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142 Analytical Techniques for Atmospheric Measurement
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Chapter 3
UV-Visible Differential Optical
Absorption Spectroscopy (DOAS)
John M.C. Plane and Alfonso Saiz-Lopez
3.1 Introduction
Differential optical absorption spectroscopy (DOAS) is the most widely used spectroscopic
technique for making measurements of atmospheric composition. It is the basis of nearly
all remote sensing measurements of chemical species in the troposphere and stratosphere,
both from the surface and airborne platforms and from satellites. It is also employed
in techniques operating in situ, such as cavity ring-down spectroscopy (CRDS). Satellite
observations of the atmosphere are an extensive subject, which are dealt with briefly in
Chapter 1. In this chapter we will describe the fundamentals of DOAS (Section 3.2),
and then explore in some detail the various DOAS techniques which are used within the
atmosphere. Note that this subject was reviewed in the mid-1990s (Platt, 1994; Plane &
Smith, 1995), so we will here focus on progress made during 1994–2005.
The light source required for a DOAS measurement in the atmosphere can be artificial,
for instance a high-pressure Xe lamp or a laser. If the light traverses the atmosphere
in a well-defined optical path of usually several kilometres, and the source is observed
directly after perhaps a single reflection, then this operating mode is known as long-path
DOAS (LP-DOAS) or active DOAS. The light can also be injected into a multi-pass cell
(usually in the White or Herriott configuration) where multiple reflections are used to
obtain a sufficiently long optical path to produce measurable absorption. An extension
of this scheme is cavity ring-down spectroscopy. All of these techniques are discussed
in Section 3.3. Note that we will not cover two other well-established applications that
employ differential spectroscopy, tunable diode laser absorption spectroscopy (TDLAS,
covered in Chapter 2) and differential absorption lidar (DIAL).
As an alternative to an artificial light source, an extra-terrestrial light source (sun,
moon or stars) can be employed for making the so-called passive DOAS measurements.
In the case of sunlight, either direct observations of the sun or indirect observations of
sunlight that has been scattered by air molecules (Rayleigh scattering) or particles (Mie
scattering), or reflected by the earth’s surface, can be used. Passive DOAS measurements
determine the column abundance of a species, often over very long paths. In Section 3.4
we will discuss examples of passive DOAS measurements from a variety of platforms,
and methods for retrieving spatial information from column abundance measurements.
Before beginning this review of the DOAS technique, it is worth considering in general
terms the desirable characteristics of an analytical technique for making atmospheric
148 Analytical Techniques for Atmospheric Measurement
measurements. First, the technique must have sufficient sensitivity to work at the low
ambient concentration levels at which many chemically interesting species occur in the
atmosphere. In this chapter we will mostly describe measurements of radical species,
which are usually highly reactive and thus occur at mixing ratios of the order of a few
parts per trillion (1 ppt is equivalent to a concentration of 24 ×10
7
molecules cm
−3
at a
pressure of 1 atm and temperature of 293 K). Therefore, sub-ppt detection limits are often
desirable. Second, the instrument needs to measure a particular species with high speci-
ficity, in order to avoid interferences from other compounds present in the atmosphere.
Third, being able to make measurements with high time resolution is necessary when
studying species that are not spatially homogeneous, usually because they have short
chemical lifetimes in the atmosphere. Finally, deploying an instrument that is largely
automated and simple to use is a considerable advantage when making measurements
in remote locations and over long periods. As we will see in this chapter, many of these
characteristics apply to DOAS instruments.
3.2 Principles of differential optical absorption
spectroscopy
3.2.1 The Beer–Lambert law
In absorption spectroscopy the Beer–Lambert law describes the exponential decrease in
the intensity of light passing through a sample containing an absorbing species i:
I
tr
= I
0
exp
⎛
⎝
−
l
0
i
P T
c
i
dl
⎞
⎠
(3.1)
where I
tr
is the light intensity, at wavelength , that is transmitted through the sample;
I
0
is the intensity of light incident on the sample; l is the path length of the light
through the sample containing species i with a concentration c
i
and an absorption
cross-section
i
P T at pressure P and temperature T . Hereafter,
i
P T will
be denoted as
i
. Note that
i
is defined with respect to base e; although this is
now the more widespread convention, some authors define
i
to base 10. The natural
logarithm of I
0
/I
tr
then defines the optical density (OD), which is the sum of the
optical densities of n contributing absorbers at wavelength :
OD = ln
I
0
I
tr
=
n
i=1
OD
i
=
n
i=1
l
0
i
c
i
dl (3.2)
The average concentration of species i over the light path of length l is then given by:
c
i
=
OD
i
i
l
(3.3)
UV-Visible Differential Optical Absorption Spectroscopy 149
In addition to molecular absorption, light is also extinguished in the atmosphere by
Rayleigh and Mie scattering (Platt, 1994; Plane & Smith, 1995). The Rayleigh extinction
coefficient describes elastic scattering of photons by air molecules:
R
=
R
−4
c
air
(3.4)
where
R
is 44 ×10
−16
cm
2
nm
4
for air, and c
air
is the molecular concentration of air
(24 ×10
19
molecule cm
−3
at 293 K and 1 atm pressure). Thus, the light intensity at a
wavelength of 300 nm is reduced by 12% per kilometre of light path, compared with
