UV-visible observations of atmospheric O4 absorptions using direct moonlight and zenith-scattered sunlight for clear-sky and cloudy sky conditions
Abstract
[1] Atmospheric observations of the O4 absorption bands at 360.5, 380.2, 477.3, 532.2, 577.2 and 630.0 nm are presented for different atmospheric conditions (clear and cloudy skies) and viewing geometries (direct and zenith-scattered light observations). From the observations of direct moonlight it was possible to derive absolute O4 absorption cross sections for atmospheric conditions. We found that the relative shape of the observed absorption bands is similar to those of the O4 spectrum measured by Greenblatt et al. [1990] in the laboratory. However, in general (except for the absorption band at 380.2 nm), the O4 absorption cross sections derived in this study are larger by several percent compared to those of the other (mainly laboratory) observations. Using the observations of zenith-scattered light, we investigated the radiative transport through the atmosphere. Our observations under cloudy sky conditions confirmed that the light path enhancement due to multiple Mie scattering on cloud droplets is independent of wavelength. From the observations under clear-sky conditions we studied the influence of Mie scattering on aerosol. It was not possible to describe the selected clear-sky measurements by taking into account only Rayleigh scattering. We found that the comparison of the O4 measurements with model results for different sets of assumed aerosol extinctions provides a new, very sensitive tool to derive aerosol parameters from zenith sky ground-based measurements.
1. Introduction
[2] The absorption of oxygen molecules in the UV-vis-near-IR spectral range corresponds to different types of chemical bonds and transitions. Besides the discrete structured ro-vibrational bands of the electronic transition of the O2 molecule and the structured bands of the bound van der Waal's molecule O4, oxygen shows also broad unstructured absorptions due to the collision induced oxygen dimer (O2)2 [see, e.g., Greenblatt et al., 1990; Solomon et al., 1998, and references therein]. These broadband absorptions were first described by Janssen [1885, 1886] who also showed that the intensity of these bands varies with the square of the oxygen pressure. Since then several studies investigated the (O2)2 absorptions and it was shown that under atmospheric conditions these bands contain no fine structure [see Solomon et al., 1998; Naus and Ubachs, 1999, and references therein]. Greenblatt et al. [1990] concluded from the weak temperature dependence of the (O2)2 absorption that (under atmospheric conditions) they are most probably related to an oxygen collision complex rather than to a bound dimer. Nevertheless, within this study we will use the term O4 for this (O2)2 collision complex since it was conventionally used in previous studies.
[3] The O4 absorption bands in the UV-vis spectral regions analyzed in this study (360.5–630.0 nm) belong to transitions between dimers of ground state oxygen molecules and electronically excited states of both of the involved oxygen molecules of O4 (see Table 1). At larger wavelengths, transitions with only one excited oxygen molecule also occur, and under atmospheric conditions, N2 can also serve as a collision partner [see, e.g., Solomon et al., 1998, and references therein].
| Wavelength, nm | Upper State of Transition (From 3Σg− + 3Σg− Ground State) |
|---|---|
| 343.4a | 1Σg+ + 1Σg+ (ν = 2) |
| 360.5 | 1Σg+ + 1Σg+ (ν = 1) |
| 380.2 | 1Σg+ + 1Σg+ (ν = 0) |
| 446.7a | 1Σg+ + 1Δg (ν = 1) |
| 477.3 | 1Σg+ + 1Δg (ν = 0) |
| 532.2 | 1Δg + 1Δg (ν = 2) |
| 577.2 | 1Δg + 1Δg (ν = 1) |
| 630.0 | 1Δg + 1Δg (ν = 0) |
- a These wavelengths were not investigated in this study.
