7

Aerosols

INTRODUCTION

Aerosols are suspensions of solid or liquid particles in a gas. The particles that compose the atmospheric aerosol range in size from nanometers (in the case of large clusters of molecules) to tens of micrometers (in the case of wind-driven sand). Some aerosols (e.g., sea salt and terpene haze) occur naturally and some (e.g., smoke) are man-made (anthropogenic). Aerosols represent one of the greatest uncertainties in climate modeling, and they can affect climate in two ways: (1) by absorbing or scattering both shortwave and longwave radiation, they alter the radiative properties of the atmosphere (the direct effect) and (2) by serving as cloud condensation nuclei (CCN), they play a critical role in the cloud formation process, changing the radiative properties of the clouds and, possibly, their physical structure and precipitation (the indirect effect).

Anthropogenic aerosols could therefore to some extent offset the global warming due to greenhouse gases (GHGs). To fully understand how aerosols affect climate, their characteristics (composition, size distribution, and optical properties) must be measured on a global scale. Aerosols reside mainly in the two lowest layers of the atmosphere, the troposphere and the stratosphere.

BASIC SCIENCE ISSUES

Tropospheric Aerosols

Tropospheric aerosols, a substantial proportion of which are anthropogenic, form a much more complex system than aerosols in the stratosphere. The aerosols may be surface-derived from both land and ocean or formed in the atmosphere as a result of gas-to-particle conversion or cloud cycling. Once in the atmosphere, they may be transported away from their place of origin, sometimes over great distances. They may be removed from the atmosphere by both dry processes (sedimentation) and wet (rainout). In the troposphere the aerosol concentration generally decreases with altitude, reaching its lowest values in the upper troposphere.

The composition of tropospheric aerosols is variable; mixtures are formed both internally (within a single particle) and externally (between particles). For the purpose of behavioral description and modeling, tropospheric aerosols are commonly classified according to their composition and source, because they vary significantly in concentration and composition by region (source). They have horizontal spatial scales ranging from about 1 km to



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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN 7 Aerosols INTRODUCTION Aerosols are suspensions of solid or liquid particles in a gas. The particles that compose the atmospheric aerosol range in size from nanometers (in the case of large clusters of molecules) to tens of micrometers (in the case of wind-driven sand). Some aerosols (e.g., sea salt and terpene haze) occur naturally and some (e.g., smoke) are man-made (anthropogenic). Aerosols represent one of the greatest uncertainties in climate modeling, and they can affect climate in two ways: (1) by absorbing or scattering both shortwave and longwave radiation, they alter the radiative properties of the atmosphere (the direct effect) and (2) by serving as cloud condensation nuclei (CCN), they play a critical role in the cloud formation process, changing the radiative properties of the clouds and, possibly, their physical structure and precipitation (the indirect effect). Anthropogenic aerosols could therefore to some extent offset the global warming due to greenhouse gases (GHGs). To fully understand how aerosols affect climate, their characteristics (composition, size distribution, and optical properties) must be measured on a global scale. Aerosols reside mainly in the two lowest layers of the atmosphere, the troposphere and the stratosphere. BASIC SCIENCE ISSUES Tropospheric Aerosols Tropospheric aerosols, a substantial proportion of which are anthropogenic, form a much more complex system than aerosols in the stratosphere. The aerosols may be surface-derived from both land and ocean or formed in the atmosphere as a result of gas-to-particle conversion or cloud cycling. Once in the atmosphere, they may be transported away from their place of origin, sometimes over great distances. They may be removed from the atmosphere by both dry processes (sedimentation) and wet (rainout). In the troposphere the aerosol concentration generally decreases with altitude, reaching its lowest values in the upper troposphere. The composition of tropospheric aerosols is variable; mixtures are formed both internally (within a single particle) and externally (between particles). For the purpose of behavioral description and modeling, tropospheric aerosols are commonly classified according to their composition and source, because they vary significantly in concentration and composition by region (source). They have horizontal spatial scales ranging from about 1 km to

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN a few thousand kilometers. The highly visible haze that persists in all the industrialized regions of the world is composed mainly of sulfates and organic compounds from emissions of sulfur dioxide (SO2); organic gases (e.g., terpenes); and organic matter and soot (carbon black) from biomass burning. The flux of SO2 emissions has increased exponentially over the past century to 65 to 80 teragrams (1 Tg = 1012 g) per year, mainly from the smelting of metal ores and the burning of fossil fuels, which has led to increased emissions of greenhouse gases, aerosol particles, and aerosol precursor gases as well. The ocean is a significant source of natural tropospheric aerosols. Air-sea exchange of particulate matter contributes to the global cycles of carbon, nitrogen, and sulfur aerosols (an example of the last-mentioned is the dimethyl sulfide (DMS) produced by phytoplankton). Ocean water and sea salt are transferred to the atmosphere through air bubbles at the sea surface. As the water evaporates, the salt is left suspended in the atmosphere. Haywood et al. (1999) suggest that naturally occurring sea salt is the leading aerosol contributor to the global-mean clear-sky radiation balance over oceans. Other significant sources of natural tropospheric aerosols are volcanic eruptions and windblown dust from arid and semiarid regions. While the direct and indirect radiative effects of sulfates are important, other tropospheric aerosols may contribute significantly to the global radiative balance. Among these are carbonaceous compounds and mineral dust. Carbonaceous compounds are present in the atmosphere in the form of elemental carbon (EC) or organic carbon (OC). A significant portion of ambient EC is soot directly emitted as a product of incomplete fossil fuel combustion. An important property of EC is its large share of the imaginary part of the refractive index at visible wavelengths. This property makes it a very good absorber of shortwave radiation and could decrease the single-scattering albedo of an aerosol to below the critical point, causing the aerosol to have a net heating effect instead of a cooling effect. Simple radiative-transfer calculations using a box model show that EC/SO4 ratios of 0.05 and 0.10 result in a positive forcing (heating) of +0.03 and +0.34 Wm−2, respectively. In comparison, the direct sulfate forcing has been estimated at -0.43 Wm−2 for the northern hemisphere. OC is either directly emitted (primary OC) or formed in the atmosphere (secondary OC) by the condensation of volatile organic carbons (VOCs). The largest sources of anthropogenic organic carbon include biomass burning, dust from paved roads, industrial emissions, and combustion for domestic purposes (e.g., cooking of food and burning of wood in stoves and fireplaces). Because a number of secondary and primary forms of OC are hygroscopic and have size distributions and optical properties similar to those of sulfate particles, these OC particles are likely to force the climate as much as, or even more than, sulfate particles. The biggest obstacle to determining the effect of these particles for use in climate models is the lack of well-defined, spectrally resolved refractive indices for determining fundamental optical properties (single-scattering albedo, asymmetry factor, and extinction efficiency). The refractive indices of these particles have not been well characterized because ambient OC is made up of more than 300 different compounds. As a result, the composition is highly variable from particle to particle depending on location, source, and meteorological conditions. Based on our current knowledge, any estimate of a set of optical properties for all OCs would entail great uncertainty. Penner et al. (1994) estimate up to −0.8 Wm−2(direct and indirect) forcing as a result of anthropogenic organic aerosols produced by biomass burning. There is evidence that organic aerosols play a key role in cloud nucleation and thus are responsible for a significant share of cloud albedo enhancement in regions affected by anthropogenic pollutants. Based on measurements made on El Yunque peak in eastern Puerto Rico, 37 percent (by number) of the total CCN were found to be sulfate particles and the remaining 63 percent were OC. Some OC particles are strongly hydrophilic and readily act as CCN. Others may be intrinsically inactive as nuclei but become active by the condensation of a thin coating of sulfuric acid. Mineral dust absorbs and scatters solar radiation and absorbs terrestrial (infrared) radiation. Although there has been considerable interest in sulfate aerosols over the last two decades, our knowledge of the distributions, global burdens, and effects on climate change of elemental carbon, organic carbon, and mineral dust is meager compared to our knowledge of sulfate aerosols. Although tropospheric aerosols are chemically complex and may be strongly influenced by local emissions, one persistent feature, worldwide, is the strong presence of sulfate. It is difficult to calculate the effect of tropospheric aerosols on Earth's climate because data are lacking for many places around the world and there is no clear understanding of the processes that link gas emissions with particle formation and growth. All the estimates

