Rotational Raman scattering in the Earth's atmosphere ... - SRON

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2 Chapter 1 Table 1.1: Instrument characteristics of spectrometers mentioned in the text. The satellite instruments passively sense the sunlight that is backscattered by the Earth at 700–800 km altitude and move in a near-polar orbit. This orbit is sun-synchronous, which means that it has a fixed orientation with respect to the Sun. The Earth rotates underneath the orbit so that global coverage is obtained in one day for OMI and GOME-2, in three days for GOME, and in six days for SCIAMACHY. The footprint size is the projected area on the Earth’s surface that corresponds to the instrument field-of-view (across track × along track). All instruments perform (near-)nadir measurements. SCIAMACHY performs limb and occultation observations as well. More information on GOME on the European Remote Sensing 2 (ERS- 2) satellite, SCIAMACHY on the Environmental Satellite (ENVISAT), and OMI on the Earth Observing System Aura satellite (EOS-Aura) and GOME-2 on the METOP-A satellite can be found in Burrows et al. [1999b], Bovensmann et al. [1999], Levelt et al. [2006], Callies et al. [2000], respectively. Spectrometer Satellite Launch Spectral range Spectral resolution Footprint size year [nm] [nm] [km 2 ] GOME ERS-2 1995 240–790 0.17–0.35 320×40 SCIAMACHY ENVISAT 2002 220–2400 0.22–1.50 60×30 OMI EOS-AURA 2004 270–500 0.45–1.00 24×13 GOME-2 METOP-A 2006 240–790 0.22–0.53 80×40 These instruments measure sunlight that is backscattered by the Earth. The measured spectra contain a wealth of information about atmospheric constituents and properties of the Earth’s surface. In addition to N 2 , O 2 , and Ar the Earth’s atmosphere contains many other gases. Despite their low concentrations these trace gases have a substantial influence on the physical and chemical properties of the atmosphere. The main purpose of the GOME, SCIAMACHY, OMI, and GOME-2 instruments is to determine trace gas abundances on a global scale. To reveal this information it is common to take the ratio of the Earth radiance spectrum (lower curve in Fig. 1.1) and a daily measured solar irradiance spectrum (upper curve in Fig. 1.1), which is called a reflectivity spectrum (Fig. 1.2). An advantage of using the reflectivity spectrum is that many calibration errors that are present in both the Earth and solar measurement cancel out. The reflectivity spectrum clearly displays how the incident solar light has been altered by its interaction with the Earth’s surface and its passage through the Earth’s atmosphere. For example, the absorption of sunlight by ozone (O 3 ) at wavelengths shorter than 320 nm is clearly visible in the reflectivity spectrum. Absorption features of other trace gases are much weaker. For example, nitrogen dioxide (NO 2 ) causes absorption features in the order of 1% near 420–460 nm, but these are hardly visible in Fig. 1.2. The reflectivity spectrum shows also the effects of scattering of sunlight by the Earth’s surface, clouds and air molecules. The clear-sky scene observation in Fig. 1.2 shows the effect of enhanced surface reflection by vegetation at wavelengths longer than 700 nm. Furthermore, the effect of light scattering by air molecules is seen as a decay of the clear-sky reflectivity spectrum between approximately 320 and 500 nm. Finally, the highly reflective property of clouds enhances the reflectivity spectrum at nearly all wavelengths (upper curve in Fig. 1.2).
