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2004
Earth Feature Story Special: Africa
to Atlantic, Dust to Dust
The
other side of the world could be closer than you think. Little pieces of Earth,
particles of dust, can travel halfway around the globe and alter air quality,
affecting animals, plants and weather. Summer storms from the planet's deserts
kick up literally millions of tons of dust, and winds send it flying to far-flung
destinations where it clogs our lungs, changes soil chemistry, deposits minerals
in bodies of water and reflects the Sun's rays back into space.
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| On
July 24, 2003, a Saharan dust storm was draped over the Atlantic Ocean and the
Canary Islands off northwest Africa. This image of the event was captured by the
Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite.
Credit: NASA | | | In
July 2000 alone, nearly 8 million tons of dust from Africa's Sahara desert reached
as far west as Puerto Rico. "If you figure that a pickup truck weighs 1 metric
ton, that dust weighed as much as 8 million pickups," says NASA aerosol researcher
Dr. Peter Colarco. Colarco works to create computer simulations that forecast
the movement of dust given current weather conditions. NASA and its partner agencies
want to know what drives the movement of this dust and how exactly the dust changes
our environment, not only so scientists can better understand the health of our
planet, but also so they can predict where dust will imminently affect people.
Take
the case of Africa. Winds blow twenty percent of dust from a Saharan storm out
over the Atlantic Ocean, and twenty percent of that, or four percent of a single
storm's dust, reaches all the way to the west side of the Atlantic. The remainder
settles out into the ocean or washes out of the air with rainfall. Scientists
think that the July 2000 measurements, made in Puerto Rico, equaled about one-fifth
of the total year's dust deposits. If these estimates hold true over the long
term, then the entire state of Florida receives about three feet of dust every
million years. Coupled
with normal urban pollution from factories and cars on a summer day, dust can
aggravate asthma and, in large enough quantities, potentially push cities in its
path out of compliance with air quality standards such as those set by the United
States' Environmental Protection Agency (EPA).
For the past 20 years, “people [thought] of dust in the air residing between
2 and 5 kilometers above Earth, where it blocks the Sun but not much else,"
Colarco adds. This may still hold true for summer dust storms from Africa, but
new research shows dust transport can be much more complicated.
From
Particle to Prediction
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| Dust
Magnified 12,000 times. Credit: USGS | | |
For
the first time, in July 2000, scientists gathered in the Caribbean to study dust
that travels there from across the ocean. In order to understand the properties
of dust that affect its transport, such as the mineral ingredients of the particles,
scientists met in Puerto Rico for the Puerto
RIco Dust Experiment (PRIDE). PRIDE provided an important framework for evaluating
our understanding of dust transport mechanisms across the tropical North Atlantic
Ocean. PRIDE operated from June 28 to July 24, 2000, from the Roosevelt
Roads Naval Station on the eastern side of Puerto Rico, and provided data
used in Colarco’s research.
PRIDE was a joint US Navy/NASA field program
led by the Navy's Space and Naval
Warfare Systems Center, San Diego (SPAWAR San Diego), the University
of Miami, and NASA Goddard Space Flight Center. Other organizations that took
part in the PRIDE experiment include: the NASA
Ames Research Center, the Naval Research
Laboratory, University of Maryland-Baltimore
County, University of Alabama-Huntsville,
University of Colorado, SRI
International, the Bay Area Environmental
Research Institute and Science Systems and
Applications, Inc. Their combined surface, airborne, and satellite studies
aimed to discover how the dust reflects light over land and water, moves over
time, and affects clouds. The results would then be used to evaluate current theories
of dust movement and create new models, if need be. Besides
quantifying the amount of dust from Africa that reaches North America, the work
of the PRIDE researchers led Colarco, then a graduate student doing NASA-funded
research at the University of Colorado, and his collaborators to several important
new ideas about where and how the dust travels. Their findings support a hypothesis
that rather than individual Saharan storms sending clouds of dust across the ocean,
a "reservoir" of dust particles hovers over Africa, and the dust at
Puerto Rico came almost solely from Africa, with little from Arabia or Asia.
Thus, these new ideas suggest that only when winds and other transport conditions
are right over Africa does dust get pushed out over the Atlantic, independent
of dust storms underway. So isolated storms may not be responsible for the dust
blowing across the ocean. Every
summer, cool, moist air flows from the Mediterranean Sea over the Sahara. The
hot, dry desert warms the air, which moves upward, taking dust with it. Scientists
have thought this dust-laden air layer, known as the Saharan Air Layer (SAL),
stretches from the surface to about four miles above the Earth while it's over
land. Once over the ocean, they thought coastal trade winds cut out the bottom
of the layer, leaving a dust void from the surface to about a mile above the surface.
