OZ8. Measuring Ozone

Accurate measurements of stratospheric ozone identified the ozone problem in the mid-1980s. 

Accurate measurements are still needed to learn more about the complicated atmospheric chemistry involved and to monitor how the ozone layer responds as CFCs are phased out. The longest continuous record of ozone measurements extends almost 80 years. But despite this long data record, certain questions about the behavior of tropospheric and stratospheric ozone could not be answered from this one type of measurement. Because stratospheric ozone is so far above the Earth’s surface, advances in transportation and communication technology were needed to take the instruments to where the ozone was located. 

I. Ground Stations

Since 1881, when Hartley and Cornu discovered the unexpected loss of UV section in the solar spectrum and related this to a possible ozone-rich layer somewhere in the atmosphere, scientists realized that by measuring the UV radiation, they could learn about ozone. However, it wasn’t until the invention of Dobson’s spectrophotometer in 1924 that repeatable, accurate, and precise UV measurements were possible. Measuring intensities of several wavelengths of UV from both direct and reflected sunlight, these spectrophotometers could provide both total column ozone amounts and vertical distributions of ozone concentration, which showed that the altitude of the maximum concentration of ozone was much lower than thought. 

The station using a Dobson spectrophotometer in Arosa, Switzerland, has been collecting data since the mid-1920s, which is the longest continuous record of ozone measurements. Although new, more sophisticated techniques to measure ozone have been invented, these consistent, long-term (and safe) measurements from the same location are very valuable. Because of their established data records, Dobson spectrophotometer measurements are often used to calibrate data obtained by other methods, including satellites. Unfortunately, since the technique depends on natural light sources coming through the atmosphere, the measurements are strongly affected by aerosols (tiny, naturally occurring atmospheric particles, such as salt particles and dust) and pollutants in the atmosphere.

A second ground instrument was developed to measure chemical constituents in the atmosphere: the Light Detection and Ranging (LIDAR) instrument. Lidar is similar to radar (Radio Detection And Ranging) and can be thought of as a laser radar. Although radar was invented 1924, it wasn’t until after World War II when technology developed that made lidars possible. A lidar instrument projects two laser beams into the atmosphere—one with a wavelength of light that is absorbed by ozone and one that is not absorbed by ozone. The instrument measures the light that is reflected back towards the instrument. By comparing the two beams the instrument measures the concentration of ozone. 

Lidars are able to measure continuously, compared to the Dobson spectrophotometer, which needs a source of natural light. The disadvantage of lidars is that they are very expensive and require significant expertise to maintain and operate. Current research is focused on creating automated, low power lidars that will be placed in remote locations around the globe.

II. Airbourne Measurements

Airborne vehicles, such as balloons, airplanes, and rockets provide the opportunity to carry instruments directly to the area of interest in order to make measurements. These measurements are the most accurate possible, since they can involve direct samples of ozone. However, there are limitations—the measurements are made only over localized regions and cannot provide a global picture of ozone distribution. There is also a high cost of human involvement.

Balloons

Balloons have been used for a long time to carry instruments or people into the sky. The first human flight took place in a hot air balloon in Paris in 1783. The radiosonde, a helium-filled balloon carrying weather instruments that transmit information to ground observers by radio, was first tested in the early 1930s, and the first U.S. radiosonde network was established in 1937. By 1960, ozone-measuring equipment could be packaged with the standard radiosonde equipment to create the ozonesonde. These instruments can measure ozone concentration up to altitudes as high as 40 km before the balloon bursts. 

Many lightweight devices have been invented to measure ozone from balloons. Most common are several varieties of devices that measure the electrical current produced during chemical reactions between atmospheric ozone and potassium iodide that is carried aloft. Another type of ozonesonde carries a laser aloft and measures the absorption of laser light projected from the balloon and reflected back to the sensor from a mirror hanging beneath it. 

Ozonesondes provide detailed information on the vertical distribution of ozone throughout much of the stratosphere. However, since balloons are unpowered, the location of measurements cannot be controlled. Also, there are problems with the quality and comparability of the ozonesonde data resulting from the variety of sensors being used around the globe and the diverse calibration and operating procedures each country has adopted for the ozonesondes under their responsibility. 

The term sounding comes from the maritime practice of calculating the depth of water with a weighted line, called a sounding line. The term is used today to describe many kinds of remote measurements that have a vertical component to the measurement, such as the vertical temperature profile (also called a temperature sounding). A sonde is an instrument that measures a sounding in the atmosphere or in the oceans.

Aircraft

Airplanes are used to make detailed measurements of ozone levels and related chemicals in the troposphere and lower stratosphere. Airplanes can carry a large number of instruments capable of measuring ozone, chemicals related to the production and destruction of ozone, and atmospheric conditions that affect ozone. Large airplanes, like NASA’s DC-8, can carry many people to perform sophisticated experiments. 

