UV Differential Absorption Lidar (DIAL): Measuring Ozone and
Aerosols in the Atmosphere
The ozone hole, the greenhouse
effect, and global warming are concerns at the center of people's
interest about the atmosphere. What exactly is causing these
atmospheric conditions? How are they changing? How serious are
One way to answer these questions
is to understand the characteristics of the atmosphere's
components, such as clouds, aerosols (suspended particles), and
ozone (a colorless, gaseous form of oxygen). Scientists have used
laser radar or lidar (light detection and ranging) since the 1960s
to study atmospheric particles and clouds. A lidar is an instrument
that uses short pulses of laser light to detect particles or gases
in the atmosphere much like radar bounces radio waves off rain in
clouds. A telescope collects and measures reflected laser
radiation, leading to a map of the atmosphere's structure.
Researchers can then determine the location, distribution, and
nature of atmospheric particles and clouds and, under special
circumstances, molecular species.
Different types of lidars measure
different atmospheric properties. Scientists know that different
molecules absorb light only at certain wavelengths. They can then
tune laser pulses to different wavelengths to target the type of
atmospheric gases they want to study. To measure ozone as well as
aerosols, NASA Langley scientists use a specialized lidar called
the airborne Ultraviolet (UV) Differential Absorption Lidar (DIAL).
These researchers have used ground-based DIAL systems since the
mid-1960s and airborne UV DIAL systems since the early 1980s.
What is the Airborne UV DIAL System?
The NASA Langley
airborne UV DIAL system is a lidar instrument that sends pulses of
laser radiation at different wavelengths into the atmosphere to
measure ozone and also simultaneously measure aerosols and clouds.
The laser beams are pointed both upwards and downwards out of the
aircraft. The UV DIAL system uses five laser (or lidar) wavelengths
in three different regions of the electromagnetic spectrum (Fig.
1): two in the UV region for ozone measurements, two in the visible
region, and one in the near infrared (IR) region. IR and visible
wavelengths both measure aerosols and clouds. Comparing these two
wavelengths can reveal information about the size distribution of
aerosols. The two UV wavelengths determine the profile of ozone by
analyzing the absorption differences due to ozone between the two
lidar returns. From this measurement, scientists can determine the
location and amount of aerosols, clouds, and ozone along the
line-of-sight of the UV DIAL system.
Why measure ozone and aerosols?
Ozone and aerosols impact the
daily lives of animals and plants on Earth. In the troposphere,
ozone can damage human health and harm vegetation. In the
stratosphere, however, ozone absorbs almost all of the UV radiation
shorter than about 3 x 10 -5 centimeters (cm) -- one hundredth of a
meter -- from the sun before it reaches the Earth's surface. UV-B
radiation is the portion of the UV spectrum between 2.8 x 10 -5 --
3.2 x 10 -5 cm, which is near peak ozone absorption of 2.5x10 -5
cm. UV-B can cause melanoma and other skin cancers, cataracts, and
immune deficiencies if stratospheric ozone does not absorb it. UV-B
radiation, however, is also helpful. It assists in the production
of vitamin D from cholesterol, which helps build strong bones and
prevent rickets, many types of cancers, and multiple sclerosis.
Aerosols also affect the
atmosphere in different ways. They can scatter sunlight into space,
which cools the Earth, and also change the size of cloud particles,
which affects when and how often it rains. Aerosols also have an
important relationship to ozone. In the stratosphere, they can
become sites for chemical reactions that convert chlorine from an
inactive form to an active form that destroys ozone. Scientists
also use aerosol distributions to trace atmospheric pollution. They
can, for example, follow urban and industrial pollution or biomass
burn plumes over oceans for thousands of miles away from their
What are other uses for UV DIAL?
Airborne UV DIAL capabilities are
also beneficial to other investigations aboard the aircraft.
Scientists can gain a much broader understanding of the chemistry,
composition, and nature of the atmosphere if they use UV DIAL to
aid other instruments rather than if they used each instrument
separately. Since UV DIAL takes measurements from above and below
the aircraft, it creates an atmospheric map of the surrounding area
(Fig. 2). Using this map, scientists can then locate other
atmospheric events like a plume from a fire and navigate to the
plume, allowing other instruments to study it.
Fig. 2: Examples of atmospheric maps. The top
image represents the outflow of aerosols associated with biomass
burning in the western part of central Africa. The bottom image is
a map of ozone distribution associated with the same burning plume
from Africa (below 6 kilometers) and another outflow from Brazil
(above 6 kilometers). The images were taken as the plane flew up
the west central coast of Africa. The color scale from pink (low
concentration) to black (high concentration) indicates the
intensity of aerosol scattering and the concentration of ozone in
the atmosphere. White areas indicate regions inaccessible to
measurements because of a lack of data or clouds. For example, the
white area below 1 km in the top image resulted because stratus
clouds prevented the laser beam from measuring below them.
Where do scientists use the Airborne UV DIAL System?
Scientists use the UV DIAL system to
study ozone in both the troposphere (atmospheric layer that humans
live in, which has an upper boundary of 8- 18 kilometers, depending
on location, above Earth's surface) and the stratosphere (layer
above the troposphere with an upper boundary of 50 kilometers)
(Fig. 3). In one six-week mission every year or two, scientists use
the UV DIAL system to study the troposphere over remote areas such
as the Atlantic and Pacific Oceans and parts of Africa, Brazil, and
Canada. Researchers have studied topics such as the effects of
biomass burning on the atmosphere and how pollution is transported
over long distances.
Every three to five years scientists
use the UV DIAL system in an airborne field experiment to study
ozone in the stratosphere, particularly in the polar regions where
the most chemical ozone depletion occurs. Scientists have also used
this system to study the effects that the eruption of Mount
Pinatubo in the Philippines in 1991 had on reducing ozone in the
tropical stratosphere. The UV DIAL system is also used to compare
with ozone measurements made by other instruments, including those
For more UV DIAL information, please contact:
NASA Langley Research Center
Office of Public Affairs
Mail Stop 115
Hampton, VA 23681-2199
Or see the Lidar Home Page at
http://asd-www.larc.nasa.gov/lidar/lidar.html for additional