Detection of Minor Tropospheric Constituents in the Near UV by Fourier Transform Spectroscopy
A contribution to subproject TOP AS
A.C. Vandaelel, P.e. Simoni, M. Carleer2 and R. Colin2 Ilnstitut d'Aeronomie Spatiale de Belgique, 3 avo Circulaire,
1180 Brussels, Belgium;
2Universite Libre de Bruxelles, Laboratoire de Chimie Physique Moleculaire, CP 160, 50 avo F.D. Roosevelt, 1050 Brussels, Belgium
The experimental set up is schematised in Fig. 1 and has already been described else- where [1]. The source is either a high pressure Xenon lamp or a Tungsten filament.
The spectra are recorded with a Bruker IFSI20HR Fourier Transform spectrograph with a dispersion of 7.7 cm- I and analysed in the 26000-38000 cm- I (260-380 nm) and 14000-30000 cm- I (330-700 nm) spectral regions.
Concentrations of S02' N02 and 03 have been routinely measured since October 1990. Their concentrations are determined using the Beer-Lambert law:
I(A) = 10(A)e-ntJ.U(A)d
where 1 is the measured intensity, 10 the measured intensity from which all absorp- tion structures have been removed, n the concentration, d the optical path length and Au the differential absorption cross section of the molecule.
The differential cross sections of S02 and N02 have been measured in the labora- tory [2]. Cross sections of 03 have been taken from the literature [3].
The detection limits of the different constituents are listed in Table 1. It should be noted that the use of a Fourier Transform spectrometer lowers the minimum detecta- ble limit compared to conventional grating spectrometers, since the signal to noise ratio is better.
Diurnal variations of S02' N02 and 03 concentrations in an urban site (Campus of the "Universite Libre de Bruxelles", ULB) for May 22nd 1991 and February 27th 1992 are presented in Fig. 2. The correlation between N02 and 03 concentrations can clearly be seen. NO concentrations have been deduced during the day from the measured 03 and N02 concentrations, using the Leighton photochemical reaction scheme:
N02 + hv
°
+ 02 + MNO + 03 The concentration is given by:
[NO]
NO +
°
03 + M N02 + 02
J(N02) [N02]
k3 [03]
J(N02)
k2 k3
1400 . . .
where k
3[cm3 molec-Is- I] = 2.0 10-12 exp( -
T )
[4] and the photodlssoclatlOnThe Proceedings oj EUROTRAC Symposium '92 edited by P.M. Borrell et al., pp. 234-238
©1993 SPB Academic Publishing bv, The Hague, The Netherlands
Detection of minor tropospheric constituents
---394m--- ET
- -- -I-i-~e?~~
FOURIER TRAHSFORH SPECTROMETER IIRUKER I'S120 HR
235
Figure 1. Experimental set up: S = High Pressure Xenon Lamp or Tungsten filament, ET = Emitting Telescope (30 cm 0), RT = Receiving Telescope (30 cm 0), M = Long Focal Retroreflector Mirror.
Table 1. Detection limits
v SIN Detection limit
(cm-I ) (ppb)
S02 33340 3200 0.1
N02 28710 500 5.8
22300 4000 0.3
°3 35305 1700 1.6
N03 15106 2700 5 x 10-3
rate J(N02) is calculated using the empirical relation given by Parrish [5].
The calculated NO concentrations for February 19th 1991 are plotted in Fig. 3. On the same plot are represented the NO concentration values measured by the Institute for Hygiene and Epidemiology (IHE, Brussels). This station is located at Uccle, 3 km from the campus of the ULB and uses a chemical technique to measure NO.
The DOAS technique is based on the determination of a "reference spectrum" 10 derived mathematically from the experimental spectrum. Numerous methods for determining 10 exist. The three following have been used in this work.
1. Fourier Transform filtering where the lower frequency portion of the power spec- trum of the experimental data is removed.
