Measurement of N02 and S02 Absorption Cross Sections

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Measurement of N02 and S02 Absorption Cross Sections

A contribution to subproject TOP AS

M. Carleer

^{l ,}

R. Colin

^{l ,}

A.C. Vandaele2 and P.C. Simon2

I

Universite Libre de Bruxelles, Laboratoire de Chimie Physique Mo/eculaire, CP 160, 50 avo F.D. Roosevelt, 1050 Brussels, Belgium;

2

lnstitut d'Aeronomie Spatiale de Belgique,

3

avo Circulaire, 1180 Brussels, Belgium

Absorption cross sections of S02 and N02 have been measured with a Fourier Transform Spectrometer in the UV and visible regions at different resolutions (2, 4, 8 and 16 cm -

I)

and at room temperature. The purpose for measuring a new set of cross sections was to record them in the same experimental conditions and on the same instrument used for recording atmospheric spectra [1) in order to eliminate any instrumental effects in the fitting procedure.

The source consists either of a "ozone free" xenon lamp or a tungsten filament.

The light is focused onto the entrance aperture of a Bruker IFS120HR Fourier Trans- form spectrometer, passes through the absorption cell, before hitting the detector, which is either a silicon diode or a solar blind UV -diode, depending on the region studied. The spectra are recorded in the wavenumber ranges 26000-39000 cm-

^I

(260-380 nm) and 14000-30000 cm-

^I

(330-700 nm). They are measured during the forward and backward movements of the mobile mirror, in double sided mode;

each spectrum is an average of 4000 interferograms.

The gas, either S02 or N02, is introduced in small quantities in a 20 cm long absorption cell; air is added to reach a total pressure of 1 atmosphere. The partial pressure of the analysed gas is measured with a Baratron gauge and the temperature is monitored with a conventional sensor.

The cross sections of S02 and N02 are obtained using the Beer-Lambert law

I

IO(A)

a(A) =

nd In I(A)

where n denotes the concentration of the gas,

d

the length of the cell, lis the measured intensity of the filled cell and

10

the measured intensity when the cell is empty. When possible, two blanks were taken before and after each measurement and

10

is then taken as the mean value of these two blanks.

Differential cross sections are obtained from the measurements using Fourier transform filtering [1). Absolute and differential cross sections of S02 and N02 at the resolution of 16 em -

^I

are plotted in Figs. 1 to 5. The accuracy of the differential cross sections of S02 is of the order of

± 2070

and of the order of

±

5% for N0

_2.

The absolute cross sections measured in this work have been compared to data from the literature (Figs. 1 and 2). The results concerning N02 have been compared to the data of Schneider (2). They are in good agreement in the wavenumber range 30800- 34000 cm -

I

(better than 5

%),

however discrepancies appear in the ranges 20000- 30800 em -

I

and 34000-42300 cm -

I

(more than 10%). In the latter region this could be due to the fact that the signal to noise ratio of the spectrum of the present work is very low, due to the decrease of the sensitivity of the detector in this

The Proceedings of EUROTRAC Symposium '92 edited by P.M. Borrell et al., pp. 419-422

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420

C/l

c 0

u ~

Q) U

C/l Q)

0

C/l E

/Jl

o """"-

L N

U E

c ^U

.~ IX)

~..-o L 0 /Jl

.n «

<II!

o

:J

"D C/l Q) [t:

2.0 .8 .5 1.4 1.2 1.0 0.8 0.5 0.4 0.2 0.0 10 5 o -5 -10

M. Carteer et at.

Thomsen

This work

/

39000 38000 37000 35000 35000 34000 33000 32000 31000 Wavenumber (em -1 )

Figure 1. Absolute absorption cross sections of S02 between 39000 and 31000 cm -I. The values of Thomsen [3] have been displaced by 7 X 10-¹⁹cm²/molec for comparison purposes.

1.2

C/l

g

1.0

~

U U

~ ~ 0.8

~ o

E

o """"- 0.5

~ NE

c U

.<:? '" 0.4

0.

L 0

0 0.2

.n C/l

«

0.0

<II! 50

o

:J U

C/l Q) [t:

30 o -30

36000 34000 32000 30000 28000 26000 24000 22000 20000 18000

-1

Wavenumber (em )

Figure 2. Absolute absorption cross sections of N02 between ³⁶⁰⁰⁰and 18000 cm -I. The values of Schneider et al. [2] have been displaced by 4 x 10-¹⁹cm2/molec for comparison purposes.

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Measurement oj N02 and S02 absorption cross sections

,....

0 Q)

a ... E

'"'

_E

0

~ 0

8.0 6.0 4.0 2.0 0.0 -2.0 -4.0

38000 36000 34000

Wavenumber (em-') Figure 3. Differential absorption cross sections of S02'

10.0 7.5

~

u

.,

₅_.0

0

E 2.5 ...

N

E 0 0.0

0

'"

⁰ ^-2^.⁵

-5.0 -7.5

421

32000 30000

34000 33000 32000 31000 30000 29000 28000 27000 26000 Wavenumber (em -1)

Figure 4. Differential absorption cross sections of N0₂in the UV range.

3.0

~ 0 2.0

~ 0

E 1.0 ...

N

E

0 0.0

~ 0

-1.0 -2.0

26000 25000 24000 23000 22000 21000 20000 19000 18000 Woven umber (em -1)

Figure 5. Differential absorption cross sections of N02 in the visible range.

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422

M. Carteer et al.

region.

It

appears that the cross sections measured by Schneider

[2]

could present some anomalies regarding the wavelength calibration. This could explain the discrepancies observed in the 20000-30800 cm

^-1

wavenumber region. The comparison between the cross sections of S02 obtained in this work and those of Thomsen [3], shows that the data are in good agreement (better than

50/0).

Acknowledgments

This project has been supported by the Belgian State - Prime Minister's Service - Science Policy Office and the "Fonds National de la Recherche Scientifique".

References

I. Vandaele, A.C., Simon, P.c., Carleer, M. and Colin, R. this volume (1992) p. 234.

2. Schneider, W., Moortgat, G., Tyndall, G. and Burrows, J. 1. Photochem. Photobiol. 40A (1987) 195.

3. Thomsen O. GKSS 901£36, GKSS-Forschungszentrum, Hamburg 1990.