Scientists have been studying the concentration of ozone in the atmosphere since the 1920s. Since then, instruments have evolved from ground based spectrometers to balloons, aircraft, rockets, and satellites. Developments in ozone instrumentation have enabled measurements to expand from the atmosphere above an isolated ground station to daily global coverage and profiles of ozone in the atmosphere.
Ground Based MeasurementsGround stations have been measuring ozone levels for most of this century. Extremely low ozone levels were first observed from Faraday Base, Antarctica. They provide long term data of both total column ozone and ozone distribution with altitude, but only over a small area. Instruments that are commonly used to measure atmospheric ozone from the ground (not surface ozone) are the Dobson spectrophotometer and Light Detection and Ranging (LIDAR).
Developed in 1924, the Dobson spectrophotometer is the earliest instrument used to measure ozone, and modern versions continue to provide data. As of 1993, there were 71 Dobson stations worldwide. They are the only long term source of ozone data, with one station in Arosa, Switzerland, providing continuous measurements since the 1920s. Unfortunately, the Dobson method is strongly affected by aerosols and pollutants in the atmosphere, and measurements are provided only over a small area. Dobson spectrophotometer measurements are often used to calibrate data obtained by other methods, including satellites.
Dobson spectrophotometers can be used to measure both total column ozone and profiles of ozone in the atmosphere. Total ozone measurements are made by comparing a frequency of the ultraviolet spectrum strongly absorbed by ozone with one that is not. Measurements can be based on light from the sun, moon, or stars. Different techniques enable measurements to be taken in varying weather conditions and throughout the day.
The vertical distribution of ozone is derived using the Umkehr method. This method relies on the intensities of reflected, rather than direct, UV light. Ozone distribution is derived from the change in the ratio of two UV frequencies with time as the sun sets. An Umkehr measurement takes about three hours, and provides data up to an altitude of 48 km, with the most accurate information for altitudes above 30 km.
Light Detection and Ranging (LIDAR) is an ozone measurement technique that relies on absorption of laser light by ozone. A telescope is used to collect ultraviolet light that is scattered by two laser beams - one of which is absorbed by ozone (308 nm) and the other is not (351 nm). By comparing the intensity of light scattered from each laser, a profile of ozone concentration vs. altitude is measured from 10 km to 50 km. LIDAR data are used to study stratospheric change and provide ground truth to correlate UARS ozone profiles.
Airborne MeasurementsAirborne measurements of ozone provide a direct (or in situ) method of determining ozone concentrations in the atmosphere. Balloons, rockets, and aircraft carry instruments into the atmosphere, resulting in the most accurate and detailed methods of measuring ozone. However, the measurements are made only over localized regions and cannot provide a global picture of ozone distribution.
Balloons have been used almost as long as ground devices to measure ozone. They can measure the change in ozone concentration with altitude as high as 40 km and provide several days of continuous coverage.Many devices are used to measure ozone from balloons. These include:
Electrochemical Concentration Cells (ECCs), which measure current produced by chemical reactions with ozone. This method is most common. Photospectroscopy, which uses film or electronic sensors sensitive to UV light to measure wavelengths affected by ozone. Laser In Situ Sensors, which measure absorption of laser light projected from the balloon and reflected back to the sensor from a mirror slung beneath it.
Several instruments can be carried at once, so simultaneous measurements of many parameters can be conducted. Since balloons are unpowered, flight paths cannot be controlled.
Rockets measure profiles of ozone levels from the ground to an altitude of 75 km by using photospectroscopy. Spectra are recorded instantaneously at various altitudes on film, or continuously by photoelectric sensors. Ozone concentrations are calculated from the recorded intensities of UV light. Rockets provide all weather capability, but are limited by their short life and narrow geographic range.
Airplanes are used to make detailed measurements of ozone levels and related chemicals in the troposphere and lower stratosphere. Typical missions include 10 or more instruments capable of measuring ozone, chemicals related to the production and destruction of ozone, and atmospheric conditions that affect ozone. Airplanes are capable of studying chemical reactions and transport phenomenon which no other platform can study. In 1987 the Airborne Antarctic Ozone Experiment determined that the ozone hole over Antarctica, which had been discovered by satellite measurements, was caused by anthropogenic chlorine. Measurements from aircraft are restricted by concerns for pilot safety, range, and flight duration, and are not continuous. They are most useful for the detailed study of reaction and transport phenomenon in a small area.