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN to date of the global effects of anthropogenic aerosols have been based solely on coupled radiative and chemical transport models. Stratospheric Aerosols Stratospheric aerosol particles are composed mostly of sulfuric acid (H2SO4) and water (H2O) droplets less than a micron in diameter. They are present globally between the tropopause and about 30 km, undergo seasonal variations, and are greatly influenced by large volcanic eruptions. During volcanically quiescent periods, the vertical distribution of the stratospheric aerosol particles relative to tropopause height is very similar at all latitudes, with mass mixing ratios and number densities on the order of 1 ppbm and 10 particles cm−3, respectively. The predominant source of stratospheric sulfate aerosols is strong, sulfur-rich volcanic eruptions, which are by nature highly intermittent and unpredictable. The flux of volcanic sulfur averaged over the last 200 years has been estimated at about 1 Tg yr−1, with lower and upper bounds of 0.3 and 3 Tg yr−1 (Pyle et al., 1996). A minimum flux of 0.5 to 1 Tg yr−1 for the past 9,000 years has been derived from ice core sulfate data. The volcanic input into the stratosphere has been unusually high during the past 15 years, with the occurrence of two relatively large sulfur-rich eruptions: 3.5 Tg from El Chichon (1982) and 9 Tg from Mt. Pinatubo (1991). Carbonyl sulfide (OCS) oxidation is believed to be the main nonvolcanic source of stratospheric sulfur (Crutzen, 1976). Recent estimates of this source range from 0.03 Tg yr−1 (Chin and Davis, 1995) to 0.049 Tg yr−1 (Weisenstein et al., 1997). Although most OCS sources are natural, there are some indications that anthropogenic emissions may be substantial and increasing (Zander et al., 1988; Khalil and Rasmussen, 1984; Hofmann, 1990). However, historical data on industrial releases suggest that anthropogenic emissions of OCS and its precursor, carbon disulfide (CS2), were relatively constant between 1977 and 1992 (Chin and Davis, 1993). Furthermore, no statistically significant trend in lower stratospheric OCS was inferred from spaceborne observations made in 1985 and 1994 (Rinsland et al., 1996). The main mechanism for removal of stratospheric aerosols is a combination of gravitational settling and stratospheric-tropospheric exchange. Typically, about one-third is removed in a year. Another major class of stratospheric particles is the polar stratospheric clouds (PSCs) (McCormick et al., 1982) observed in cold regions of the lower polar stratosphere, primarily during winter. Based on their optical properties, PSCs have been further divided into distinct subclasses: type 1 PSCs are thought to be relatively small and rich in nitric acid (HNO3), and type 2 PSCs are larger, primarily H2O ice particles. Typical mass mixing ratios for type 1 and type 2 PSCs are 10 and 1,000 ppbm, respectively (Drdla, 1996). Since the discovery of the stratospheric aerosol layer in 1957 (Junge et al., 1961), there has been much speculation about the stability of the layer and the background source of the H2SO4 that is the primary component of the aerosol. The measurements by Junge et al. (1961) were made at the end of a long period free of volcanic eruptions (Stothers, 1996) but were not extensive enough to establish a baseline. There are four periods in the modern (post-1970) measurement era during which the influence of volcanic eruptions has been at a minimum: 1974, 1979, 1989 to early 1991, and the present. Many studies have focused on these data periods in an attempt to clarify the processes that sustain the background nonvolcanic stratospheric aerosol layer and to explain the cause(s) for changes observed from one period of minimum volcanic activity to another. Both tropospheric and stratospheric aerosols play an important role in global climate change. Natural variations of aerosols, especially those due to episodic eruptions of large volcanoes, are recognized as a significant forcer of climate; that is, they alter the planetary radiation balance and thus tend to cause global temperature change. In addition, there are several ways in which humans are altering atmospheric aerosols and thereby possibly affecting climate. The concern here is with radiative forcing of climate due to changing aerosols, both direct and indirect. These climate forcings are not well determined, especially the forcing by anthropogenic aerosols. Findings from recent studies suggest that anthropogenic aerosols, primarily sulfates, organics, and carbon black, induce a significant radiative forcing opposite in sign to radiative forcing by anthropogenic GHGs. However, the GHG and anthropogenic aerosol forcings have very different spatial and temporal scales. In industrialized areas, aerosol forcing can be much larger than GHG forcing. According to the Intergovernmental Panel on Climate Change (IPCC), the negative direct effect, worldwide, of tropospheric aerosol forcing is about

