SIST EN 16253:2013

SLOVENSKI STANDARD
oSIST prEN 16253:2011
01-junij-2011
.DNRYRVW]UDND3UL]HPQHPHULWYH]XQDQMHJD]UDND]GLIHUHQFLDOQRRSWLþQR
DEVRUEFLMVNRVSHNWURVNRSLMR'2$60HULWYH]XQDQMHJD]UDNDLQUD]SUãHQLKHPLVLM
Air quality - Atmospheric measurements near ground with Differential Optical Absorption
Spectroscopy (DOAS) - Ambient air and diffuse emission measurements
Luftqualität - Messungen in der bodennahen Atmosphäre mit der Differentiellen
Optischen Absorptionsspektroskopie (DOAS) - Immissionsmessungen und Messungen
von diffusen Emissionen
Qualité de l'air - Mesurages atmosphériques à proximité du sol par Spectroscopie
d'Absorption Optique Différentielle (DOAS) - Mesurages de l'air ambiant et des
émissions diffuses
Ta slovenski standard je istoveten z: prEN 16253
ICS:
13.040.20 Kakovost okoljskega zraka Ambient atmospheres
oSIST prEN 16253:2011 en,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
oSIST prEN 16253:2011

---------------------- Page: 2 ----------------------
oSIST prEN 16253:2011


EUROPEAN STANDARD
DRAFT
prEN 16253
NORME EUROPÉENNE

EUROPÄISCHE NORM

March 2011
ICS
English Version
Air quality - Atmospheric measurements near ground with
Differential Optical Absorption Spectroscopy (DOAS) - Ambient
air and diffuse emission measurements
Qualité de l'air - Mesurages atmosphériques à proximité du Luftqualität - Messungen in der bodennahen Atmosphäre
sol par Spectroscopie d'Absorption Optique Différentielle mit der Differentiellen Optischen Absorptionsspektroskopie
(DOAS) - Mesurages de l'air ambiant et des émissions (DOAS) - Immissionsmessungen und Messungen von
diffuses diffusen Emissionen
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee CEN/TC 264.

If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations which
stipulate the conditions for giving this European Standard the status of a national standard without any alteration.

This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other language
made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland,
Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to
provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without notice and
shall not be referred to as a European Standard.


EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2011 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 16253:2011: E
worldwide for CEN national Members.

---------------------- Page: 3 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
Contents Page
Foreword .3
Introduction .4
1 Scope .5
2 Terms and definitions .5
3 Symbols and abbreviations .6
4 Principle .7
5 Measurement procedure . 14
6 Measurement planning . 17
7 Procedure in the field . 18
8 Calibration methods . 20
9 Quality assurance . 21
Annex A (informative) Components of the measurement system . 23
Annex B (informative) Influence of scattered solar radiation . 30
Annex C (informative) Examples of implementations of the DOAS technique . 31
Annex D (informative) Performance characteristics . 39
Annex E (informative) SI and common symbols and units in spectroscopy . 44
Annex F (informative) Application examples . 45
Annex G (informative) Calibration with complete spectral modelling . 72
Annex H (informative) Example of sample form for a measurement record . 74
Bibliography . 78

2

---------------------- Page: 4 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
Foreword
This document (prEN 16253:2011) has been prepared by Technical Committee CEN/TC 264 “Air quality”, the
secretariat of which is held by DIN.
This document is currently submitted to the CEN Enquiry.
3

---------------------- Page: 5 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
Introduction
Differential Optical Absorption Spectroscopy (DOAS) has been successfully progressed, starting in the late
1970s, from a laboratory based method to a versatile remote sensing technique for atmospheric trace gases.
In the DOAS measuring process, the absorption of radiation in the ultraviolet, visible or infrared spectral range
by gaseous constituents is measured along an open monitoring path between a radiation source and a
spectrometer, and the integral concentration over the monitoring path is determined.
DOAS systems support direct multi-constituent measurements. They provide alternative measuring
techniques in that they can handle a large number of measuring tasks which cannot be adequately addressed
by in-situ techniques based on point measurements. Examples of such tasks include the monitoring of diffuse
emissions from area sources such as urban settlements, traffic routes, sewage treatment plants and
industrially or agriculturally used surface areas; the minimisation of production losses through a detection of
leaks in equipment zones or pipeline systems; or ambient air monitoring in any of the above mentioned
applications.
With an appropriate measuring set-up, the local air pollution can usually be assessed very quickly.
Measurements can be taken effectively even in areas which are difficult or impossible to access, or where the
direct presence of personnel or equipment would be hazardous. The measurement in the open atmosphere
eliminates potential losses by sample handling.
4

