3.2 DOAS
3.2.1 Types of DOAS experiments
There are two main families of DOAS assemblies, with different goals and capabilities:
• Active systems, of which a simple illustration is presented in Fig. 3.6, are char-acterized by relying on an artificial light source for their measurements. A spec-trometer at the end of the light path performs spectroscopic detection. Active DOAS techniques are very similar to traditional in-lab absorption spectroscopy techniques [70];
• Passive DOAS techniques, illustrated in Fig. 3.7, use natural light sources, such as the Sun and the moon, in their measurement process. An electromagnetic radiation collection system is pointed in certain elevation and azimuth angles and sends the captured light into a spectrometer, connected to a computer. The system returns the total value of the light absorption in its path [60, 70].
CH A P T E R 3 . T H E O R E T I CA L BACKG R O U N D
Start
s= 0 δ=k X= LSQ(τ0, A) χ2= chisq(τ0,X) χ2o=χ2
so=s n=n+ 1 s+=s+δ s−=s−δ τ+= shift(τ0, s+) τ−= shift(τ0, s−) X+= LSQ(τ+0, A) X−= LSQ(τ−0, A) χ2+= chisq(τ+0,X+) χ2−= chisq(τ−0,X−) χ2= min(χ2+, χ2−)
n < nlim
&&
χ2> χ2min
χ2< χ2o
s=so
δ= 0.1·δ
End F
T
F T
Figure 3.5: Simplified schematic flowchart of the DOAS algorithm, including the non-linear part.
Figure 3.6: Active DOAS schematic.
Within the two main DOAS families, there are several types of possible experiment.
Differences in the design of these assemblies originate from a number of different target requirements. Active or passive applications can differ with relation to their intended spectral range, light throughput or resolution, among others.
In active experiments, the choice of the light source is the most critical aspect of the whole experimental design. Active DOAS radiation sources must be stable, have a very high throughput (these experiments are often conducted over long optical paths) and must have an adequate cost to purchase, maintain and operate. This is especially true in long-running experiments, which must remain working for months or even years.
3 . 2 . D OA S
Figure 3.7: Passive DOAS schematic.
Moreover, the spectral range of the emitted light is also of central importance, because most trace gases have very particular spectral cross sections. The spectral structure of the emitted light is also an important feature to consider, for similar reasons.
The sun and the moon are the two most important light sources when it comes to passive DOAS applications. Sunlight can be used directly or after a scattering event, the latter being the more common. In these experiments, instead of pointing directly at the sun, the collector is pointed at a certain point in the atmosphere, entering the system after it has been scattered. There are many possible geometries to a scattered sunlight DOAS experiment. Some of them are schematically represented in Figure 3.8.
142 6 Differential Absorption Spectroscopy
I I
Det.
1. Long-Path DOAS (LP-DOAS)
I0
Lamp + Det.
2. Vertical Profiling LP-DOAS Reflectors
Light source Retro-reflector
3. Tomographic DOAS 4. Folded-Path DOAS
Det.
5. Direct Sunlight DOAS
Det.
6. Balloon-borne (direct sunlight) DOAS
LPMA/DOAS Gondola + Balloon
SCIAMACHY
7. Satellite-borne DOAS - Occultation 8. Zenith Scattered Light (ZSL-DOAS)
Det.
9. Multi-Axis DOAS (MAX-DOAS)
Det.
10. Airborne Multi-Axis DOAS (AMAX-DOAS)
Fig. 6.4. The DOAS principle can be applied in a wide variety of light path ar-rangements and observation modes using artificial (1–4) as well as natural direct (5–7) or scattered (8–14) light sources. Measurements can be done from the ground, balloons, aircrafts, and from space
(a) Zenith Scattered Sunlight
142 6 Differential Absorption Spectroscopy
I I
Det.
1. Long-Path DOAS (LP-DOAS)
I0
Lamp + Det.
2. Vertical Profiling LP-DOAS Reflectors
Light source Retro-reflector
3. Tomographic DOAS 4. Folded-Path DOAS
Det.
5. Direct Sunlight DOAS
Det.