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN 20 percent of the effect of forcing from GHGs (with an uncertainty range of 0 to 40 percent); the indirect effect of aerosols is highly uncertain but has been estimated to be even larger than their direct effect. GHG forcing exists during the day and at night, whether it is clear or cloudy, and attains a maximum in the hottest, driest locations on Earth. Anthropogenic aerosol forcing exists mainly during the day, attains a maximum in clear conditions, and—because the aerosols have relatively short residence times—is concentrated near aerosol source regions. Negative forcings as large as 40 to 60 Wm−2 have been reported to occur at midday. The National Research Council (NRC) report Aerosol Radiative Forcing and Climate Change (1996) recommended that the uncertainties in calculated aerosol forcing at the top of the atmosphere be reduced to within ±15 percent, both globally and locally. Locally, this would imply an uncertainty in forcing of less than 1.5 Wm−2. Estimates of climate forcings by GHGs, aerosols, and other forcers have been reported in a number of publications (e.g., IPCC, 1995). Figure 7.1 shows the climate forcings by several different agents (Hansen et al., 1998). GHG forcing is estimated at 2.3±0.25 Wm−2, while tropospheric aerosols are estimated to force climate on a global scale −0.4±0.3 Wm−2. Indirect effects are estimated at −0.5 to −2 Wm−2. Because of the great spatial variability in tropospheric aerosol concentrations that results from the aerosols' short lifetimes, there are many regions, principally near major industrial areas, where aerosol negative forcing exceeds the greenhouse positive forcing (e.g., Charlson et al., 1992). Recent studies have shown that (1) aerosol effects appear to be present in the global and regional 20th-century temperature record and (2) the inclusion of aerosol effects in numerical model predictions improves agreement with observed temperatures, in both timing and spatial patterns (Karl et al., 1995). Volcanic aerosol forcing, albeit episodic, is estimated at 0.2 to −0.5 Wm−2. About 1 year after the Mt. Pinatubo eruption, for example, forcing was estimated at about −3 Wm−2, which is greater than GHG forcing. Also shown in Figure 7.1 is an estimate of negative forcing by lower stratospheric ozone depletion of about −0.2±0.1 Wm−2. Mid- to lower-stratospheric ozone depletion is thought to be caused primarily by heterogeneous reactions on stratospheric volcanic aerosol particles and therefore represents an indirect effect of aerosols in addition to changes in clouds, mentioned earlier. Heterogeneous reactions on the surfaces of PSCs and volcanic aerosols have been shown to be the key to understanding ozone depletion (Solomon et al., 1986, 1996; McElroy et al. 1986). As can be seen from Figure 7.1, aerosols have been one of the greatest sources of uncertainty in the interpretation of climate change during the past century and in the projection of future climate change. In addition to their radiative effects and their effects on stratospheric chemistry, aerosols are also important to tropospheric chemistry, air quality, acid deposition, visibility, and cloud and precipitation processes. FIGURE 7.1 Estimates of climate forcing by greenhouse gases, other anthropogenic forcers, and natural forcers (Hansen et al., 1998).

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN Some attention has also recently focused on carbon black soot aerosols in the stratosphere, again because of their potential heterogeneous chemical reactivity. Measurements (Blake and Kato, 1995; Pueschel, 1996) and model calculations (Bekki, 1995) indicate that aircraft emissions are the most important source of soot in the stratosphere. Maximum soot concentrations on the order of 1 ng m−3 are found at northern midlatitudes around aircraft cruise altitudes (about 10 km). Most of the soot particles end up embedded in H2SO4/H2O solutions via coagulation with H2SO4/H2O aerosols and, possibly, the condensation of gaseous H2SO4. In the troposphere, black carbon aerosol has been detected near industrial regions and even in regions considered remote from anthropogenic sources. In contrast to sulfate and organic aerosol, black carbon aerosol is a strong absorber of solar radiation and can lead to localized warming rather than cooling. OBSERVING STRATEGY During the past 20 to 25 years, characteristics of stratospheric aerosol have become much better known as a result of in situ measurements from balloons and aircraft as well as remote sensing by lidar and satellites. Model studies have paralleled the observational program. The satellite measurements by the Stratospheric Aerosol Measurement (SAM) II, Satellite Aerosol and Gas Experiment (SAGE) I, and SAGE II have given the most complete global picture of these characteristics. Ground-based lidar networks, although mainly in the northern mid-to high latitudes, have also contributed to this more global view and the Polar Ozone and Aerosol Measurement (POAM) II and Halogen Occulation Experiment (HALOE) instruments have recently added to this stratospheric aerosol data set (Bevilacqua, 1997; Russell et al., 1993). However, the SAM II/SAGE series of measurements is the only global source of upper tropospheric aerosol information (Kent et al., 1988) and the longest global stratospheric aerosol database available. Whereas the stratospheric aerosol is more homogeneous in composition and controlled primarily by episodic volcanic eruptions, the tropospheric aerosol is more heterogeneous in composition and location and is controlled by the myriad of aerosol sources in each region. Global data on tropospheric aerosol are sparse. Tropospheric aerosols may be observed from space by measuring solar radiation scattered back from the atmosphere (Kaufman, 1995). Such measurements reveal most clearly the larger and lower-altitude aerosol concentrations such as desert dust clouds and pollution episodes. Converting these measurements to quantitative estimates of aerosol concentration is made difficult by the problem of separating the aerosol signature from that of the background and by the presence of multiple scattering. Because of these factors, tropospheric aerosol studies from space are largely confined to studies over the oceans (Griggs, 1975; Rao et al., 1989; Durkee et al., 1991), although several techniques have been used to retrieve aerosol properties over land (Kaufman, 1995; Holben et al., 1992). The SAM/SAGE series of satellites, using solar occultation with a limb-viewing geometry, has a much greater sensitivity to the presence of aerosols and provides data that are in many ways complementary to those obtained from nadir viewing instruments, although it primarily yields data in the stratosphere and upper troposphere. Nadir viewing instruments lack vertical resolution but possess good horizontal resolution, while SAM/SAGE instruments have good vertical resolution (~1 km) and a horizontal resolution of about 200 km. Intensive field programs can also provide the data needed to reduce uncertainties and improve the performance of climate prediction models. The International Global Atmospheric Chemistry (IGAC) Program is coordinating four such field programs, including the Tropospheric Aerosol Radiation Forcing Observation Experiment (TARFOX) and the Aerosol Characterization Experiments (ACE-1, ACE-2, and ACE-3) (IGAC, 1996). TARFOX was designed to reduce the uncertainty of aerosol effects on atmospheric radiation by measuring and analyzing aerosol properties and effects on the United States eastern seaboard, where one of the world's largest plumes of urban and industrial haze moves from the continent out over the Atlantic Ocean. The TARFOX intensive field campaign was conducted July 10 through July 31, 1996. It included coordinated measurements from four satellites (GOES-8, NOAA-14, ERS-2, and Landsat), four aircraft (ER-2, C-130, C-131A, and a modified Cessna), land sites, and ships. A variety of aerosol conditions was sampled, ranging from relatively clean, behind frontal passages, to moderately polluted, with aerosol optical depths exceeding 0.5 at mid-visible wavelengths. Gradients of aerosol optical thickness were sampled to aid in separating aerosol effects from other radiative effects and to more tightly constrain closure tests, including those of satellite retrievals. Early results

