CSC Poster 2006

How to quantify gases in air with an open-path FTIR
or a variable pathlength cell
Denis Bussières, Dépt. Sc. Fondamentales, Université du Québec à Chicoutimi, Saguenay, QC
Since the middle of the 90’s several FTIR instruments are available to measure and
quantify gases in the open atmosphere. A typical setup (monostatic) looks like this :
IR
beam
Telescope
From the good old Beer-Lambert’s law(2) :
Abs = ε n l
Where ε is in units of cm2 molec-1
n in units of molec cm-3
l in units of cm
Mirror
The sum of the line intensities over a whole band has to be done to get a comparable
value(3) :
FTIR
Σ ε x FWHM (cm-1)
Σ
→
S (cm molec-1)
Here the FWHM was used instead of the Doppler halfwidth for a matter of availability
(medium resolution spectra).
Before going outside, it is easier to get spectra in the lab and make sure of the results.
So, spectra were taken with a Bruker IFS66 FTIR at 0,5 cm-1 resolution (aperture of 2,5
mm) and a Wilks « variable path » cell (0,75-21,75 m) equipped with BaF2 windows.
0.9
2
0.75m
NO2 spectra (22.5ppmv)
3.75m
NH3 (630-1244 cm-1)
SO2 (1311-1400 cm-1)
NO2 (1429-1837 cm-1)
0.75m
6.75m
6.75m
0.7
1.5
9.75m
0.5
Absorbance
Absorbance
NH3 spectra (~1300 ppmv)
Table 1.
0.3
0.1
Σ S
(cm molec-1)
Hitran database
This work
8.78 x 10-20
6.49 x 10-20 *
3.08 x 10-17
2.71 x 10-17
5.69 x 10-17
4.72 x 10-17
1500
2000
2500
1
Then the line intensities (S) is related to the Einstein A coefficient (4) :
0.5
700
3000
-26 %
-12 %
-17 %
* From 700 cm-1 only
ΣS=
0
1000
-0.1
Difference
800
900
1000
1100
1200
A gu
A
-1
Wavenumbers (cm )
-0.5
Wavenumbers (cm -1 )
Figure 1.
gl 8 ¶ nr3 c ν2
Figure 2.
where A : Einstein A coefficient (spontaneous emission)
The way the spectra were taken, the reference spectrum was made with the cell empty
(pressure < 1 Torr) then the gas inserted into the cell and completed to atmospheric
pressure with N2.
gu and gl : degeneracies of upper and lower states (assumed to be ~1)
nr : refraction index (taken as 1.00027)
ν : mean wavenumber of the transition (950, 1362 and 1617 cm-1 respectively)
By doing so, a series of spectra were taken by varying the optical pathlength in the cell
(from 0,75 to 11,25 m). These spectra showed a moving baseline due to pathlength
difference between the gas spectrum and the reference spectrum (see Figure 1).
Table 2.
When not respecting the Beer-Lambert law(1) (see Figures 1 and 2), spectra may be
distorted and care was taken to avoid it.
NH3
SO2
NO2
A usual feature in open atmosphere spectra is the presence of bands in the 1400-1800,
2250 and 3500-4000 cm-1 regions (see Figure 6). These bands are due to the ubiquitous
presence of H2O and CO2 which were avoided in the lab setup.
Absorbance (baseline corrected)
0.8
0.6
Absorbance
0.10
0.4
0.2
0.08
0.06
0.04
0.02
0.00
0
1230
1270
-0.02
1330
-0.2
1430
1530
1630
1730
1830
1930
Wavenumbers (cm-1)
1290
1310
1330
1350
-1
Wavenumbers (cm )
1370
1390
0.08
R2 = 0.9648
0.07
0.06
0.05
0
2
4
6
8
Pathlength (m)
10
Figure 5.
The baseline was flattened close to zero absorbance by taking reference points on each
side of the region of importance and assuming a linear variation with the wavenumbers.
As a simple verification, the mean baseline absorbance value between 1260 and 1270
cm-1 had a value lower than 1 milliabsorbance (see Figure 4).
One peak of SO2, at 1373.8
pathlength as expected.
A warm car exhaust sample (Volvo 2000) was
aspirated into the cell (through 5µ filter) and
diluted with N2 to give spectra in Figure 6.
y = 0.0031x + 0.0517
Figure 4.
cm-1,
- 99.9 %
- 9 %
- 30 %
Respecting the Beer-Lambert law allows the quantification of gases in agreement with
accepted values (HITRAN). Ammonia result is three orders of magnitude off the reference
value because of a distorted spectrum by too high absorbance.
0.09
2030
Figure 3.
Difference
Absorbance at 1373.8 cm-1 vs. pathlength (1.18 ppmv SO2)
SO2 spectrum corrected (at 9.75m et ~1.18 ppm)
1250
Einstein A coefficient (s-1)
Hitran database
This work
33
0.044
47
38
161
93
is shown in Figure 5 to increase linearly with optical
12
Spectra of H2O and CO2 with the exact same
treatment would be needed to subtract them
from the ones on Figure 6 to be able to get
the trace contaminant in the exhaust.
Warm car exhaust diluted 621 and 3122 times in N 2 (at 2,25 m)
1.1
320 ppmv
1610 ppmv
0.9
Absorbance
0.75m
3.75m
5.25m
SO2 spectra (~1.18ppmv)
Absorbance (baseline corrected
1
c : speed of light (2.9979x1010 cm s-1)
0.7
0.5
mainly H2 O
CO2
CO
very tiny
0.3
H2 O
CH4 C2H2 and C2H4
all lost in the grass
CO2
0.1
700
1200
1700
2200 Wavenumbers
2700(cm-1)
3200
3700
Figure 6.
Sincere thanks to Dr. G. Harris and his team members at the Center for Atmospheric Chemistry, York University for
receiving me in his lab and helping me for this work.
Thanks to the Université du Québec à Chicoutimi for supporting me financially through this work.
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(4) Newman, S.M., Lane, I.C., Orr-Ewing, A.J., Newnham, D.A. and Ballard, J., J.Chem.Phys., 110, 22, pp.10749-10757 (1999).