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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5 Atmospheric Composition: Ozone Depletion and global Pollution Although on average there are only four molecules of In contrast to the stratosphere, the elevated ozone ozone for every 10 million molecules of air, it is central to levels in the troposphere are problematic. The gas-phase atmospheric composition for several reasons. Stratospheric composition of the troposphere is very complex, bound by ozone absorbs ultraviolet (UV) radiation; thus, it shields the the tropopause above and the ocean and land surface below lower atmosphere and Earth’s surface from UV radiation (Figure 5.1). In tropical and midlatitudes especially, the that is harmful to living organisms. This absorption of UV ocean and land surfaces are active sources of trace gases that radiation warms the stratosphere and plays a major role in are linked through physical and chemical transformations to establishing the temperature structure of the atmosphere other tropospheric species. Tropospheric ozone together with (Box 5.1), while its infrared (IR) absorption and emission other radicals (most importantly hydroxide) contributes to are also important in Earth’s energy balance. No less impor- the oxidative capacity. This oxidative capacity is important tant are the chemical and photochemical reactions of ozone to the removal of the most reactive air pollutants, which with other species, which regulate the trace gas structure of cleanses the atmosphere. Similar to its stratospheric counter- the stratosphere and troposphere. Given ozone’s importance part, tropospheric ozone levels are strongly influenced by to the atmospheric composition and temperature structure photochemical reactions involving nitrogen oxides and and as Earth’s UV shield, it is critical to measure its global hydrogen oxides and by ozone inputs from the stratosphere. distribution; understand its trends and the mechanisms that In the presence of elevated nitrogen oxides from localized control its distribution; and understand its interactions with sources of pollution, hydrocarbons—even the ubiquitous atmospheric chemistry, dynamics, and the climate system. methane and carbon monoxide—can perturb the natural A delicate balance of photochemical reactions among ozone balance toward unhealthy levels. Ozone’s powerful oxygen (molecular and atomic), nitrogen oxides, hydrogen oxidizing capacity threatens human health, agricultural pro- oxides, and halogenated oxides maintains the “ozone layer” ductivity, and natural ecosystems. In addition, ozone in the in the stratosphere, where 90 percent of the total ozone free troposphere is a powerful greenhouse gas. In contrast to column resides. A rapid increase in anthropogenic halogen- the stratosphere, halogen chemistry is relatively benign in the containing gases (collectively called “halocarbons,” which troposphere, with a noteworthy exception in polar regions. includes chlorofluorocarbons [CFCs]) over the past 50 years Stratospheric ozone studies have benefited greatly from resulted in a huge perturbation to the natural stratospheric a long history of space observations. These investigations ozone balance. represent one of the best-known examples of satellite “suc- Satellite-generated data have made a vital contribution cesses” in Earth observations. The role that satellites have to understanding the threat of stratospheric ozone depletion, played in tropospheric ozone is more complex but no less observing stratospheric dynamics, and determining the cause important. As this chapter documents, satellites discovered for the Antarctic ozone hole. Satellite observations provided ozone “pollution” in the remote tropics 20 years ago. Follow- the first global measurements of stratospheric ozone and on exploration with newer satellites and ground-based and temperatures, expanding and revolutionizing our understand- aircraft instrumentation has shown that climate dynamics in ing of the atmosphere above the tropopause. This knowl- the tropics and stratospheric forcing can be as significant as edge confirmed the dangers associated with the release of photochemical reactions. Today, a new generation of satellite anthropogenic CFCs and other halocarbons and helped shape instrumentation, described later in this chapter, is mapping international policies to minimize their use and release. 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 al- titude up to the stratopause, at about 50 km altitude. The temperature again decreases in the overlying mesosphere up to the mesopause (~80 km), above FIGURE 5.1 Schematic of biogeochemical cycling with human which it rises in the thermosphere under the influ- contributions included, illustrating the major gas-phase constituents ence of solar radiation. The temperature structure in in the lower atmosphere that are measured from space (shaded in the stratosphere tends to suppress vertical motions, gray). NO2 and NO are encircled to represent equilibrium; their leading to more horizontal winds. 