Abstract. Carbon dioxide (CO2) gas is one of the main greenhouse gases (following water
vapor) and has a significant impact on the Earth’s climate. Determation of greenhouse gas
concentration (of CO2 in particular) will improve our understanding of the Earth’s climate
changes. In this study, we use absorption spectra to determine the concentration of CO2 in the
atmosphere. The absorption spectra of CO2 corresponding to three transitions in the midinfrared region were recorded at room temperature using a diode laser system. Ambient air was
charged into the absorption cell (Whitetype) with an optical path of 24 to 40 m and total
pressure of 4 Torr to 300 Torr. The absorption spectra were analyzed using a line by line
process and the Voigt profile. Spectroscopic parameters (line intensity) were used with the
2012 HITRAN databases to determine the concentration of CO2 in the laboratory atmosphere.
The results show that the concentration of CO2 in the atmosphere exceeded 400 ppm.
7 trang 
Chia sẻ: thanhle95  Lượt xem: 196  Lượt tải: 0
Bạn đang xem nội dung tài liệu Determination of carbon dioxide concentration in the atmosphere from absorption spectra, để tải tài liệu về máy bạn click vào nút DOWNLOAD ở trên
135
JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/23541059.20170041
Mathematical and Physical Sci. 2017, Vol. 62, Iss. 8, pp. 135141
This paper is available online at
DETERMINATION OF CARBON DIOXIDE CONCENTRATION
IN THE ATMOSPHERE FROM ABSORPTION SPECTRA
Nguyen Thi Thuy, Nguyen Thi Huyen Trang and Ngo Ngoc Hoa
Faculty of Physics, Hanoi National University of Education
Abstract. Carbon dioxide (CO2) gas is one of the main greenhouse gases (following water
vapor) and has a significant impact on the Earth’s climate. Determation of greenhouse gas
concentration (of CO2 in particular) will improve our understanding of the Earth’s climate
changes. In this study, we use absorption spectra to determine the concentration of CO2 in the
atmosphere. The absorption spectra of CO2 corresponding to three transitions in the mid
infrared region were recorded at room temperature using a diode laser system. Ambient air was
charged into the absorption cell (Whitetype) with an optical path of 24 to 40 m and total
pressure of 4 Torr to 300 Torr. The absorption spectra were analyzed using a line by line
process and the Voigt profile. Spectroscopic parameters (line intensity) were used with the
2012 HITRAN databases to determine the concentration of CO2 in the laboratory atmosphere.
The results show that the concentration of CO2 in the atmosphere exceeded 400 ppm.
Keywords: Atmosphere, intensity, concentration of carbon dioxide, voigt profile.
1. Introduction
The rapid development of industrialization and mechanization has resulted in nonstop air
pollution. High concentrations of pollutants such as CO2, NO2, hydrocarbons cause significant
health and the environmental problems. Of these carbon dioxide is one of the most important
elements in the Earth’s atmosphere, the second most prevalent greenhouse gas following water
vapor in the atmosphere, and therefore, it contributes greatly to planet warming and has a strong
impact on global climate change [1, 2]. As an inevitable consequence of human activities
(combustion of fossil fuels), the amount of CO2 in the atmosphere has rapidly increased since the
beginning of the industrial revolution. According to annual statistical data, the concentration of
CO2 increased about 3 parts per million (ppm) between (February 2014, 397 ppm and February
2015, 400 ppm) [3]. The current global atmospheric CO2 level is greater than 400 ppm [4, 5],
which is a problem of global concern.
The absorption or emission spectra of CO2 can give information about the characteristics of
this molecule. For example the reversion of absorption spectra shows the concentration of CO2 in
the atmosphere. These factors have promoted the study and determination of the concentration of
CO2 in the atmosphere from studies of CO2 spectra in the lab, such as Fourier transform interferometer
Received May 3, 2017. Accepted August 15, 2017.
Contact Ngo Ngoc Hoa, email: hoa.nn@hnue.edu.vn
Nguyen Thi Thuy, Nguyen Thi Huyen Trang and Ngo Ngoc Hoa
136
[6, 7], laser diode system [8], CRDS system [9] or by satellite for remote sensing methods OCO
(The Orbit Carbon Observatory) [10].
To determie the CO2 concentration in the atmosphere, we need to know the spectroscopic
parameters (position line, line intensity, line broadening and lineshape). To describe the
experimental line shape, the Voigt profile is used in most available studies, the Voigt profile being
a convolution of two broadening mechanism, one of which produces a Gaussian profile (usually,
as a result of Doppler broadening), the other would producing a Lorentz profile (as a result of
pressure broadening). When performing the calculations with Voigt profile, the Doppler width
being generally fixed to its theoretical value.