6. Balloon-borne (direct sunlight) DOAS
LPMA/DOAS Gondola + Balloon
SCIAMACHY
7. Satellite-borne DOAS - Occultation 8. Zenith Scattered Light (ZSL-DOAS)
Det.
9. Multi-Axis DOAS (MAX-DOAS)
Det.
10. Airborne Multi-Axis DOAS (AMAX-DOAS)
Fig. 6.4. The DOAS principle can be applied in a wide variety of light path ar-rangements and observation modes using artificial (1–4) as well as natural direct (5–7) or scattered (8–14) light sources. Measurements can be done from the ground, balloons, aircrafts, and from space
(b) MultiAxis-DOAS (MAX-DOAS)
142 6 Differential Absorption Spectroscopy
I I
Det.
1. Long-Path DOAS (LP-DOAS)
I0
Lamp + Det.
2. Vertical Profiling LP-DOAS Reflectors
Light source Retro-reflector
3. Tomographic DOAS 4. Folded-Path DOAS
Det.
5. Direct Sunlight DOAS
Det.
6. Balloon-borne (direct sunlight) DOAS
LPMA/DOAS Gondola + Balloon
SCIAMACHY
7. Satellite-borne DOAS - Occultation 8. Zenith Scattered Light (ZSL-DOAS)
Det.
9. Multi-Axis DOAS (MAX-DOAS)
Det.
10. Airborne Multi-Axis DOAS (AMAX-DOAS)
Fig. 6.4. The DOAS principle can be applied in a wide variety of light path ar-rangements and observation modes using artificial (1–4) as well as natural direct (5–7) or scattered (8–14) light sources. Measurements can be done from the ground, balloons, aircrafts, and from space
(c) Airborne MAX-DOAS
6.4 Experimental Setups of DOAS Measurements 143
12. Satellite-borne DOAS -Nadir Geometry 11. Imaging DOAS
Det.
13. Satellite-borne DOAS - Scattered Light Limb Geometry
Det.
14. Determination of the Photon Path length L (in Clouds) ‘inverse DOAS’
Fig. 6.4. Continued
light in the atmosphere. Spectroscopic detection is achieved by a spectrom-eter at the end of the light path. In general, active DOAS is very similar to classical absorption spectroscopy, as employed in laboratory spectral pho-tometers. However, the low trace gas concentrations in the atmosphere require very long light paths (up to tens of kilometres in length, see above), making the implementation of these instruments challenging (see Chap. 7 for details).
Active DOAS applications are typically employed to study tropospheric com-position and chemistry, with light paths that are often parallel to the ground.
In addition, active DOAS systems are also used in smog and aerosol chamber experiments.
The earliest applications of active DOAS, i.e. the measurement of OH radicals (Perner et al., 1976), used a laser as the light source along one single path (Fig. 6.4, Plate 1). This long-path DOAS setup is today most com-monly used with broadband light sources, such as xenon-arc lamps, to mea-sure trace gases such as O
3, NO
2, SO
2, etc. (e.g. Stutz and Platt, 1997a,b).
Expansion of this method involves folding the light beam once by using retro-reflectors on one end of the light path (Axelsson et al., 1990). This setup simplifies the field deployment of long-path DOAS instruments. In ad-dition, applications that use multiple retro-reflector setups to probe on dif-ferent air masses are possible. Figure 6.4, Plate 2, shows the setup that is used to perform vertical profiling in the boundary layer with one DOAS sys-tem. An expansion that is currently under development is the use of mul-tiple crossing light paths to perform tomographic measurements (Fig. 6.4,
(d) Imaging DOAS
Figure 3.8: Several examples of possible passive DOAS experiment geometries. All these examples use only the light from an astronomical source to detect many atmo-spheric trace gases. All these examples were taken from [70].
Although instrumentally simpler than their active counterpart, passive DOAS applications require more care in the retrieval process. In this kind of application, light sources are extremely far away. Additionally, they are normally highly structured.