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;  = 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 cal relatives (nitrogen dioxide, bromide, carbon monoxide, dioxide. SOURCE: Drawing by A.M. Thompson and K.M. Dough- formaldehyde). erty, Pennsylvania State University. UNDERSTANDINg AND REMOVINg THE THREAT OF STRATOSPHERIC OZONE DEPLETION Until about 1964 it was thought that the Chapman (1930) These gases would reduce the amounts of stratospheric scheme, based only on forms of oxygen, could explain the ozone below the natural background level, letting more UV stratospheric abundance of ozone (Wayne 1985). Subse- radiation reach the surface, causing increased incidence of quently, improved laboratory measurements of reaction human skin cancer as well as damage to other biological rate coefficients showed that this approach overestimated processes. Subsequent studies showed that bromine, which the amount of stratospheric ozone by a factor of 2. Further has natural and anthropogenic sources, could also cause laboratory measurements showed that reactions involving significant ozone depletion (Wofsy et al. 1975, Yung et al. compounds of hydrogen, chlorine, and bromine from natural 1980). Because of the dire nature of these predictions, it was sources could enter into catalytic cycles that would speed crucial to develop a better understanding of this region of the up the rate of ozone destruction, decreasing estimates of atmosphere as quickly as possible. ozone amounts and bringing them into better agreement with ground-based observations. OBSERVINg STRATOSPHERIC DYNAMICS A very alarming fact was that two of these gases had large and potentially rapidly increasing anthropogenic To predict the ozone distribution and its changes in the sources. A projected fleet of 500 commercial supersonic stratosphere, it is also necessary to understand atmospheric airplanes flying many hours each day was expected to inject motions. These are closely linked to radiative heating and large amounts of nitrogen oxides into the lower stratosphere, cooling, which depends on the atmospheric composition, with deleterious effects (Crutzen 1970, Johnston 1971). Ulti- notably the ozone distribution (Craig 1965). Understanding mately this fleet did not materialize. However, the techniques this interacting system of chemistry dynamics, and radiation and models developed to address the former problem were requires global observations, unavailable from ground-based ready to be applied to the next threat to the ozone layer: chlo- measurements, as well as the synergistic use of models to rine, which was being released in significant amounts by the incorporate this information and allow accurate and trust- photolysis of CFCs in the stratosphere. The chlorine released worthy predictions to be made. from CFCs was also predicted to cause a serious reduction The stratosphere was first identified in 1899, when in ozone (Cicerone et al. 1974, Molina and Rowland 1974). balloonborne measurements showed that the atmospheric

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 ATMOSPHERIC COMPOSITION: OZONE DEPLETION AND GLOBAL POLLUTION temperature did not continue to decrease with altitude but perature measurements with higher vertical resolution. These became constant or even increased above a height termed the more detailed pictures revealed a wider range of atmospheric tropopause. By the end of World War II, data from operational motions, including waves in the tropics with short vertical weather balloons, which could in the best case reach altitudes wavelengths, and provided more detail on planetary wave of 30 km, provided a picture of stratospheric temperatures and activities. The accurate and densely spaced measurements of winds up to this altitude but with low spatial and temporal temperature and the derived estimates of wind speed made it resolution. After World War II, rocket soundings at about a possible to study global transport in more detail. To avoid the dozen locations extended these measurements to 65 km and uncertainties associated with conventional fluid dynamics, the- occasionally higher. These data were enough to delineate the oreticians recast their equations to essentially follow air par- global variation of the vertical temperature and wind distri- cels, leading ultimately to simple but accurate approximations. bution but yielded little information on features with smaller These showed how planetary waves, propagating up from the temporal or horizontal scales and provided little information troposphere, could interact with the eastward wind motions, on the southern hemisphere’s atmosphere. Only the roughest and thereby change the mean vertical and poleward circulation picture of motions at the longest horizontal scales was avail- (Matsuno 1971, Andrews and McIntyre 1976). This explained able, and many studies were constrained by data availability to the phenomenon of “sudden stratospheric warming,” in which limited altitude and geographic regions. Craig (1965) presents temperatures in the polar stratosphere at an altitude of 30 km a good discussion of knowledge and speculation at that time. can increase by 30°C or more in a few days. Similarly, a limited number