In this paper, we use absorption spectra to determine the concentration of CO2 in the
laboratory atmosphere. It is organized into 4 sections as follows: introduction, theoretical
foundations and experimental process, results and discussion, and conclusions and future studies.
2. Content
2.1. Theoretical foundations
2.1.1. Integrated intensity
Absorption spectra are used as an analytical method for determination of certain substances
present in a sample. There are many experimental techniques for measuring absorption spectra.
The most common method is direct measurement by comparing the intensity of radiation before I0
(ω) and after IL (ω) through the sample. The intensity IL (ω) of the beam after through the sample
with optical path L depends on the interaction between matter and radiation. The relation between
I(ω) and Io(ω) follows the BeerLambert law [11, 12].
αL
o
I
=e ,
I
(1)
where, α is the absorption coefficient and L is the optical path. The absorption coefficient can be
written as [11, 13]:
oα=β×f ωω , (2)
where, is a normalization function describing the lineshape spectra:
+
o
0
f ωω dω=1,
(3)
Integrating both sides of the equation (1), we have:
+ +
o
0 0
αdω= f ωω ×βdω=β,
(4)
Thus, β called the integrated intensity depends on many factors such as pressure of activated
molecules, optical path and transition. To make application easier, integrated intensity is generally
calculated for one unit concentration and one unit optical path and is called the standard integrated
intensity. Standard integrated intensities (S) for many kinds of molecules and many transitions are
stocked in HITRAN (https://www.cfa.harvard.edu/hitran/). The HITRAN database 2012 contains
spectroscopic parameters for thousand of transitions and for 34 different kinds of molecules [14].
Standard integrate intensity is determined by equation [13]:
β
S= ,
L×n
(5)
Determination of carbon dioxide concentration in the atmosphere from absorption spectra
137
If the integrated intensity β is determined by (cm1), the concentration n of activated
molecules is in molecule/cm
3
, optical path L is in cm then standard integrated intensity S will be
in cm
1
molec
1
cm
2
[14].
2.1.2. Line shape
We then take into account both broadening mechanisms, Doppler and Lorentzian broadening,
with the hypothesis that the two mechanisms are completely independent (but not taking into
account the Dicke narrowing effect and the speed dependence of the collisional parameters due to
our experimental uncertainties). The corresponding absorption coefficients for an isolated line are
given as [11]:
VP o D
D
β ln2
α σ Re W σσ ,Γ ,Γ ,
Γπ
(6)
where 0 and D are the unperturbed spectral position of the transition and the Doppler width, is
the collisional halfwidth (HWHM). The complex probability function W is given by:
2t
o D
o
D D
i e
W σσ ,Γ ,Γ dt
π ln2 ln2
σσ t+iΓ
Γ Γ
(7)
And (in cm
1
) is given by expressions [11]:
7
D o
T
Γ 3.58 10 σ ,
M
(8)
where 0 is number wave of unperturbed spectral position of transition (cm
1
), M is molar mass
(g/mol) and T is temperature (K).
In this work, we used the Voigt profile to fit the absorption spectra of CO2, the integrated
intensity and the standard integrated intensity in HITRAN database 2012 [14] to determine the
concentration of CO2 in atmosphere.
2.2. Measurement procedure
The absorption spectra of CO2 corresponding to three transitions in the midinfrared region
were measured at room temperature by using a diode laser system described in detail in [15, 13].
Ambient air was charged into the absorption cell (whitetype) with an optical path of 20 to 40 m at
a total pressure from 4 to 300 Torr. The obtained spectra are fitted line by line using the Voigt
profile. Three transitions and the experimental conditions are shown in Table 1.
Table 1. Experimental conditions for measurement of CO2 absorption spectra
Transition (cm
1
)
[14]
Standard integrated
intensity (cm
1
.molec

1
.cm
2
) [12]
Total
pressure
(Torr)
Optical
length (cm)
Temperature
(K)
2251.681043 cm
1
1,050.10
20
4 – 50 4000 295
2257.508986 cm
1
1,876.10
20
3 – 250 2400 295
2261.277760 cm
1
2,507.10
20
4 – 200 2400 295
Nguyen Thi Thuy, Nguyen Thi Huyen Trang and Ngo Ngoc Hoa
138
2.3. Results and discussion
2.3.1. The fitting results of the experimental spectra
The fitting results of experimental spectra are given by Figure 1. Figure (1a) and (1b)
respectively show results for transition at wave number 2251.681043 cm
1
and 2257.508986 cm
1
.
