This image shows two MODIS-Aqua products for Oct. 17, 2008, over the persistent Southeast Pacific stratocumulus deck, off the coasts of Chile and Peru. UI researchers and their colleagues have developed a new technique to evaluate how aerosol pollutants affect clouds, thereby giving scientists the ability to examine clouds and determine particle concentrations in the atmosphere below. Satellite retrievals courtesy of NASA Goddard Space Flight Center; image courtesy of Pablo Saide, Greg Carmichael, Scott Spak, Matthew Janechek, and Nicholas Thornburg, University of Iowa. |
Until
now, scientists who study air pollution using satellite imagery have
been limited by weather. Clouds, in particular, provide much less
information than a sunny day.
University
of Iowa scientists have created a technique to help satellites “see”
through the clouds and better estimate the concentration of pollutants,
such as soot. The finding is important, because, like GPS systems,
clouds block remote-sensing satellites’ ability to detect, and thus
calculate, the concentration of pollution nearer to the ground. This
includes particles (commonly known as soot) that reduce air quality and
affect weather and climate.
The results of the study are published July 9 in the online early edition of the journal Proceedings of the National Academy of Sciences (PNAS).
“Particles
in the atmosphere (aerosols) interact with clouds changing their
properties. With this technique, we can use remote sensing observations
from satellites to estimate these cloud properties in order to correct
predictions of particle concentrations. This is possible due to a
numerical model that describes these aerosol-clouds interactions,” says
Pablo Saide, environmental engineering doctoral student and researcher
at the UI Center for Global and Regional Research (CGRER).
Scott
Spak, co-author and assistant professor of civil and environmental
engineering in the UI College of Engineering, adds that the new
technique is expected to find immediate application across a wide range
of activities. Examples include air quality forecasting, numerical
weather prediction, climate projections, oceanic and anthropogenic
emissions estimation, and health effects studies.
But the ability to see pollution “through the clouds” is also expected to have “on the ground” health results.
“Unlike
previous methods, this technique can directly improve predictions of
near-surface, fine-mode aerosols—such as coal-fired electric generating
plants and wood-fueled cooking fires—responsible for human health
impacts and low-cloud radiative forcing (solar heating),” says Greg
Carmichael, co-author, professor of chemical and biochemical
engineering, and CGRER co-director. “This technique is also
complementary to previous methods used, allowing the observing system to
‘see aerosols’ even under cloudy conditions.”
Here’s how the technique works:
- Existing
weather satellites observe warm, single-layer clouds, such as the
stratocumulus clouds that form off the west coasts of Africa, North
America, and South America. These clouds are thought to be the main
factors contributing to climate cooling. - Researchers
calculate the number of droplets in the clouds using the satellite
data, which are compared to a model estimate provided by the UI program. - As
airborne particles interact with clouds changing their properties,
model estimates of particles are corrected so that the model will
generate a better agreement with the satellite number of droplets. - Particles
interacting with clouds are usually below clouds, thus, in some cases,
the model corrections can be attributed to manmade emissions.
The
researchers conducted their study using National Science Foundation
(NSF) aircraft measurements to make simultaneous cloud and particle
observations, which verified satellite observations and the mathematical
formulas used to determine the pollution concentrations in the air.
All
three UI researchers agree that their new technique for seeing through
clouds to make ground observations is likely to generate growing
interest as the need to infer ground air pollution levels, and the need
to mitigate the human hazards posed, grows larger.
In
addition to UI researchers, paper co-authors include Patrick Minnis of
NASA Langley Research Center, Hampton, Va., and Kirk Ayers of Science
Systems and Applications Inc., Hampton, Va.
The
PNAS article is titled “Improving aerosol distributions below clouds by
assimilating satellite-retrieved cloud droplet number.”
The research was funded by NSF and NASA.
