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Introduction

First atmospheric profiles of the collisional complex of (Otex2html_wrap_inline205)tex2html_wrap_inline205, or brief Otex2html_wrap_inline209, are reported. The information derived from (Otex2html_wrap_inline205)tex2html_wrap_inline205 profiles is particularly important since (a) it may help to derive cloud top heights and atmospheric optical pathlengths from ground-based as well as satellite-borne (GOME) UV/VIS instruments, (b) (Otex2html_wrap_inline205)tex2html_wrap_inline205 is known as an absorber of solar radiation, (c) its absorption structures interfere with other absorbers in the UV/VIS spectral range and thus a detailed knowledge of its profile is required for the retrieval of stratospheric BrO, OClO, NOtex2html_wrap_inline205, Htex2html_wrap_inline205O, and ozone, and (d) (Otex2html_wrap_inline205)tex2html_wrap_inline205-profiles measured at different atmospheric temperatures and pressures may provide new insight into the collision process of (Otex2html_wrap_inline205 + Otex2html_wrap_inline205) and the thermochemistry of (Otex2html_wrap_inline205)tex2html_wrap_inline205. In particular, in some optical properties of the (Otex2html_wrap_inline205)tex2html_wrap_inline205-complex under ambient conditions (pressures tex2html_wrap_inline239 1atm and temperatures tex2html_wrap_inline239 300K), which are - due to the weak (Otex2html_wrap_inline205)tex2html_wrap_inline205 absorption - not easily accessable in the laboratory. Observation Geometry

  figure152
Figure: Observation geometry of balloon soundings of atmospheric Otex2html_wrap_inline209.

Observations

The (Otex2html_wrap_inline205)tex2html_wrap_inline205-absorption features were retrieved from direct sunlight spectra by the well known DOAS-technique (Differential Optical Absorption Spectrometry). The spectra, were recorded during two stratospheric balloon flights of the LPMA/DOAS gondola (Laboratoire Physique Moléculaire et Application). The balloon flights were conducted from León/Spain in Nov. 96 and from Kiruna/Sweden in Feb. 97. The instrumental details are described on the posters of Ferlemann et al. (Poster 198), the DOAS-retrieval technique, by Harder et al. (Poster 75), and the observation geometry in figure 1. The retrieved atmopheric absorption features of (Otex2html_wrap_inline205)tex2html_wrap_inline205 are shown in figure 2 (trace e). Using a profile retrieval algorithm (also described in detail by Ferlemann et al., Poster 198), the atmospheric profiles of the three (Otex2html_wrap_inline205)tex2html_wrap_inline205-absorption (tex2html_wrap_inline261(H), H=height) at 477.3 nm, 532.2 nm, and 577.2 nm were derived (Figure 3).

Differential Optical Densities

  figure158
Figure: Atmospheric spectra as recorded during a balloon ascend from Kiruna on 14 Feb. 1997. Trace (a); Direct sunlight spectrum recorded at an altitude of 6.2 km (SZA = 83tex2html_wrap_inline263). Trace (b); Direct sunlight spectrum recorded at an altitude of 31 km (SZA = 89.7tex2html_wrap_inline263). Trace (c); Ratio of both spectra; Trace (d); Retrieved absorption signature of the Otex2html_wrap_inline267-Chappuis band. Trace (e); Retrieved absorption signature of atmospheric (Otex2html_wrap_inline205)tex2html_wrap_inline205.

Measurements

The (Otex2html_wrap_inline205)tex2html_wrap_inline205 absorption coefficient (tex2html_wrap_inline277) (at 447.3 nm, 532.2 nm, and 577.2 nm) are determined from the measured height profiles of the atmospheric (Otex2html_wrap_inline205)tex2html_wrap_inline205 absorptions (tex2html_wrap_inline283)


eqnarray60

whereby the height profile of the oxygen molecular concentration (ntex2html_wrap_inline285(H)) was calculated from the measured atmospheric temperatures (T) and pressures (P).

It is found, that

(a) within the error bars of the study neither the band shapes nor the integrated band strengths (S) depend on the atmospheric pressure (as illustrated in Figure 4 for the 477.3 nm absorption).