[4] Atmospheric absorptions of O4 are important in different fields of atmospheric radiative transfer modeling: First, the atmospheric absorption of solar radiation by O4 is expected to be responsible for about 1–2% of the total atmospheric absorption of the solar radiation [Pfeilsticker et al., 1997; Solomon et al., 1998; Mlawer et al., 1998; Zender, 1999]. Second, since the atmospheric column of oxygen varies only slightly (depending on pressure), observations of the atmospheric absorptions of O2 and in particular of O4 can serve as an indicator for variations of the radiative transport through the atmosphere. O4 and O2 observations by ground-based, and satellite instruments have been used, for example, to detect and characterize clouds and aerosols [Kuze and Chance, 1994; Van Roozendael et al., 1994; Erle et al., 1995; Wagner et al., 1998a, 1998b; Pfeilsticker, 1999; Pfeilsticker et al., 1998, 1999; Veitel et al., 1998; Koelemeijer and Stammes, 1999; Wagner, 1999]. Although most commonly the observation of the atmospheric O2 absorption (e.g., the oxygen-A band) is used for cloud retrieval algorithms [see, e.g., Kuze and Chance, 1994; Koelemeijer and Stammes, 1999] they are subject to several general shortcomings: First, when the highly fine structured O2 absorption is measured by low resolving instruments (as often used for atmospheric absorption spectroscopy [Van de Hulst, 1957; Platt et al., 1979, 1997; Noxon et al., 1979; Solomon et al., 1987; Platt, 1994]), saturation effects can make the measured O2 absorption insensitive to changes of the column density along the light path [Veitel et al., 1998; Heidinger and Stephens, 2000; Funk, 2001]. Second, the atmospheric light paths for the observation of scattered radiation depend strongly on the absorption strength of atmospheric species. In particular, the shape of the measured (low resolution) O2 absorption band can become significantly different from that of direct light observations [Platt et al., 1997; Marquard et al., 2000; Wagner et al., 2000]. Third, inelastic scattering processes (Raman scattering) on air molecules can cause a so-called ‘filling-in’ of the narrow O2 absorption lines [Grainger and Ring, 1962; Fish and Jones, 1995; Sioris and Evans, 2000].
[5] All these shortcomings can be almost completely avoided by studying the atmospheric O4 absorptions, which are relatively broad (FWHM in the range of a few nm [Greenblatt et al., 1990]) and show no spectral fine structure. Thus they can be spectrally resolved by a typical DOAS instrument [Platt, 1994]. In addition, the O4 absorption bands cover the whole UV and visible spectral range from about 340 to 630 nm. Thus O4 absorption measurements are very well suited for the investigation of the radiative transport through the atmosphere.
[6] O4 absorption cross sections in the UV and visible spectral range were measured under laboratory conditions in many studies (see, for example, Salow and Steiner [1936], Herman [1939], Dianov-Klokov [1964], Greenblatt et al. [1990], Newnham and Ballard [1998], and Naus and Ubachs [1999]; an excellent overview is given by Solomon et al. [1998]). Most of these measurements were performed at high pressures (>1000 mbar) and temperatures different from those of the troposphere where most of the atmospheric O4 absorption takes place. Several studies also presented measurements of the O4 absorption in the atmosphere. Perner and Platt [1980] reported O4 absorption cross sections (for the bands at 343.4, 360.5, 380.2, 446.7, 477.3, 577.2 nm) for surface near atmospheric conditions derived by long path absorption DOAS measurements using an artificial light source. Volkamer [1996] derived O4 absorption cross sections (for the bands at 343.4, 360.5, 380.2, 477.3, 532.2, 577.2 and 630.0 nm) under similar conditions using a multi-path reflection cell. O4 absorption cross sections for the total atmospheric column were derived by Mlaver et al. [1998] for the bands in the near-infrared (1140 and 1260 nm) using solar radiation. From the observation of direct moonlight, Solomon et al. [1998] showed that the O4 absorption of the atmospheric column (at 630.0 nm) is consistent with laboratory measurements by Greenblatt et al. [1990]. Osterkamp et al. [1998] determined O4 absorption cross sections using atmospheric balloon soundings under varying atmospheric conditions (7–500 mbar, 203–260 K) for the absorption bands at 432, 477.3, and 577.2 nm [see also Pfeilsticker et al., 2001].
