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 they?
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 continental sources.
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?
Or see the Lidar Home Page at http://asd-www.larc.nasa.gov/lidar/lidar.html for additional information