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN from TARFOX show, among other things, the unexpected importance of carbonaceous compounds and water condensed on aerosols in the U.S. East Coast haze plume, chemical apportionment of the aerosol optical depth, aerosol-induced changes in upwelling and downwelling shortwave radiative fluxes, and generally good agreement between measured flux changes and those calculated from measured aerosol properties (Russell et al., 1999). The Atmospheric Radiation Measurement (ARM) program is a multilaboratory, interagency program created in 1989 with funding from the U.S. Department of Energy (DOE). The ARM program is part of DOE's effort to resolve scientific uncertainties about global climate change, with a specific focus on improving the performance of general circulation models used for climate research and prediction. These improved models will help scientists better understand the influences of human activities on Earth's climate. The Aerosol Observation System (AOS) is part of the aerosol component of the ARM program. There are three AOS sites with a suite of instruments for characterizing tropospheric aerosols. Each site has a variety of optical particle counters, a single-channel nephelometer, a three-channel nephelometer, a light-absorption photometer, and a condensation nuclei counter, all mounted on a 10-m tower. A Raman lidar and a micropulse lidar are the other instruments for measuring aerosols. The AErosol RObotic NETwork (AERONET) is an optical ground-based aerosol monitoring network set up by NASA and developed to support the earth satellite systems of NASA, the Centre Nationale d'Etudes Spatiales (CNES) of France, and the National Space Development Agency (NASDA) of Japan (Holben et al., 1998). AERONET consists of identical automatic Sun-sky scanning spectral radiometers. Data from this program provide globally distributed near-real-time observations of aerosol spectral optical depths, aerosol size distributions, and precipitable water in diverse aerosol regimes. The main goal of AERONET is to provide algorithm validation of satellite aerosol retrievals, as well as to characterize aerosol properties unavailable from satellite sensors. The recent, current, and future approved satellite instruments should be sufficient for monitoring most stratospheric aerosol properties important to climate and chemistry. These include three SAGE III instruments, the first scheduled for launch in late 2000 (multiple copies are required to ensure global coverage, since the location at which occultation technique measurements are made depends on spacecraft orbit characteristics). SAGE III is described below in this chapter. The global tropospheric aerosol measurements, however, depend primarily on the success of the satellite-borne MODIS instrument and the ESSP PICASSO-CENA mission and their ability to retrieve aerosol optical depth in Earth's boundary layer (the first few kilometers of altitude). MODIS is the centerpiece of NASA's Earth Observing System (EOS), now aboard Terra (formerly the EOS AM-1 platform), launched on December 18, 1999, and on the EOS-PM platform, to be launched in late 2000. The techniques being used will build on those used to retrieve aerosol optical depth from the AVHRR instrument. With many more wavelength channels, the MODIS science team plans to measure a number of aerosol properties, such as particle size and optical depth. PICASSO-CENA, which will be launched in 2003, uses the lidar technique to achieve high resolution of aerosol characteristics and optical depths. It builds on the proof-of-principle Lidar In-Space Technology Experiment (LITE) flown on the shuttle for 10 days in 1994 (McCormick et al., 1995). Current Spacecraft Instruments Table 7.1 summarizes the aerosol measurement capabilities, uncertainties, and vertical resolution of recently flown or now-being-flown instruments designed to collect aerosol information. SAGE II NASA's SAGE II has provided dependable stratospheric constituent measurements since October 1984. The SAGE II instrument aboard the Earth Radiation Budget Satellite (ERBS) was launched by the space shuttle Challenger into a 610-km circular orbit with a 57-degree inclination. The SAGE II instrument is a nearly self-calibrating, limb-scanning Sun photometer that measures vertical profiles of aerosol extinction during spacecraft sunrise and sunset. These are measured at four wavelengths: 1.02, 0.525, 0.453, and 0.385 µm. The gaseous absorbers nitrogen dioxide (NO2), ozone, and water are measured at 0.448 (and 0.453), 0.600, and 0.940 µm, respectively (Mauldin et al., 1985). The SAGE II instrument takes 15 sunset and 15 sunrise measurements each day with a vertical resolution of 1 km. The latitudinal spacing is roughly 0.5 degrees between measurements

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN depending on latitude, while the longitudinal spacing is approximately 24 degrees. The SAGE II observations cover approximately 75 degrees S to 75 degrees N over a year, providing near-global coverage. The National Centers for Environmental Prediction/Climate Prediction Center (NCEP) supplies the SAGE II database with temperature and pressure data to develop heights at each measurement location. More information can be found on the SAGE II program and early data applications in McCormick (1987), on the instruments in Mauldin et al. (1985), and on the inversion algorithm in Chu et al. (1989). The predecessor instrument, SAGE I, flew aboard the Application Explorer Mission II spacecraft from 1979 through 1981. It had four channels, centered at 0.385, 0.45, 0.6, and 1.02 µm. TABLE 7.1 Aerosol Measurement Capabilities of Recent or Currently-Being-Flown Spaceborne Instruments Instrumenta Measurement Inferred Property Uncertainty (%) Vertical Resolution SAGE II Transmission from the 0.385, 0.453, 0.525, and 1.020 µm wavelengths Aerosol extinction coefficient, aerosol optical depth, PSCb frequency of occurrence 10 to 30 5 1 km HALOE Transmission from the 2.45, 3.40, 3.46, and 5.26 µm wavelengths Aerosol extinction coefficient 15 to 20 2 km ILAS Transmission from the 0.780 µm wavelength Aerosol extinction coefficient, PSCb frequency of occurrence NVc 2 km POLDER Polarization and directionality from Earth's reflectance Optical depth over ocean NVc 2 km POAM II Transmission from the 0.442, 0.448, and 1.06 µm wavelengths Aerosol extinction coefficient, PSCb frequency of occurrence, PMCd frequency of occurrence 20 to 35 1 km AVHRR Upwelling radiance from the 0.63 µm wavelength Aerosol optical depth at the 0.5 mm wavelength over oceans 25   LITE Backscatter from the 0.355, 0.532, and 1.064 µm wavelengths Aerosol backscatter coefficient 10 15 m aAcronyms for instruments are defined in Appendix B. bPSC, polar stratospheric cloud. cNV, not validated. dPMC, polar mesospheric cloud. HALOE NASA's HALOE instrument was launched aboard the Upper Atmosphere Research Satellite (UARS) on September 12, 1991, by the space shuttle Discovery into a 585-km, near-circular orbit with a 57-degree inclination. Like SAGE, the HALOE instrument uses the solar occultation technique to measure vertical profiles of O3, HCl, HF, CH4, H2O, NO, and NO2, extinction due to aerosols, and temperature versus pressure. However, because it uses broadband and gas-filter radiometry methods (Russell et al., 1993) in the spectral range between 2.45 and 10.04 µm, it can provide stratospheric microphysical aerosol information when there are high aerosol loadings, such as occurs during volcanic eruptions (Hervig et al., 1998). Like SAGE II, the HALOE instrument measures approximately 15 sunrise and 15 sunset measurements each day, with similar latitudinal and longitudinal sampling but lower vertical resolution, of approximately 2 km. ILAS The Improved Limb Atmospheric Spectrometer (ILAS), another occultation instrument, was launched aboard the Japanese Advanced Earth Observation Satellite (ADEOS) on August 17, 1996, into an 800-km, Sun-synchronous