[7] Here we provide a set of O4 absorption cross sections derived for the total atmospheric column (for Arctic winter conditions) including six O4 bands from 360.5 to 630.0 nm. These O4 absorption cross sections were obtained from direct moonlight observations, for which the atmospheric radiative transport can be modeled with high accuracy (because of the well defined geometry of the absorption path). Preliminary results of these observations were already presented by Wagner et al. [1996]. Our data set complements the observations from Mlawer et al. [1998] and Solomon et al. [1998] derived for a similar observation geometry. In this study (sections 2.2 and 2.3) we apply this O4 cross sections to the interpretation of zenith-scattered light measurements performed with the same instruments for both clear-sky and cloudy sky conditions. From the observations under a heavy cloud cover it was possible to characterize the influence of Mie scattering; from the observations under clear sky the influence of atmospheric aerosols on the radiative transfer was investigated. We find that O4 observations of zenith-scattered light provide a new, very sensitive method for the investigation of atmospheric aerosols.
2. Instrumental Setup and Data Evaluation
2.1. Instruments
[8] Two grating spectrometers were used to measure spectra of direct moonlight and zenith-scattered sunlight in the UV (318–384 nm) and visible (375 nm to 688 nm) spectral ranges. The spectral resolution (FWHM) ranges from 0.3–0.5 nm (4.5–7 pixels) for the UV instrument and from 1.1–3.1 nm (3.5–10 pixels) for the instrument of the visible spectral range. The spectral resolution of the visible spectrometer is <2 nm at wavelengths <600 nm. The spectral resolution of our instruments is thus much smaller than the widths of the O4 absorption bands, which ranges from about 4 nm (FWHM) in the UV to about 14 nm in the red spectral region [Greenblatt et al., 1990]. Thus our instruments are well suited to resolve the atmospheric O4 absorption bands over the whole spectral range from 360.5 to 630.0 nm investigated in this study. More detailed information on our instrumental set-up is given by Stutz [1991], Fiedler et al. [1993], Otten et al. [1998], and Wagner et al. [1998a].
2.2. Spectral Evaluation
[9] The measured spectra were analyzed using the DOAS technique [Platt, 1994]. To remove the strong Fraunhofer structures the spectra were divided by a spectrum taken at low zenith angle ϑ (this spectrum is in the following called Fraunhofer spectrum). Several reference spectra (see below) of the trace gases which show structured absorptions in the respective wavelength regions (including the O4 cross section) as well as a polynomial (of degree up to 7, see Table 2) are fitted to the logarithm of this ratio spectrum using a non linear least squares algorithm [Stutz and Platt, 1996]. The polynomial accounts for broadband absorption structures as well as for atmospheric Mie and Rayleigh scattering. For the correction of the ‘filling in of the Fraunhofer lines in the spectra of scattered sunlight (the so-called Ring effect [Grainger and Ring, 1962; Bussemer, 1993; Fish and Jones, 1995]) one or two Ring spectra are also included into the fitting routine. The first Ring spectrum was calculated assuming that Rayleigh scattering was the dominant atmospheric scattering process, the second Ring spectrum assuming that Mie scattering was the dominant atmospheric scattering process. Compared to the first Ring spectrum the amplitude of the second Ring spectrum increases towards smaller wavelengths. This reflects the strong difference of the wavelength dependence of Raman scattering (and Rayleigh scattering) compared to that of Mie scattering [Wagner, 1999]. Using two Ring spectra in the DOAS analysis of scattered light spectra minimizes the errors of the fitting results, especially when large wavelength ranges are analyzed. In this study we analyze the atmospheric O4 absorptions in relatively small wavelength ranges around the different individual bands (360.5, 380.2, 477.3, 577.2, and 630.0 nm) as well as over larger wavelength ranges including several bands (see Figure 1). The selected trace gas reference spectra and the parameters of the different DOAS analysis are listed in Table 2. The following trace gas reference spectra were used: (1) an O4 absorption cross section [Greenblatt et al., 1990] for 296 K, which was interpolated to the instrumental dispersion, (2) up to two H2O absorption spectra (for two column densities of 1 and 20 × 1022 molec/cm2) derived from the HITRAN database [Rothman, 1992] and convoluted to the spectral resolution of our instruments, (3) an O3 absorption cross section for 223 K [Burrows et al., 1999] convoluted to the spectral resolution of our instruments, and (4) a NO2 absorption cross section for 238 K [Harder et al., 1997] convoluted to the spectral resolution of our instruments. The wavelength calibration was performed by fitting the measured spectra to a highly resolved solar spectrum [Kurucz et al., 1984], which was convoluted to the spectral resolution of our instruments. Examples of the DOAS O4 analysis including several bands are shown in Figure 1. For measurements with strongly enhanced O4 absorptions (e.g., under heavy clouds, see section 2.2) it was possible to compare the wavelength calibration of the O4 absorption cross sections to the O4 absorptions in the atmosphere. We found slight shifts of the positions of the different bands (between +0.8 nm and −0.34 nm, see Table 3) and we used this new wavelength calibration for the O4 analysis in our study. However, the influence of this new calibration on the derived O4 absorptions was negligible (less than 0.5%).