---------------------- Page: 6 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
1 Scope
This document describes the operation of active DOAS measuring systems with continuous radiation source,
the calibration procedures and applications in determining gaseous constituents (e.g., NO , SO , O , BTX, Hg)
2 2 3
in ambient air or in diffuse emissions.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
active DOAS
DOAS with artificial radiation source
2.2
background spectrum
spectrum taken by the DOAS system with the lamp switched off
NOTE The background spectrum results mainly from scattered sunlight.
2.3
complete spectral modelling
process of using synthetic spectra to match with observed experimental spectra
2.4
dark spectrum
spectrum which identifies the thermal effects of the detector when no radiation is admitted to the detector
2.5
electronic offset spectrum
spectrum which identifies the electronic effects of the detector when no radiation is admitted to the detector
2.6
instrument line shape (ILS)
mathematical function which describes the effect of the instrument's response on a monochromatic line
2.7
intensity
radiant power per unit solid angle (non-collimated beam) or per unit area (collimated beam)
2.8
lamp spectrum
spectrum which is achieved by admitting direct light from the lamp to the spectrometer
2.9
monitoring path
actual path in space over which the pollutant concentration is measured and averaged
2.10
open-path measurement
measurement which is performed in the open atmosphere
2.11
path length
distance that the radiation travels in the open atmosphere
5

---------------------- Page: 7 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
2.12
reference spectrum
spectrum of the absorbance versus wavenumber for a pure gaseous sample under defined measurement
conditions and known and traceable concentrations
2.13
signal-to-noise ratio
ratio between the signal strength and the root mean square noise
3 Symbols and abbreviations
3.1 Symbols
a(λ) specific absorption coefficient at wavelength λ;
a (λ) specific absorption coefficient of constituent i at wavelength λ;
i
a (λ) portion of the specific absorption coefficient which varies little with the wavelength;
0i
′
a (λ) portion of the specific absorption coefficient which varies strongly with the wavelength;
i
a Mie scattering coefficient;
M
a Rayleigh scattering coefficient;
R
c mass concentration;
c aerosol mass concentration;
AE
c mass concentration of constituent i;
i
c density of air;
LM
d coefficient j of a polynomial;
j
D(λ) optical density;
D'(λ) differential optical density;
i index number;
I(λ, l) intensity of received radiation of wavelength λ after a path-length l;
I (λ) intensity of emitted radiation of wavelength λ;
0
'
I (λ,l) differential initial intensity;
0
I (λ) modelled intensity;
mod
l length of the monitoring path;
M molar mass of component i;
i
6