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN polar orbit with a 98.6-degree inclination (it provides only polar coverage). It measures vertical profiles of O3, HNO3, N2O, NO2, CH4, H2O, CFC-11, CFC-12, N2O5, and aerosol extinction in the infrared band between 6.21 and 11.77 µm and temperature, pressure, and aerosol extinction in a visible band centered near 0.78 µm. ILAS measures approximately 14 sunrise and 14 sunset measurements each day with a vertical resolution of 2 km. The sunrise measurements occur entirely at high northern latitudes (55 to 72 degrees N), while the sunset events occur entirely at high southern latitudes (65 to 88 degrees S). POLDER The French instrument Polarization and Directionality of the Earth 's Reflectances (POLDER) was also launched aboard the ADEOS spacecraft. POLDER measures the polarization, directional, and spectral characteristics of the solar light reflected by aerosols, clouds, oceans and land surfaces. The POLDER instrument is a push-broom-type, wide field-of-view, multiband imaging radiometer and polarimeter designed to measure data in eight narrow spectral bands in the visible and near infrared (0.443, 0.490, 0.565, 0.665, 0.763, 0.765, 0.865, and 0.910 µm). A scientific goal of the POLDER experiment was to determine the physical and optical properties of aerosols so as to classify them and study their variability and cycle (Herman et al., 1997). The ILAS and POLDER instruments operated successfully until June 30, 1997, when the ADEOS satellite malfunctioned (SPARC, 1998). The ILAS II and POLDER instruments are planned as part of the ADEOS II payload. At the time of this writing, the ILAS and POLDER aerosol data are not considered to have been validated. POAM II The Naval Research Laboratory's POAM II was launched aboard the French satellite Système Pour l'Observation de la Terre (SPOT) 3 on September 25, 1993, into an 833-km Sun-synchronous polar orbit with a 98.7-degree inclination. The POAM II instrument is also a solar occultation instrument. It is designed as a simpler SAGE II instrument to measure vertical profiles of aerosols, O3, NO2, and H2O in nine channels between approximately 0.35 and 1.06 µm, with 1 km vertical resolution (Glaccum et al., 1996). The instrument performs 14 sunrise and 14 sunset measurements each day; because it is in a Sun-synchronous orbit like ILAS, all sunrise events occur entirely at high northern latitudes (55 to 71 degrees N) and all sunset events occur entirely at high southern latitudes (63 to 88 degrees S). The spacing is approximately equal in longitude (~25.4 degrees) for successive sunrise and sunset events and varies slowly in latitude, with the lowest latitudes measured at the solstices and highest latitudes at the equinoxes. The POAM II database is supplemented by temperature, pressure and potential vorticity for each altitude per measurement location provided by the NCEP. The instrument operated successfully until November 14, 1996, when the SPOT 3 satellite malfunctioned (Bevilacqua, 1997). Aerosol-related products include vertical profiles of polar region aerosols, PSCs, and polar mesospheric clouds (PMCs) (Randall et al., 1996; Fromm et al., 1997; Debrestian et al., 1997). The POAM III instrument was launched in March 1998 aboard the SPOT 4 satellite and is an improved version of POAM II. It has been operational, although the initial aerosol data are considered unvalidated. AVHRR The AVHRR instrument flies on the NOAA series of polar-orbiting, Sun-synchronous satellites. The orbital period (time to complete one full orbit around Earth) is approximately 100 minutes, so there are approximately 14 full orbits per day. The nominal altitude of NOAA platforms is about 830 km. The AVHRR is a cross-track scanning system, with a scanning rate of 360 scans per minute. Current AVHRR instruments take data in five narrow-band channels (0.63, 0.83, 3.7, 10.8, and 12 µm). The instantaneous field-of-view for each channel is about 1.4 milliradians, which for a satellite altitude of 830 km leads to a satellite subpoint resolution of approximately 1.1 km. For each scan line (6 per second), the AVHRR takes 2,048 samples per channel that span a viewing angle of ±55 degrees from the nadir (Rao et al., 1989). The aerosol product is optical depth at 0.5 µm wavelength, derived from the 0.63 µm wavelength reflectance data (Stowe et al., 1992).

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN LITE The LITE instrument is a three-wavelength (1064, 532, and 256 nm) backscatter lidar developed by NASA and flown on the space shuttle Discovery for 10 days in September 1994. The goals of the LITE mission were to validate key lidar technologies for spaceborne applications, to explore the applications of space lidar, and to gain operational experience that would benefit the development of future systems on free-flying satellite platforms. The performance of the lidar was excellent, as the data gathered presented the first highly detailed global view of the nadir-viewed vertical structure of cloud and aerosol from Earth's surface through the middle stratosphere. TOMS The Nimbus-7 and Meteor-3 Total Ozone Mapping Spectrometer (TOMS) instruments, using measured 340-nm and 380-nm radiances, produce daily global maps of ultraviolet (UV)-absorbing aerosols. The same information is currently obtained from the Earth Probe TOMS and was obtained from ADEOS TOMS using the 331-nm and 360-nm wavelength channels. Biomass burning, dust storms, volcanic ash clouds, and even oil fires have been detected by TOMS. Work has been progressing on detecting aerosols that do not absorb UV. Torres et al. (1998) have developed techniques to infer aerosol column optical depths and single-scattering albedo, which includes UV-absorbing aerosols from TOMS measurements in the near-ultraviolet (330 to 400 nm). The main constraint on the ability of these techniques to infer aerosol characteristics is the dependence on external information, such as the type and altitude of the absorbing aerosol present at particular locations and the reflectivity of the surface. Another shortcoming is that they can be applied only in cloud-free conditions. Future Spacecraft Instruments SAGE III The SAGE III series of instruments is part of the EOS program, with the first instrument launch scheduled for December 2000. The SAGE III instrument contains 12 spectral channels over the wavelength region 0.28 to 1.54 µm and is essentially an improved version of its predecessors, SAGE I and II. Whereas previous instruments in the SAGE series used single silicon diodes, the SAGE III instrument uses an 800-pixel charge-coupled device (CCD) linear array detector. The CCD is designed to measure (1) aerosol extinction coefficients centered at wavelengths 0.385, 0.450, 0.521, 0.676, 0.756, 0.869, and 1.0195 µm, (2) absorption features of O3, NO2, and H2O, and (3) both temperature and molecular density profiles from O 2 A-band measurements near 0.760 µm (McCormick et al., 1999). The SAGE III instrument will also have a channel centered near 1.54 µm to improve the size discrimination of larger aerosol particles and to separate cloud and aerosol signals. A unique feature of the SAGE III instrument is the implementation of a lunar occultation mode to additionally measure the active nighttime chemical species NO3 and chlorine dioxide (OClO). Three SAGE III instruments will enhance and extend the existing database of stratospheric constituents from the SAGE I and II data sets as far back as 1979, when SAGE I was launched. MODIS The Moderate-Resolution Imaging Spectroradiometer (MODIS) instrument, to be launched on both the EOS-AM and EOS-PM satellites, measures upwelling scattered radiation in 36 discrete wavelength bands from the visible to the thermal infrared (i.e., 0.4 to 14.5 µm) and will view Earth's entire surface every 1 to 2 days. It uses a conventional imaging radiometer concept, consisting of a cross-track scan mirror and collecting optics, and a set of linear detector arrays. With a spatial resolution of 250 m, 500 m, or 1 km at nadir, MODIS will provide aerosol products in the form of optical thickness, particle size, and mass transport (Esaias et al., 1998; Tanré et al., 1997).