The lowest panels are ten times the difference between the experimental spectra and the fitting
results using Voigt profile. We used a straight line for the base line, half width Doppler
broadening is fixed to its theoretical values calculated from equation (8), we have determined the
spectroscopic parameters for each transition: peak position, halfwidth Lorentzian, integrated
intensity together with two parameters describing the base line.
0.0
0.5
1.0
1.5
0.0
0.5
1.0
1.5
2.0
0.06 0.03 0.00 0.03 0.06
0.3
0.0
0.3
0.04 0.02 0.00 0.02 0.04
0.3
0.0
0.3
60.530 Torr
42.510 Torr
20.510 Torr
15.150 Torr
10.624 Torr
(b)(a)
A
b
s
o
rp
ti
o
n
50.000 Torr
45.040 Torr
35.060 Torr
17.980 Torr
10.058 Torr
1
0
x
(o
b
s
c
a
lc
)
Relative Wavenumber (cm
1
)
Figure 1. The absorption spectrum of atmospheric CO2 at room temperature measured
different pressures corresponding to two transitions at wave number 2251.681043 cm
1
(a) and 2257.508986 cm
1
(b)
The lowest panels are ten times the difference between experimental spectra
and fitting results using Voigt function
The fitting results show that the difference between the experimental spectra and calculations
is clearly higher than the noise levels of our measurements. The W form of residues (figure 1(a))
shows the narrowing effect of the experimental lines. In other words, the Voigt profile neglects
the other kinetic process which may change lineshape as the Dicke narrowing effect [11, 16] or
speed dependence of collisional parameters [17, 18].
In figure (1b), we also observed two transitions near the studied transition. In the fitting
process, we did not calculate these two transitions. Therefore the results of integrated intensity,
concentration of gas may be affected by these near transitions.
Determination of carbon dioxide concentration in the atmosphere from absorption spectra
139
2.3.2. Results determine concentration CO2 in atmosphere
From formula (5):
B
r
βk Tβ
S= ,
L×n L×P
(9)
where P is a particular pressure of CO2 in atmosphere, kB is the Boltzmann constant. With mr is
the molecular ratio of total CO2 molecules in the atmosphere, formula (9) can be written as:
r
B
L.m
β=S P,
k T
(10)
P representing total pressure. Therefore, when T is constant, the integrated intensity is linearly
proportional to total pressure.
Figure 2. Integrated intensity is expressed as total pressure to transition at wave
number2251.681043 cm
1
(a) and 2261.277760 cm
1
(b)
Figure 2 describes the dependence of integrated intensity on the total pressure for two
transitions at wave number 2251.681043 cm
1
and 2261.277760 cm
1
. The results show a linear
dependence of integrated intensity via the total pressure. After the linear fitting process, from the
slope together with the parameters S, L, T shows in Table 1, we have determined the molecular
ratio of total CO2 molecules in the atmosphere corresponding to three transitions, the results
shown in Table 2.
Table 2. The molecules ratio were determined from three transitions in this work
Transition Molecule ratio
mr(ppm)
2251.681043 cm
1
4,74.10
6
cm
1
/Pa 459
2257.508986 cm
1
4,22.10
6
cm
1
/Pa 381
2261.277760 cm
1
6,38.10
6
cm
1
/Pa 432
Average 420 30
Nguyen Thi Thuy, Nguyen Thi Huyen Trang and Ngo Ngoc Hoa
140
The results in Table 2 show that the ratio of CO2 molecules determined from the transition at
2257.508986 cm
1
has the smallest value and differs with the results of the other two transitions.
This may be due to the influence of two near transitions of which we have not taken into account
in the fitting process for this transition.
The average value of CO2 molecules ratio in the atmosphere is higher than the value from [3].
This result may be because, the values of standard integrated intensity S from [14] is usually
smaller than the value determined from diode laser system by about 5% [13] and therefore the
value of mr will be higher than the real value by about 5%.
3. Conclusions
In this work, we used absorption spectra of three different transitions of O
16
C
12
O
16
in the
atmosphere in a laboratory, the absorption spectra recorded at room temperature to determine CO2
molecules ratio in the atmosphere. The results show that this value was about 420 ppm.
The fitting result from experimental spectra corresponding to the transition at 2257.508986
cm
1
may be affected by two near transitions. Therefore, in future study, we will take into account
these transitions.
