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Über dieses Buch

This book gives a much needed explanation of the basic physical principles of radiative transfer and remote sensing, and presents all the instruments and retrieval algorithms in a homogenous manner. The editors provide, for the first time, an easy path from theory to practical algorithms in one easily accessible volume, making the connection between theoretical radiative transfer and individual practical solutions to retrieve aerosol information from remote sensing, and providing the specifics and intercomparison of all current and historical retrieval methods.



1. Introduction

Aerosols are the solid and liquid particles suspended in the atmosphere. They can be seen as the amorphous haze that decreases visibility on polluted days, or the well-defined plumes of particles that rise out of a burning fire. These particles can be either natural (lofted desert and soil dust, sea salt particles, volcanic emissions, wildfire smoke, biogenic emissions) or anthropogenic (industrial emissions, biomass burning for agriculture, or land use changes that accelerate erosion and evaporation of lakes). As most aerosols are produced at the Earth’s surface, they are generally concentrated in the lower layers of the troposphere, and near the production sources. However particles can reach higher levels (4–6km) and can be transported over long distances. Aerosols are removed from the atmosphere by dry deposition, scavenging by precipitation, and evaporation. At the stratosphere level, they are abundant only after major volcanic eruptions, and they are mainly formed by gas-toparticle conversion. Although much less numerous than tropospheric aerosols, they may have an important impact, due to their long residence time and to their spread all around the globe.
Jacqueline Lenoble, Lorraine A. Remer, Didier Tanré

2. Absorption and scattering by molecules and particles

The Earth’s atmosphere absorbs, scatters, and emits electromagnetic radiation. Although air molecules are the primary actors in these processes, aerosol particles are also present ubiquitously (see Chapter 1) and modify the radiation field. In fact, this modification constitutes the very physical basis of aerosol remote sensing. Whenever clouds are present, they have a much larger influence on radiation which largely overshadows the aerosol impact. Therefore, in aerosol remote sensing, one often has to limit observations to cloudless conditions and screen cloudy pixels.
Jacqueline Lenoble, Michael I. Mishchenko, Maurice Herman

3. Radiative transfer in the Earth’s atmosphere

In Chapter 2, we analyzed the scattering and absorption properties of air molecules and aerosol particles. These properties create a specific signature of aerosol impact on radiation and this signature provides the basis for aerosol remote sensing. We have seen that this signature depends on the shape, size, and chemical nature of the particle, so that by measuring the result of an incident beam’s interaction with an aerosol particle, we can derive information about that particle. The problem becomes challenging because generally we have to deal with an ensemble of different particles, mixed within the air molecules, not just a single particle.
Jacqueline Lenoble, Maurice Herman

4. Direct observation of the sun for aerosol retrieval

The simplest remote sensing methods rely on the observation of the extinction of radiation, as it is defined in Chapter 2, in Eqs 2.4 and 2.5 (Beer exponential extinction law), which we recall here as Eqs 4.1 and 4.2:
$$I(S_2)=I(S_1)\, {\rm exp}(-\tau_e),$$
Colette Brogniez, Jacqueline Lenoble, Glenn Shaw

5. Determination of aerosol optical properties from inverse methods

Light scattering by atmospheric aerosols modifies the diffuse and direct solar radiation observed at the Earth’s surface as well as from spaceborne radiometers. When the atmospheric characteristics are known, including the vertical distribution of aerosols and their optical and microphysical properties, the diffuse radiation field can be computed using the methods outlined in Chapter 3. Ground-based measurements of diffuse and direct solar radiation have been used to infer the size distribution of aerosol particles in the intervening atmosphere as well as their optical properties. This is often accomplished by comparing measurements with computations for a wide range of aerosol parameters. This method is referred to as look-up table procedures whereby a solution is obtained by comparing measurements with theoretical calculations.
Michael D. King, Oleg Dubovik

6. Passive shortwave remote sensing from the Ground

Deducing and attempting to quantify properties of aerosols and trace atmospheric gases from ground-based sensors on the Earth’s surface has a long and interesting history that provided the foundation for the general field now known as “Remote Sensing”. The method utilizes measurements of the attenuation of light passing through the atmosphere, often taken at multiple wavelengths and sometimes broken down into spectral high resolution. Today, the method based on measuring attenuation of the direct beam is supplemented with measures of the intensity (radiance) or polarization of scattered light from the sun taken at different angles. Using the direct beam measurements to infer aerosol optical thickness is described in Chapter 4. The inversion methods applied to both direct beam and to angular measurements of the diffuse light are described in Chapter 5. In this chapter, we provide a history of ground-based aerosol remote sensing networks, leaving the formalism of the retrievals and inversions to the previous chapters. Not described in detail in this book are methods to retrieve trace gas measurements using absorption features measured by groundbased sensors, even though the history of trace gas measurements and aerosol measurements go hand-in-hand. Remote sensing by ground-based sensors can provide information about the spectral aerosol optical thickness, particle size distribution, aerosol absorption and degree of nonsphericity of the particles. The techniques often impose great demands on precision and accuracy. The history of ground-based remote sensing networks is plagued with good intentions that were stymied by the lack of consistent and reliable calibration that could assure the necessary precision and accuracy.
Glenn E. Shaw, Brent N. Holben, Lorraine A. Remer