only 1% at 600 nm. The Mie extinction coefficient describes elastic scattering processes
by aerosols:
M
=
M
−n
(3.5)
where
M
and n (which varies between 1 and 4) depend on the aerosol size distribution
and chemical composition. In the evaluation of the transmitted intensity, Rayleigh and
Mie scattering can be treated analogously to molecular absorption, so that Equation 3.1
becomes:
I
tr
= I
0
exp
⎛
⎝
−
l
0
n
i=1
i
c
i
+
R
+
M
dl
⎞
⎠
(3.6a)
Averaging over the optical pathlength l yields:
I
tr
= I
0
exp
−l
n
i=1
i
c
i
+
R
+
M
(3.6b)
3.2.2 Differential absorption spectroscopy
In the atmosphere the majority of molecular species are present continuously, and
Rayleigh and Mie scattering always occur, so the unattenuated atmospheric spectrum I
0
cannot be obtained free of any extinction. Hence, the Beer–Lambert law (Equation 3.6b)
cannot be applied directly. Instead, differential absorption can be used if the absorption
spectrum of the species of interest contains marked structure within a fairly narrow wavelength
range (typically 10 nm or less). This structure will appear in the atmospheric absorption
spectrum, and can be used as a ‘fingerprint’ of the molecular species. The differential
absorption cross-section of the molecule,
i
, is defined by considering the absolute
cross-section
i
as the sum of
i
, which varies rapidly with wavelength, and a
slowly varying component
s
i
(Platt, 1994; Plane & Smith, 1995):
i
=
i
+
s
i
(3.7)
The partitioning of the absolute absorption cross-section into its differential and broad-
band components is depicted in Figure 3.1, for the case of NO
2
over a wavelength
150 Analytical Techniques for Atmospheric Measurement
4
6
8
Wavelength (nm)
350 360 370 380 390
–1
0
1
σ'/10
–19
cm
2
σ'/10
–19
cm
2
σ(λ)
σ
s
(λ)
σ′(λ)
σ′(λ) = σ(λ)
– σ
s
(λ)
Figure 3.1 Absolute and differential absorption cross sections for NO
2
in the 350–390 nm region. In the
top panel a 3
rd
degree polynomial (smooth line) is fitted to the absolute cross-section, , to account for
the slowly varying trend of the spectrum
s
. In the lower panel the differential absorption cross-section
, obtained after removing
s
is shown.
range commonly used by DOAS instruments. Note that the differential structure is much
smaller than the absolute cross-section; nevertheless, there is sufficient structure for fitting
to atmospheric absorption spectra. Substituting Equation 3.7 into 3.6b and including
a factor F
ins
which incorporates instrument-related factors such as the wavelength
dependence of the optical transmission in the spectrometer and the quantum efficiency
of the detector elements yields:
I
tr
= I
0
exp
−l
n
i=1
i
c
i
exp
−l
n
i=1
s
i
c
i
+
R
+
R
F
ins
(3.8)
The first exponential term in Equation 3.8 describes light extinction by narrow-band
molecular absorption features, whereas the second exponential term and the instrument
factor describe broad-band fluctuations. We can now define the measured intensity in
the absence of differential absorption by I
0
:
I
tr
= I
0
exp
−l
n
i=1
i
c
i
(3.9)
UV-Visible Differential Optical Absorption Spectroscopy 151
Hence, the differential optical density OD
is given by:
OD
= ln
I
0
I
tr
= l
n
i=1
i
c
i
(3.10)
By analogy with Equation 3.3, the concentration of a trace gas is now obtained from
the differential absorption cross-section and differential optical density:
c
i
=
OD
i
i
l
(3.11)
3.2.3 Wavelength regions and differential absorption
cross-sections
In this section we discuss the criteria for selecting the wavelength region in which
to observe a particular atmospheric species by DOAS. The first consideration is
the wavelength dependence of light transmission through the atmosphere. At short
wavelengths in the UV <290nm the optical transmission of the atmosphere is
significantly limited by the Hartley bands of O
3
, molecular absorption by O
2
and
Rayleigh scattering. These factors restrict DOAS measurements between about 220
and 290 nm to optical paths of less than about 1 km. In the infrared, optical trans-
mission in the atmosphere is limited by the vibrational bands of molecules with strong
absorption features such as H
2
O, O
3
and CO
2
. In addition, absorption coefficients in this
spectral region tend to be much weaker when compared with those in the UV/visible
(Okabe, 1978). Nevertheless, one advantage of working in the infrared is that individual
ro-vibrational transitions can often be resolved because Lorentz-broadening is negligible
at the range of pressures in the atmosphere (Okabe, 1978).
A second consideration when choosing a wavelength region is that the absorption
spectrum of the molecule of interest contains significant absorption structures – for
DOAS this is more important than the size of the absolute absorption cross-section.
However, the absorption bands in the UV/visible region arise from the Lorentz-broadened
ro-vibrational lines of electronic transitions (Okabe, 1978), so that the largest differential
structures often occur at wavelengths where there is also significant absolute absorption.
Figure 3.2 illustrates the differential absorption cross-sections in the UV/visible region
for a number of important atmospheric species, over the wavelength intervals commonly
used by DOAS instruments (Platt, 1994; Plane & Smith, 1995).
Another criterion to be considered is the spectral characteristics of the light source.
For example, the NO
2
molecule exhibits a larger differential (and absolute) absorption
cross-section in the 400–450 nm region compared with the 350–390 nm region. However,
measuring NO
2
in the former spectral region is complicated by the pressure-broadened
Xe lines in the spectra of high pressure Xe arc lamps usually employed in LP-DOAS
instruments, so that the near-UV region illustrated in Figure 3.1 is usually preferred
for measuring NO
2
, especially for low concentrations in clean air (Platt, 1994; Plane &
Smith, 1995).