| O4 Band, nm | Wavelength Range, nm | Pixel Range | Reference Spectrab | Degree of Polynomial |
|---|---|---|---|---|
| 360.5 | 347.1–375.3 | 420–820 | O4, O3, NO2, Ring | 3 |
| 380.2 | 365.4–388.7 | 680–1010 | O4, NO2, Ring | 3 |
| 477.3 | 431.3–526.1 | 550–850 | O4, O3, NO2, H2O, Ring | 3 |
| 532.2 | 488.7–556.7 | 450–670 | O4, O3, NO2, H2O, Ring | 3 |
| 577.2 | 538.4–616.4 | 250–510 | O4, O3, NO2, H2O, Ring | 3 |
| 630.0 | 586.8–654.2 | 120–350 | O4, O3, O2, NO2, H2O, Ring | 3 |
| 360.5, 380.2c | 347.1–388.7 | 420–1010 | O4, O3, NO2, 2× Ring | 5 |
| 477.3, 532.2, 577.2, 630.0c | 415.1–682.9 | 20–900 | O4, O3, O2, NO2, 2× H2O, Ring | 7 |
- a The results of the single band evaluations were used for the Langley plots. The multiband evaluations were used to determine the O4 absorption spectra for atmospheric conditions (Figure 1).
- b For the analysis of the moonlight measurements no Ring spectra were included in the spectral analysis.
- c For these DOAS algorithms the (logarithm of the) Fraunhofer spectrum was included in the fitting procedure, and the reference spectra were fitted to the logarithm of the measured spectrum instead of to the logarithm of the ratio formed by the measured spectrum and the Fraunhofer spectrum.
| O4 Absorption Band | Spectral Shift Necessary to Match the Atmospheric O4 Absorptions, nm |
|---|---|
| 360.5 nm | +0.56 ± 0.13 |
| 380.2 nm | +0.80 ± 0.18 |
| 477.3 nm | +0.05 ± 0.14 |
| 532.2 nm | −0.15 ± 0.25 |
| 577.2 nm | −0.36 ± 0.10 |
| 630.0 nm | −0.15 ± 0.14 |
- a The shifts of the different bands were determined with respect to the strongly enhanced O4 absorptions under heavy clouds (see Figure 1b and section 2.2). The ‘best shift’ was determined for the minimum of the residual structure. The range of uncertainty of the shifts was (somewhat arbitrarily) set to the values for which the residual structures increased up to more than 20% compared to the minimum value.
[12] For satellite measurements (e.g., by the GOME instrument aboard ERS-2 [Bednarz, 1995; European Space Agency, 1996]) the situation can, however, be completely different because usually an extraterrestrial sun spectrum can serve as Fraunhofer reference spectrum. Such spectra contain no atmospheric absorptions and the VCD can easily be obtained from a single observation [see, e.g., Wagner et al., 1998b; Wagner, 1999].
2.3. Modeling of the Radiative Transport
[13] In this study the AMFs for direct light and for zenith-scattered light observations are calculated using the radiative transfer model AMFTRAN [Marquard, 1998; Marquard et al., 2000]. It is based on a multiple scattering Monte Carlo code for full spherical geometry taking into account atmospheric refraction. Multiple scattering is calculated for Molecules (Rayleigh scattering), aerosol particles (Mie scattering) and for the Earth's surface (reflection). Aerosols are characterized by their extinction and scattering properties.