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN MISR The Multi-angle Imaging Spectroradiometer (MISR) is a satellite instrument also designed to measure scattered sunlight upwelling from Earth and scheduled for launch into a polar orbit aboard NASA's EOS-AM. The MISR instrument uses nine individual CCD-based push-broom cameras to view Earth at nine different view angles: one at nadir and eight symmetrical views at 26.1, 45.6, 60.0, and 70.5 degrees forward and aft of nadir. Each camera will obtain images at four spectral bands centered at 443, 555, 670, and 865 nm with a horizontal resolution of 275 m, 550 m, or 1.1 km. The MISR data will be used to produce aerosol optical depth (Diner et al., 1998). EOSP The Earth Observing Scanning Polarimeter (EOSP) instrument proposes to measure radiance and linear polarization of reflected sunlight in 12 spectral bands from 0.41 to 2.25 µm. EOSP data will provide information on the global aerosol and cloud distribution and on such properties as optical depth, phase, particle size, and cloud-top pressure. At this time, EOSP is not funded for development and flight. PICASSO-CENA The Pathfinder Instruments for Clouds and Aerosols using Spaceborne Observations-Climatologie Etendue des Nuages et des Aerosols (PICASSO-CENA) instruments include a dual-wavelength (530 nm and 1060 nm), polarization-sensitive lidar, an oxygen A-band spectrometer operating over the O2 absorption region at 763 to 769 nm with 0.5 cm−1 resolution and an imaging infrared radiometer operating at two wavelengths, 10.5 and 12 µm. In addition, a high-resolution, widefield camera will be boresighted with the lidar and the other instruments. PICASSO-CENA will fly in formation with the EOS-PM spacecraft, providing joint measurements within 6 minutes of each other. It is designed to address the overdependence of our present understanding of the climate system on theoretical models by providing data that will reduce uncertainties in aerosol and cloud forcing. SCHIAMACHY One non-U.S, instrument that will measure aerosols is the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY), to be launched by the European Space Agency (ESA) on the ENVISAT-1 in June 2001. The SCIAMACHY instrument is a spectrometer designed to measure sunlight transmitted, reflected, and scattered by Earth 's atmosphere or surface in the ultraviolet, visible, and near infrared wavelength region (0.240 to 2.38 µm) at moderate spectral resolution. It is expected to provide aerosol optical depths and profiles of extinction and scattering with accuracies not yet determined. Measurement Requirements and Capabilities Uncertainties in aerosol radiative forcing must be quantified and reduced, or at least limited to ±1.5 Wm−2 if models are to provide accurate predictions of regional climate change. There is a consensus that anthropogenic sulfate aerosols produce a direct forcing that is a substantial fraction of the forcing from GHGs (NRC, 1996). At least three key parameters must be better quantified if the uncertainties in direct aerosol forcing are to be reduced: (1) optical depth (a measure of total column content), (2) single scattering albedo (the fraction of attenuated radiation that is scattered rather than absorbed), and (3) aerosol source strength (an essential input for models). Anthropogenic aerosol sources are located over land, and their forcings there are the largest. Since current spacecraft instruments cannot make tropospheric aerosol measurements over land, on a global scale, the uncertainties are large. Table 7.2 lists the expected optical depth measurement uncertainties for MODIS, MISR, and a number of other spaceborne sensors. Figure 7.2 (NRC, 1996) gives the optical depth accuracies needed for different optical depth values to reduce uncertainties in aerosol forcing to ±15 percent. Comparison of Table 7.2 and Figure 7.2 shows that only PICASSO-CENA is capable of meeting this optical depth requirement.

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN TABLE 7.2 Optical Depth Measurements and Radiative Forcing Uncertainty from Current and Future Satellite Instruments Instrumenta Optical Depth Resolution/Uncertainty Forcingb Resolution/Uncertainty (Wm−2) GOME/ERS-2 0.05 1.5 SeaWiFS 0.03 over ocean 0.9 AVHRR/NOAA-K 0.05 over ocean 1.5 MODIS/TERRA 0.03 over ocean 0.9 MISR/TERRA 0.05 over ocean 1.5 EOSP/TERRA2 0.03 0.9 PICASSO-CENA 0.005 0.15 Global-mean, annually averaged aerosol forcing   0.4 aAcronyms for instruments are defined in Appendix B. bForcing estimates based on Δfr = 30 Δτ Wm−2 (Harshvardhan, 1993). FIGURE 7.2 National Research Council (1996) estimates of optical depth accuracies versus optical depth values needed to reduce uncertainties in aerosol forcing to ±15 percent. Reprinted from NRC (1996). Single-scattering albedo cannot currently be measured from space, and even in situ measurements are sparse and uncertain. New techniques or combinations of measurement techniques, both spaceborne and in situ, are needed to narrow significantly the uncertainty in the resulting aerosol forcing. Source strengths are also not well-known globally. They can be estimated from optical depth measurements but will remain a challenge for the currently approved nonlidar spaceborne experiments. Similarly, it will be difficult for spaceborne instruments to quantify indirect aerosol forcing. Again, a combination of instruments like PICASSO-CENA and EOS PM flying

