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Earth Observations from Space: The First 50 Years of Scientific Achievements
5
Atmospheric Composition: Ozone Depletion and Global Pollution
Although on average there are only four molecules of ozone for every 10 million molecules of air, it is central to atmospheric composition for several reasons. Stratospheric ozone absorbs ultraviolet (UV) radiation; thus, it shields the lower atmosphere and Earth’s surface from UV radiation that is harmful to living organisms. This absorption of UV radiation warms the stratosphere and plays a major role in establishing the temperature structure of the atmosphere (Box 5.1), while its infrared (IR) absorption and emission are also important in Earth’s energy balance. No less important are the chemical and photochemical reactions of ozone with other species, which regulate the trace gas structure of the stratosphere and troposphere. Given ozone’s importance to the atmospheric composition and temperature structure and as Earth’s UV shield, it is critical to measure its global distribution; understand its trends and the mechanisms that control its distribution; and understand its interactions with atmospheric chemistry, dynamics, and the climate system.
A delicate balance of photochemical reactions among oxygen (molecular and atomic), nitrogen oxides, hydrogen oxides, and halogenated oxides maintains the “ozone layer” in the stratosphere, where 90 percent of the total ozone column resides. A rapid increase in anthropogenic halogen-containing gases (collectively called “halocarbons,” which includes chlorofluorocarbons [CFCs]) over the past 50 years resulted in a huge perturbation to the natural stratospheric ozone balance.
Satellite-generated data have made a vital contribution to understanding the threat of stratospheric ozone depletion, observing stratospheric dynamics, and determining the cause for the Antarctic ozone hole. Satellite observations provided the first global measurements of stratospheric ozone and temperatures, expanding and revolutionizing our understanding of the atmosphere above the tropopause. This knowledge confirmed the dangers associated with the release of anthropogenic CFCs and other halocarbons and helped shape international policies to minimize their use and release.
In contrast to the stratosphere, the elevated ozone levels in the troposphere are problematic. The gas-phase composition of the troposphere is very complex, bound by the tropopause above and the ocean and land surface below (Figure 5.1). In tropical and midlatitudes especially, the ocean and land surfaces are active sources of trace gases that are linked through physical and chemical transformations to other tropospheric species. Tropospheric ozone together with other radicals (most importantly hydroxide) contributes to the oxidative capacity. This oxidative capacity is important to the removal of the most reactive air pollutants, which cleanses the atmosphere. Similar to its stratospheric counterpart, tropospheric ozone levels are strongly influenced by photochemical reactions involving nitrogen oxides and hydrogen oxides and by ozone inputs from the stratosphere. In the presence of elevated nitrogen oxides from localized sources of pollution, hydrocarbons—even the ubiquitous methane and carbon monoxide—can perturb the natural ozone balance toward unhealthy levels. Ozone’s powerful oxidizing capacity threatens human health, agricultural productivity, and natural ecosystems. In addition, ozone in the free troposphere is a powerful greenhouse gas. In contrast to the stratosphere, halogen chemistry is relatively benign in the troposphere, with a noteworthy exception in polar regions.
Stratospheric ozone studies have benefited greatly from a long history of space observations. These investigations represent one of the best-known examples of satellite “successes” in Earth observations. The role that satellites have played in tropospheric ozone is more complex but no less important. As this chapter documents, satellites discovered ozone “pollution” in the remote tropics 20 years ago. Follow-on exploration with newer satellites and ground-based and aircraft instrumentation has shown that climate dynamics in the tropics and stratospheric forcing can be as significant as photochemical reactions. Today, a new generation of satellite instrumentation, described later in this chapter, is mapping tropospheric ozone globally along with its photochemi-
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Earth Observations from Space: The First 50 Years of Scientific Achievements
BOX 5.1
Atmospheric Structure
The atmosphere is often divided into layers to differentiate regions with different characteristics. The lowest layer is called the troposphere, from Greek words indicating a region of overturning. In this region the temperature generally decreases with altitude to a height called the tropopause. Above this altitude lies the stratosphere, where the temperature remains constant or increases with altitude up to the stratopause, at about 50 km altitude. The temperature again decreases in the overlying mesosphere up to the mesopause (~80 km), above which it rises in the thermosphere under the influence of solar radiation. The temperature structure in the stratosphere tends to suppress vertical motions, leading to more horizontal winds.
cal relatives (nitrogen dioxide, bromide, carbon monoxide, formaldehyde).
UNDERSTANDING AND REMOVING THE THREAT OF STRATOSPHERIC OZONE DEPLETION
Until about 1964 it was thought that the Chapman (1930) scheme, based only on forms of oxygen, could explain the stratospheric abundance of ozone (Wayne 1985). Subsequently, improved laboratory measurements of reaction rate coefficients showed that this approach overestimated the amount of stratospheric ozone by a factor of 2. Further laboratory measurements showed that reactions involving compounds of hydrogen, chlorine, and bromine from natural sources could enter into catalytic cycles that would speed up the rate of ozone destruction, decreasing estimates of ozone amounts and bringing them into better agreement with ground-based observations.
