A NASA-funded study provides direct measurements confirming
aerosols, tiny particles in the atmosphere, may be changing how much
carbon plants and ecosystems absorb from or release to the air.

The research is important for understanding climate change and the
various factors that influence how much carbon gets transferred from
the air into below ground carbon sinks. Carbon dioxide acts as a heat-
trapping greenhouse gas in the atmosphere. The study appeared in a
recent issue of Geophysical Research Letters.

The study reported the effects of aerosols on overall carbon exchange
might be more significant than clouds. Cloud cover tended to reflect
the sun’s radiation back out to space, reducing the overall amount of
light to Earth’s surface. As a result, less sunlight on plants caused
less photosynthesis.

The study, which benefited from NASA satellite data, focused on six
sites around the country. The sites represented a wide variety of
landscapes, including forests, crops, and grassland. When aerosol
levels were high, the amount of carbon absorbed by an ecosystem
increased for forest and croplands, and it decreased for grasslands.

Lead author Dev Niyogi, a research assistant professor at North
Carolina State University, Raleigh, N.C., and colleagues, suggested the
effect of aerosols on the overall exchange of carbon dioxide by
ecosystems may be greater than the effects of clouds on these

“We were very excited to find direct observational evidence that one
variable, the amount of aerosols in the atmosphere, can have such a
significant effect on something so complex as an ecosystem’s carbon
exchange,” Niyogi said.

The researchers used data from NASA’s AERONET (AErosol RObotic NETwork)
and the AmeriFlux network. AERONET provided data on the amount of
aerosols in the air. From AmeriFlux, Niyogi and colleagues were able to
measure the exchange of carbon dioxide between the air and an

But aerosols did not dramatically cut the amount of radiation that
reached Earth’s surface. Instead, aerosols scattered sunlight allowing
more radiation to penetrate to the lower layers of leaves. This less
concentrated radiation due to aerosols allowed for more leaves to
photosynthesize at a higher rate. During photosynthesis, plants absorb
carbon from the air.

In grasslands the top layers of leaves are not as dense as with crops
and forests, causing the ground to heat more. When the ground heats,
the soil gives more off carbon dioxide, thus reducing the net effect.

The study also benefited from the Moderate Resolution Imaging
Spectroradiometer (MODIS) in NASA’s Terra satellite. It was used at
regular intervals to provide broader geophysical context to the more
continuous data available from AERONET. MODIS data were also used to
assess the vegetation health and map leaf area for each site, and to
interpret the net ecosystem exchange.

For each site, the researchers analyzed how carbon cycled in each
ecosystem on cloudy and cloud-free days. They examined carbon exchange
levels for high and low levels of scattered sunlight as well as high
and low levels of aerosols. Measurements were taken during afternoons
in the peak growing season from June through August. Years of available
data varied for each site.

AERONET is a ground-based aerosol-monitoring network and data archive.
It was initiated and supported by NASA’s Earth Observing System. It was
expanded into a consortium with many non-NASA institutions. NASA
provides equipment and standardization to institutions that participate
in the program. Data from AERONET provides near real-time observations
of aerosols. AmeriFlux is a multi-institutional network supported by
several federal agencies that provides ongoing data of ecosystem level
exchanges of carbon dioxide, water, energy and other factors from daily
to yearly time scales.

For more information and images about this release on the Internet,


For information about NASA and agency programs on the Web, visit: