The United States satellite ozone measurement program for ozone (NASA and NOAA) has measured ozone distribution by season, latitude, and longitude, and has observed long-term changes over more than 20 years using a variety of satellite instruments. The instruments in use today will over the next five to ten years be replaced by a new generation of improved, more sophisticated instruments. These will be employed to continuously monitor the atmosphere from operational meteorological satellites in the United States.
We've already discussed some of these instruments in the context of their viewing geometry, such as the Total Ozone Mapping Spectrometer (TOMS), the Solar Backscatter Ultraviolet (SBUV) series, and the Stratospheric Aerosol and Gas Experiment (SAGE) series. Other scheduled future missions include the on-going Upper Atmospheric Research Satellite (UARS); the Earth Observing System-Chemistry (EOS-Chem) platform; the two separate SAGE III missions; the National Polar-orbiting Operational Environmental Satellite System (NPOESS), scheduled for launch in 2007; the European ENVISAT and EUMETSAT's Meteorological Operational (METOP) series missions, beginning in 2003; and the Japanese ADEOS-II mission; as well as future Space Shuttle missions. We will review each of these ozone monitoring missions in this section.
6.1.1 Timeline of Ozone Monitoring Satellite Missions -- Although atmospheric constituent measurements began in 1970, routine measurements did not begin until 1978 with the launch of the Nimbus-7 satellite. It carried both a TOMS and an SBUV instrument. Figure 7.03 illustrates a timeline of satellite atmospheric measurements since November 1978 when Nimbus-7 was launched. It runs through the present (1998), showing the various SBUV, SAGE, TOMS-like, and GOME instruments currently in use. The timeline is also projected into the future to 2010 in order to show the new generation of SBUV, SAGE, and TOMS instruments slated to be launched into orbit on future missions such as EOS-Chem, NPOESS, and ENVISAT-1.
6.1.2 UARS Measurements -- The Upper Atmospheric Research Satellite (UARS) was launched in 1991. The spacecraft has already exceeded its design life, and plans are now focused on continuing measurements for a long time period (a delicate task of balancing aging instrument and satellite problems). Figure 7.04 shows a schematic diagram of the UARS observatory, including the various instrumentation it carries.
The remote sensing instruments are attached to the satellite such that their viewing axes are not obscured by solar panels or telemetry antennas. The Halogen Occultation Experiment (HALOE) continues to provide profile measurements of ozone and other important gases, notably the halogen reservoir species hydrogen chloride (HCl) and hydrogen fluoride (HF). The Microwave Limb Sounder (MLS) provides stratospheric ClO, HNO3, and O3, as well as upper tropospheric H2O. The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) and Solar Stellar Irradiance Comparison Experiment (SOLSTICE) provide spectrally resolved UV radiation, and the Active Cavity Radiometer Irradiance Monitor (ACRIM) measures total solar irradiance.
6.1.3 Space Shuttle Measurements -- NASA is making limited use of the Space Shuttle for atmospheric chemistry measurements. The Shuttle Solar Backscatter Ultraviolet (SSBUV) instrument flew eight times on the Shuttle to provide ozone calibration data for the SBUV/2, TOMS, and UARS ozone instruments. NASA sponsored the German Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere-Shuttle Pallet Satellite (CRISTA-SPAS) payload launched in August 1997. CRISTA-SPAS provided unique data in several areas, including small-scale dynamical variability in trace constituent composition important in ozone chemistry. Also launched was the Middle Atmosphere High Resolution Spectrographic Investigation (MAHRSI) instrument. It measures the amount of hydroxyl radicals in the upper and middle stratosphere. The Space Shuttle is also being used as a platform to demonstrate advance techniques, such as limb scattering for observing ozone and atmospheric chemistry from space.
6.1.4 EOS-Chem Measurements -- The chemistry platform of the Earth Observing System (EOS-Chem), to be launched in December 2002, will have four complementary instruments which utilize different wavelength regions and viewing techniques to measure ozone and related atmospheric constituents. They will focus on the stratosphere and upper troposphere. We've already discussed in Section 4.1.3 (C), the High Resolution Dynamics Limb Sounder (HiRDLS) and the Tropospheric Emission Spectrometer (TES). The HiRDLS is an infrared emission spectrometer which will also measure small-scale dynamical variability in trace constituent distributions. The TES will measure profiles of ozone and several other important constituents in the troposphere, and will also measure a number of species in the lower stratosphere. The improved Microwave Limb Sounder (MLS) aboard the EOS-Chem will measure concentrations of many radical and reservoir species. The Ozone Monitoring Instrument (OMI) is a Dutch designed, next generation, TOMS and SBUV type of instrument that will provide daily maps and profiles of total ozone, aerosols, UV-B radiation, and other atmospheric constituents.