A very alarming fact was that two of these gases had large and potentially rapidly increasing anthropogenic sources. A projected fleet of 500 commercial supersonic airplanes flying many hours each day was expected to inject large amounts of nitrogen oxides into the lower stratosphere, with deleterious effects (Crutzen 1970, Johnston 1971). Ultimately this fleet did not materialize. However, the techniques and models developed to address the former problem were ready to be applied to the next threat to the ozone layer: chlorine, which was being released in significant amounts by the photolysis of CFCs in the stratosphere. The chlorine released from CFCs was also predicted to cause a serious reduction in ozone (Cicerone et al. 1974, Molina and Rowland 1974).
FIGURE 5.1 Schematic of biogeochemical cycling with human contributions included, illustrating the major gas-phase constituents in the lower atmosphere that are measured from space (shaded in gray). NO2 and NO are encircled to represent equilibrium; their sum is referred to as NOx. NOTE: BrO = bromine monoxide; CH4 = methane; CO = carbon monoxide; h = Planck’s constant; H2O = water; HCHO = formaldehyde; HO2 = hydroperoxyl; v = photon frequency; NO = nitric oxide; NO2 = nitrogen dioxide; O(1D) = electronically excited oxygen atoms; O2 = molecular oxygen; O3 = ozone; OH = hydroxide; RH = hydrocarbon species; SO2 = sulfur dioxide. SOURCE: Drawing by A.M. Thompson and K.M. Dougherty, Pennsylvania State University.
These gases would reduce the amounts of stratospheric ozone below the natural background level, letting more UV radiation reach the surface, causing increased incidence of human skin cancer as well as damage to other biological processes. Subsequent studies showed that bromine, which has natural and anthropogenic sources, could also cause significant ozone depletion (Wofsy et al. 1975, Yung et al. 1980). Because of the dire nature of these predictions, it was crucial to develop a better understanding of this region of the atmosphere as quickly as possible.
OBSERVING STRATOSPHERIC DYNAMICS
To predict the ozone distribution and its changes in the stratosphere, it is also necessary to understand atmospheric motions. These are closely linked to radiative heating and cooling, which depends on the atmospheric composition, notably the ozone distribution (Craig 1965). Understanding this interacting system of chemistry dynamics, and radiation requires global observations, unavailable from ground-based measurements, as well as the synergistic use of models to incorporate this information and allow accurate and trustworthy predictions to be made.
The stratosphere was first identified in 1899, when balloonborne measurements showed that the atmospheric
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temperature did not continue to decrease with altitude but became constant or even increased above a height termed the tropopause. By the end of World War II, data from operational weather balloons, which could in the best case reach altitudes of 30 km, provided a picture of stratospheric temperatures and winds up to this altitude but with low spatial and temporal resolution. After World War II, rocket soundings at about a dozen locations extended these measurements to 65 km and occasionally higher. These data were enough to delineate the global variation of the vertical temperature and wind distribution but yielded little information on features with smaller temporal or horizontal scales and provided little information on the southern hemisphere’s atmosphere. Only the roughest picture of motions at the longest horizontal scales was available, and many studies were constrained by data availability to limited altitude and geographic regions. Craig (1965) presents a good discussion of knowledge and speculation at that time.
Similarly, a limited number of ground-based instruments in the pre-satellite era, mainly UV spectrophotometers, could provide only a rudimentary view of the vertical, latitudinal, and seasonal variations of the ozone distribution, with large uncertainties (Goody 1954). One particular puzzle was the nature of the atmospheric motions that transport ozone from high altitudes in the tropics, where it is produced by solar UV radiation and atmospheric chemistry, to low altitudes in polar regions and midlatitudes, where processes destroying it dominate (Craig 1965).
It was recognized that the transport of ozone and other gases, as well as heat and momentum, was important. Initially, such questions were addressed in terms of conventional fluid dynamics (Hunt and Manabe 1968). However, these early estimates were so uncertain that it was often impossible even to determine whether the ozone transport was northward or southward (NRC 1979).
The understanding of atmospheric dynamics was revolutionized beginning in 1969, when satellite instruments were launched to measure temperature and ozone in the stratosphere. The first downward- or nadir-looking temperature sounders on Nimbus 3 (Box 5.2) demonstrated that remote sounding techniques could provide global observations of atmospheric temperatures from the surface to mesospheric altitudes. Although the soundings from these and subsequent nadir-looking instruments had low vertical resolution, they were sufficient to allow the heights of atmospheric pressure surfaces to be calculated. The balance between the slopes of these heights and Earth’s rotation allowed scientists to make accurate calculations of stratospheric winds (Smith and Bailey 1985). These winds could be separated into winds in the east-west direction (along parallels of latitude) and into north-south wavelike perturbations. A review of early results clearly showed that the stratosphere and mesosphere were dynamically very active, with large (planetary)-scale waves propagating from the troposphere into the stratosphere and mesosphere in the winter hemisphere (Hirota and Barnett 1977).
The advent of horizon-viewing sounders provided temperature measurements with higher vertical resolution. These more detailed pictures revealed a wider range of atmospheric motions, including waves in the tropics with short vertical wavelengths, and provided more detail on planetary wave activities. The accurate and densely spaced measurements of temperature and the derived estimates of wind speed made it possible to study global transport in more detail. To avoid the uncertainties associated with conventional fluid dynamics, theoreticians recast their equations to essentially follow air parcels, leading ultimately to simple but accurate approximations. These showed how planetary waves, propagating up from the troposphere, could interact with the eastward wind motions, and thereby change the mean vertical and poleward circulation (Matsuno 1971, Andrews and McIntyre 1976). This explained the phenomenon of “sudden stratospheric warming,” in which temperatures in the polar stratosphere at an altitude of 30 km can increase by 30°C or more in a few days.
One particular scientific achievement should be noted. Brewer (1949) and Dobson (1956) had independently postulated a mean north-south overturning circulation in the stratosphere, in which air rises from the troposphere into the stratosphere in the tropics and then travels to high latitudes (in both hemispheres) where it returns to the troposphere. Observations of distributions of methane, nitrous oxide, ozone, and water vapor (all from the limb sounders) were used to test and validate these (then-novel) ideas and theoretical approaches to the calculation of net transport of these gases (Andrews et al. 1987). A related triumph was the observation of the tropical “tape recorder” (Mote et al. 1996), which significantly advanced and confirmed scientific understanding of stratospheric dynamics and motions (Box 5.3).
Remote sounding also provided information on the composition of the stratosphere. The first instruments to shed light on the global distribution of stratospheric ozone were the backscattered ultraviolet (BUV) on Nimbus 4 and the solar backscattered ultraviolet (SBUV) and the Total Ozone Mapping Spectrometer (TOMS) on Nimbus 7, which provided global measurements of the total ozone in a vertical column. In addition, the Limb Radiance Inversion Radiometer (LRIR), the Limb Infrared Monitor of the Stratosphere (LIMS), and the Stratospheric and Mesospheric Sounder (SAMS) on Nimbus 6 and 7 retrieved the distributions of ozone, water vapor, nitrogen dioxide, nitric acid, nitrous oxide, and methane. The power of these measurements is shown by the observations of nitrogen dioxide and nitric acid, present in concentrations on the order of only 10 parts per billion by volume.
DETERMINING THE CAUSES OF ANTARCTIC OZONE DEPLETION
The advances in our understanding of the cause and dynamics of the Antarctic ozone hole exemplify the productive interactions of satellite observations with in situ and ground-based observations and numerical models. As
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Earth Observations from Space: The First 50 Years of Scientific Achievements
BOX 5.2
Remote Sensing of the Stratospherea
Earth and its atmosphere emit infrared (or heat) and microwave radiation to space, which can only be detected from Earth-orbiting sensors. The emerging signal at wavelengths corresponding to the absorbing bands of atmospheric gases will depend on the vertical distribution of the gas, the strength of its absorption, and the temperature at a level where the chance of a photon escaping to space is about 37 percent. Because the fraction of carbon dioxide with altitude is known and nearly constant in the atmosphere, Louis Kaplan (1959) suggested that the vertical temperature profile could be retrieved by measuring the radiation emerging from the atmosphere as a function of wavelength in the 15-μm bands of carbon dioxide. Satellite Infrared Spectrometer (SIRS-A, a filter radiometer) and the Infrared Interferometer Spectrometer (IRIS-A) on Nimbus 3 (launched in 1969) were the first two instruments to demonstrate that remote sounding techniques could provide global observations of atmospheric temperatures from the surface into the stratosphere. Improvements of these instruments were accompanied on Nimbus 4 (1970) by the Selective Chopper Radiometer (SCR), which filtered the radiance through cells of carbon dioxide and permitted temperatures into the mesosphere to be determined.
Soundings from these and subsequent downward-looking (sometimes referred to as nadir-viewing) instruments had low vertical resolution. By looking at the horizon or limb with a narrow field of view, much higher vertical resolution (2-4 km) can be achieved (Gille and House 1971). The first instrument for infrared measurements was the Limb Radiance Inversion Radiometer (LRIR) on Nimbus 6 (1975). The Limb Infrared Monitor of the Stratosphere (LIMS) and the Stratospheric and Mesospheric Sounder (SAMS) followed on Nimbus 7 (1978). An additional advantage of this technique is that it allows measurement of trace gases, from cloud tops into the mesosphere, also with high vertical resolution. In 1991 the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) exploited the limb sounding geometry to measure additional species that emit measurable signals in the millimeter-wavelength part of the microwave spectrum.
Some of the sunlight striking the atmosphere is scattered back toward space and absorbed by atmospheric gases in its path. The spectral distribution of this backscattered radiation is affected by the amounts and vertical distributions of the absorbing gases. The nadir-viewing backscattered ultraviolet (BUV) instrument on Nimbus 4 was the first orbiting instrument to make use of these principles to determine the ozone distribution as a function of altitude and the total amount in a vertical column. The solar backscattered ultraviolet (SBUV) and the Total Ozone Mapping Spectrometer (TOMS) instruments were first flown on Nimbus 7. TOMS, by scanning from side to side, provided maps with complete global coverage of the total column amounts of ozone. Although designed for a year’s operation, it lasted from 1978 until 1993, providing an excellent and consistent long-term data record of ozone columns.
Occultation measurements are another method for sounding the atmosphere. In this case the instrument measures light from the Sun as it sets behind the atmosphere as seen from the satellite. Trace gases absorb the radiation as it passes through the atmosphere, and aerosols scatter sunlight from the beam. The vertical distributions of these gases and aerosols can be determined by measuring the decrease of sunlight as a function of altitude in spectral bands where atmospheric gases absorb. The second such experiment was the Stratospheric Aerosol and Gas Experiment. The Halogen Occultation Experiment (HALOE) on UARS was designed to measure trace gases that were otherwise difficult to observe.
aDescribed by Houghton et al. (1984).
satellite observations were clarifying the structure, dynamics, and composition of the stratosphere, modeling activities were advancing rapidly. After about a decade of evolution, these models were predicting relatively modest but still important decreases in ozone concentrations, centered near 40 km altitude (Wuebbles et al. 1983). Later data confirmed these expectations (Solomon 1999).
However, in 1984 the world was startled by the discovery of a much larger than predicted ozone decrease over Antarctica at a much lower altitude, near 20 km in the lower stratosphere (Farman et al. 1985). This feature quickly became known as the Antarctic ozone hole. This unexpected phenomenon called into question the theoretical understand-
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BOX 5.3
Discovery of the Tropical Tape Recorder
The discovery of the so-called tape recorder (Mote et al. 1996) represents a remarkable scientific achievement in understanding stratospheric dynamics and motions. The temperature of the tropical tropopause controls the fraction of water vapor in the air at the tropopause, near 16 km. Colder temperatures during a northern hemisphere winter “freeze dry” the air to a greater extent than the warmer temperatures later in the year. Figure 5.2, based on data from the Microwave Limb Sounder (MLS) on Aura, shows periods of dry air (negative departures from the mean) alternating with periods of more moist air (positive departures) at each level. These create alternating bands sloping upward, confirming the rising motion in the tropics. A closer look indicates that the slope of the bands, proportional to the upward velocity, varies with season. This varies with the convergence of wave activity in the upper stratosphere, as expected from theory. The tropopause thus acts like a recording head, with the temperatures “recording” the time-varying water vapor amounts on the air.
FIGURE 5.2 Time series of zonal mean water vapor profile measurements by the Microwave Limb Sounder on the Aura satellite. The colors represent a percentage change relative to the 15° S-15° N mean at each pressure level. The upward progression with time above the 140-hPa level (~14 km altitude) shows the vertical motion consistent with theoretical predictions. SOURCE: Figure courtesy of Jonathan Jiang, National Aeronautics and Space Administration, Jet Propulsion Laboratory.
ing of the mechanisms of ozone destruction and the model projections.
Satellite measurements played two important roles in unraveling the questions of ozone depletion:
measurements of trace species that lead to or catalyze ozone destruction contributed to confirming the causes of the depletion
measurements of stratospheric ozone concentrations and distribution, and their changes over time, enabled comparison to model predictions.
Theory predicted that the tropical upwelling—discussed in Box 5.2—carried CFCs and other halocarbons from the tropospheric source into the stratosphere, where they were present at only a few molecules per 10 billion atmospheric molecules. In the stratosphere, solar UV breaks CFCs apart, releasing chlorine molecules that react to produce relatively inert hydrochloric acid (HCl). Laboratory investigations showed that HCl could react on the surface of polar stratospheric clouds (discovered by the Stratospheric Aerosol and Gas Experiment [SAGE] measurements; McCormick et al. 1982) releasing approximately 1 part per billion of chlorine
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monoxide (ClO). ClO effectively catalyzes ozone destruction in the presence of sunlight.
Images from the second generation of infrared limb sounders confirmed the transport of these extremely small amounts of CFCs into the stratosphere and also the presence of the predicted compound chlorine nitrate (ClONO2; Nightengale et al. 1996, Mergenthaler et al. 1996). In addition, Halogen Occultation Experiment (HALOE) measurements demonstrated the amounts and distribution of HCl (Russell et al. 1996). The picture was complete when sensitive microwave measurements confirmed measurements of earlier ER-2 flights of the direct anticorrelation of ClO. MLS added information on the global extent of the presence of high concentrations of ClO in the high-latitude lower stratosphere in spring, where the ozone hole formed (Figure 5.3; Waters et al. 1993). Thus, the ozone values had decreased at the same time and locations where high values of ClO occurred over Antarctica in the southern spring.
More recently, spaceborne measurements of bromine oxide (BrO) have been made by instruments measuring reflected UV-visible radiation (McElroy et al. 1986, Tung et al. 1986, Sinnhuber et al. 2005) and microwave emissions (Livesey et al. 2006). BrO is even more effective in the photolytical destruction of ozone; 50 times more effective than ClO on a molecule per molecule basis. The satellite data are broadly consistent with current understanding of bromine chemistry, indicating that at the observed concentrations BrO plays a significant role in the budget of lower-stratospheric ozone. Taken together, these and related data on other species confirmed the chemistry in the coupled models of the stratosphere, greatly improving their utility and trustworthiness as tools to guide policymakers.
Although satellite instruments did not discover the severely disturbed ozone conditions in southern polar regions, satellite observations from the BUV series of instruments provided unique detailed maps of the Antarctic ozone hole (Figure 5.4). The monthly mean ozone column over Antarctica provides information on the evolution of the Antarctic ozone hole from the first measurements in 1970 until 2005. These maps allowed tracking of its size and depth every year, providing the most extensive information on its annual growth, extent, and decay crucial to ozone assessments and to the amendments to the Montreal Protocol (WMO 2006). In the first measurements a crescent of higher ozone can be observed, generally centered south of Australia, with a lower amount over Antarctica itself. With the passage of time, the
FIGURE 5.3 Chlorine monoxide (ClO; left panel) and stratospheric ozone (O3; right panel) columns over the southern hemisphere measured by the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) for days during the austral springs of 1991 and 1992. These images show that high ClO concentrations coincide in space and time with low O3 concentrations confirming ground-based measurements and the proposed mechanisms for ozone depletion. The white circle over the pole indicates area where no data is available. SOURCE: Waters et al. (1993). Reprinted with permission from Macmillian Publishers Ltd., copyright 1993.
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FIGURE 5.4 October monthly mean total ozone column over the southern hemisphere for 8 selected years between 1970 and 2005. These show large interannual variations, with the hole generally becoming larger and deeper until recent years. SOURCE: Data provided by R. McPeters, NASA GSFC; modified by John Gille.
amount over Antarctica decreased sharply. The lowest ozone values were observed in 1995, with a slight increase since then. Analysis of the area and extent of the ozone hole for the years 2005 and 2006 compared to the mean from 1979 to 2005 indicates that the extent of ozone depletion over Antarctica is greater during austral spring in the most recent years compared to the mean (Figure 5.5). The maximum extent is usually reached near the end of September.
OZONE DEPLETION OVER THE NORTHERN HEMISPHERE
Although the Antarctic ozone hole is the better-known phenomenon that has illuminated and confirmed the theory of halogen-catalyzed chemical ozone destruction, a more important question from a societal point of view is the effect of anthropogenic chlorine, bromine, and other gases on the ozone concentrations over heavily populated nonpolar latitudes. Because ground-based measurements could never provide the necessary coverage or sampling frequency, satellite measurements are essential in determining the extent of global ozone. The measurements of globally averaged ozone are sufficiently stable and precise to be able to detect a 3 percent decrease in the northern hemisphere midlatitudes from 1979 to 1997 and a 6 percent decrease in the southern hemisphere over the same period.
Since 1997, in response to international regulations, concentrations of chlorine-containing gases in the atmosphere have decreased and the rate of depletion of stratospheric ozone has slowed (WMO-UNEP 2006). Data in that report provide some indications of the beginning of a recovery (Yang et al. 2006; Figure 5.6). To facilitate the search for trends due to halogen-induced destruction, variations due to seasonal, solar, and quasi-biennial effects have been removed from the ozone time series. These corrected ozone values, shown in Figure 5.6, display a decrease from 1979 until the mid-1990s, after which they seem to increase.