Introduction 3 Extracting the wealth of information that is contained in the measured reflectivity spectra requires models that accurately describe the measurement with all its spectral features. In this thesis we focus on improving the modeling of the spectral features that are associated with light scattering by molecules. Before we discuss the details of molecular light scattering processes (Section 1.3), we will point out some of these spectral signatures. The inset in Fig. 1.2 shows a zoom-in on a typical clear-sky GOME reflectivity spectrum in the wavelength range 350–400 nm. The fact that light with short wavelengths is more strongly scattered by molecules than light with longer wavelengths is clearly seen in this particular wavelength range. The spectral continuum is commonly described by a purely elastic light scattering process called Rayleigh scattering. However, a closer inspection shows spectral fine-structure that is also caused by scattering by molecules. In each scattering event approximately 4% of the light is inelastically scattered by molecules due to a process called Raman scattering (e.g. [Joiner et al., 1995]). This type of inelastic scattering causes spectral fine-structure in the reflectivity spectrum, which is also referred to as the Ring effect as will be explained in the next section. This thesis deals with the development of accurate radiative transfer models that adequately describe the full reflectivity spectrum, with emphasis on including the Ring effect. With the help of such accurate models the retrieval of trace gas concentrations from satellite-based reflectivity spectra can be improved. 1.2 The Ring effect: a short historical background Filling-in of Fraunhofer lines – In 1802 William Wollaston noticed lines of low intensity in a solar spectrum that was measured on Earth. He figured – incorrectly – that these had to be the division lines between the different colors. Joseph von Fraunhofer rediscovered the lines in 1814 and studied them in greater detail. By 1815 he had catalogued more than 300 of them, assigning Roman letters to the most prominent ones. Each Fraunhofer line, as was found later by Kirchhoff and Bunsen (1861), can be attributed to a specific chemical element. Most Fraunhofer lines in a solar spectrum that is measured on the ground are absorption lines of chemical elements in the upper layers of the Sun, but some of them turned out to be of terrestrial origin. For example, the A Fraunhofer line near 760 nm is a strong absorption band of O 2 , which was later referred to as the O 2 A absorption band. In 1961 Grainger and Ring compared a diffuse skylight spectrum (i.e. a spectrum of the sky measured during the day, but pointing away from the solar disk) with a direct lunar spectrum. They discovered that the H Fraunhofer line near 397 nm was less deep in the diffuse skylight spectrum than in the direct lunar spectrum [Grainger and Ring, 1962]. Grainger and Ring proposed that daylight airglow might be responsible for this, but this hypothesis was later rejected [Brinkmann, 1968, Kattawar et al., 1981]. Since then several mechanisms have been proposed to explain the filling-in, for example by aerosol fluorescence [Noxon and Goody, 1965] and surface reflection effects such as plant fluorescence [Sioris et al., 2003]. Nowadays scientists concur that the filling-in of Fraunhofer lines in a daytime spectrum is predominately caused by inelastic scattering by N 2 and O 2 molecules in the Earth’s atmosphere [Joiner et al., 1995, Fish and Jones, 1995, Burrows et al., 1996, Chance
Page 1 and 2: VRIJE UNIVERSITEIT Rotational Raman
Page 5 and 6: Contents 1 Introduction 1 1.1 Obser
Page 7: 1 Introduction 1.1 Observing skylig
Page 11 and 12: Introduction 5 and Spurr, 1997, Vou
Page 13 and 14: Introduction 7 energy: E rot (v,J)
Page 15 and 16: Introduction 9 Figure 1.4: Probabil
Page 17 and 18: Introduction 11 scattered radiance
Page 19 and 20: Introduction 13 approximation in wh
Page 21 and 22: Introduction 15 scattering have bee
Page 23 and 24: 2 A doubling-adding method to inclu
Page 25 and 26: A doubling-adding method to include
Page 43: A doubling-adding method to include
Page 46 and 47: 40 Chapter 3 nm with a spectral res
Page 48 and 49: 42 Chapter 3 contributions of lower
Page 50 and 51: 44 Chapter 3 if we use the effectiv
Page 52 and 53: 46 Chapter 3 This in turn results i
Page 54 and 55: 48 Chapter 3 This may be shown in a
Page 56 and 57: 50 Chapter 3 The forward and adjoin
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52 Chapter 3 Here, ∆ = diag [1, 1
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54 Chapter 3 Figure 3.3: Relative R
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56 Chapter 3 biases the simulation
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58 Chapter 3 3.4.2 Approximation me
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60 Chapter 3 where I ram,app and I
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62 Chapter 3 3.5 Simulation of pola
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64 Chapter 3 Figure 3.10: Relative
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66 Chapter 3 dependence on the vert
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68 Chapter 3 Figure 3.13: Same as F
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70 Chapter 3 3.6 Summary A vector r
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72 Chapter 3 where λ is the wavele
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74 Chapter 3 where S l is the expan
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76 Chapter 3 3.C Appendix: Evaluati
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4 Accurate modeling of spectral fin
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5 Retrieval of cloud properties fro
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Retrieval of cloud properties from
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References Aben, I., F. Helderman,
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References 121 Ellery, A., D. Wynn-
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References 123 Lacis, A. A., J. Cho
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References 125 Schulz, F., K. Stamn
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Summary A spectrum of sunlight that
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Summary 129 Earth radiance spectrum
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Samenvatting Het door de aardatmosf
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Samenvatting 133 met een vector-mod
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List of publications Peer-reviewed
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