[14] AMFs for direct light observations are independent of wavelength; they only depend on the (relative) atmospheric concentration profile of the considered species. In the case of O4 the profile can be easily determined from temperature and pressure profiles measured, for example, by radiosondes. Because of the well-defined geometry of direct light observations the accuracy of such AMFs is much better than for scattered light observations. From internal comparisons between different radiative transport models we estimate the accuracy of the AMFs for direct light O4 observations used in this study to about ≤1%.
[15] In contrast, the AMFs for scattered light observations are subject to relatively large uncertainties. Because of the wavelength dependence of Rayleigh and Mie scattering and the atmospheric trace gases (mainly O3) they strongly vary with wavelength. In addition they depend on the atmospheric trace gas concentration profile and on the atmospheric aerosol load. They especially depend strongly on clouds: for (mainly) stratospheric absorbers the effect of an increasing altitude of the scattering surface (from ground to cloud top height) and the change of the albedo are the dominant effects [Wagner et al., 1998a; Richter and Burrows, 2002]. For tropospheric absorbers like O4, however, the changes can become even much larger due to multiple Mie scattering inside clouds and/or reflection between several cloud layers and the earth's surface. Compared to clear-sky conditions these changes can be up to several hundred percent [Erle et al., 1995; Wagner et al., 1998a; Pfeilsticker et al., 1999].
[16] To date, realistic 3-dimensional modeling of clouds is very difficult, and in addition, appropriate cloud parameters for a given atmospheric situation are generally not available. Thus in this study zenith-scattered light measurements of O4 are quantitatively compared with modeled AMFs only for clear-sky conditions. However, also for clear-sky conditions Mie scattering on aerosols can significantly affect the AMFs for tropospheric absorbers like O4. Thus the comparison between measured and modeled AMFs for clear-sky conditions allows us (1) to test the modeling of the atmospheric radiative transfer in general and (2) to determine the atmospheric aerosol extinction profile which yields the best agreement between the O4 measurements and the modeled O4 AMFs (see section 2.3.3).
2.4. Direct Light Observations
[17] According to the astronomical moon parameters direct moonlight observations were possible within two periods during the measurement campaign in January to March 1994 in Kiruna (northern Sweden, 68°N, 21°E). Please note that poleward from the polar cycle the moon stays above the horizon for several days around full moon. After the moon was sufficiently bright (after 22.01. and 18.02) our moon observations started; they ended after the moon set below the horizon on (04.02. and 04.03.). The most sensitive measurements were possible around 28.01 and 25.02. when the moon was full (31.01. and 28.02.) and the range of LZAs includes small as well as large values. For the quantitative determination of the O4 cross sections we chose the observation during the night from January 28 to 29 because during this night the height profiles of the atmospheric pressure and temperature above the measurement site were measured by an ozonesonde (launched shortly after midnight). Also during this night a large number of individual measurements at different LZAs was possible.
2.4.1. Spectra
[18] In Figure 1a the result of a spectral evaluation of all six O4 absorption bands between 360.5 and 630.0 nm is shown for the night of 28./29.01. The O4 absorption spectrum from Greenblatt et al. [1990] is scaled to the O4 absorptions in the measured spectrum. The reference spectra of all trace gases which show structured absorptions in the respective wavelength ranges were simultaneously fitted over the whole wavelength ranges (347–389 nm for the UV and 415–681 nm in the visible spectral range) as described in section 1.2. This is possible because for direct light measurements the AMF does not depend on the wavelength, and correspondingly the atmospheric O4 absorptions are in general well described by the laboratory cross section measured by Greenblatt et al. [1990] over the whole wavelength range. Also some small deviations are found: The relative strengths of the different O4 bands in the atmosphere seem to be slightly different from those of the Greenblatt spectrum. The bands at 360.5 and 577.2 nm are by about 5–6% larger compared to those at 360.5 and 477.3 nm, respectively. The band at 630.0 nm is about 1–2% larger compared to that at 477.3 nm.