In conclusion, satellite observations provided the first measurements of the global vertical, horizontal, and temporal distributions of ozone and dynamical variables in the stratosphere, permitting the monitoring of their long-term changes. By allowing verification of the factors causing those changes, satellite observations were critical in confirming the seriousness of the danger posed by the release of anthropogenic halocarbons and, thus, in leading to the international agreement to protect the ozone layer. “The evolution of scientific understanding of ozone depletion and related policy decisions has since been heralded as one of the most remarkable environmental success stories of the 20th century” (NRC 2007b). It has created conditions for the recovery of the ozone layer to preindustrial conditions and removed a major hazard to human health and the biosphere.
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FIGURE 5.5 The area of the ozone hole with less than 220 dobson units as deduced from TOMS and the Ozone Monitoring Instrument (OMI) data for 2006 (red) and 2005 (blue). The thick black line indicates the 1979-2005 mean, with the light-blue area giving the 10th-90th percentiles over that period and the blue-green area giving the 30th-70th percentiles. The thin gray line shows the maximum over this period. The area in 2006 was occasionally the highest on record. SOURCE: WMO (2006). World Meteorological Organization, copyright 2006.
TROPOSPHERIC OZONE AND TRACE GASES
The troposphere presents special challenges to passive satellite detection because of clouds and diminished viewing by limb or occultation methods in the lower atmosphere. Nonetheless, space-based studies of tropospheric ozone began in the mid-1980s, and other critical trace species became measurable from 1995 onward with the launch of the European Space Agency’s (ESA) Global Ozone Monitoring Experiment (GOME; later SCIAMACHY, GOME II) and a constellation of the National Aeronautics and Space Administration’s (NASA) Earth Observing System (EOS) and related satellite instruments (Table 5.1). The chemical species listed are those of sufficient concentration, lifetime, and distribution to be detectable from space. Given the lifetimes and interactions of tropospheric gases and aerosols (discussed in Chapter 4), the variability of these species with synoptic processes reveal as much about tropospheric transport as chemistry. Furthermore, a few other processes (e.g., lightning, which is detected from space) are proxies from which trace gas concentrations are often inferred.
Tropospheric Ozone in the Tropics: “First Success”
Tropical tropospheric ozone deserves special mention because it has been derived from instruments designed to measure total and stratospheric ozone and because its time series, dating from the Nimbus era (above) is sufficiently long that trends and climate signals are detectable in the satellite record. Differencing the total and the stratospheric ozone column amount to deduce a “tropospheric residual” is possible due to the observation that stratospheric ozone is zonally invariant in the tropics and changes slowly over week-to-month timescales. Thus, a stratospheric profiler like (SAGE, later SBUV, HALOE, and MLS) is used to determine the stratospheric ozone column to be subtracted from a BUV-based total ozone instrument such as TOMS or OMI.
The first maps of the so-called tropospheric ozone resid-
FIGURE 5.6 Left panel: Time series of monthly average ozone based on merged TOMS-SBUV total columns between 60° S and 60° N for 1979-2005, with effects due to other causes removed. The trend line indicates the ozone trend calculated from the data for 1979-1996 (solid line) and projected linearly thereafter. Right panel: Cumulative sum of differences from the mean trend line (percent). The solid straight line indicates the line fitted to the ozone trend calculated from the data for 1979-1996 and projected linearly thereafter (dotted line). The solid black line rising above the green line is an indication that ozone recovery has begun. SOURCE: Yang et al. (2006). Reprinted with permission by American Geophysical Union, copyright 2006.
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TABLE 5.1 Space-Based Studies of Tropospheric Ozone and Other Critical Trace Species
Chemical Species
Instrument/Technique
References
Tropospheric O3
TOMS-SAGE differencing
Fishman et al. (1991)
TOMS-SBUV differencing
Fishman et al. (1996, 2005)
OMI-HALOE differencing (on Upper Atmosphere Research Satellite [UARS])
Ziemke et al. (1998)
TOMS/OMI-MLS differencing
Ziemke et al. (1998)
Chandra et al. (2003)
“Cloud Slicing”
Ziemke et al. (2001)
Ziemke and Chandra (2005)
OMI Assimilation
Stajner et al. (2006)
Pierce et al. (2007)
TES (Tropospheric Emission Spectrometer)
Worden et al. (2007)
CO
TES
Rinsland et al. (2006)
MOPITT (Measurement of Pollution in the Troposphere, on Terra Satellite)
Lamarque et al. (2003)
Deeter et al. (2003)
Yurganov et al. (2004)
AIRS (Atmospheric Infrared Sounder)
McMillan et al. (2005)
NO2
GOME (I, 1995; II, 2005)
Richter and Burrows (2002)
SO2
and
Eisinger and Burrows (1998)
BrO
SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY)
Carn et al. (2005)
HCHO
Richter et al. (1998)
Hollwedel et al. (2004)
Chance et al. (2000)
ual were seasonal averages in the tropics and subtropics that revealed a distinctive zonal wave-one pattern in tropospheric ozone in the southern hemisphere (Fishman and Larsen 1987, Fishman et al. 1991, 2003; Figure 5.7). The minimum in the ozone residual occurs in the central-western Pacific where photochemical sources are few and convection associated with the Walker circulation maintains a low-ozone column throughout the troposphere (Kley et al. 1996, Thompson et al. 2003). The South Atlantic maximum is characterized by a tropospheric ozone column with an amplitude approximately 10-15 Dobson units (DU) greater than that over the Pacific.
