Introduction

First atmospheric profiles of the collisional complex of (O), or brief O, are reported. The information derived from (O) 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) (O) 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, NO, HO, and ozone, and (d) (O)-profiles measured at different atmospheric temperatures and pressures may provide new insight into the collision process of (O + O) and the thermochemistry of (O). In particular, in some optical properties of the (O)-complex under ambient conditions (pressures 1atm and temperatures 300K), which are - due to the weak (O) absorption - not easily accessable in the laboratory. Observation Geometry

  
Figure: Observation geometry of balloon soundings of atmospheric O.

Observations

The (O)-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 (O) 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 (O)-absorption ((H), H=height) at 477.3 nm, 532.2 nm, and 577.2 nm were derived (Figure 3).

Differential Optical Densities

  
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 = 83). Trace (b); Direct sunlight spectrum recorded at an altitude of 31 km (SZA = 89.7). Trace (c); Ratio of both spectra; Trace (d); Retrieved absorption signature of the O-Chappuis band. Trace (e); Retrieved absorption signature of atmospheric (O).

Measurements

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

whereby the height profile of the oxygen molecular concentration (n(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 (O)-absorptions show no 'fine' structure with respect to wavelength.

(c) the (O) absorption coefficient decreases by about 20 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((T))=ln()-H/(RT), (see Figure 5)) (an Arrhenius type temperature dependence). Accordingly, from the slope of the temperature dependence the 'binding' enthalpy ( H) of the (O) can be calculated (Table 2).

Profiles

  
Figure: Height profile of the atmospheric (O)-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 (O) 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 (O) 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 O-O 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 (O)-complex as a dimer or 'van der Waals' molecule. However, when accordingly an entropie change ( S) due to the formation of this hypothetical O 'van de Waals' molecule is also taken into account (about 125 J/(mol K), then the concentration of the oxygen dimer would change by about 70 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 (O) is more likely a collisional complex. Although this asymmetry is also found here, this model would more point to a T 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 (O) 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 ( H = (- 1038 466) J/mol) our findings lead us to conclude that (O) is most likely a metastable complex, whereby the (O)-absorptions may occur in collisional-induced electronic transitions.

Lines

  
Figure: Variation of the O absorption band shape (at 477.3 nm) with the atmospheric observation height, and a comparison with the 'low pressure' (O)-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 (O) absorption coefficient () with literature data (the data of the present study are interpolated to 296 K).

Volkamer Greenblatt et.al. Wagner Perner Dianov- Herman Salow This study

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) 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' O absorptions bands and comparison with literature data.

Study Absorption line Formation enthalpy Comment

(nm) (J/Mol) Present study 477.3

532.2 not considered 577.2 Summary

Long and Ewing,(1973) T = 90 K Orlando et al.,(1991) Horowitz et al.,(1989)

Summary and Conclusions

First atmospheric profiles of the 'visible' (O) absorption are presented. Such an information is particularly important to investigate the formation mechanism of the collisional complex of (O) 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 (O) in the visible wavelength range are not dependent on the pressure, but on temperature. From their temperature dependence a formation enthalpy of ( H = (- 1038 466) J/mol) could be derived, which is in fair agreement with previous results.

Results

  
Figure: Temperature dependence of the (O) collisional pair absorption cross section ((T) = measured optical density/(oxygen partial pressure)) for the 477.3 nm, 532.2 nm, and 577.2 nm band, as derived from the measured atmospheric temperatures, pressure and the measured O absorption profiles. Note that within the range of the temperatures encountered the natural logarithm of each set of measured ((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 (O) 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 (O) formation ( H) see Table 2.

References

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  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

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