of ground-based instruments One particular scientific achievement should be noted. in the pre-satellite era, mainly UV spectrophotometers, could Brewer (1949) and Dobson (1956) had independently provide only a rudimentary view of the vertical, latitudinal, postulated a mean north-south overturning circulation in and seasonal variations of the ozone distribution, with large the stratosphere, in which air rises from the troposphere uncertainties (Goody 1954). One particular puzzle was the into the stratosphere in the tropics and then travels to high nature of the atmospheric motions that transport ozone from latitudes (in both hemispheres) where it returns to the tro- high altitudes in the tropics, where it is produced by solar posphere. Observations of distributions of methane, nitrous UV radiation and atmospheric chemistry, to low altitudes in oxide, ozone, and water vapor (all from the limb sounders) polar regions and midlatitudes, where processes destroying were used to test and validate these (then-novel) ideas and it dominate (Craig 1965). theoretical approaches to the calculation of net transport of It was recognized that the transport of ozone and other these gases (Andrews et al. 1987). A related triumph was the gases, as well as heat and momentum, was important. Initially, observation of the tropical “tape recorder” (Mote et al. 1996), such questions were addressed in terms of conventional fluid which significantly advanced and confirmed scientific under- dynamics (Hunt and Manabe 1968). However, these early standing of stratospheric dynamics and motions (Box 5.3). estimates were so uncertain that it was often impossible even Remote sounding also provided information on the to determine whether the ozone transport was northward or composition of the stratosphere. The first instruments to southward (NRC 1979). shed light on the global distribution of stratospheric ozone The understanding of atmospheric dynamics was revolu- were the backscattered ultraviolet (BUV) on Nimbus 4 tionized beginning in 1969, when satellite instruments were and the solar backscattered ultraviolet (SBUV) and the launched to measure temperature and ozone in the stratosphere. Total Ozone Mapping Spectrometer (TOMS) on Nimbus 7, The first downward- or nadir-looking temperature sounders on which provided global measurements of the total ozone in Nimbus 3 (Box 5.2) demonstrated that remote sounding tech- a vertical column. In addition, the Limb Radiance Inver- niques could provide global observations of atmospheric tem- sion Radiometer (LRIR), the Limb Infrared Monitor of the peratures from the surface to mesospheric altitudes. Although Stratosphere (LIMS), and the Stratospheric and Mesospheric the soundings from these and subsequent nadir-looking instru- Sounder (SAMS) on Nimbus 6 and 7 retrieved the distribu- ments had low vertical resolution, they were sufficient to allow tions of ozone, water vapor, nitrogen dioxide, nitric acid, the heights of atmospheric pressure surfaces to be calculated. nitrous oxide, and methane. The power of these measure- The balance between the slopes of these heights and Earth’s ments is shown by the observations of nitrogen dioxide and rotation allowed scientists to make accurate calculations of nitric acid, present in concentrations on the order of only stratospheric winds (Smith and Bailey 1985). These winds 10 parts per billion by volume. could be separated into winds in the east-west direction (along parallels of latitude) and into north-south wavelike perturba- DETERMININg THE CAUSES OF ANTARCTIC OZONE tions. A review of early results clearly showed that the strato- DEPLETION sphere and mesosphere were dynamically very active, with large (planetary)-scale waves propagating from the troposphere The advances in our understanding of the cause and into the stratosphere and mesosphere in the winter hemisphere dynamics of the Antarctic ozone hole exemplify the pro- (Hirota and Barnett 1977). ductive interactions of satellite observations with in situ The advent of horizon-viewing sounders provided tem- and ground-based observations and numerical models. As

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0 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 Inter- ferometer 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 resolu- tion (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. a Described by Houghton et al. (1984). satellite observations were clarifying the structure, dynamics, However, in 1984 the world was startled by the dis- and composition of the stratosphere, modeling activities were covery of a much larger than predicted ozone decrease over advancing rapidly. After about a decade of evolution, these Antarctica at a much lower altitude, near 20 km in the lower models were predicting relatively modest but still important stratosphere (Farman et al. 1985). This feature quickly decreases in ozone concentrations, centered near 40 km became known as the Antarctic ozone hole. This unexpected altitude (Wuebbles et al. 1983). Later data confirmed these phenomenon called into question the theoretical understand- expectations (Solomon 1999).