Interestingly, the Atlantic tropospheric ozone maximum is largest at the end of the southern hemisphere biomass burning season, from August through November. A fire-ozone linkage was established through the 1992 Southern Africa Fire-Atmosphere Research Initiative Transport and Atmospheric Chemistry near the Equator-Atlantic ground, multiaircraft and balloon campaigns (van Wilgen et al. 1997), using ozone, ozone precursor, and free radical measurements over South America and southern Africa (Fishman et al. 1996, Jacob et al. 1996). The synergism of satellite and in situ measurements in these experiments, with aircraft flying toward satellite-observed ozone maxima, ushered in a new era for tropospheric chemistry—just as 5 years earlier, airborne ozone depletion missions targeted regions where the TOMS satellite pinpointed column ozone loss.
Further studies with satellites have shown the South Atlantic ozone maximum to be more complex than initially assumed. First, the late burning season overlaps the start of the tropical rains, suggesting that biogenic nitrogen oxide from wet soils (Harris et al. 1996) and lightning nitrogen oxide (Moxim and Levy 2000) also contribute to the ozone burden in September and October. The location and amount of lightning have been observable only with Optical Transient Detector on MicroLab-1 and the Tropical Rainfall Measuring Mission (TRMM) Lightning Imaging Sensor (Christian et al. 1989, Bocippio et al. 2000). Second, closer inspection of tropospheric ozone maps showed the South Atlantic maximum was year-round, exemplified by the so-called tropical ozone paradox, named for the persistence of the maximum in January and February when biomass burning was a maximum north of the Intertropical Convergence Zone (Thompson et al. 2000). The causes of the paradox were analyzed with sondes (Jenkins et al. 2003, Chatfield et al. 2004, Jenkins and Ryu 2004), aircraft data (Sauvage et al. 2006), and other satellites, principally the Measurements of Pollution in the Troposphere (MOPITT; Edwards et al. 2003).
Tropospheric Views Since 1995
Breakthroughs in our understanding of tropospheric composition escalated after the 1995 launch of GOME, with its 2002 follow-on mission, the Scanning Imaging
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FIGURE 5.7 Seasonally averaged tropospheric ozone column, so-called residual amounts, that show high ozone in northern midlatitude spring and during the late biomass burning season over South America and Africa. SOURCE: After Fishman et al. (2003). Reprinted with permission from the European Geosciences Union, copyright 2003.
Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY), and with the EOS constellation of satellites (1999, 2002, 2004), each of which has instruments sensing lower-atmospheric trace gases, aerosols, and clouds. Multiple methods and sensors have been used to measure most of these constituents (Table 5.1).
Tropical Ozone
Refined satellite products, including several using the ozone residual concept, showed greater complexity in tropospheric ozone, notably in the tropics. During the 1997-1998 El Niño-Southern Oscillation (ENSO), upper-tropospheric ozone increased and water vapor decreased due to enhanced subsidence from the lower stratosphere (Chandra et al. 1998). A time series of tropospheric ozone derived from TOMS back to 1980 showed signatures of ENSO events in the 1980s (Thompson et al. 2001). Pollution from the Indonesian fires, instigated by the 1997-1998 ENSO drought, created tropospheric ozone that TOMS followed across the Indian Ocean. New aerosol products (from TOMS and the Sea-Viewing Wide Field-of-View Sensor [SeaWiFS]) project tracked pollution day to day, showing that during the worst health episodes, smoke and ozone were decoupled (Thompson et al. 2001; Figure 5.8).
A dedicated tropical ozonesonde validation network for satellite instruments has pinpointed ozone interactions with dynamics (Thompson et al. 2003). Profiles from soundings combined with the global view afforded by SAGE, HALOE, MLS, and Atmospheric Infrared Sounder (AIRS) instruments, have characterized the natural modulation of water vapor and ozone in the tropical tropopause region. This has fostered the growth of a subdiscipline of “tropical tropopause layer” studies, including cirrus clouds as well as trace gases (Gettelman et al. 2002, Folkins and Martin 2005, Dessler and Minschwaner 2007, Takashima and Shiotani 2007).
