the severity of climate change.

In 2004, the Extended-MODIS Validation Experiment (EVE) evaluated the data on mineral dust properties from three instruments: the Ames Airborne Tracking Sunphotometer (AATS), mounted on a CIRPAS Twin-Otter aircraft that flew over the dark ocean off the coast of Monterey, Calif., and two MODIS sensors mounted on satellites – Terra and Aqua – that collected data from the same area. Research findings were published in the Geophysical Research Letters (Vol. 33, L18814, 2006).

aerosol optical depth “Our findings showed that dust measurements from MODIS-Terra did a better job of matching the AATS results than did MODIS-Aqua,” explained Jens Redemann, the lead author of the paper at NASA Ames Research Center, in California's Silicon Valley. "Our sample was small, but if true, a statement could be made about the reliability of these instruments for measuring mineral dust aerosols.”

Left: Top image - True color image created from three bands of MODIS-Aqua on April 21, 2004, off of Monterey Bay. Lower image - False color image of aerosol optical depth created from the image on the left. Note that in this case the aerosol is not visible to the naked eye. Both images show the aircraft flight track from which suborbital measurements were obtained for comparison to the MODIS observations.

Aerosols are formed by the disintegration of solids and liquids, or by the conversion of gases to particles, such as desert soil erosion or the exhaust from heavy traffic, respectively. Once airborne, theses tiny particles can transform themselves and take on different shapes and sizes. They are suspended in gas and may be solid, liquid or a mixture of both. Variations in their chemical composition, structural properties, and the occurring microphysics can change their biochemical behavior and nucleating properties, which makes them difficult to study.

Visibility of atmospheric aerosols is determined by their optical properties, which depend on particle size distribution, their refractive index, and the wavelength of the light. Mineral dust is among the most difficult aerosols to measure quantitatively from space.

The EVE-2004 campaign used two types of instruments mounted on three different vehicles. The AATS is a 14-channel instrument that measures a sun beam’s transmission through the atmosphere at different wavelengths (colors of light). The MODIS is a 36-channel optical scanner that measures both visible and near-infrared wavelengths. It has spatial resolution, ranging from 250 meters to 1 kilometer.

“Technically, AOD is a measurement that describes how much the intensity of a beam of light is reduced when traversing the medium that contains the aerosol. First, AOD is proportional to aerosol number, the more particles the larger the optical depth. Second, AOD also depends on particle size, or the larger the aerosols, the larger the optical depth. The term aerosol optical depth is also related to the concept of ‘visibility’ – the larger the AOD, the hazier the atmosphere and lower the visibility,” explained Redemann.

The objectives of EVE-2004 were: First, collect data on aerosol properties from MODIS scanners attached to satellites Terra and Aqua, and compare with the AATS data aboard the CIRPAS aircraft. Second, compare the two sets of data from the MODIS- Aqua and MODIS-Terra. Finally, compare the spatial variability depicted by MODIS with that shown by AATS.

“The observational challenges are due to the lack of knowledge of dust particle shape and absorption, and from spatial structure of dust plumes, which makes it difficult for certain cloud masking techniques to differentiate dust from cirrus clouds.”

"The significance of our study is that it can give people an idea of the accuracy of the MODIS sensor to monitor and measure such tiny particles that can have such a huge effect on the Earth’s atmosphere,” explained Redemann. “The cause of the differences in performance between MODIS-Terra and MODIS-Aqua could be instrument calibration and needs to be explored further.”

What’s next? “We will probably extend the study to include more data in different geographic regimes. Also, the role of clouds on the aerosol observations will be evaluated,” said Redemann.

Sometimes little things can have big consequences.

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Ruth Marlaire
NASA Ames Research Center