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 ATMOSPHERIC COMPOSITION: OZONE DEPLETION AND GLOBAL POLLUTION 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, con- firming 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 Theory predicted that the tropical upwelling—discussed projections. in Box 5.2—carried CFCs and other halocarbons from the Satellite measurements played two important roles in tropospheric source into the stratosphere, where they were unraveling the questions of ozone depletion: present at only a few molecules per 10 billion atmospheric molecules. In the stratosphere, solar UV breaks CFCs apart, 1. measurements of trace species that lead to or cata- releasing chlorine molecules that react to produce relatively lyze ozone destruction contributed to confirming the causes inert hydrochloric acid (HCl). Laboratory investigations of the depletion showed that HCl could react on the surface of polar strato- 2. measurements of stratospheric ozone concentra- spheric clouds (discovered by the Stratospheric Aerosol and tions and distribution, and their changes over time, enabled Gas Experiment [SAGE] measurements; McCormick et al. comparison to model predictions. 1982) releasing approximately 1 part per billion of chlorine

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 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS monoxide (ClO). ClO effectively catalyzes ozone destruction photolytical destruction of ozone; 50 times more effective in the presence of sunlight. than ClO on a molecule per molecule basis. The satellite data Images from the second generation of infrared limb are broadly consistent with current understanding of bromine sounders confirmed the transport of these extremely small chemistry, indicating that at the observed concentrations BrO amounts of CFCs into the stratosphere and also the pres- plays a significant role in the budget of lower-stratospheric ence of the predicted compound chlorine nitrate (ClONO2; ozone. Taken together, these and related data on other species Nightengale et al. 1996, Mergenthaler et al. 1996). In addi- confirmed the chemistry in the coupled models of the strato- tion, Halogen Occultation Experiment (HALOE) measure- sphere, greatly improving their utility and trustworthiness as ments demonstrated the amounts and distribution of HCl tools to guide policymakers. (Russell et al. 1996). The picture was complete when sensi- Although satellite instruments did not discover the tive microwave measurements confirmed measurements of severely disturbed ozone conditions in southern polar regions, earlier ER-2 flights of the direct anticorrelation of ClO. MLS satellite observations from the BUV series of instruments added information on the global extent of the presence of provided unique detailed maps of the Antarctic ozone hole high concentrations of ClO in the high-latitude lower strato- (Figure 5.4). The monthly mean ozone column over Antarctica sphere in spring, where the ozone hole formed (Figure 5.3; provides information on the evolution of the Antarctic ozone Waters et al. 1993). Thus, the ozone values had decreased hole from the first measurements in 1970 until 2005. These at the same time and locations where high values of ClO maps allowed tracking of its size and depth every year, occurred over Antarctica in the southern spring. providing the most extensive information on its annual More recently, spaceborne measurements of bromine growth, extent, and decay crucial to ozone assessments and oxide (BrO) have been made by instruments measuring to the amendments to the Montreal Protocol (WMO 2006). reflected UV-visible radiation (McElroy et al. 1986, Tung et In the first measurements a crescent of higher ozone can be al. 1986, Sinnhuber et al. 2005) and microwave emissions observed, generally centered south of Australia, with a lower (Livesey et al. 2006). BrO is even more effective in the amount over Antarctica itself. With the passage of time, the 21 September 1991 CIO O3 FIGURE 5.3 Chlorine monoxide (ClO; left panel) and stratospheric ozone (O3; right panel) columns over the southern hemi- sphere 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 20 September 1992 measurements and the proposed mecha- nisms for ozone depletion. The white circle over the pole indicates area where no data CIO (1019 molecules m-2) is available. SOURCE: Waters et al. (1993). 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Reprinted with permission from Macmillian O3 (Dobson Units) Publishers Ltd., copyright 1993. 120 140 160 180 200 220 240 260 280 300 320 340

