2.3 Other Instruments
2.3.7 sage
sage (Stratospheric Gas and Aerosol Experiment) stands for a series of space-borne occultation instruments. It started in the 1970s with the sam experiments (Stratospheric Aerosol Measurement) and continued with sage onboard Applica-tions Explorer Mission B (19791981), sage-ii on erbs (Earth Radiation Bud-get Satellite launched in 1984), and sage-iii on Meteor-3M (launched in Decem-ber 2001). sage-iii is a multi-channel grating spectrometer covering the uv-vis-nirwavelength range similar to sciamachy. All other instruments up to sage-ii were broadband instruments.
sage-iii is the rst instrument that performed also lunar occultation measure-ments. sage-iii is planned to be extended by an instrument on the international space station in 2004. Results of sage-iii are also used for sciamachy validation.
A detailed description of retrieval algorithms and the instrumental design can be found in [McCormick et al., 2002]. Results of the sage-iii instrument are com-pared with sciamachy occultation results in Chapter 5. The sage-ii instrument is described in [Mauldin et al., 1985].
30 CHAPTER 2. SCIAMACHY
poam-ii poam-iii
vertical 0.01◦ 0.013◦
ifov horizontal 1◦ 0.81◦
Orbit 833 km
sun-synchronous, polar 98.7◦ inclination
833 km
sun-synchronous, polar 98.7◦ inclination Spatial Coverage 54◦71◦ North (sunrise)
63◦88◦ South (sunset) 54◦71◦ North (sunrise) 63◦88◦ South (sunset)
Wavelength channelsa
1: 252.3 nm (4.4 nm) 2: 441.6 nm (2.0 nm) 3: 448.1 nm (2.1 nm) 4: 601.4 nm (14.3 nm)
5: 761.2 nm (2.2 nm) 6: 781 nm (16.7 nm)
7: 921 nm (2.1 nm) 8: 936.4 nm (2.3 nm) 9: 1060.3 nm (11.1 nm)
1: 354 nm (9.7 nm) 2: 439.6 nm (2.1 nm) 3: 442.2 nm (2.1 nm) 4: 603 nm (17.7 nm) 5: 761.3 nm (2.3 nm) 6: 779 nm (10.2 nm) 7: 922.4 nm (2.6 nm) 8: 935.9 nm (2.6 nm) 9: 1018 nm (11.6 nm) Main targets O3, NO2, H2O, aerosols,
temperature O3, NO2, H2O, aerosols, temperature
aEach channel has one detector for a specic wavelength with a spectral width given in parentheses.
Table 2.7: Characteristics of recent poam instruments.
2.3. OTHER INSTRUMENTS 31
sage-ii sage-iii
vertical 0.00833◦ 0.00833◦
ifov horizontal 0.04167◦ 0.04167◦ (lunar: 0.0833◦)
Orbit 650 km
non sun-synchronous 57◦ inclination
1020 km
sun-synchronous, polar 97.5◦ inclination Spatial Coveragea 80◦ North to 80◦ South 48◦80◦ North (sunrise)
34◦58◦ South (sunset)
Wavelength channelsb
385 nm 448 nm 453 nm 525 nm 600 nm 940 nm 1020 nm
287293 nm 382386 nm 432450 nm 518522 nm 560596 nm 753771 nm 867871 nm 933960 nm 10191025 nm 15301560 nm Main targets O3, NO2, H2O, aerosols O3, NO2, NO3, OClO,
H2O, aerosols, clouds, pressure, temperature
aLunar occultation of sage-iii is highly variable and covers almost all latitudes.
bIn sage-ii, each channel has one detector for a specic wavelength with a spectral bandwidth of 1020 nm. At 448 nm and 453 nm the width is 3 nm and 2 nm, respectively. sage-iii has various detector systems with a spectral resolution of approximately 0.7 nm and 10 nm beyond 1000 nm.
Table 2.8: Characteristics of recent sage instruments.
32 CHAPTER 2. SCIAMACHY
Chapter 3
Simulation of Measurements
Basic part of each trace gas retrieval is the so-called forward model. The forward model maps the concentration proles into the wavelength space with the spectro-scopic data. It is needed by every retrieval method. Furthermore, it reects the knowledge about the radiative transfer through the atmosphere. In this chapter, the radiative transfer model for occulation geometry will be introduced. Finally, some studies concerning the theoretical sensitivity of sciamachy will be discussed.
3.1 Atmospheric Constitution
For a sophisticated radiative transfer model that can be used for complex trievals, a broad understanding of atmospheric properties is required. Every re-trieval method needs an a priori knowledge about trace gas mixing and aerosol loading.
