BOULDER—Summertime ozone near the Earth’s surface forms in most major
U.S. cities when sunlight and heat mix with car exhaust and other pollution,
causing health officials to issue "ozone alerts." But in other
parts of the world, such as the tropical Atlantic, ozone near the Earth’s
surface appears to originate naturally in ways that have left scientists
puzzled. Now, using four satellites, scientists can tell where low-level
ozone pollution comes from and whether it is manmade or natural.

Atmospheric scientist David Edwards and his colleagues from the National
Center for Atmospheric Research (NCAR), along with collaborators in Canada
and Europe, have studied this problem using satellite data from three
NASA spacecraft, one satellite from the European Space Agency (ESA), and
a computer model from NCAR. They were surprised to find that much more
low-atmosphere ozone over the tropical Atlantic develops as a result of
lightning than of agricultural or fossil fuel burning. Their findings
appeared in a recent issue of the Journal of Geophysical Research –
Atmospheres, a publication of the American Geophysical Union (AGU).

The formation of ozone involves several factors, including lightning
and pollution from agriculture and fossil fuel burning. The scientists
used multiple satellites to look at each factor in turn. NASA satellites
included Terra, Earth Probe/TOMS, and the Tropical Rainfall Measuring
Mission (TRMM). ESA’s ERS-2 satellite was also used, along with NCAR’s
MOZART-2 (Model for OZone And Related chemical Tracers) computer model,
which simulated the chemical composition of the atmosphere. NCAR’s primary
sponsor is the National Science Foundation (NSF).

The different satellite instruments detected fires, lightning flashes,
and the resulting pollution and ozone in the atmosphere. The computer
model helped tie all the pieces together.

The scientists found that in the early part of the year, intense fires
set by farmers for land-clearing and traditional cultivation in northwest
Africa, just south of the Sahara Desert, resulted in large amounts of
pollution, which was tracked by the satellites as it spread over the Atlantic
towards South America. This pollution greatly increased ozone at low altitudes
near the fires.

However, when Edwards and his colleagues examined areas of elevated
ozone levels measured by satellites and aircraft over the Atlantic Ocean
south of the equator, they were surprised to find that this ozone was
caused mainly by lightning rather than the fires.

In other parts of the world, especially near cities, ozone near Earth’s
surface is often produced from pollution as a result of industrial fossil-fuel
burning and cars. Understanding where the pollution comes from in each
case is important for improving our air quality.

Fires create smoke and carbon monoxide; lightning creates nitrogen oxides
(NOx). All of these come together with other unstable compounds in a chemical
soup, in which sunlight helps trigger the reaction that helps form ozone.

The Measurements of Pollution in the Troposphere (MOPITT) instrument
aboard NASA’s Terra satellite is a joint NASA/Canadian Space Agency mission
that measured carbon monoxide concentrations at various levels of the
atmosphere. The TOMS instrument on EP/TOMS measured tropical tropospheric
ozone over the mid-Atlantic. The TRMM satellite counted the number of
fires in a region using its Visible/Infrared Scanner (VIRS), and also
catalogued lightning flash data from its Lightning Imaging Sensor (LIS).
The satellite data were then interpreted using the MOZART-2 computer model.

Previously, scientists used TOMS observations to get a general idea
of where tropospheric ozone levels were high, but it was often difficult
to say where the ozone came from and which pollution source or natural
process led to its creation. Only recently has the four-satellite combination
enabled scientists to make this distinction.

This research was funded by NASA’s Earth Science Enterprise (ESE) in
cooperation with NSF. NASA’s ESE is dedicated to understanding the Earth
as an integrated system and applying Earth System Science to improve prediction
of climate, weather and natural hazards using the unique vantage point
of space.