---------------------- Page: 8 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
p atmospheric pressure;
R molar gas constant (= 8,3145 J/(mol⋅K));
S(λ) intensity of scattered solar radiation of wavelength λ;
T ambient temperature;
x mixing ratio of component i.
i
τ(λ) attenuation factor of the optical system.
3.2 Abbreviations
DOAS Differential Optical Absorption Spectroscopy
IR Infrared
UV Ultraviolet
UV/VIS Ultraviolet/Visible
4 Principle
4.1 General
The DOAS measurement is based on the principle whereby the atmospheric concentration of gaseous
constituents is quantified on the basis of their characteristic absorption of radiation. The radiation spectrum
examined for this purpose ranges from near ultraviolet to near infrared (approx. 250 nm to 2500 nm).
Accordingly, the analysed absorption of radiation will be based on electronic transitions in molecules and,
possibly, atoms and in the near infrared on molecular vibrational transitions.
The method shows high selectivity and sensitivity due to the following combination of features:
 The measurement of radiation intensities is conducted with a high spectral resolution (0,1 nm to 1 nm)
over a broad spectral range comprising numerous vibrational bands of one or more electronic transition(s).
 Reference spectra are fitted to the measured spectra by the least squares method. Thus, the
characteristic absorption structures of the target compounds are employed to identify the measured
compounds. Superimposed absorption structures of other constituents may be separated.
 Since the structured spectral absorption is analysed, unusually low optical densities (in some cases below
–3
10 ) can be identified. This fact, in conjunction with the long monitoring paths (usually from ca. 100 m to
ca. 800 m, depending on the compounds to be measured) in the open atmosphere, yields low limits of
detection for the trace gases.
 Quasi-continuous absorptions resulting from absorption processes by particles and droplets (e.g.,
radiation attenuation due to aerosol dispersion or decreasing transmittance of the optical system) as well
as moderate fluctuations of the radiation intensity will not affect the result over a wide measurement
range because in this technique differential absorption is used rather than the absolute absorption.
4.2 Configuration of the measurement system
Open-path techniques measure the 'concentration × path-length' product of one or more species in the
atmosphere within a defined, extended optical path. The concentration of the species is derived from this
7

---------------------- Page: 9 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
measurement value. Two of the basic configurations for an open-path monitoring system are given in Figures
1 and 2.
In the bistatic system (Figure 1) the transmitter and the detector are separated at the two ends of the optical
path. The monostatic system (Figure 2) operates by transmitting the optical beam into the atmosphere to a
passive retroreflector which returns the beam to the detector.

Key
1 DOAS spectrometer
2 telescope for radiation collection
3 ambient air
4 monitoring path
5 radiation source with collimating optics
Figure 1 ― Bistatic arrangement for DOAS remote sensing

Key
3 ambient air
4 monitoring path
6 DOAS spectrometer including radiation source
7 telescope for transmission and collection of radiation
8 retro-reflector
Figure 2 ― Monostatic arrangement for DOAS remote sensing
In the bistatic measurement set-up, the radiation source (5) and the DOAS spectrometer (1) are spatially
separated. The two instrumental parts are oriented in such a way that the radiation emitted from the radiation
8

---------------------- Page: 10 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
source and collimated by a parabolic mirror is collected by the DOAS spectrometer telescope (2). The
monitoring path length is the distance between collimating and receiving optics.
For a monostatic measurement set-up, transmitting and receiving optics are an integral part of the DOAS
spectrometer (6), which also includes the radiation source and a beam splitter serving to separate the
received and transmitted beams. By means of a retroreflector (8) the radiation beam passes twice through the
measurement volume. The monitoring path length in this case is twice the distance between the
transmitter/receiver and the retroreflector optics.
4.3 The Beer-Lambert law
When radiation passes through a medium, e.g., the atmosphere, it undergoes a change in intensity that can
be expressed by means of the Beer-Lambert law:
I (λ,l)= I (λ)⋅ exp()− a(λ)⋅ c⋅ l (1)
0
where
I(λ, l) is the intensity of the radiation of wavelength λ incident on the receiver after passing the atmosphere
along the monitoring path l;
I (λ) is the intensity of the radiation of wavelength λ emitted by the radiation source;
0
3 –1 –1
a(λ) is the specific absorption coefficient of the medium at wavelength λ in (µg/m ) ·m ;
3
c is the concentration of the measured constituent in µg/m ;
l is the length of the monitoring path in m.
The radiation absorption produces changes in the energy state of the absorbing gaseous species. In the
UV/VIS range considered here, this implies a change in the rotational and vibrational state of the gaseous
species, in addition to the change in their electronic state. As a general rule, the rotational bands are not
resolved, so what is measured essentially is the vibrational structure of the electronic transitions [1].
By introducing the optical density D(λ)
 I()λ 
0
D()λ = ln  (2)
 