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN in formation, supported by carefully designed in situ measurements and aerosol modeling, will probably be able to reduce the large uncertainties in indirect aerosol forcing. The National Polar-orbiting Operational Environmental Satellite System (NPOESS) program has proposed using the angstrom turbidity coefficient α as a representative aerosol size parameter (Table 7.3). The size parameter is valid only if the aerosol particle size distribution is given by an inverse power law, such as a Junge distribution. This is a reasonable assumption for volcanically unperturbed stratospheric conditions and very clean tropospheric conditions. During episodic periods of increased volcanic aerosol loading, the stratospheric size distribution becomes multimodal, and these size distributions will produce complex and nonlinear spectral shapes (Russell et al., 1996). The same is true for tropospheric aerosols near industrial regions. Thus, the size parameter α will not be useful for a volcanically perturbed stratosphere or a regionally polluted (industrial) troposphere, both of which must be taken into account in order to understand aerosol climate forcing. The currently planned and funded satellite instruments described above have the potential to yield information on tropospheric and stratospheric aerosols. Without lidar or some other new approach, these systems are clearly limited in their application to understanding climate forcing. The techniques employed by the nadir-viewing passive systems are expected to work reliably over the oceans but to experience significant degradation over land and to provide no data on stratospheric aerosols in the presence of clouds. In addition, retrieval schemes for passive nadir instruments almost always require extensive a priori modeling of the aerosol's surface properties and of the aerosols themselves. Finally, because optical depths of the order of 0.05 must be measured to an accuracy of 10 to 20 percent, only SAGE III (~5 percent) in the stratosphere and spaceborne lidar in the troposphere, with supporting in situ measurements, can meet the accuracy demands at low and moderate optical depths. Nonetheless, the current and future spaceborne instruments, such as MODIS and MISR, are expected to enhance overall understanding of tropospheric aerosol characteristics globally. CALIBRATION AND VALIDATION STRATEGY The integrity of all satellite-derived data is based on calibrating and validating the sensors. Calibration is the process of quantitatively defining the system responses to known, controlled signal inputs and is critical for climate research. Prelaunch calibrations are required to establish a baseline for sensor operation. Postlaunch or onboard calibrations are also a necessary component of sensor characterization. Validation involves evaluating the algorithms required to extract physical quantities from calibrated and well-characterized instrument products and is a means of quantifying the overall system calibration, including all the data-processing algorithms. Validation and calibration campaigns are crucial to analyzing potential inherent and systematic biases of newly launched and existing satellite systems. These campaigns are also important for algorithm development and quality control to ascertain potential biases and confidence in the retrieved quantities. Validation measurements must be maintained at some level of activity throughout the lifetime of the satellite experiment. TABLE 7.3 Selected Environmental Data Record Requirements for the Measurement of Aerosol Size Parameters (α) System Capability Threshold Objective Vertical coverage Surface to 30 km Surface to 50 km Horizontal coverage Over ocean only Global Vertical resolution Total column   From 0 to 2 km   0.25 km From 2 to 5 km   0.5 km >5 km   1 km Mapping uncertainty 4 km 1 km Measurement range −1 to +3 −2 to +4 Measurement precision 0.3 0.1 Measurement accuracy ±0.3 over ocean ±0.1 Refresh 6 hours 4 hours; 2 hours during daylight Long-term stability (%) 0.1 0.03 SOURCE: Extracted from NOAA (1997) and IORD-1. IORD-1 and other documentationrelated to the NPOESS program are available online at <http://npoesslib.ipo.noaa.gov/ElectLib.htm>.

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN In the committee's view, a data validation plan should be organized to assess anticipated systematic uncertainties in the NPOESS aerosol data products and to better understand the assumptions underlying the measurement technique. The plan should be patterned after other successful validation programs. It should be based on intercomparisons with correlative measurements by in situ and remote sensors on the ground and aboard aircraft, balloons, and spacecraft (including EOS platforms). Intercomparisons are made with two kinds of data: planned measurements by in situ and remote sensors on a single-event basis and data from other sensors on a statistical or target-of-opportunity basis. Uncertainties in funding and in the long-term availability of aircraft and other forms of support require that the validation plan be a working document, periodically updated to reflect necessary changes. Because there is no accepted technique for calibrating atmospheric aerosol extinction measurements, previous validation programs (e.g., SAM II and SAGE I and II) have relied on intercomparisons of satellite measurements with observations from lidars, Sun photometers, dustsondes, and particle samplers. Aerosol extinction is a derived product for these sensors, often requiring additional information such as a backscatter-to-extinction conversion factor or a knowledge of the aerosol composition or refractive index. Unfortunately, these derivations can introduce greater uncertainty into the intercomparison, so it is desirable to acquire an array of correlative measurements to gain closure on the retrieved products. For example, a more direct intercomparison will be possible between the inferred aerosol surface area observations and measurements of the size distribution from particle samplers. Another important component of a validation plan is intercomparison of complementary observations by other measurement programs. This activity will allow ongoing intercomparisons to occur immediately following launch and to continue for the lifetime of the mission at little cost. It will further develop an important cross-reference data set for assessing biases between in situ and remote sensing instruments around the world. Satellite observations will also provide an important database for assessing instrument biases and precision. A commitment to long-term validation must be made so that global change can be quantified. Box 7.1 Findings In order to fulfill the need for a global data set of aerosol measurements, the committee believes that a limb occultation instrument operating in visible and near-infrared wavelengths, such as the Satellite Aerosol and Gas Experiment (SAGE) III in a highly inclined orbit, will provide the required stratospheric aerosol data record both in the transition period and in the National Polar-orbiting Operational Environmental Satellite System era. In addition, in the committee's estimation, spaceborne lidar complemented by other passive sensors offers the greatest likelihood of producing the data set required to understand the impact of tropospheric (and anthropogenic) aerosols on climate. The combination of a carefully planned in situ measurement network, a coordinated aircraft campaign, and simultaneous measurements from a synchronized satellite orbit using, e.g., the Moderate-Resolution Imaging Spectroradiometer (MODIS), and instruments boresighted with a lidar (e.g., an oxygen A-band instrument) on the same platform appears to be the ideal approach for depicting global tropospheric aerosols and reducing uncertainties in aerosol forcing to the required levels.

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN DATA MANAGEMENT Reprocessing Issues Usually, it becomes necessary to reprocess the entire data set and associated time series as the data record length increases. This ensures the integrity, consistency, and continuity of a high-quality data set suitable for climate research. As the satellite-based instruments age and (possibly) change their characteristics, as spacecraft orbits change, and as episodic events such as volcanic eruptions occur, the retrieval algorithms become outdated and it becomes evident that there is a need to continually compare measurements with those from newer instruments and updated retrieval algorithms. Funding must be made available for these activities. Raw data must also be stored so that new inversion techniques can later be tried or studied by inversion specialists. These needs become even more important for climate studies. To be usable by the science community, data (that is, derived products and satellite ephemeris data) must also be archived using readily accessible formats. Algorithm Status Algorithms used for studies of long-term change must be stable and capable of validation. Many satellite data sets are based either on empirical relationships (statistically determined from previous climatology) or on a priori assumptions, rather than on physically robust models. While the empirical approach is valuable, it may become obsolete after an episodic event such as a volcanic eruption. For example, Thomason (1991) used empirical relationships between the 0.525, 0.940, and 1.02 µm extinction measurements to seek a size distribution parameterization that gave the best water vapor retrieval at 0.940 µm. He showed that the SAGE II measurements of the pre-Pinatubo stratospheric aerosol could be modeled with size distributions in the form of a segmented power law while preserving the wavelength dependence of the measured extinction. The main problem with using a priori assumptions, such as a specific form of an aerosol size distribution, is that they may preclude other forms that may be more realistic or suitable. Required Ancillary Data The NPOESS instruments must provide meteorological or other data required for interpretation, validation, four-dimensional assimilation, and trajectory analyses. At a minimum, meteorological data such as temperature, pressure, and potential vorticity should be included as part of the data set. These three variables, for example, are useful for comparing satellite sensors with different vertical coordinates and for studying the transport mechanisms of the derived products on quasi-conserved surfaces. EVOLUTION STRATEGY There should be a continuing effort to develop and incorporate new technologies for the space-based observation of aerosols. Areas for research and development should include the following: New instrument techniques, Miniaturization, Use of lightweight components, Onboard data handling, Flexibility in instrument operations in space (for those that can be reconfigured), More capable detectors, noise-free and with higher quantum efficiency, More efficient and long-lived lasers or other such devices, and New output wavelengths.