The HiRDLS, MLS, and TES instruments on-board the EOS-Chem satellite all use the limb emission viewing technique to make measurements of thermal radiation, though the TES also has nadir viewing capabilities. Figure 7.05 illustrates the constituents that will be measured by these three instruments onboard EOS-Chem. The importance of these constituents to ozone concentrations was discussed in Chapter 5.
6.1.5 SAGE III Measurements -- Using both a solar and a lunar occultation viewing technique, the Stratospheric Aerosol and Gas Experiment (SAGE) III instrument will use both a solar and a lunar occultation viewing technique, to provide vertical profiles of ozone, nitrogen dioxide, and water vapor, as well as aerosols in the upper troposphere, stratosphere, and mesosphere. Measurements will be in the UV, visible, and infrared regions of the spectrum.
SAGE III is slated for flight aboard a Russian Meteor-3M spacecraft in 1999 and the International Space Station in 2002. These are in complementary orbits. The sun-synchronous polar orbit of Meteor-3M will allow the SAGE III instrument to make measurements at high latitudes using the solar occultation technique. The inclined orbit (51.6°) of the Space Station will provide solar occultation coverage from the tropics to mid-latitudes. Lunar occultations will cover a broad range of latitudes, and will provide the opportunity for measurements of NO3 and OClO, which are only present in significant amounts at nighttime due to their rapid photolysis during the day.
6.1.6 NPOESS Measurements -- NASA is closely cooperating with NOAA on the development of ozone-measuring instruments for the National Polar-Orbiting Operational Environmental Satellite System (NPOESS) in which Department of Defense, NOAA, and NASA have been directed to combine their operational measurements into a single satellite system. The major need in this area is to find a replacement for the NOAA-14 SBUV/2 instrument, which is unable to measure profiles with sufficient resolution and to measure the ozone profile below the peak in the ozone layer. The replacement instrument (or instruments) will have capability for both total ozone and ozone vertical profile measurements, exceeding the capabilities of the SBUV/2.
6.1.7 ENVISAT-1 Measurements -- The European Space Agency's ENVISAT-1, an advanced polar-orbiting Earth observation satellite, is scheduled for launch in May 2000. It will contain several instruments, including SCIAMACHY, GOMOS, and MIPAS designed to measure atmospheric constituents such as ozone. Refer back to Section 4.1.4 for a discussion of the SCIAMACHY instrument. The GOMOS instrument is discussed in Section 4.1.2 (C). The MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is a Fourier transform spectrometer for measurement of high resolution emission spectra of trace gas species at the Earth's limb. It operates in the near- and midinfrared range.
6.1.8 EUMETSAT Measurements -- Starting in early 2003, The European Space Agency and the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) will launch a series of polar orbiting satellites. They are designed to replace the NOAA "morning meteorological satellites" that cross the equator on a descending orbit before noon local time. The first two Meteorological Operational (METOP) satellites are slated to carry new Global Ozone Monitoring Experiment (GOME) instruments. As noted in Section 4.1.1 (iv), a GOME instrument is now flying on the European Remote Sensing-2 (ERS-2) satellite.
6.1.9 ADEOS-II Measurements -- In early 1999, National Space Development Agency of Japan (NASDA) will launch ADEOS-II (short for Advanced Earth Observing Satellite-II). It will carry the Improved Limb Atmospheric Spectrometer-II (ILAS-II) to monitor high-latitude stratospheric ozone using solar occultation. ILAS-II will also measure vertical profiles of species related to ozone depletion such as nitrogen dioxide, nitrous oxide, methane, chlorine nitrate, and chlorofluorocarbons. The first ADEOS, which carried an array of instruments, including an ILAS and a TOMS instrument, malfunctioned after less than a year because of solar array failure.
High flying balloons provide an important tool for probing the atmosphere. Figure 7.06 shows a picture of an upper atmospheric research balloon being launched.