()
I λ
 
and from Equation (1), the concentration c of the absorbing gaseous species is
D()λ
c= (3)
a()λ ⋅ l
I(λ)
NOTE The term extinction is also widely used for D(λ). The quotient is defined as the transmittance.
I (λ)
0
4.4 Extended Beer-Lambert law
In atmospheric measurements, radiation is attenuated not merely by molecular absorption effects. It also
disappears from the monitoring path due to scattering by air molecules (Rayleigh scattering) and to absorption
and scattering by aerosol particles (Mie scattering). Apart from these attenuation effects, Rayleigh scattering
of solar radiation leads to an increase in the radiation intensity incident on the detector. This additional
9

---------------------- Page: 11 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
contribution shall be determined and taken into account, as appropriate (see Annex B). Considering all these
effects the absorption law (1) takes the following form:
 
( ) () ()( ) () () ()
I λ,l = I λ ⋅ exp − a λ ⋅ c − a λ ⋅ c ⋅l + − a λ ⋅ c ⋅ l + S λ (4)
0 R LM M AE i i
∑
 
i
 
where
I(λ, l) is the intensity of the radiation of wavelength λ incident on the receiver after passing the atmosphere
along the monitoring path l;
I (λ) is the intensity of the radiation of wavelength λ emitted by the radiation source;
0
3 -1 -1
a (λ) is the Rayleigh scattering coefficient in (µg/m ) ·m ;
R
3
c is the density of the ambient air in µg/m ;
LM
3 -1 -1
a (λ) is the Mie scattering coefficient in (µg/m ) ·m ;
M
3
c is the aerosol concentration in µg/m ;
AE
l is the length of the monitoring path in m;
3 -1 -1
a (λ) is the specific absorption coefficient of constituent i at wavelength λ in (µg/m ) ·m ;
i
3
c is the concentration of constituent i in µg/m ;
i
S(λ) is the intensity of scattered solar radiation.
NOTE The concentration of the constituent i in a mixed medium is expressed as a mass concentration (c =m /V). It
i
i
can also be expressed as a mixing ratio x = n /n = V /V in ppm or ppb using the following conversion formula:
i i tot i tot
p⋅ M
i
c = x ⋅
i i
T⋅ R
where
3 3
c is the mass concentration of constituent i in µg/m (or mg/m )
i
x is the mixing ratio of constituent i in ppb (or ppm)
i
p is the atmospheric pressure in Pa
T is the ambient temperature in K
M is the molar mass of component i in kg/mol
i
R is the molar gas constant (= 8,3145 J/(mol⋅K))
Reference conditions (pressure, temperature) for ambient air measurements are usually 1013 hPa and 20 °C.
Equation (4) does not yet allow an analysable correlation between I(λ, l) and I (λ) to be established, i.e.,
0
determining a concentration averaged over the monitoring path. This is due to the difficulty of determining the
intensity I (λ) incident on the absorber and to the variability of light scattering effects in the atmosphere as
0
expressed by a (λ) and a (λ).
R M
10

---------------------- Page: 12 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
4.5 Differential optical density
The DOAS method relies only on the high-frequency (narrowband) part of the absorption structure. For this
purpose, the absorption coefficient a (λ) is considered the sum of two components as shown in Figure 3:
i
′
a()λ = a (λ)+ a()λ (5)
i 0i i
where
a (λ) is the portion of the specific absorption coefficient which varies little with the wavelength;
0i
′
a (λ) is the portion of the specific absorption coefficient which varies strongly with the wavelength.
i
′
The parameters a (λ) and a()λ denote the low-frequency and high-frequency portion of the absorption
0i
i
coefficient of the gaseous component i (see Figure 3).
Equation (4) can then be modified as follows:
   
   
I(λ,l)= I()λ ⋅ exp(− a ()λ ⋅ c − a ()λ ⋅ c )⋅ l+ − a ()λ ⋅ c ⋅ l ⋅ exp − a'()λ ⋅ c ⋅l + S()λ (6)
0 R LM M AE ∑ 0i i ∑ i i
   
 i   i 
The differentiation with respect to the absorption coefficient a (λ) permits definition of the differential initial
i
′
intensity I (λ) which corresponds to the initial intensity I (λ) after attenuation through Rayleigh and Mie
0
0
scattering and through the continuous portion of the component-related absorption. Furthermore it is assumed
that the intensity of the scattered solar radiation S(λ) (see Equation (4)) will be determined and subtracted.
Additionally, the attenuation factor τ(λ) is introduced which allows for the broad wavelength-dependent
transmission of the entire optical system (radiation source, telescopes, spectrometer (cf. Annex A)), including
the spectral sensitivity of the detector. Thus, Equation (6) can be modified as follows:
 