Not all aircraft can carry large numbers of instruments or people, but some can be quite important based on their flight capabilities, such as being able to reach very high altitudes. In the case of the ER-2, where space is very limited, automated and miniaturized sensors are used while taking advantage of primary benefit of using aircraft: the ability to control where they fly. This allows specific altitudes and locations of the atmosphere to be studied. All of these benefits come at a cost—research with aircraft is quite expensive, and, long term, continuous collection of data is not possible. Aircraft are most useful for studying the detailed chemical reactions over a relatively brief period of time. 

Technology is being advanced to create Unmanned Aerial Vehicles that can carry instruments weighing hundreds of pounds to altitudes exceeding 27 km for several days at a time. These aircraft will be quite valuable for future data collection because pilot/crew safety is not an issue, the aircraft and instruments are recoverable, and the flight path and altitude controllable.

Rockets

By the early 1960s sounding rockets were used to measure profiles of ozone levels from the ground to an altitude of 75 km. Many countries have their own rocket programs, some starting in the 1960s. A wide variety of rockets have been developed to carry instruments of various weights to specific altitudes. Rockets can be flown in most weather conditions, and costs can be lowered if surplus rocket engines are available from the military. Rockets do not fly on a daily or weekly basis because it often takes a minimum of six months to build the instruments and rockets. Teams of researchers and technical support staff are needed to develop the sensors and rocket, launch and record the data, and recover the rocket and sensor. While rockets can go very high in the atmosphere, they can collect data over a small area for a very limited period of time.

III. Satellites

Ever since the Soviet Union launched the Sputnik satellite in 1957, scientists realized that vast regions of Earth’s atmosphere and surface could be measured by instruments in orbit. NASA’s first Earth observing satellite, TIROS-1 (Television Infrared Observation Satellite), was launched on April 1, 1960. The purpose of TIROS-1 was to improve weather forecasting with pictures from space. Two onboard television sensors stored images onto magnetic tape for later broadcast to receivers on the ground. Although TIROS-1 lasted only 78 days, it proved to be a huge success in providing valuable information about the Earth and its atmosphere, regardless of the weather conditions on the ground.

LIMS was designed to measure ozone concentration at different altitudes by measuring the intensity of infrared (IR) radiation emitted by the ozone as observed edgewise at the atmosphere, looking sideways toward the Earth. This technique is called limb scanning. LIMS also measured water vapor, NO₂, HNO₃, and temperature.

It was the TOMS data that NASA reanalyzed to confirm the local ozone depletion observed by the British and Japanese ground stations during the mid 1980s. The data also showed the size of the region experiencing ozone depletion. NIMBUS-7 lasted longer than any other Earth observing satellite, providing data until mid-1993. TOMS has provided almost continuous, global coverage of total column ozone measurements since 1978 until today by being flown on later satellite platforms.

History of ozone measuring satellites:
https://rammb.cira.colostate.edu/dev/hillger/ozone-monitoring.htm

Space Shuttle

Similar to aircraft, NASA’s space shuttle can carry a large number of instruments to high altitudes along with people to operate the equipment. Since 1989, the Shuttle has periodically conducted ozone experiments using a backscatter-measuring instrument. Because of the unique abilities of the shuttle, the instrument is carefully calibrated before launch, during flight, and after landing, as well as controlling where measurements are made. These controls allow ozone data to be correlated between the TOMS and other satellite ozone measurements. These frequent flights are used to ensure the accuracy of ozone measurements from satellites.

NASA Ozone Watch page:
https://ozonewatch.gsfc.nasa.gov

Ozone Monitoring Instrument (OMI) on NASA’s Aura Mission satellite:
https://aura.gsfc.nasa.gov/omi.html

Each of the pixels that makes up a satellite image has the same color across the entire area. You could think of a pixel as a tile or dot of a solid color. The larger the pixel, the less detail within the image that you can see since the details get averaged into the color of the larger tile. Also, larger pixels are more noticeable than smaller pixels. Look at a computer monitor or television screen with a magnifying glass. Can you see the pixels of color? From a distance, the pictures on a computer or television appear smooth, but up close, you can see the pixels.

The example on these two pages dealt with horizontal resolution—pixel size in the east/west and north/south directions. However, ozone concentration changes with altitude as well as horizontally. How well does a satellite sensor “see” these details? When discussing vertical resolution, think of layers, such as that of a birthday cake. This birthday cake is very special—each layer is a different flavor from all the others, but the baker will only make the cake a certain height so it can fit into the oven. The thicker the layers, the fewer the flavors making up the cake. Similarly, if the satellite sensor has a coarse vertical resolution (it can see only a few layers), the fewer details, or “flavors”, of ozone that can be observed.

See Staying current for this chapter.