Such balloon launches form an essential part of high altitude atmospheric research. There are three major advantages of the balloon program involving (1) the broad range of altitudes they can reach for either in-situ or remote sensing measurements; (2) the additional, correlative data source they represent beyond satellite measurements; and (3) the testing platform for new equipment that they provide.
6.2.1 Extensive Altitude Range -- The first major advantage of the balloon program for atmospheric research is the extensive altitude range they can cover. Balloons provide a unique way of covering a broad range of altitudes for in-situ or remote sensing measurements in the stratosphere. Of particular interest is the 22-40 km region, which is higher than the altitude range of current aircraft such as the ER-2. In some cases, a combination of instruments on a single balloon gondola can provide a complete set of measurements. Balloons filled with 7500 cubic meters of helium can carry a 50 kilogram payload to an altitude of 40 kilometers. These measurements can be utilized together with process models to carry out detailed tests of photochemical theory. Data gathered by balloon platforms can also be used together with data gathered by aircraft platforms. Measurements from the Observation of the Middle Stratosphere (OMS) experiment, carried on a balloon platform, were made in association with the Stratospheric Tracers of Atmospheric Transport (STRAT) study, which was carried out on ER-2 aircraft platforms.
6.2.2 Additional, Correlative Data Source -- The second advantage is that balloon instruments provide the opportunity for additional, correlative data for satellite based measurements, including both validation ("atmospheric truth") and complementary data (for example, measurement of species not measured from the space based instrument). Balloon instruments formed a critical part of the correlative measurements program for the Upper Atmosphere Research Satellite (UARS), and current plans are to use some of NASA's balloon instruments in support of the validation program for SAGE III, EOS-Chem, and the European Space Agency ENVISAT-1.
6.2.3 Venue For Testing -- The third and final application of balloon based platforms is that they constitute an important and inexpensive venue for testing instruments under development. These can be either potential instruments for unmanned aerial vehicles (UAV) or, in some cases, for satellite based remote sensing instruments.
Aircraft observations have made major contributions to our understanding of the stratosphere. NASA has two primary aircraft for these observations; the high altitude ER-2 (NASA's version of the famous U-2 spy plane), and a long range DC-8 (a four engine airliner converted to science use). The ER- 2 can reach altitudes over 20 km (~70,000 ft.), while the DC-8 has a range of about 10,000 km. The aircraft carry both in-situ and remote sensing (active and passive) instruments.
6.3.1 Specific Aircraft Missions -- These aircraft have been employed on a number of missions devoted to investigating specific aspects of the stratosphere over the last decade. A partial list of the missions is given below.
These missions have investigated a variety to topics related to stratospheric and tropospheric ozone. STEP and STRAT examined how material is transported between the stratosphere and troposphere, both into the tropical stratosphere and out of the extratropical stratosphere. The AAOE and ASHOE/MAESA missions studied the Antarctic ozone hole, in the context of the radiation, dynamics, and photochemical evolution of the southern hemisphere. AASE I and II, TOTE/VOTE, SPADE, and POLARIS investigated Arctic and northern midlatitude ozone losses. The SONEX mission investigated the impact of aircraft on ozone levels in the upper troposphere.
6.3.2 ER-2 Payload -- The ER-2 primarily carries a large payload of in-situ instruments that measure an extensive spectrum of radical trace gases, such as NOx, HOx, ClO, BrO, and other longer lived gases, such as ozone, methane, nitrous oxide, sulfur hexafluoride, and the chlorofluorocarbons. These instruments are state-of-the-art science packages that operate autonomously during flight with minimal actions by the ER-2 pilot. Each instrument is maintained by a separate team of experimentalists. During the 1997 POLARIS mission, 19 instruments were aboard the ER-2. This set of instruments provided a complete set of observations for assessing the chemical makeup of the stratosphere. Measurements of selected gases within the important families of stratospheric gases (such as nitrogen, hydrogen, chlorine, bromine, and oxygen) provide an accurate assessment of chemical species in the stratosphere. The set of observations taken during POLARIS revealed that the summer polar stratospheric ozone levels were dominated by catalytic loss due to nitrogen radicals (see Chapter 5). Making such determinations is critically dependent on in-situ field observations, theoretical models, and laboratory measurements.