 
I' (λ)= I (λ)⋅exp()− a (λ)⋅ c − a (λ)⋅ c ⋅l+ − a (λ)⋅ c ⋅l ⋅τ (λ) (7)
0 0 R LM M AE ∑ 0i i
 
 i 
where
′
I (λ) is the differential initial intensity;
0
τ(λ) is the attenuation factor of the optical system.
The optical density determined on the basis of the differential initial intensity I′ (λ) is referred to as the
0
differential optical density D'(λ):
′
I (λ)
0
′
D' (λ)= ln = a()λ ⋅ c ⋅ l (8)
i i
∑
I(λ)
This procedure ensures that DOAS spectra can be properly analyzed, as the optical density definition is
expanded by taking into account the influence of the continuous absorption structures, i.e., those which do not
′
vary much with the wavelength. By relying on the concept of differential initial intensity I (λ) , DOAS solves
0
the problem that the intensity of initial radiation I (λ) emitted by a radiation source is impossible to determined
0
from a measured spectrum due to absorption and scattering effects. Figure 3 illustrates the difference
′
between the intensities I(λ), I (λ), I (λ) .
0
0
11

---------------------- Page: 13 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)

Key
1 Mie extinction
2 Rayleigh extinction
3 continuous absorption component
X wavelength
Y1 intensity
Y2 absorption coefficient
Figure 3 ― Intensities I(λλ), I (λλ) and I ’(λλ) in an absorption spectrum (upper panel) and associated
λλ λλ λλ
0 0
absorption coefficients a(λλλλ), a (λλλλ) and a’(λλλλ) (lower panel)
0
I′ (λ) can be determined by interpolation between the shoulder values I(λ ) and I(λ ). D′(λ) is then obtained
1 2
0
from the quotient of the intensities I′ (λ) (centre of band) and I(λ) according to Equation (9).
0
I′ (λ)  λ −λ
0 0 1
D' (λ)= ln = ln I(λ )+()I(λ )− I(λ ) ⋅ − lnI(λ ) (9)
1 2 1 0
 
I(λ) λ −λ
 2 1
12

---------------------- Page: 14 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
However, this method is used only in simple cases (one single strong, dominant absorber). Usually, the
analysis is conducted by mathematical modelling of I(λ) with the aim of minimizing the deviation between I(λ)
and I (λ), where I (λ) is the modelled intensity as a function of wavelength (i.e., a modelled spectrum).
mod mod
 
 ′ 
I (λ)= P(λ)⋅exp− a (λ)⋅c ⋅l (10)
mod ∑ i i
 
 i 
P(λ) describes the combined influences of all influencing variables which vary little with wavelength, such as
τ(λ) or broadband absorbers (cf. Equation (7)), where it is not necessary to determine all coefficients. It
suffices to approximate P(λ) to the wavelength using a suitable smooth function, e.g., a higher-order
polynomial:
k
j
P(λ)≈ d ⋅λ (11)
∑ j
j=0
e.g., a 5th order polynomial [1]) or a low-pass filtered spectrum of I(λ) (e.g., with the aid of a Bessel filter [2]),
see Figure 4.
The deviation between I(λ) and I (λ) is usually minimized by the least squares method:
mod
2
2
χ =()I (λ)− I(λ) = minimal (12)
mod
Thus the desired concentrations c of the absorbing constituents averaged over the monitoring path, the
i
coefficients determining P(λ) (e.g., the coefficients d of the polynomial), and such additional coefficients as
j
may be involved (e.g., those describing wavelength shifts) are obtained.
NOTE The polynomial coefficients d (Equation (11)) can be useful for quality control purposes. Major variations of
j
these coefficients during the measurement indicate pronounced changes in atmospheric or system conditions.
The advantage of this analytical procedure is that the differential optical densities of all bands of the given
constituents will be taken into account in determining the average concentration across the spectral range
selected for the analysis.
A selected spectral range may contain the absorption structures of several constituents. It is possible to
distinguish between them and to determine their concentrations c independently. In addition, potential shifts in
i
the wavelength scale can be corrected. A detailed description of this method is given in [3].
13