Atmospheric Layers
It is useful to divide the atmosphere into dierent layers. This is commonly done by the characteristic behaviour of the vertical temperature gradient. Temperature is plotted versus altitude in Figure 3.1. The values are averaged globally. Since pressure, which decreases exponentially with height, is an important quantity to many applications, it is sometimes convenient as an alternative height scale. The atmospheric layers can be divided into two main dierent classes, characterised by temperature increase and decrease with height, respectively:
Troposphere The lowest layer of the atmosphere extending from the surface up
33
34 CHAPTER 3. SIMULATION OF MEASUREMENTS
Figure 3.1: Temperature vs. altitude. It is the basis of a commonly used classica-tion into layers. The logarithmic pressure scale on the right is often used as height scale. Data are taken from [NASA, 1976].
to 1015 km is characterised by decreasing temperature with height and rapid vertical mixing due to solar irradiance and heating of the surface. All weather phenomena we observe every day take place within the troposphere.
Stratosphere The next layer extends up to approximately 50 km. Temperature increases with height as a result of the strong absorption in the ozone layer.
Therefore, vertical mixing is slow.
Mesosphere It extends up to circa 90 km and is characterised by decreasing tem-perature and rapid vertical mixing. It ends with the coldest point in the atmosphere.
Thermosphere Here the temperature increases extremely due to absorption of short wavelength radiation by N2 and O2. Nevertheless, vertical mixing is rapid.
Exosphere The outermost region beginning roughly at 500 km is the Exosphere from which molecules with high translational energies can escape into space.
The boundaries between the layers are called tropopause, stratopause, and mesopause, respectively. They are dened by the points of inection in Figure 3.1.
Sometimes, the upper mesosphere and the lower thermosphere are called iono-sphere indicating a huge amount of ionised particles. The upper region dominated
3.1. ATMOSPHERIC CONSTITUTION 35
by the Earth's magnetic eld is also called magnetosphere. It is our protection against the solar wind.
Atmospheric Gases
At a global average, the dry Earth's atmosphere is mainly composed of the follow-ing gases:
Constituent Mixing ratio [ppm]
Nitrogen (N2) 780,840 Oxygen (O2) 209,460
Argon 9,340
Carbondioxid (CO2) 355
Neon 18
Helium 5.2
Methan (CH4) 1.72
Krypton 1.1
Ozone (O3) 0.010.1
Nitrogenoxides (NOx) 10−610−2
Most of them are well mixed, i.e. their mixing ratios are almost constant throughout the atmosphere, as they are chemically inert. Other gases that are more active such as ozone show a certain height distribution. Nitrogenoxides are also locally related to combustion on the Earth's surface.
The calculation of vertical concentration proles is the major task of occulta-tion measurements. To obtain them properly, a priori knowledge about the height distribution is required. Throughout this thesis, the U.S. standard climatology from 1976 is used [NASA, 1976] as well as information provided by Anderson et al.
[1986] and model calculations from the Max-Planck-Institute, Mainz [Brühl and Crutzen, 1991]. They contain global averages and estimations of pressure, tem-perature, and trace gas proles. Standard proles for O3 and NO2 are shown in Figure 3.2. Here, they are given in volume mixing ratios as well as in absolute concentrations. Considering trace gases to be approximately ideal gases, the vol-ume mixing ratio is also the mixing ratio in terms of molecule numbers. It is a common unit within atmospheric sciences. In the case of spectroscopic measure-ments, absolute concentrations are needed for calculation of optical thicknesses, as will be discussed in Section 3.2. Particle number density N can be calculated from volume mixing ratio vmr, pressure p, and temperature T via the ideal gas law:
N =vmr· p
RT ·NA, (3.1)
36 CHAPTER 3. SIMULATION OF MEASUREMENTS
Figure 3.2: Globally averaged vertical proles of O3 and NO2 [NASA, 1976] given in volume mixing ratio and absolute concentrations.
where R = 8.31441 J
mol·K is the molar gas constant and NA = 6.022·1023mol−1 the Avogadro constant.
Aerosols
Aerosols are generally considered to be particles in the range from a few nanome-ters to tens of micromenanome-ters. They can be liquid or solid. The technical denition of aerosols contains a suspension of particles in a gas. In atmospheric sciences, the expression focusses only on the particles. For occultation measurements, mainly the region of the upper troposphere and the lower stratosphere is of interest. The aerosol background in this region is mainly provided by carbonyl sulde (OCS).
Its life time is long enough to diuse from onground sources into the stratosphere.
There it is dissociated by uv-radiation and forms sulfuric acid solutions. Oc-casionally, large volcanic eruptions inject considerable amounts of SO2 into the stratosphere, resulting also in sulfuric acid aerosols. In the lower troposphere, one