6.3.3 DC-8 Payload -- The DC-8 also carries a large payload of instruments that can make observations of the stratosphere. In contrast to the ER-2, which principally carries an in-situ payload, the DC-8 carries both remote sounding instruments and in-situ instruments. In the recent TOTE/VOTE experiment, the DC-8 used lidar instruments to obtain observations of temperature, ozone, aerosols, water, and methane across the polar vortex with flights between Alaska and Iceland. These remote observations provide a very detailed picture of stratospheric ozone levels that is simply unavailable from satellite or ground observations.
6.3.4 Using Aircraft and Satellite Data To Complement Each Other -- The aircraft and satellite data sets complement one another. Aircraft data are obtained from highly calibrated instruments at a very high precision, accuracy, and spatial resolution. In principle, such observations are superior for understanding the stratosphere. But aircraft observations provide only snapshot pictures of the stratosphere. It is not possible to have such high quality, global data on a continuous basis. Though satellite data have coarser resolution, they can provide long term, global observations (depending on the orbital characteristics of the satellite).
Aircraft observations are used to validate satellite measurements. For example, multiyear TOMS total ozone satellite observations have shown that total ozone levels have decreased in the northern polar region during the summer. During the POLARIS campaign, ER-2 observations confirmed this decrease and demonstrated that the decrease resulted from NOx catalytic destruction of ozone. For their part, satellite measurements extend observations to times and regions that are unobservable by aircraft. An example is the ER-2 observations of the Antarctic ozone hole in 1987 during the AAOE mission. This mission demonstrated that the ozone hole resulted from chlorine radicals that had been activated on polar stratospheric clouds. Satellite measurements have since been able to observe the development of the ozone hole each year at the same time of the year. Both of these examples illustrate that while aircraft can only make snap shot of the stratosphere, they are necessary to understand the overall conditions monitored by satellites.
Ground based measurements of atmospheric trace constituents depend on the use of a global network of high quality research instruments. Such instruments help determine variability and long-term trends in these trace gases, including ozone. The two major efforts in this area are (1) the Advanced Global Atmospheric Gases Experiment (AGAGE), formerly known as the Atmospheric Lifetime Experiment - Global Atmospheric Gases Experiment (ALE-GAGE), and (2) the Network for the Detection of Stratospheric Change (NDSC). AGAGE provides critical monitoring of the surface concentrations of both manmade and naturally produced trace gases, while NDSC monitors the chemical composition of the stratosphere. The network of AGAGE and NDSC stations are shown in Figure 7.07.
Figure 7.07 includes AGAGE ground measurement sites and NDSC primary and complementary sites. The period of operation for Irish stations was 1978-1983 for Adrigole and since 1987 for Mace Head. The Cape Meares, Oregon station operated only from 1980 to 1989.
6.4.1 AGAGE Monitoring Network -- The AGAGE monitoring network uses gas chromatography coupled with mass spectrometry (two types of in-situ techniques) to measure the concentration of natural and manmade (anthropogenic) atmospheric gases at Earth's surface.
The original emphasis in AGAGE was on halogen containing species (principally chlorinated species), though nitrous oxide and methane were also measured. This has been expanded to include bromine-containing species like halons and methyl bromide, as well as hydrochlorofluorocarbons (HCFCs). AGAGE investigators have a significant modeling program to ensure that their full global data set is used. AGAGE sites are included in Figure 7.07.
6.4.2 NDSC Monitoring Network -- NDSC is an internationally directed and sponsored program with major support from NASA, NOAA, and foreign partners, in which a number of remote-sensing instruments meeting mutually agreed upon standards have been deployed at sites around the world. Major instrument categories deployed at the NDSC's "primary sites" include Fourier transform interferometers, millimeter wave emission spectrometers, visible and UV absorption spectrometers, and lidars. Together these instruments provide measurements of column amounts or profiles of atmospherically important species. In addition to the primary NDSC sites, complementary sites involving a less complete set of instruments are also participating. Primary NDSC sites and a summary of complementary sites are also given in Figure 7.07.
Since consistency of calibration, both among various instruments used and over long time periods, is a hallmark of the NDSC program, there will be a continuing program of instrument comparisons through laboratory and field efforts. NDSC has a program for related modeling investigations which ensures that the data it gathers will be fully utilized. NDSC instruments have also been involved in validation measurements for space based instruments, and provide measurements in support of aircraft campaigns.