Other Trace Gases in the Troposphere
The reach of pyrogenic pollution is sometimes surprising. The air quality community has used satellite measurements of cardon monoxide (CO), ozone, and smoke to discriminate local and imported pollution for regulatory purposes (Morris et al. 2006, Pierce et al. 2007), especially in the case of boreal fires. An important feature of satellite CO instrumentation is that detection is strongest within midtropospheric layers where the gas has been introduced by convection. Indirectly, then, regions of maximum convective activity are identified through chemical measurement.
The power of spaceborne CO measurements was proven with the Measurement of Air Pollution from Satellites (MAPS) Shuttle instrument (1984-1994; Connors et al. 1999) but only since MOPITT was launched on the Terra platform have global observations of this key “ozone precur-
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FIGURE 5.8 Tracking pollution using data from NASA’s TOMS satellite instrument. In 1997 smoke from Indonesian fires remained stagnant over Southeast Asia while smog (tropospheric, low-level ozone) spread more rapidly across the Indian Ocean toward India. This situation was exacerbated by ENSO, which had already increased the thickness of smog over the region. At the same time, additional smog from African fires streamed over the Indian Ocean and combined with the smog from Indonesia in mid-October (lower right), creating an aerial canopy of pollutants. SOURCE: NASA.
sor” constituent been available (Figure 5.9). Ozone in the free troposphere has a lifetime of weeks to a month or more; for CO the photochemical lifetime is several months. MOPITT CO shows transhemispheric transport properties similar to ozone in the “paradox” region of the South Atlantic (Edwards et al. 2003). An AIRS product (McMillan et al. 2005) also tracks CO from industrial activity, and boreal and tropical fires over thousands of kilometers. Transboundary and transoceanic pollution among industrialized regions shows expected patterns.
Nitrogen oxides1 (NOx) are released by combustion along with carbon monoxide. However, the chemical NOx lifetime is much shorter (hours), so sources are readily identified. GOME and OMI NO2 appears most intense in industrial regions compared to biomass burning, but it shows up during the tropical rainy season when soil release is expected to make a significant contribution (Jaeglé et al. 2004). Models must be used to infer NOx from lightning, and the conversion
1
More specifically, nitrogen dioxide (NO2) in equilibrium with the prime emittant nitric oxide (NO). The sum, NO + NO2, is designated as NOx.
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FIGURE 5.9 The seasonally changing global distribution of CO pollution observed by Terra MOPITT at an altitude of 700 hPa (about 3 km). Averages are shown separately for March 2006 (top) and September 2006 (bottom). High CO pollution levels are shown in red. In addition to chemical production, northern hemisphere pollution sources are predominantly urban and industrial, while high CO in the tropics and southern hemisphere often results from biomass burning. NOTE: ppbv = parts per billion by volume. SOURCE: Modified from Edwards et al. (2006). Reprinted with permission by American Geophysical Union, copyright 2006.
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of lightning flashes to tropospheric NO2 release has been parameterized in several ways.
Satellites have allowed mapping of other important tropospheric trace gases and have been essential in solving the mystery of “polar sunrise” tropospheric ozone depletion. Since the 1980s, Arctic ozone has been known to disappear at the surface in the first few weeks of spring (Barrie et al. 1988). Organic halogen in some form was originally implicated, but the mechanisms were unclear until GOME detection of BrO (Richter et al. 1998, Hollwedel et al. 2004) in the first sunlit days (Figure 5.10). Reactions with highly saline surface associated with annual sea ice are now believed to be the source of airborne labile halogen compounds, which cause the surface ozone depletion (Rankin et al. 2002). The same phenomenon is detected at the edge of the Antarctic continent in austral spring.
For many years satellite measurements of stratospheric composition have advanced our understanding of the chemistry and dynamics of the region above the tropopause. As this region continues to respond to changes in halocarbon concentrations and global temperature, the measurements will continue to be vital to monitoring the health of the planet. Furthermore, the present growth of greenhouse gases leads not only to warming of the troposphere but also to cooling of the stratosphere, which is predicted to affect the rate and extent of ozone recovery. Continuation of the types of measurements described above is essential to monitoring the progress of ozone recovery and to further the understanding of the complex role of ozone in the climate system.
Although satellite measurements of tropospheric species are more difficult, rapid advances in measurements of tropospheric composition are providing insights into the sources, mechanisms, and transport of many species. Combined with data assimilation schemes, continuing tropospheric chemistry observations from satellites will lead to a better understanding of the factors affecting air quality and the ability to predict its interactions with the stratosphere and climate system.
FIGURE 5.10 BrO from GOME (April monthly averages, 1996-2002) SCIAMACHY (2003-2007). Total column BrO includes more or less uniform stratospheric and free tropospheric contributions. The majority signal is from boundary layer BrO that forms from heterogeneous processes associated with the annual sea ice (Richter et al. 1998, Hollwedel et al. 2004). SOURCE: Figure courtesy of A. Richter and J.P. Burrows, University of Bremen, Germany.
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