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 ATMOSPHERIC COMPOSITION: OZONE DEPLETION AND GLOBAL POLLUTION October 1970 October 1979 October 1982 October 1986 500 450 400 DOBSON UNITS 350 300 250 NIMBUS 7 TOMS NIMBUS 7 TOMS NIMBUS 7 TOMS NIMBUS 4 BUV 200 150 October 1990 October 1995 October 2000 October 2005 100 NOAA–9 SBUV– Earth Probe TOMS OMI OMI NIMBUS 7 TOMS 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 Since 1997, in response to international regulations, con- 5-4 values were observed in 1995, with a slight increase since centrations of chlorine-containing gases in the atmosphere then. Analysis of the area and extent of the ozone hole for the have decreased and the rate of depletion of stratospheric years 2005 and 2006 compared to the mean from 1979 to 2005 ozone has slowed (WMO-UNEP 2006). Data in that report indicates that the extent of ozone depletion over Antarctica is provide some indications of the beginning of a recovery greater during austral spring in the most recent years compared (Yang et al. 2006; Figure 5.6). To facilitate the search for to the mean (Figure 5.5). The maximum extent is usually trends due to halogen-induced destruction, variations due to reached near the end of September. 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 OZONE DEPLETION OVER THE NORTHERN mid-1990s, after which they seem to increase. HEMISPHERE In conclusion, satellite observations provided the first Although the Antarctic ozone hole is the better-known measurements of the global vertical, horizontal, and temporal phenomenon that has illuminated and confirmed the theory distributions of ozone and dynamical variables in the strato- of halogen-catalyzed chemical ozone destruction, a more sphere, permitting the monitoring of their long-term changes. important question from a societal point of view is the effect By allowing verification of the factors causing those changes, of anthropogenic chlorine, bromine, and other gases on satellite observations were critical in confirming the serious- the ozone concentrations over heavily populated nonpolar ness of the danger posed by the release of anthropogenic latitudes. Because ground-based measurements could never halocarbons and, thus, in leading to the international agree- provide the necessary coverage or sampling frequency, sat- ment to protect the ozone layer. “The evolution of scientific ellite measurements are essential in determining the extent understanding of ozone depletion and related policy deci- of global ozone. The measurements of globally averaged sions has since been heralded as one of the most remarkable ozone are sufficiently stable and precise to be able to detect environmental success stories of the 20th century” (NRC a 3 percent decrease in the northern hemisphere midlatitudes 2007b). It has created conditions for the recovery of the from 1979 to 1997 and a 6 percent decrease in the southern ozone layer to preindustrial conditions and removed a major hemisphere over the same period. hazard to human health and the biosphere.

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 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS 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 Moni- toring 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 life- times 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. 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 Tropospheric Ozone in the Tropics: “First Success” black line indicates the 1979-2005 mean, with the light-blue area giving the 10th-90th percentiles over that period and the blue-green Tropical tropospheric ozone deserves special mention area giving the 30th-70th percentiles. The thin gray line shows the because it has been derived from instruments designed to maximum over this period. The area in 2006 was occasionally the measure total and stratospheric ozone and because its time highest on record. SOURCE: WMO (2006). World Meteorological series, dating from the Nimbus era (above) is sufficiently Organization, copyright 2006. 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 TROPOSPHERIC OZONE AND TRACE gASES week-to-month timescales. Thus, a stratospheric profiler like The troposphere presents special challenges to passive (SAGE, later SBUV, HALOE, and MLS) is used to determine satellite detection because of clouds and diminished view- the stratospheric ozone column to be subtracted from a BUV- ing by limb or occultation methods in the lower atmosphere. based total ozone instrument such as TOMS or OMI. Nonetheless, space-based studies of tropospheric ozone 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 permis- sion by American Geophysical Union, copyright 2006.