---------------------- Page: 15 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)

Key
X wavelength λ in nm
Y1 intensity I in relative units
Y2 log I /I
1 2
Figure 4 ― Top: Measured raw spectrum and fitted fifth degree polynomial (smoothed line). Bottom:
Quotient of raw spectrum and fitted spectrum produces the high-pass filtered spectrum, fitted SO
2
reference spectrum for comparison (smoother line)
5 Measurement procedure
5.1 General
DOAS measurements are based on the principle of recording and analysing absorption bands of the
constituents of interest (see Clause 4). In addition, the following parameters shall be measured or recorded:
 monitoring path length, determined e.g. using a geographical map, a range finder, GPS data or a tape
measure (short distances);
14

---------------------- Page: 16 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
 density (temperature and pressure) of the air column, if appropriate (depending on the measurement
task), in order to express the results under standard conditions;
 instrument status data (e.g. received relative radiation intensity (e.g. in order to prevent saturation),
detector temperature (relevant only for diode laser systems)).
Beam alignment and blocking or optical path deflection systems may be necessary, especially for automatic
operation. These systems shall be controlled as appropriate.
DOAS measuring procedures depend on the instrument type. In addition to the atmospheric raw spectrum, the
system shall generally determine at least the background, dark, electronic offset, lamp, reference compound
spectra and, in some cases, the scattered solar radiation spectrum [8].
5.2 Principle
The general procedure for active long-path DOAS systems is outlined in Figure 5. Depending on the
instrument technique not all of these steps and not all of the spectra are necessary.

Figure 5 — Flow chart of a DOAS measurement and its evaluation
15

---------------------- Page: 17 ----------------------
oSIST prEN 16253:2011
prEN 16253:2011 (E)
The true optical density D'(k) is achieved by processing the following spectra:
 the atmospheric raw spectrum, I (k), k numbers the spectral interval covered by detector pixel number or

the channels, being the aggregate spectrum of the lamp, solar scattered light and trace gas absorption
spectra (the standard procedure is to add several (N ) individual spectra having the integration time (t )
M M
in each case);
 the lamp spectrum I (k) recorded by a direct scan of the lamp radiation excluding the atmosphere;
L
 optional: in case of CCD array as detector the dark spectrum (including electronic offset) I (k), wherein
D
I is obtained by averaging the sum of N added spectra (I (k) = I (k)/N );
D1 D D1 D D
 optional: the background spectrum I (k) with an integration time t , since stray solar radiation may
B B
interfere with the measurement.
EXAMPLE In a specific DOAS technique the true optical density D'(k) is calculated as follows:
 
t
M
()I (k)− N ⋅ I (k) −()I (k)− N ⋅ I (k)
M M D1 B B D1
 
t
 B 
D' ( k )=−ln (13)
()I (k)− N ⋅ I (k)
L L D1
(N and N respectively, indicate the number of background and lamp spectra added up).
,
B L
As I (k) is an averaged dark spectrum, the factors N , N , N are introduced, in order to correlate the correct intensities
D1 M B L
within this algorithm for the raw atmospheric spectrum, the lamp spectrum, the dark spectrum and the background
spectrum. For the same reason the ratio t /t is introduced, as I and I might have been recorded with different
M B M B
measurement times.
The true optical density D'(k) can be processed as follows:
1) A least-squares fit is carried out with a series of reference spectra according to 4.5.
2) The resulting fit parameters indicating the optical densities of the respective trace gas are used to
calculate the column density of this trace gas (as well as its standard deviation).
3) Dividing the column density by the length of the monitoring path gives the trace gas concentration, which
can be
...