The Dirt on Atmospheric Dust
June 22, 2016
This data map shows the presence of aerosols in the atmosphere based on data from the AVHRR and VIIRS sensors aboard NOAA polar-orbiting spacecraft.
It might seem small, but atmospheric dust is a big deal. Consisting (mostly) of tiny pieces of metal oxides, clays and carbonates, dust is the single largest component of the aerosols in Earth’s atmosphere, and it likely has a significant impact on the Earth’s climate, as it effects a wide range of phenomena, including from temperatures in the Atlantic Ocean to the rate of snowmelt in the southwestern U.S. Dust may also affect hurricanes, as recent research based on data sets dating back to the 1950s suggests an inverse relationship between dust in the tropical North Atlantic and the number of Atlantic hurricanes during the past several decades.
Yet, while its impact on Earth’s ecosystems is easy to detect, its presence in satellite imagery may not be.
If you’re wondering how a satellite travelling 22,236 (or 512) miles above the Earth’s surface can even detect something as small as dust in the first place, it’s because dust, like other aerosols in the atmosphere, reflects or absorbs light. Satellite sensors, such as the GOES I-M Imager aboard the GOES-13 and -15 satellites, the Advanced Very High Resolution Radiometer (AVHRR) aboard the NOAA-series satellites, and the Visible Infrared Imager Radiometer Suite (VIIRS) aboard Suomi NPP, can detect these areas of reflection and absorption, thus indicating varying amount of aerosol in the atmosphere.
In visible satellite imagery such as this image from GOES-East, it can be hard to distinguish dust clouds from the background.
In the map of AVHRR and VIIRS aerosol optical thickness data (shown above), areas of the atmosphere with thick aerosol layers (i.e., areas in which a lot of light is reflected or absorbed) are colored in deep orange, whereas areas with low aerosol optical thickness are colored light yellow. (Note the large plumes of aerosols from sand, dust and salt-spray moving westward, off the coast of Africa.)
Dust Detection Difficulties
Of course, just because satellites can detect dust in the atmosphere does not mean scientists can always detect it in satellite imagery. Why? More often than not it comes down to color. For example, in visible satellite imagery, which is grayscale (see GOES image), it can be difficult to see dust clouds over light-colored backgrounds or distinguish them from other cloud types. In response, scientists have tried to take advantage of the technological advances in satellite instrumentation to create different types of satellite imagery to overcome this dust-detection difficulty.
For example, Suomi NPP’s VIIRS instrument, a more advanced instrument compared to its predecessors, allows scientists to create photorealistic images of Earth by combining three (red, green, and blue) of VIIRS’s 22 channels. In these so-called "false-color" images, such as in the one of China’s Taklimakan Desert, dust clouds show up quite clearly, even over lighter areas. Unfortunately, these types of images are only available in the daytime, which presents a problem for scientists trying to track the development and distribution of atmospheric dust clouds over time.
When applied to this VIIRS image, EUMETSAT’s Dust RGB methodology shows dust clouds in bright pink. (animation courtesy of EUMETSAT)
Taking a different approach, the scientists at the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) have developed a method of processing satellite imagery specifically designed to aid the detection of atmospheric dust. Aptly named "Dust RGB," and available both day and night, this method combines three of a satellite imager’s red, green and blue channels so that dust clouds appear pink or red (depending on the time of day the image was produced). Nevertheless, even though this type of imagery allows analysts to detect low-level dust clouds at night, they may still be difficult to see in Dust RGB images because of the small amount of contrast between dust clouds and terrestrial areas.
Taking yet another approach, the scientists at Cooperative Institute for Research in the Atmosphere have developed an algorithm known as "DEBRA," for Dynamic Enhanced Background Reduction Algorithm. As its name implies, DEBRA works by reducing the background in an image (be it a polar-orbiting or geostationary satellite) to grayscale, thereby enhancing the appearance of dust clouds, which appear yellow. The intensity of the yellow corresponds to confidence that a given pixel contains dust.
CIRA’s DEBRA algorithm reduces the background in a satellite image to grayscale, thereby enhancing the appearance of dust clouds, which appear yellow.
Scientists at the Advanced Satellite Products Branch in NOAA's Center for Satellite Applications and Research have suggested several new spectral bands that have been included on Advance Baseline Imager, an instrument that will fly aboard the GOES-R, the next generation of NOAA’s geostationary weather satellites scheduled to launch this fall. These improved spectral and temporal attributes will greatly improve dust detection and characterization.
What It Is and Where It Comes From
Improving our ability to detect dust in the atmosphere is beneficial because just how much dust enters the atmosphere each year is unclear – projections range from 200 to 5,000 teragrams a year (a teragram, Tg, equals one trillion grams). Scientists estimate that, on average, about 20 Tg of dust are suspended in the atmosphere at any given time, but seasonal variability is common. Inter-annual variability is also a factor, as ocean-related weather phenomena such as the North Atlantic Oscillation and El Niño have been associated with greater Saharan dust transport across the Atlantic.
And speaking of the Sahara, Lake Chad, which sits just below it in the north-central part of Africa, is the Earth’s largest single source of atmospheric dust. In fact, about half of the dust suspended in Earth’s atmosphere originates in North Africa, due to both the abundance of dust sources there and the region's position under the subtropical jet stream, which carries dust around the world. The rest is said to come from just a handful of other well-known dust-producing regions, including northwestern China’s Taklimakan Desert, parts of Arabia, Iran, the shore of the Caspian Sea, the Lake Eyre Basin in Australia, and the area around Utah’s Great Salt Lake.
This true-color image of northwestern China’s Taklimakan Desert from Suomi NPP shows a dust cloud over a light background. The Taklimakan is among the handful of well-known dust-producing regions around the globe.
Atmospheric dust arises from these locations because they all share a common trait: they all sit in low-elevation basins near or surrounded by mountains, which feed rivers that deposit large amounts of sediment in these low-lying areas. These particle-producing places also tend to be completely flat and devoid of significant (or any) vegetation cover, two features that allow winds to build momentum and drive more dust into the atmosphere.
How long dust hangs around in the atmosphere depends on the size of the individual particles. Particles with radii between 0.1 and 1.0 micrometers (a micrometer is one-millionth of a meter) can stay aloft for 20 or more days. Larger dust particles with radii between five and 10 micrometers usually fall out of the sky within 24 hours.