The results also show that there were other molecular kinetic mechanisms affecting the line
shape spectrum. Therefore, in future studies, we will use different models that take into account
molecular dynamic mechanisms such as: Dicke narrow effect and the speed dependence of
collisional parameters to describe refined lineshape absorption spectra.
Acknowledgments. The authors are pleased to acknowledge the financial support of this research
by the Vietnam Ministry of Education & Training for project No. B2016SPH21.
REFERENCES
[1] Donald M. Hunten, 1993. Atmospheric evolution of the terrestrial planets. Science, 259 915.
[2] I. Pouchet, V. Zéninari, B. Parvitte, G. Durry, 2004. Diode laser spectroscopy of CO2 in the
1.6μm region for the in situ sensing of the middle atmosphere. Journal of Quantitative
Spectroscopy and Radiative Transfer 83 619.
[3]
[4] NOAA, Record annual increase of carbon dioxide observed at Mauna Loa for 2015
[5] NIES, Wholeatmosphere monthly mean CO2 concentration based on GOSAT observations
 Recent data
[6] A. PredoiCross, A.V. Unni, W. Liu, I. Schofield, C. Holladay, A.R.W. McKellar, D.
Hurtmans, 2007. Line shape parameters measurement and computations for selfbroadened
carbon dioxide transitions in the 30012 00001 and 30013 00001 bands, line mixing,
and speed dependence. Journal of Molecular Spectroscopy 245 34.
[7] N.H. Ngo, X. Landsheere, E. Pangui, S.B. Morales, J.M. Hartmann, 2014. Self
broadening of
16
O
12
C
16
O3band lines. Journal of Molecular Spectroscopy 306 33.
[8] I. Pouchet, V. Zéninari, B. Parvitte, G. Durry, 2004. Diode laser spectroscopy of CO2 in the
1.6 μm region for the in situ sensing of the middle atmosphere, Journal of Quantitative
Spectroscopy and Radiative Transfer 83 619.
[9] B.V. Perevalov, A. Campargue, B. Gao, S. Kassi, S.A.Tashkun, V.I. Perevalov, 2008. New
CWCRDS measurements and global modeling of
12
C
16
O2 absolute line intensities in the 1.6
m region. Journal of Molecular Spectroscopy 252 190.
Determination of carbon dioxide concentration in the atmosphere from absorption spectra
141
[10] Z. Kuang, et al., 2002. Spaceborne measurements of atmospheric CO2 by highresolution
NIR spectrometry of reflected sunlight: an introductory study Geophys. Res. Lett. 29 1716.
[11] J. M. Hartmann, C. Boulet and D. Robert, 2008. Collisional effects on molecular
spectra.Laboratory experiments and models, consequences for applications, Elsevier,
Amsterdam.
[12] A.P. Thorne, U. Litzén, and S. Johansson, 1999. Spectrophysics: Principles and
Applications. Springer.
[13] N. H. Ngo, N. Ibrahim, X. Landsheere, H. Tran, P. Chelin, M. Schwell, and J. M. Hartmann,
2012. Intensities and shapes of H2O lines in the nearinfrared by tunable diode laser
spectroscopy. Journal of Quantitative Spectroscopy and Radiative Transfer 113 870.
[14] Rothman LS et al., 2013. The HITRAN 2012 molecular spectroscopic database. Journal of
Quantitative Spectroscopy and Radiative Transfer 130 4.
[15] N. Ibrahim, P. Chelin, J. Orphal, Y. I. Baranov, 2008. Line parameters of H2O around 0.8
m studied by tuneable diode laser. Journal of Quantitative Spectroscopy and Radiative
Transfer 109 25232536.
[16] H. Ngo, H. Tran, and R. R. Gamache, 2012. A pure H2O isolated lineshape model based
on classical molecular dynamics simulations of velocity changes and semiclassical
calculations of speeddependent collisional parameters. J. Chem. Phys. 136 154310.
[17] F. Rohart, H. Mader and H. W. Nicolaisen, 1994. Speed dependence of rotational
relaxation induced by foreign gas collisions: Studies on CH3F by millimeter wave coherent
transients. The Journal of Chemical Physics 101 64756486.
[18] H. Tran, D. Bermejo, J.L.Domenech, P. Joubert, R. R. Gamache, and J.M.Hartmann, 2007.
Collisional parameters of H2O lines: Velocity effects on the lineshape. Journal of
Quantitative Spectroscopy & Radiative Transfer 108 126145.