The first instrument to study ozone was Christian Friedrich Schonbein’s nose. While conducting an experiment on the electrolysis of water, the German-Swiss chemist noticed a pungent smell and determined the smell was the “odor of electricity,” calling it ozone from the Greek word “ozein” meaning to smell. Since this find in 1839, scientists learned that the “odor of electricity” was the product of a chemical reaction that produced O3, or ozone. Researchers have learned that Schobein’s olfactory discovery has large implications for our health and climate, and they now conduct science investigations with more than just their noses.
Celebrating its 10th anniversary on July 15, 2014, NASA’s Aura satellite has provided key measurements to improve our understanding of ozone. With its four instruments, Aura helps researchers understand how the chemistry in the atmosphere influences life on the ground.
When Schonbein smelled the pungent ozone odor, he did not anticipate the impressive versatility of an ozone molecule. Ozone has beneficial and harmful roles depending on its location in the atmosphere.
At and near the ground level, the lower troposphere and where humans live, ozone is an air pollutant. Ground-level ozone damages plants and is harmful to breathe as it can decrease lung function in humans. A few miles up at the mid-troposphere, where pollutants like carbon monoxide from power plants may linger, ozone helps purify the air by producing the hydroxyl radical, an important compound that reacts with air pollutants and helps rid those toxic chemicals from the atmosphere. Higher in the troposphere, ozone affects climate as it is a greenhouse gas, trapping heat in Earth’s atmosphere. Aura’s Tropospheric Emission Spectrometer (TES) provides measurements of greenhouses gases such as ozone and water vapor. A few miles higher, ozone plays a unique role in making life possible on Earth.
Understanding Ozone as our Sunscreen
[image-51]The majority of ozone — about 90 percent — is located between 12 to 30 miles above ground in the stratosphere, higher than where most airplanes fly. At these altitudes, ozone forms an advantageous shield, absorbing and thereby screening us from most harmful ultraviolet (UV) light from the sun. A small fraction of UV light still makes it to the surface – and is necessary for plants to grow, but for humans, too much time in the sun leads to sunburn on exposed skin. The ozone in the stratosphere forms naturally when oxygen molecules are broken apart by very high energy UV sunlight. NASA satellites have measured the ozone layer for more than three decades, and these observations are helping scientists understand more about the different factors that affect ozone concentrations.
The formation of the ozone hole is a seasonal occurrence. Every winter over the Antarctic, powerful winds spiral into a huge whirling eddy, known as the polar vortex. Inside this spiraling vortex, cold temperatures and chlorine gases from human-produced sources work together to rapidly destroy ozone when sunlight returns in the spring. Researchers have learned that year-to-year variations in cold temperatures, wind circulation and chlorine concentrations influence the ozone hole.
Scientists use Aura’s Microwave Limb Sounder (MLS) instrument to obtain a comprehensive, inside look of the ozone hole from year to year. The MLS instrument measures ozone and other trace gases, or chemicals present at low concentrations, in the stratosphere to study the chemical composition in and around the ozone hole.
In 2006 and 2011, Aura’s instruments revealed two of the largest and deepest ozone holes in the past decade. Surprisingly, however, Aura data showed that the holes were large and deep for different reasons. In 2006, ozone fell to low levels over a broad area due to reaction cycles involving chlorine monoxide, a key ozone-destroying chemical that MLS measures. In 2011, MLS measurements implied that inorganic chlorine was 10 percent lower than in 2006.
Computer simulations of the two years show that chlorine was not the only difference. The 2011 ozone hole was large and deep like 2006 in spite of lower chlorine because of meteorological differences in the two years. In 2011, the winds transported less ozone to the Antarctic.
“Changes in the ozone column can’t be understood without information on chemistry and meteorology at different levels in the stratosphere,” said Susan Strahan, atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. “This is why vertical profiles of ozone and other trace gases measured by MLS are so important to understanding the state of the ozone layer and how it may change in the future.”
Application: Measuring the Sun’s Ultraviolet Rays
Measuring the amount of ozone in the atmosphere is important for estimating the amount of UV radiation reaching Earth. Ozone absorbs UV radiation from the sun. Less ozone in the stratosphere allows more UV radiation to reach the surface. While people need some UV light as a source for Vitamin D, overexposure can cause immediate problems, such as sunburn, and long-term issues, such as skin cancer and cataracts. Wearing hats and sunglasses, limiting sun exposure and sun tanning, applying sunscreen, and watching the UV index can reduce sun-related illnesses.
Every day, the National Oceanic and Atmospheric Administration and the Environmental Protection Agency compute a UV index that indicates how much UV radiation is reaching the ground. The index has a scale of one to 10 where 10 represents the presence of very powerful sunrays that are harmful to a person’s health. The index is calculated using ozone forecasts that rely on data from several sources, including the OMI and MLS instruments on the Aura satellite.
Other factors that affect the amount of UV radiation that reaches the ground are also considered in the UV index. Clouds can block UV rays and also scatter sunlight, allowing some to reach the ground. Grass absorbs UV radiation whereas sand and snow reflect more radiation from the surface. Therefore, extra precaution should be taken at the beach and in the mountains, as the reflected UV is dangerous for people’s eyes. Small particulates called aerosols also absorb and scatter rays.
Stratospheric ozone will always be the key factor in protecting life from potentially harmful UV rays. “Ozone protects us from UV-B and UV-C rays,” said Nick Krotkov, an atmospheric scientist at Goddard. “If you don’t have stratospheric ozone, you have all the dangerous, high-energy photons here.”
Looking into the Future of the Ozone Layer
Predicting the future of our ozone layer is not an easy task and requires decades of data. Daily and even yearly changes are simply not good indicators of long-term behavior. In order to accurately assess the ozone layer’s health, scientists need to observe the stratosphere as it changes over a long period of time, similar to someone monitoring the health of retirement investments. The stratosphere does not respond on one-minute time scales.
A series of NASA satellites, including Aura, have observed the stratosphere for more than 30 years, helping scientists identify ozone variations that are apparent on a decadal timescale but undetectable on a shorter timescale. For instance, in 1987, nations around the world signed the Montreal Protocol. This international treaty banned ozone-depleting substances such as chlorofluorocarbons (CFCs). CFCs, used in refrigeration and air conditioning, are broken apart by UV radiation in the stratosphere, releasing chlorine atoms that participate in ozone-destroying chemical reactions. With help from ground-based measurements and airborne missions, Aura and its satellite predecessors observed the effect of the Montreal Protocol on the ozone layer as scientists could start to see a decline of chlorine compounds.
Still, longer data sets are needed to fully see the effect of the Montreal Protocol. NASA scientists estimate that the impact on the Antarctic ozone hole from CFC reduction will not be significantly noticeable until 2025. Aura and the Suomi National Polar-orbiting Partnership (NPP) mission, which was launched in 2011, continue to measure the ozone layer. In the upcoming decades, scientists will also study other factors that affect the ozone layer, like climate change.
Because of Aura and other satellites before it, we know ozone-destroying chemicals like CFCs have declined,” said Anne Douglass, Aura project scientist at Goddard. “Continuous, long-term monitoring of the ozone layer is also essential to understand how climate change is affecting the ozone layer as the stratospheric chlorine level goes back to normal."