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ISSUES IN THE INTEGRATION OF RESEARCH AND OPERATIONAL SATELLITE SYSTEMS FOR CLIMATE RESEARCH: I. SCIENCE AND DESIGN Progress is also needed in areas such as the following that support the measurements needed to resolve cloud-aerosol forcing issues: Research in new remote sensors and in combinations of instruments, such as lidar and radar, for vertical profiling, Formation flying, and Group flying. In general, better tropospheric measurements are also needed. BIBLIOGRAPHY Bekki, S. 1995. On the possible role of aircraft-generated soot in middle latitude ozone depletion. J. Geophys. Res. 100: 7195. Bevilacqua, R.M. 1997. Introduction to special section: Polar Ozone and Aerosol Measurement (POAM II). J. Geophys. Res. 102: 23591-23592. Blake, D.F., and K. Kato. 1995. Latitudinal distribution of black carbon soot in the upper troposphere and lower stratosphere. J. Geophys. Res. 100: 7202. Charlson, R.J., J. Langner, H. Rodhe, C.B. Loevy, and S.G. Warren. 1991. Perturbation of the northern hemisphere radiative balance by backscattering from anthropogenic sulfate aerosols. Tellus 43AB: 152-163. Charlson, R.J., S.E. Schwartz, J.M. Hales, R.D. Cess, J.A. Coakley Jr., J.E. Hansen, and D.J. Hoffman. 1992. Climate forcing by anthropogenic aerosols. Science 255: 423-430. Chin, M., and D.D. Davis. 1993. Global sources and sinks of OCS and CS2 and their distributions. Global Biogeochemical Cycles 7: 321-337. Chin, M., and D.D. Davis. 1995. A reanalysis of carbonyl sulfide as a source of stratospheric background sulfur aerosol. J. Geophys. Res. 100: 8993-9005. Chu, W.P., M.P. McCormick, J. Lenoble, C. Brogniez, and P. Pruvost. 1989. SAGE II inversion algorithm. J. Geophys. Res. 94: 8339-8351. Crutzen, P.J. 1976. The possible importance of CSO for the sulfate layer of the stratosphere . Geophys. Res. Lett. 3: 73-76. Debrestian, D.J., J.D. Lumpe, E.P. Shettle, R.M. Bevilacqua, J.J. Olivero, J.S. Hornstein, W. Glaccum, D.W. Rusch, C.E. Randall, and M.D. Fromm. 1997. An analysis of POAM II solar occultation observations of polar mesospheric clouds in the southern hemisphere. J. Geophys. Res. 102: 1971-1981. Diner, D.J., J.C. Beckert, T.H. Reilly, C.J. Bruegge, J.E. Conel, R. Kahn, J.V. Martonchik, T.P. Ackerman, R. Davies, S.A.W. Gerstl, H.R. Gordon, J-P. Muller, R. Myneni, R.J. Sellers, B. Pinty, and M.M. Verstraete. 1998. Multi-angle Imaging Spectroradiometer (MISR) instrument description and experiment overview. IEEE Trans. Geosci. Remote Sensing 36: 1072-1085. Drdla, K. 1996. Applications of a model of polar stratospheric clouds and heterogeneous chemistry. Ph.D. thesis, University of California at Los Angeles. Duce, R.A. 1995. Sources, distributions, and fluxes of mineral aerosols and their relationship to climate. Aerosol Forcing of Climate, R.J. Charlson (ed.). New York: John Wiley & Sons. Durkee, P.A., et al. 1991. Global analysis of aerosol particle characteristics. Atmos. Environ. 25A: 2457-2471. Esaias, W., M.R. Abbott, I. Barton, O.B. Brown, J.W. Campbell, K.L. Carder, D.K. Clark, R.H. Evans, F.E. Hoge, H.R. Gordon, W.M. Balch, R. Letelier, and P.J. Minnett. 1998. An overview of MODIS capabilities for ocean science observations. IEEE Trans. Geosci. Remote Sensing 36: 1250-1264. Fromm, M.D., R.M. Bevilacqua, J.D. Lumpe, E.P. Shettle, J.S. Hornstein, S.T. Massie, and K.H. Fricke. 1997. Observations of Antarctic polar stratospheric clouds by POAM II: 1994-1996. J. Geophys. Res. 102: 23659-23672. Glaccum, W., R. Lucke, R.M. Bevilacqua, E.P. Shettle, J.S. Hornstein, D.T. Chen, J.D. Lumpe, S.S. Krigman, D.J. Debrestian, M.D. Fromm, F. Dalaudier, E. Chassefiere, C. Deniel, C.E. Randall, D.W. Rusch, J.J. Olivero, C. Brogniez, J. Lenoble, and R. Kremer. 1996. The Polar Ozone and Aerosol Measurement instrument. J. Geophys. Res. 101: 14479-14487. Griggs, M. 1975. Measurement of atmospheric aerosol optical thickness over water using ERTS-1 Data. J. Air Pollut. Control Assoc. 25: 622-626. Hansen, J.E., M. Sato, A. Lacis, R. Ruedy, I. Tengen, and E. Matthews. 1998. Climate forcings in the industrial era. Proc. Natl. Acad. Sci. U.S.A. 95: 12753-12758. Harshvardhan. 1993. Aerosol-Climate Interactions. Aerosol-Cloud-Climate Interactions. P.V. Hobbs (ed.). San Diego: Academic Press, p. 81. Haywood, J.M., and K.P. Shine. 1995. The effect of anthropogenic sulfate and soot aerosol on the clear sky planetary radiation budget. Geophys. Res. Lett. 22: 603-606. Haywood, J.M., V. Ramaswamy, and B.J. Soden. 1999. Tropospheric aerosol climate forcing in clear-sky satellite observations over the oceans. Science 283:1299-1303. Herman, M., J.L. Deuzé, C. Devaux, Ph. Goloub, F.M. Bréon, and D. Tanré. 1997. Remote sensing of aerosols over land surfaces, including polarization measurements; application to some airborne POLDER measurements. J. Geophys. Res. 102: 17039-17049.

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