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5 ATMOSPHERIC COMPOSITION: OZONE DEPLETION AND GLOBAL POLLUTION 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 Atlantic ozone maximum to be more complex than initially revealed a distinctive zonal wave-one pattern in tropospheric assumed. First, the late burning season overlaps the start of ozone in the southern hemisphere (Fishman and Larsen 1987, the tropical rains, suggesting that biogenic nitrogen oxide Fishman et al. 1991, 2003; Figure 5.7). The minimum in the from wet soils (Harris et al. 1996) and lightning nitrogen ozone residual occurs in the central-western Pacific where oxide (Moxim and Levy 2000) also contribute to the ozone photochemical sources are few and convection associated burden in September and October. The location and amount with the Walker circulation maintains a low-ozone column of lightning have been observable only with Optical Tran- throughout the troposphere (Kley et al. 1996, Thompson et sient Detector on MicroLab-1 and the Tropical Rainfall al. 2003). The South Atlantic maximum is characterized by a Measuring Mission (TRMM) Lightning Imaging Sensor tropospheric ozone column with an amplitude approximately (Christian et al. 1989, Bocippio et al. 2000). Second, closer 10-15 Dobson units (DU) greater than that over the Pacific. inspection of tropospheric ozone maps showed the South Interestingly, the Atlantic tropospheric ozone maximum Atlantic maximum was year-round, exemplified by the so- is largest at the end of the southern hemisphere biomass called tropical ozone paradox, named for the persistence of burning season, from August through November. A fire- the maximum in January and February when biomass burn- ozone linkage was established through the 1992 Southern ing was a maximum north of the Intertropical Convergence Africa Fire-Atmosphere Research Initiative Transport and Zone (Thompson et al. 2000). The causes of the paradox Atmospheric Chemistry near the Equator-Atlantic ground, were analyzed with sondes (Jenkins et al. 2003, Chatfield et multiaircraft and balloon campaigns (van Wilgen et al. 1997), al. 2004, Jenkins and Ryu 2004), aircraft data (Sauvage et using ozone, ozone precursor, and free radical measurements al. 2006), and other satellites, principally the Measurements over South America and southern Africa (Fishman et al. of Pollution in the Troposphere (MOPITT; Edwards et al. 1996, Jacob et al. 1996). The synergism of satellite and in 2003). situ measurements in these experiments, with aircraft flying toward satellite-observed ozone maxima, ushered in a new Tropospheric Views Since 1995 era for tropospheric chemistry—just as 5 years earlier, air- borne ozone depletion missions targeted regions where the Breakthroughs in our understanding of tropospheric TOMS satellite pinpointed column ozone loss. composition escalated after the 1995 launch of GOME, Further studies with satellites have shown the South with its 2002 follow-on mission, the Scanning Imaging

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 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS 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 dynamics (Thompson et al. 2003). Profiles from soundings (SCIAMACHY), and with the EOS constellation of satellites combined with the global view afforded by SAGE, HALOE, (1999, 2002, 2004), each of which has instruments sensing MLS, and Atmospheric Infrared Sounder (AIRS) instru- lower-atmospheric trace gases, aerosols, and clouds. Mul- ments, have characterized the natural modulation of water tiple methods and sensors have been used to measure most vapor and ozone in the tropical tropopause region. This has of these constituents (Table 5.1). 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 Tropical Ozone Minschwaner 2007, Takashima and Shiotani 2007). Refined satellite products, including several using the ozone residual concept, showed greater complexity in tropo- Other Trace gases in the Troposphere spheric ozone, notably in the tropics. During the 1997-1998 El Niño-Southern Oscillation (ENSO), upper-tropospheric The reach of pyrogenic pollution is sometimes surpris- ozone increased and water vapor decreased due to enhanced ing. The air quality community has used satellite measure- subsidence from the lower stratosphere (Chandra et al. 1998). ments of cardon monoxide (CO), ozone, and smoke to A time series of tropospheric ozone derived from TOMS discriminate local and imported pollution for regulatory back to 1980 showed signatures of ENSO events in the 1980s purposes (Morris et al. 2006, Pierce et al. 2007), especially (Thompson et al. 2001). Pollution from the Indonesian fires, in the case of boreal fires. An important feature of satellite instigated by the 1997-1998 ENSO drought, created tropo- CO instrumentation