Satellite MeasurementsSatellites measure ozone over the entire globe every day, providing comprehensive data. In orbit, satellites are capable of observing the atmosphere in all types of weather, and over the most remote regions on Earth. They are capable of measuring total ozone levels, ozone profiles, and elements of atmospheric chemistry. In the mid-1980s wide-ranging ozone depletion over the Antarctic was first recognized from satellite data.
Early Satellite Measurements
The first measurements of ozone involving satellites were made from the ground, using the Echo I satellite to reflect solar radiation to a ground-based instrument. Early satellite observations were conducted by the Orbiting Geophysical Observatory (OGO), the Defense Meteorological Satellite Program (DMSP) , Atmospheric Explorer - 5 (AE-5), and Nimbus 3,4, and 6.
Operational from 1978 - 1993, Nimbus-7 had the longest duration of any satellite mission to observe Earth. Three instruments on the satellite measured ozone: the Limb Infrared Monitor of the Stratosphere (LIMS), the Solar Backscatter Ultraviolet (SBUV), and the Total Ozone Mapping Spectrometer (TOMS).
LIMS was a sensor designed to measure ozone profiles in the atmosphere by scanning for IR radiation emitted by ozone. It employed limb scanning, a technique of observing the edge of the atmosphere that extends from Earth's surface. The scanner was capable of measuring ozone from the bottom of the stratosphere to the top of the atmosphere with a vertical resolution of 3km. Besides ozone, LIMS observed other parameters that influence ozone chemistry: water vapor, NO2, HNO3, and temperature. Unfortunately, the sensor required external cooling, and was deactivated after 7 months of observations after the coolant was depleted.
SBUV compared incident solar radiation to radiation backscattered from the atmosphere to determine ozone levels. 12 discrete wavelengths of UV radiation were collected by the sensor, and this information was used to calculate vertical ozone profiles and total ozone. UV radiation at different wavelengths penetrates deeper into the atmosphere, thus providing information on ozone at different atmospheric pressure levels. The instrument was limited to measuring ozone profiles above the ozone maximum, and total ozone directly beneath the satellite.
TOMS shared some components with SBUV, and mapped global total ozone daily through the same method. The instrument scanned back and forth beneath the satellite and detected 6 individual frequencies of UV light in a swath 51 degrees on either side of the satellite with a resolution ranging from 50 km beneath the satellite to 280 km at the extreme scan angle. The accuracy of TOMS is estimated to be ± 5%. After British and Japanese ground stations observed local ozone depletion in the early 1980s, TOMS data confirmed the existence of a continent-wide Antarctic ozone hole. The Nimbus-7 TOMS is unique because it provided a global map of ozone levels every day from November 1, 1978 until May 6, 1993.
The Applications Explorer Mission - 2 (AEM-2) was launched on February 1979 carrying the Stratospheric Aerosol and Gas Experiment (SAGE-1). SAGE-1 determined ozone by measuring the decrease in solar intensity caused by ozone absorption as the rays passed through the atmosphere to the spacecraft during sunrise and sunset, which were observed 30 times a day by the satellite. It also measured levels of NO2, which can contribute to ozone loss. Measurements continued until November, 1981.
The Solar Mesospheric Explorer (SME) was designed to study ozone levels and ozone chemistry in the mesosphere (50-80km). It was launched in 1981 and carried three instruments that measured ozone. The UV ozone experiment measured total mesospheric ozone by observing scattered UV light. Ozone profiles were measured by the Four-Channel Infrared Radiometer and the Airglow Instrument. The satellite was shut down in April, 1989.
SAGE II was carried on the Earth Radiation Budget Satellite (ERBS). It was launched in October 1984, and is still operational. SAGE II is a more sophisticated version of SAGE I, and data from the two instruments are often combined for long-term studies. It measures ozone loss in the upper stratosphere.
The NOAA Polar-Orbiting Operational Environmental Satellites (POES) are the primary meteorological satellites operated by the United States. At an given time, two NOAA-POES satellites are active, and each covers the globe daily. In addition to providing weather data, they are capable of measuring total ozone levels. These can be inferred from measurements from the suite of instruments that make up the TIROS Operational Vertical Sounder (TOVS). The primary mission of the TOVS is to measure atmospheric temperature and humidity profiles through microwave and IR measurements. TOVS includes a 9.6 micron channel which allows for both daytime and nighttime estimates of total ozone.
More accurate ozone data are obtained from the SBUV/2 which was first carried aboard NOAA-9 in December, 1984. It is similar to the SBUV sensor aboard Nimbus-7, but does not have a TOMS component. Through measurement of backscattered UV radiation, it is capable of measuring total ozone with an accuracy of ±1%, and ozone profiles accurate to ±5% in the upper stratospheric regions above the ozone maximum. NOAA-11 (launched September, 1988), NOAA-13 (launched August, 1993, power failure after 12 days in orbit), and NOAA-14 (launched December, 1994) also carry SBUV/2 instruments. Since the failure of the TOMS/2 aboard Meteor-3-5, on December 27, 1994, the NOAA satellites have provided the only source of global ozone data.