(b) for our instrumental resolution (FWHM 1.5 nm) and for the pressures and temperatures encountered during the measurements (500 > P > 7 mbar, 260 K > T > 203 K), the (Otex2html_wrap_inline205)tex2html_wrap_inline205-absorptions show no 'fine' structure with respect to wavelength.

(c) the (Otex2html_wrap_inline205)tex2html_wrap_inline205 absorption coefficient decreases by about 20tex2html_wrap_inline303 for a change in the ambient temperature of 100 K (from T = 200 K to 300 K).

(d) the measured absorption coefficient follows an Arrhenius temperature dependence (ln(tex2html_wrap_inline277(T))=ln(tex2html_wrap_inline307)-tex2html_wrap_inline309H/(Rtex2html_wrap_inline311T), (see Figure 5)) (an Arrhenius type temperature dependence). Accordingly, from the slope of the temperature dependence the 'binding' enthalpy (tex2html_wrap_inline309 H) of the (Otex2html_wrap_inline205)tex2html_wrap_inline205 can be calculated (Table 2).

Profiles

  figure164
Figure: Height profile of the atmospheric (Otex2html_wrap_inline205)tex2html_wrap_inline205-absorptions at 477.3 nm, 532.2 nm, and 577.2 nm, as observed during the León flight (23 Nov. 1996) (Fig. 3a) and the Kiruna flight (14 Feb. 1996) (Fig. 3b). The different amounts of (Otex2html_wrap_inline205)tex2html_wrap_inline205 absorptions reflect the different band strengths. Differences between the two flights reflect different atmospheric conditions (temperature and pressure profile) encountered during the flights.

Discussion

In the existing literature it is discussed whether the complex (Otex2html_wrap_inline205)tex2html_wrap_inline205 has to be regarded either as (1) a dimer (see for example Long and Ewing, (1973), Orlando et al., 1991), or (2) as a collisional complex where the absorption may take place because of an distorted oxygen 'Hamiltonian' during the collision (Tabisz et al., 1969), or (3) a metastable dimer (properties midway between the characteristics (1) and (2)), where during the Otex2html_wrap_inline205-Otex2html_wrap_inline205 collision a common 'binding' molecular orbital may exist (without a necessary pairing of electrons) (Johnston et al., 1984, Blake and McCoy, 1987).

Our observation may help to distinguish between these scenarios.

At first glance finding (d) may suggest to regard the (Otex2html_wrap_inline205)tex2html_wrap_inline205-complex as a dimer or 'van der Waals' molecule. However, when accordingly an entropie change (tex2html_wrap_inline309 S) due to the formation of this hypothetical Otex2html_wrap_inline209 'van de Waals' molecule is also taken into account (about 125 J/(moltex2html_wrap_inline311 K), then the concentration of the oxygen dimer would change by about 70tex2html_wrap_inline303 when going from 300 K to 200 K, in disagreement with finding (c).

In contrast, McKellar et al. (1972) and Tabisz et al., (1969) concluded from the asymmetry of the band profiles to a Boltzmann relation between the high and low frequency wings of an absorption band, which they interpreted as a hint that (Otex2html_wrap_inline205)tex2html_wrap_inline205 is more likely a collisional complex. Although this asymmetry is also found here, this model would more point to a tex2html_wrap_inline351 Ttex2html_wrap_inline353 temperature dependence (12 > m> 6) of the cross sections, partly in disagreement with finding (c) and (d). In addition, within this model a 'binding' enthalpy of (Otex2html_wrap_inline205)tex2html_wrap_inline205 is hardly explainable. Therefore, and because our findings (a) and (b) are probably only valid for our limited measurement conditions (see above), and with respect of the small 'binding' enthalpy (tex2html_wrap_inline309 H = (- 1038 tex2html_wrap_inline365 466) J/mol) our findings lead us to conclude that (Otex2html_wrap_inline205)tex2html_wrap_inline205 is most likely a metastable complex, whereby the (Otex2html_wrap_inline205)tex2html_wrap_inline205-absorptions may occur in collisional-induced electronic transitions.