This means that one has to be extremely mindful when using it for the retrieval of small concentration changes. In addition to this, there is always the need to convert
37
CH A P T E R 3 . T H E O R E T I CA L BACKG R O U N D
the system’s direct measurement, a column density, into vertical densities. Since in scattered sunlight measurements, the optical path is impossible to calculate in a precise manner, this requires the use of complex radiative transfer models [36, 70].
3.2.1.1 Satellite Measurements
One particularly interesting use of passive DOAS are satellite measurements. There are three types satellite DOAS experiments:
Occultation measurements: this is a direct sunlight measurement. Light comes from the sun and traverses the Earth’s atmosphere in a tangential manner before entering the satellite’s light collector;
142 6 Differential Absorption Spectroscopy
I I
Det.
1. Long-Path DOAS (LP-DOAS)
I 0
Lamp + Det.
2. Vertical Profiling LP-DOAS Reflectors
Light source Retro-reflector
3. Tomographic DOAS 4. Folded-Path DOAS
Det.
5. Direct Sunlight DOAS
Det.
6. Balloon-borne (direct sunlight) DOAS
LPMA/DOAS Gondola + Balloon
SCIAMACHY
7. Satellite-borne DOAS - Occultation 8. Zenith Scattered Light (ZSL-DOAS)
Det.
9. Multi-Axis DOAS (MAX-DOAS)
Det.
10. Airborne Multi-Axis DOAS (AMAX-DOAS)
Fig. 6.4. The DOAS principle can be applied in a wide variety of light path ar-rangements and observation modes using artificial (1–4) as well as natural direct (5–7) or scattered (8–14) light sources. Measurements can be done from the ground, balloons, aircrafts, and from space
Figure 3.9: Occultation measurement schematic representation [70]
Limb: a scattered sunlight measurement, in which the collector is pointed towards the Earth, at an angle. Light reaches the detector after being scattered in the atmosphere, the ground, or both;
6.4 Experimental Setups of DOAS Measurements 143
12. Satellite-borne DOAS -Nadir Geometry 11. Imaging DOAS
Det.
13. Satellite-borne DOAS - Scattered Light Limb Geometry
Det.
14. Determination of the Photon Path length L (in Clouds) ‘inverse DOAS’
Fig. 6.4. Continued
light in the atmosphere. Spectroscopic detection is achieved by a spectrom-eter at the end of the light path. In general, active DOAS is very similar to classical absorption spectroscopy, as employed in laboratory spectral pho-tometers. However, the low trace gas concentrations in the atmosphere require very long light paths (up to tens of kilometres in length, see above), making the implementation of these instruments challenging (see Chap. 7 for details).
Active DOAS applications are typically employed to study tropospheric com-position and chemistry, with light paths that are often parallel to the ground.
In addition, active DOAS systems are also used in smog and aerosol chamber experiments.
The earliest applications of active DOAS, i.e. the measurement of OH radicals (Perner et al., 1976), used a laser as the light source along one single path (Fig. 6.4, Plate 1). This long-path DOAS setup is today most com-monly used with broadband light sources, such as xenon-arc lamps, to mea-sure trace gases such as O 3 , NO 2 , SO 2 , etc. (e.g. Stutz and Platt, 1997a,b).
Expansion of this method involves folding the light beam once by using retro-reflectors on one end of the light path (Axelsson et al., 1990). This setup simplifies the field deployment of long-path DOAS instruments. In ad-dition, applications that use multiple retro-reflector setups to probe on dif-ferent air masses are possible. Figure 6.4, Plate 2, shows the setup that is used to perform vertical profiling in the boundary layer with one DOAS sys-tem. An expansion that is currently under development is the use of mul-tiple crossing light paths to perform tomographic measurements (Fig. 6.4, Plate 3).
Figure 3.10: Schematic representation of the limb satellite measurement geometry.
Nadir: this is the most common measurement geometry for satellite experiments.
In this mode, light that gets reflected off the Earth’s surface is captured by the collecting device, while it is pointing directly down.
Satellite based DOAS measurements have also been important because they have given rise to new trace gas retrieval techniques. Through them, new trace gases, pre-viously unreachable through DOAS have been quantified on a global level, such as
38