Space Shuttle Measurements
NASA's space shuttle periodically conducts ozone experiments using the Shuttle Solar Backscatter Utraviolet (SSBUV) instrument. First flown in October 1989, the SSBUV is carefully calibrated before launch, during flight, and after landing. This calibration, combined with coincident measurements with other instruments, allows ozone data to be correlated between the TOMS, the SBUV/2, and the Upper Atmosphere Research Satellite (UARS) measurements. Information from these experiments is used to develop improved calibrations for existing datasets. SSBUV experiments were also conducted in October 1990, August 1991, March 1992, April 1993, March 1994, November 1994, and January 1996. These frequent flights provide a continuous method of ensuring the accuracy of satellite ozone data.
In August 1991 a Russian Meteor-3 satellite carried the successor to the Nimbus-7 TOMS into orbit. Similar to the NOAA-POES series, the Meteors are weather satellites. Meteor-3 TOMS/2 was a refurbished engineering model of the original instrument and was carried in addition to the Meteor's standard complement of instruments. It extended the TOMS dataset until December 27, 1994, when an electrical problem caused the instrument to fail.
The Upper Atmosphere Research Satellite (UARS) is the first satellite of NASA's Mission to Planet Earth and was launched in September 1991. Although it was designed to last only three years, many instruments are still operational. The goal of UARS is to study chemical constituents, solar energy input, and physical and chemical processes in the stratosphere, mesosphere, and thermosphere. The cycle of ozone creation and destruction, ozone depletion, and ozone transport are a focus of the experiments. Four instruments carried aboard the satellite measure ozone: the Cryogenic Limb Array Etalon Spectrometer (CLAES), the Improved Stratospheric and Mesospheric Sounder (ISAMS), the Halogen Occultation Experiment (HALOE), and the Microwave Limb Sounder (MLS).
CLAES measured vertical profiles of ozone, CFCs, and other chemicals in the upper atmosphere. It took 20 measurements at 4 IR frequencies simultaneously at separate altitudes from 10 to 60 km. The vertical resolution was 2.5 km. The sensitivity of the instrument required it to be cooled to -257 degrees C, and coolant ran out (as expected) on May 5, 1995.
ISAMS scanned the atmospheric limb to investigate temperature, photochemistry, water vapor, and aerosols in the stratosphere and mesosphere. It featured on-board samples of the chemicals to be studied for calibration. In combination with HALOE data, ISAMS provided detailed information on the formation and destruction of the Antarctic ozone hole. In July 1992 ISAMS failed prematurely.
HALOE detects chemicals and aerosols by solar occultation, the scattering of sunlight by the atmosphere during sunrise or sunset (the same method used by the SAGE instruments). It has a field of view of 1.6km and is sensitive to O3, HCl, HF, and other chemicals, as well as temperature. Ozone levels are measured at altitudes from 10 to 90 km. Haloe observations are instrumental in determining the influence of anthropogenic chemicals on ozone depletion.
MLS measures microwave emissions from O3, ClO, and other chemicals, and provides atmospheric pressure measurements. ClO concentrations can be measured from 20 to 60 km, with a resolution of 4 km. Data from MLS show the correlation between ClO levels and ozone depletion, an important confirmation of theories connecting ozone loss with stratospheric chlorine.
Recent & Future Ozone Probes
Several new ozone observing instruments have been recently launched or are scheduled for launch in the near future. The European Remote Sensing Satellite - 2 (ERS-2) carries the Global Ozone Monitoring Experiment (GOME) and was launched in April, 1995. It measures stratospheric and tropospheric ozone from backscattered radiation. The Earth Probe Total Ozone Mapping Spectrometer (TOMS-EP) was launched July 2, 1996. It is a single instrument designed to continue the global ozone mapping carried out by the Nimbus-7 and Meteor-3 TOMS instruments. Another TOMS was launched in 1996 aboard the Japanese ADEOS satellite. The EOS Aura satellite carrying three ozone sensors Ozone Monitoring Senso (OMI), High Resolution Dynamics Limb Sounder (HIRDLS, and a successor to the Microwave Limb Sounder (MLS) was launched in 2004.
For full list of past, current and future Ozone sensors see, http://disc.sci.gsfc.nasa.gov/ozone/additional/mission