Lines

  figure172
Figure: Variation of the Otex2html_wrap_inline209 absorption band shape (at 477.3 nm) with the atmospheric observation height, and a comparison with the 'low pressure' (Otex2html_wrap_inline205)tex2html_wrap_inline205-absorption signature as measured in the laboratory by Newnham et al. (1997). Note that within the error bars the line shape does not change with the atmospheric pressure (500 mbar > p > 7 mbar).

Tables

Table 1: Comparison of the (Otex2html_wrap_inline205)tex2html_wrap_inline205 absorption coefficient (tex2html_wrap_inline277) with literature data (the data of the present study are interpolated to 296 K).

Wavelength Collisional absorption coefficient tex2html_wrap_inline277 tex2html_wrap_inline393
Volkamer Greenblatt et.al. Wagner Perner Dianov- Herman Salow This study
(Otex2html_wrap_inline209-Band) und Platt Klokov et al.
[1996] [1990] [1996] [1980] [1964] [1939] [1936]
296 K 296 K 196 K 241 K 278 K 290 K 296 K
328.2 0.2
342.7 1.18 (9) 1.2 (1) 0.70 (24) 0.99
360.8 5.42 (7) 4.1 (4) 5.7 (6) 4.7 (6) 5.4 (15) 4.4 3.6
380.2 2.4 (2) 2.4 (2) 3.7 (4) 2.4 (5) tex2html_wrap_inline397 2.2 2.1
446.7 0.57 (6) 1.0 (12) 0.7 0.3
477.1 6.1 (3) 6.3 (6) 7.6 (13) 7.7 (8) 5.9 (18) 5.5 8.0 5.3 6.49(3)
531.7 1.3 (3) 1.0 (1) 1.5 0.4 1.1(2)
576.9 10.3 (3) 11 (1) 14.3 (15) 16 (6) 9.8 13.0 7.7 11.1(4)
630.6 5.5-6.9 (6) 7.2 (7) 9.7 (12) 6.9 6.2 5.3
628.0 12.8 (5)
1065.2 12 (1) 12.0

Table 2: Derived formation enthalpies for the investigated 'visible' Otex2html_wrap_inline209 absorptions bands and comparison with literature data.

Study Absorption line Formation enthalpy Comment
(nm) (J/Mol)
Present study
477.3 - 970 tex2html_wrap_inline365 550
532.2 (- 1373 tex2html_wrap_inline365 1019) not considered
577.2 - 1106 tex2html_wrap_inline365 365
Summary - 1038 tex2html_wrap_inline365 466
Long and Ewing,(1973) - 2218 tex2html_wrap_inline365 293 T = 90 K
Orlando et al.,(1991) - 4604 tex2html_wrap_inline365 2093 T = 225 to 356, tex2html_wrap_inline413 tex2html_wrap_inline351 6401 nm
Horowitz et al.,(1989) - 837 tex2html_wrap_inline365 1674

Summary and Conclusions

First atmospheric profiles of the 'visible' (Otex2html_wrap_inline205)tex2html_wrap_inline205 absorption are presented. Such an information is particularly important to investigate the formation mechanism of the collisional complex of (Otex2html_wrap_inline205)tex2html_wrap_inline205 at low atmospheric pressures and temperatures (unlike the conditions of many laboratory studies), which is necessary for further atmospheric applications. It is found, that the collisonal pair absorption cross sections and the line shapes of the collisional complex (Otex2html_wrap_inline205)tex2html_wrap_inline205 in the visible wavelength range are not dependent on the pressure, but on temperature. From their temperature dependence a formation enthalpy of (tex2html_wrap_inline309 H = (- 1038 tex2html_wrap_inline365 466) J/mol) could be derived, which is in fair agreement with previous results.