7. History of passive remote sensing of aerosol from space

The earliest views of aerosol from space came from Russian cosmonauts who took handheld photographs of Earth and Earth’s atmosphere through the windows of orbiting spacecraft in the early 1960s (Lazarev et al., 1987). From these photographs we could see for the first time the bluish haze that covered polluted regions and the dust emitted from deserts. The pictures showed that these hazes were inhomogeneous and temporally inconsistent, but quantitative information was missing until the first spectroscopic measurements of the atmosphere were obtained in 1970 by Soyuz-9 cosmonauts using handheld spectrometers. Thus, the era of space-based aerosol remote sensing had begun.
Omar Torres, Lorraine A. Remer

8. Recent instruments and algorithms for passive shortwave remote sensing

Passive remote sensing of aerosol using the shortwave spectrum draws on a long heritage of experience that began with three main techniques described in Chapter 7: occultation methods, dark target approaches, and spectral ultra-violet (UV) algorithms. Beginning in the 1970s, these techniques have been applied to instruments flown on a series of different satellite platforms and have produced important time series of aerosol parameters that span decades. This heritage is especially valuable given the fact that the early downward-viewing sensors used for aerosol retrieval – the Advanced Very High Resolution Radiometer (AVHRR), the Total Ozone Mapping Spectrometer (TOMS) and even the Geostationary Operational Environmental satellites (GOES) – were designed for purposes other than retrieving aerosol. However, the success of using these instruments for aerosol characterization motivated the development of sensors designed with aerosol retrievals in mind. Improved spatial resolution, narrower spectral channels, increased spectral range and density, enhanced capability in terms of multiple angular views of the same scene and polarization are some of the specific improvements designed into the sensors flying during the 2000s that were intended to provide better aerosol retrievals than AVHRR and TOMS.
Lorraine A. Remer, Colette Brogniez, Brian Cairns, N. Christina Hsu, Ralph Kahn, Piet Stammes, Didier Tanré, Omar Torres

9. Longwave passive remote sensing

The longwave spectral domain dealt with here, also called the “thermal infrared” domain, roughly covers the 3–15 μm spectral range. The main radiation source is not the sun but the Earth system, that is, the Earth’s surface (land and ocean) and the atmosphere. The infrared emission from bodies is directly linked to their temperature as described by Plank’s law: the hotter the bodies are, the higher their emission. The Earth and its atmosphere are heated by the fraction of sunlight they absorb. Their increase in temperature results in increased infrared emission to space, thus ensuring the energy balance of the Earth system.
Clémence Pierangelo, Alain Chédin, Michel Legrand

10. Active lidar remote sensing

Lidar, an acronym for “LIght Detection And Ranging”, is an active remote sensing technique analogous to radar. Lidar systems use a laser as an active radiation source. The short pulse lengths produced by a laser (approximately 20 ns) and the spectral bandwidth (1 cm-1) allow for highly-resolved ranging measurements with high signal-to-noise. Also, as in radar, lidars could be either monostatic (collocated transmitter and receiver) or bistatic (separated transmitter and receiver). Figure 10.1 illustrates an operational, monostatic lidar. Laser radiation is transmitted and scattered or absorbed by atmospheric constituents, such as clouds, aerosols, or molecules. Photons scattered back to the receiver are then collected, directed to a detector whose signal is analog-to-digitally recorded or counted as a function of altitude or range. The strength of the return signal is related to the physical and optical properties of the scatterers.
M. Patrick McCormick, Kevin R. Leavor

11. Conclusion: Results and suggestions for future research

Aerosols are a complex and important component of the atmosphere. These particles consist of various chemical compositions (homogeneous or inhomogeneous), shapes and sizes, and they affect human health, the environment, visibility, and atmospheric chemistry. A major concern is their influence on climate, directly by modifying the Earth’s radiation budget, and indirectly by modifying cloudiness, cloud properties, precipitation and atmospheric circulations. The aerosol influence does not depend only on their total amount, as it is the case for the gaseous components of the atmosphere, but on their chemical composition, shapes, and sizes.
Jacqueline Lenoble, Lorraine A. Remer, Didier Tanré


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