2. Fourier Transform filtering after having subtracted the emission peaks of the source lamp.
3. Polynomial interpolation.
Table 2. Comparison of the three methods Method I Atmos. Molecule n Eabs Ere! n spectrum (ppb) (ppb) (0J0 ) (ppb) 27/8/91 S02 19.0 0.1 0.5 19.0 10" LT
°3
8 3 38 18 N02 78 2 3 85 27/8/91 S02 10.2 0.1 l.0 10.1 11h LT°3
21 3 14 31 N02 38 2 5 45 27/8/91 S02 5.5 0.1 l.8 5.4 12hLT°3
35 3 9 45 N02 26 2 8 34 27/8/91 S02 4.0 0.1 2.5 4.0 13h LT°3
49 3 6 59 N02 11 9 18 27/8/91 S02 4.9 0.1 2.0 4.9 14h LT°3
53 2 4 63 N02 20 5 26Method 2 Eabs Ere! n (ppb) (070 ) (ppb) 0.1 0.5 16.5 3 17 36 2 2 88 0.1 l.0 8.9 2 7 28 2 4 48 0.1 l.9 4.7 2 4 33 2 6 37 0.1 2.5 3.6 2 3 34 2 11 19 0.1 2.0 4.4 2 3 30 2 8 28
Method 3 tabs (ppb) 0.2 1 3 0.2 1 3 0.1 1 3 0.2 2 3 0.2 2 3
Ere! (0J0 ) l.2 3 3 2.3 4 6 2.1 3 8 5.6 6 16 4.6 7 11
tv W 0'1
I~
~ ::l2-
~ ~ ~....
I:) :-Detection of minor tropospheric constituents
8 12 16 20 24 4
20 .-~ __ - . __ ~ __ . -__ ~2r7~FTe~b~r~u~c~r~1~9~9~2T-__ '-~'_-'--~--~
.Cl 15 ...
I '... / ... -... -... -... -...
it 10 ... / \ / \ . . . - . . . ..., ... -...
5 ... - ... - ... -... ...-... - ... __ ....-
o r-~--~--~--+-~---+---r--+-~~-+--~--+---~~ 100 80 60 40 E-~~~~
__
L-~ __ L-~~~~~~~ __ L-~~~O8 12 16 20 24 4 8 12
Universel Time (h) Figure 2. Dirunal variations of S02 (" l, N02 (.) and 03 (0).
150 . -__ - r ____ , -__ ~--~1~9~F~e~b~r~uerr~~1~9r9~1--_.----._--~._--~
~ 100
.Cl a.
a. 50
o r----r----+---~----_r~--r---_+----+---_+----_r--~400
300
.Cl a.
a.
~
200 ~ a.
100
8 12 24
Figure 3. Comparison between the calculated and the measured NO daytime concentration.
237
The three methods have been compared. Table 2 shows the concentrations of S02' N02 and 03' obtained for five different spectra. The relative and absolute errors on these concentrations are given. As can be seen, there is a great discrepancy in the con- centration values found with the different methods. As cross sections are identical for the three methods, these discrepancies can only be attributed to the method used to derive 10, A more careful study of the reasons of these discrepancies is in progress.
Acknowledgements
This project has been supported by the Belgian State - Prime Minister's Service - Science Policy Office and the "Fonds National de la Recherche Scientifique". We
238 A.C. Vandaele et al.
would like to also thank D. De Muer (Koninklijk Meteorologisch Instituut) and the Institute for Hygiene and Epidemiology for the data they have provided.
References
I. Carleer, M., Colin, R., Vandaele, A.C. and Simon, P.c. Technical Digest Ser., 18 (1991) 278.
2. Carleer, M., Colin, R., Vandaele, A.C. and Simon, P.C. This volume, pp. 417.
3. Daumont, D., Barbe, A., Brion, J. and Malicet, J. 1. Atmos. Chem. (1992) submitted.
4. DeMore W.B., Sander S.P., Golden D.M., Molina, M.J., Hampson, R.F., Kurylo, M.J., Howard, C.J. and Ravishankara, A.R., Chemical Kinetics and Photochemical Data for Use in Stratospheric Chemistry Modelling, JPL Publication No 90 (1990).
5. Parrish, D.O., Murphy, P .C., Albritton, D.L. and Fehsenfeld, F.C., Atmos. Environ. 17 (1986) 1365.