Results

  figure178
Figure: Temperature dependence of the (Otex2html_wrap_inline205)tex2html_wrap_inline205 collisional pair absorption cross section (tex2html_wrap_inline277(T) = measured optical density/(oxygen partial pressure)tex2html_wrap_inline441) for the 477.3 nm, 532.2 nm, and 577.2 nm band, as derived from the measured atmospheric temperatures, pressure and the measured Otex2html_wrap_inline209 absorption profiles. Note that within the range of the temperatures encountered the natural logarithm of each set of measured (tex2html_wrap_inline277(T)) for either of the absorption bands fall on a straight line, when plotted versus the inverse atmospheric temperature (1/T). It thus appears that the collisional complex (Otex2html_wrap_inline205)tex2html_wrap_inline205 does have a 'binding' energy, which can be obtained from the corresponding Arrhenius expression (the slope of the linear regression line). For the derived enthalpy of (Otex2html_wrap_inline205)tex2html_wrap_inline205 formation (tex2html_wrap_inline309 H) see Table 2.

References

  • Blake, A. J., and D. G. McCoy, The pressure dependence of the Herzberg photoabsorption continuum of oxygen, J. Quant. Spectrosc. and Radiat. Transfer, 38, 113-120, 1987

    Dianov-Klokov, V.I., Absorption Spectrum of Oxygen at Pressures from 2 to 35 atm in the Region from 12,600 to 3600 Å, Optics and Spectroscopy, 16, 224-227, 1964

    Greenblatt, G.D., J.J. Orlando, J.B. Burkholder and A.R. Ravishankara, Absorption measurements of oxygen between 330 and 1140nm, J. Geophys. Res. 95, 18577-18582, 1990

    Herman, L., Spectre d'absorption de l'oxygène, Ann. Phys. (Paris), 11, 548-611, 1939

    Horowitz, A., W. Schneider, and G.K. Moortgat, The role of oxygen dimer in oxygen photolysis in the Herzberg Continuum. A temperature dependence study, J. Phys. Chem., 93, 7859, 1989

    Johnston, H. S., M. Paige, and F. Yao, Oxygen absorption cross sections in the Herzberg continuum and between 206 and 327 K, J. Geophys. Res., 89, 11665, 1984

    Long, C. A., and G. Ewing, Spectroscopic investigation of van der Waals molecules, 1. Infrared and visible spectra of (Otex2html_wrap_inline205)tex2html_wrap_inline205, J. Chem. Phys., 58, 11, 4824, 1973

    McKellar, A. R., W. N. H. Rich, and H.L. Welsh, Collision-induced vibrational and electronic spectra of gaseous oxygen at low temperatures, Can. J. Phys., 50, 1-9, 1972

    Newnham, D.A., J. Ballard, and M.S. Page, Otex2html_wrap_inline209 Absorption Cross-Section (Otex2html_wrap_inline205 Removed): 1000hPa Oxygen at 283K, ESA study 11340/95/NL/CN, private communication

    Orlando, J.J., G.S. Tyndall, K.E. Nickerson, and J.G. Calvert, The Temperature Dependence of Collision-Induced Absorption by Oxygen Near 6tex2html_wrap_inline465m, J. Geophys. Res. 96, 20755-20760, 1991

    Perner, D., and U. Platt, Absorption of light in the atmosphere by collision pairs of oxygen (Otex2html_wrap_inline205)tex2html_wrap_inline205, Geophys. Res. Lett., 7, 1053-1056, 1980

    Salow, H., and W. Steiner, Durch die Wechselwirkungskräfte bedingten Absorptionsspektren des Sauerstoffs, 1. Absorptionsbanden des Otex2html_wrap_inline205-Otex2html_wrap_inline205 Moleküls, Z. Physik, 99, 137-158, 1936

    Tabisz, G. C., E.J. Allin, and H.L. Welsh, Interpretation of the visible and near-infrared spectra of compressed oxgen as collion-induced electronic transitions, Can. J. Physics, 47, 2859, 1969

    Volkamer, R., Absorption von Sauerstoff im Herzberg I System, und Anwendungen auf die Aromatenmessungen am EUropean PHoto REactor, (EUPHORE), Diplomarbeit an der Universtät Heidelberg, 1996

    Wagner, T., Poster presented at XXI General Assembly of the European Geophysical Society, The Hague, 6 - 10 May, 1996




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Richard Fitzenberger
Fri Apr 24 18:47:41 MEST 1998