At the same time, air circulation causes the dust to sink, lowering the top of
the layer. Then, scientists believed, by the time the SAL reaches the Caribbean,
rainfall has washed a large portion of dust into the ocean, and the layer ends
up only about one mile thick. At this point, the bottom hovers at two miles above
the Earth's surface, with the dust relatively evenly distributed throughout. However,
the work of PRIDE researchers and Colarco cast doubt on such a simple profile
of the dust layer. By comparing different proposed models of dust transit to the
data collected in Puerto Rico, Colarco, then part of the Goddard Earth Sciences
and Technology Center in collaboration with the University of Maryland, Baltimore
County, simulated the distribution of dust vertically above the Earth's surface.
He determined that the classic, evenly distributed Saharan Air Layer theory only
proves true some of the time. At times, the layer varied more in distribution:
either dust concentrated more heavily near the Earth's surface or it mixed evenly
from the surface to just over 2 miles up, without the void at the surface and
with a lower top than the classic SAL. The lower-level dust mostly traveled across
the ocean several miles above the planet's surface, then settled down, rather
than starting low and moving across the ocean closer to the surface. Gravity and
downward moving winds caused much of the settling to lower levels in the clouds,
though low-level rainfall also plays a role in this process. Colarco’s
theories have competition not only from the earlier models but also from alternatives
proposed by other PRIDE researchers, such as a wind-based mechanism for the varied
dust distribution in the air layer. As with many scientific endeavors, it seems
once again more information, while answering some questions, also leads to more
questions. With satellites and ground stations continuing to take measurements,
scientists will be able to compare the theories to the data and ultimately resolve
the true mechanism. Who
Wants to Know Where the Dust Goes?
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The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) instrument captured the Earth
on June 28, 2000. Dust stretches across the Atlantic from Africa almost to North
and South America. Credit: NASA/Orbimage
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Where
the dust settles interests not only atmospheric and Earth scientists but also
marine biologists, oceanographers, and meteorologists. Some evidence points to
dust storms accelerating the loss
of coral reefs. Dust may be more than just dust: researchers such as Eugene
Shinn and colleagues at the United States Geological
Survey (USGS) have begun preliminary studies showing that these Saharan transports
can also carry to the Caribbean microscopic
organisms such as a fungus known to cause a coral-reef-killing disease. On
the other hand, the minerals in dust, such as iron, aluminum, calcium, magnesium,
and potassium in the case of the Saharan dust, alter the chemical composition
of the ocean it settles into to the benefit of some other species. Iron and aluminum,
especially, provide necessary nutrients to the tiny plant life, phytoplankton,
that form the bottom of the ocean food chain. Increasing the population of this
rung of the ladder can potentially explode the populations of larger species that
feed on the phytoplankton, changing the balance of oceanic life. Or this iron
influx could increase the populations of harmful
species of algae such as those that cause red tides, killing birds and other
wildlife, shifting the marine balance in another direction.
Finally,
when it gets back to land, that dust can impact air quality, respiratory health,
and climate processes that even more directly impact human life. This haze wreaks
havoc with lung function, hitting asthmatics especially hard. People wishing to
exercise outdoors find it harder to breathe on "bad air" days. These
reversible, short-term exposure consequences have long-term counterparts such
as chronic coughing and lung cancer, says the World
Resources Institute, and environmental research and policy firm, basing their
findings on World Health Organization work.
In the United States, the Environmental Protection Agency (EPA) tracks particles
in the air as one of its pollution measures. Joseph Prospero of the University
of Miami tracks dust impact specifically on the city of Miami at the southern
tip of Florida. With its large urban population, Miami faces pollution problems
similar to those of other major U.S. cities. Urban pollution typically results
from factories and vehicles in the city and immediate surroundings; however, Miami
faces the additional onslaught of summer dust storms as a detriment to local air
quality. Prospero postulates that the storms could bring enough particles to Miami
that, on a summer day already high in particulate matter from local sources, the
added dust could put Miami out of compliance with EPA standards for the day.
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|
| A
massive sandstorm blowing off the northwest African desert has blanketed hundreds
of thousands of square miles of the eastern Atlantic Ocean with a dense cloud
of Saharan sand. True color image from MODIS, Feb. 26, 2000. Credit: NASA |
| | However,
the EPA's dust tracking does not spell disaster for Miami and other affected cities.
The cities pushed out of compliance due to such "uncontrollable natural source"
events face no consequences. In fact, it is thanks to NASA's satellites that the
EPA can track these events at all; the agency's judgment of whether or not the
elevated particulate levels result from dust storms or other unavoidable events
come either from direct visual evidence or from NASA satellite images. Images
from NASA's MODerate resolution Imaging
Spectroradiometer (MODIS) aboard the Terra
and Aqua satellites, available daily,
usually make it "pretty easy" for agency experts to see that a dust
storm is on its way, says EPA Associate Director for Science/Policy John Bachmann.