is that detection is strongest within spheric ozone that TOMS followed across the Indian Ocean. midtropospheric layers where the gas has been introduced by New aerosol products (from TOMS and the Sea-Viewing convection. Indirectly, then, regions of maximum convective Wide Field-of-View Sensor [SeaWiFS]) project tracked activity are identified through chemical measurement. pollution day to day, showing that during the worst health The power of spaceborne CO measurements was episodes, smoke and ozone were decoupled (Thompson et proven with the Measurement of Air Pollution from Satel- al. 2001; Figure 5.8). lites (MAPS) Shuttle instrument (1984-1994; Connors et al. A dedicated tropical ozonesonde validation network for 1999) but only since MOPITT was launched on the Terra satellite instruments has pinpointed ozone interactions with platform have global observations of this key “ozone precur-

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 ATMOSPHERIC COMPOSITION: OZONE DEPLETION AND GLOBAL POLLUTION a b c d 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 the5-8 a,b,c,d over the region. At the same time, additional smog from thickness of smog 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 Nitrogen oxides1 (NOx) are released by combustion troposphere has a lifetime of weeks to a month or more; for along with carbon monoxide. However, the chemical NOx CO the photochemical lifetime is several months. MOPITT lifetime is much shorter (hours), so sources are readily identi- CO shows transhemispheric transport properties similar to fied. GOME and OMI NO2 appears most intense in industrial ozone in the “paradox” region of the South Atlantic (Edwards regions compared to biomass burning, but it shows up during et al. 2003). An AIRS product (McMillan et al. 2005) also the tropical rainy season when soil release is expected to tracks CO from industrial activity, and boreal and tropi- make a significant contribution (Jaeglé et al. 2004). Models cal fires over thousands of kilometers. Transboundary and must be used to infer NOx from lightning, and the conversion transoceanic pollution among industrialized regions shows expected patterns. More specifically, nitrogen dioxide (NO2) in equilibrium with the prime 1 emittant nitric oxide (NO). The sum, NO + NO2, is designated as NOx.

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 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS CO 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 5-9 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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 ATMOSPHERIC COMPOSITION: OZONE DEPLETION AND GLOBAL POLLUTION of lightning flashes to tropospheric NO2 release has been this region continues to respond to changes in halocarbon parameterized in several ways. concentrations and global temperature, the measurements Satellites have allowed mapping of other important will continue to be vital to monitoring the health of the tropospheric trace gases and have been essential in solving planet. Furthermore, the present growth of greenhouse gases the mystery of “polar sunrise” tropospheric ozone depletion. leads not only to warming of the troposphere but also to cool- Since the 1980s, Arctic ozone has been known to disappear ing of the stratosphere, which is predicted to affect the rate at the surface in the first few weeks of spring (Barrie et al. and extent of ozone recovery. Continuation of the types of 1988). Organic halogen in some form was originally impli- measurements described above is essential to monitoring the cated, but the mechanisms were unclear until GOME detec- progress of ozone recovery and to further the understanding tion of BrO (Richter et al. 1998, Hollwedel et al. 2004) in the of the complex role of ozone in the climate system. first sunlit days (Figure 5.10). Reactions with highly saline Although satellite measurements of tropospheric species surface associated with annual sea ice are now believed to are more difficult, rapid advances in measurements of tropo- be the source of airborne labile halogen compounds, which spheric composition are providing insights into the sources, cause the surface ozone depletion (Rankin et al. 2002). The mechanisms, and transport of many species. Combined same phenomenon is detected at the edge of the Antarctic with data assimilation schemes, continuing tropospheric continent in austral spring. chemistry observations from satellites will lead to a better For many years satellite measurements of stratospheric understanding of the factors affecting air quality and the composition have advanced our understanding of the chem- ability to predict its interactions with the stratosphere and istry and dynamics of the region above the tropopause. As 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.