Short- and long-term climatological and environmental effects accompany these
health effects. Dust changes the relative amounts of sunlight reflected back to
space and absorbed by our atmosphere. Scientists find this generally results in
a warmer atmosphere and cooler surface, which alters weather, both regionally
and eventually globally. The chemicals in dust deposited into waters and on land
can alter water and soil chemistry, acting as fertilizer for some and poison for
other animals and plants. Finally, from a more aesthetic, quality of life standpoint,
dust in the atmosphere creates a haze, reducing visibility for sky watchers and
others looking for a scenic view. The
Far-Flung Future of Dust Research Understanding
these properties of dust will allow researchers to track dust movement more precisely
with satellites, which NASA uses to monitor our planet continuously. Ultimately,
NASA's observational scientists want to pass on their observation data to scientists
who specialize in modeling systems. Those scientists create computer simulations
designed to predict where dust will go, given certain climate conditions. Weather
forecasters do the same thing - use computer models based on years of observation
to predict amounts of rainfall, wind speeds and directions, and temperatures.
Scientists continue to collect data to create more accurate models and more precise
forecasts.
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| This
is the data image from TOMS of the massive sandstorm blowing off the northwest
African desert seen in the image above. (Click your browser's refresh button to
view animated GIF.) Credit: NASA | | |
Once the scientists working in Puerto Rico gathered their on-site data, Colarco
tested his models of dust movement to see if they accurately reflected the real
data. Colarco's modeling research combines rules that scientists think govern
the movements of winds, rain, and chemicals through our atmosphere into a simulation
that he expects will give scientists an idea of where dust coming off Africa will
go, given the current weather conditions. The more real data he gets to strengthen
these rules and how they interact, the more accurate his simulation and forecast
is. His models rely on a model
of dust emission from another NASA/university collaboration, led by Paul Ginoux,
then of the Georgia Institute of Technology,
now at Princeton University working with
the NOAA Geophysical Fluid Dynamics Laboratory.
Colarco compared his model's predictions with data from PRIDE and satellites
such as NASA's Total Ozone Mapping Spectrometer
(TOMS) aboard the Earth Probe satellite and the National
Oceanic and Atmospheric Administration's (NOAA's) Advanced Very High Resolution
Radiometer (AVHRR) aboard the NOAA-14 polar orbiting satellite. Colarco's
models proved very accurate in predicting the dust plume's position off of Africa,
but moved the plume southward of its position determined by satellites as the
dust moved across the ocean, sometimes missing the dust's arrival at Puerto Rico
completely. When describing the vertical height of the dust at Puerto Rico, again
the model worked fairly well, but sometimes placed the dust too high, possibly
due to the coarse vertical resolution of the model. Finally, model estimates of
the amount of dust deposition in Puerto Rico came close to the observed results
from ground measurements.
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| Click
on image to view animation. During
spring 2001, TOMS watched a huge dust storm travel halfway around the world from
China to the United States. Scientists use this data to study how regional ecosystems
impact air quality and climate. Credit: NASA | | |
Obviously,
scientists have reason to keep their eyes on the skies, watching for those plumes
of dust rolling in, and hopefully eventually forecasting them. MODIS captures
images in visible wavelengths, where dust is easily spotted over water. Over land,
however, the dust is as bright as the Earth’s surface and disappears from
visible images. TOMS uses ultraviolet wavelengths to image the Earth, helping
with this problem, but it doesn’t take pictures as close-up as MODIS. Aura,
the latest satellite to measure ozone and aerosols in NASA’s Earth-observing
arsenal, should continue TOMS’ pictures at higher resolution when it launches
in 2004.
Satellites, however, must rely on indirect measurements of properties
of dust, as they can't physically sample the dust. By using a vast array of tools
such as those involved in PRIDE, Earth scientists can verify satellite data in
order to ultimately develop a near-daily global picture of aerosol transport.
This record will add to NASA’s years of observations, creating a multi-year
picture of global aerosols, part of the larger issue of Earth observation, monitoring
the planet for its continued well being.
Colarco’s research appeared in the Journal of Geophysical Research –
Atmospheres on July 12, 2003 and August 6, 2003. Colarco, who published the findings
while working for the Goddard Earth Sciences and Technology Center, is now a NASA
scientist at the University of Maryland’s Earth System Science Interdisciplinary
Center.
Contact
Information: K.
A. Stofer
Goddard Space Flight Center
Greenbelt, MD 20771
Phone: (301)
286-5687 Back
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