2.4.2. O4 Absorption Cross Sections
[21] The errors of our O4 absorption cross sections result from different contributions: (1) the errors of the spectral retrieval, (2) the errors of the calculated AMFs, (3) the errors of the Langley-plots, and (4) the errors of determining the O4 VCD* from the radiosonde data. The 1-σ-errors of the spectral analysis are between <1% and several 10%. They are displayed as error bars in Figure 3. The error of the AMF for direct light observations is assumed to be <1% (see also section 1.3). From the Langley-plots the error of the slope of the fitted line was determined by varying the slope within the error bars of the measurements. The error of the O4 VCD* calculated from the pressure and temperature data of the radiosonde is estimated to be about 2%. For the O4 bands at 360.5, 380.2, and 532.2 nm the total error is dominated by the error of the slope of the Langley-plot. It varies between 9% for 360.5 nm and 30% for 532.2 nm. For all other bands the error is dominated by the uncertainty of the O4 VCD*. The total error of these bands is about 3%.
[22] In Figure 3 and Table 4 the temperature dependent O4 absorption cross sections for the six O4 bands are compared to those of previous studies. The lines in Figure 3 represent a least squares fit to all values (except those of Perner and Platt [1980] because their uncertainties significantly exceed those of the other measurements). Please note that many measurements errors as well as the spread of the data points is large indicating the poor knowledge about the temperature dependence. For all considered O4 absorption bands an increase of the peak absorption with decreasing temperature is found: about 13%/100K at 477.3 and 532.2 nm, about 20% at 360.5 and 577.2 nm, and about 33% at 380.2 and 630.0 nm [see also Pfeilsticker et al., 2001]. Except for the band at 380.2 nm the O4 absorption cross sections derived in this study are slightly larger compared to the line fitted to all measurements.
| O4 Band, nm | This Work (242K) | Greenblatt et al. [1990] (296K) | Greenblatt et al. [1990] (196K) | Perner and Platt [1980] (279K) | Volkamer [1996] (296K) | Osterkamp et al. [1998] (204K) | Osterkamp et al. [1998] (256K) | Newnham and Ballard [1998] (223K) | Newnham and Ballard [1998] (283K) | Naus and Ubachs [1999] (294K) |
|---|---|---|---|---|---|---|---|---|---|---|
| 360.5 | 5.70 ± 0.5 | 4.1 ± 0.4 | 5.7 ± 0.6 | 5.4 ± 1.5 | 5.42 ± 0.7 | – | – | – | – | – |
| 380.2 | 2.44 ± 0.4 | 2.4 ± 0.2 | 3.7 ± 0.4 | <1.4 | 2.4 ± 0.2 | – | – | – | – | – |
| 477.3 | 7.80 ± 0.2 | 6.3 ± 0.6 | 7.6 ± 1.3 | 5.9 ± 1.8 | 6.1 ± 0.3 | 7.9 ± 0.3 | 7.0 ± 0.3 | 6.99 ± 0.35 | 8.34 ± 0.83 | – |
| 532.2 | 1.74 ± 0.5 | – | – | – | 1.3 ± 0.3 | 1.4 ± 0.2 | 1.2 ± 0.2 | 1.31 ± 0.2 | 1.23 ± 0.38 | – |
| 577.2 | 13.50 ± 0.4 | 11 ± 1 | – | 16 ± 6 | 10.3 ± 0.3 | 12.2 ± 0.4 | 13.6 ± 0.4 | 12.61 ± 0.11 | 11.75 ± 0.2 | 11.41 ± 0.5 |
| 630.0 | 9.61 ± 0.3 | 7.2 ± 0.7 | – | – | 6.2 ± 0.6 | – | – | 8.8 ± 0.13 | 7.9 ± 0.15 | 7.55 ± 0.5 |
- a Here σ* values are given in 1046 cm5/molecule2.
[23] Our measurements add new information on the O4 absorption cross sections for a temperature region (around 242 K), where only very few previous measurements exists. This temperature (and also pressure) best represents the conditions for O4 absorption measurements of the whole atmospheric O4 column performed from ground-based or space-borne instruments. Thus they are well suited for the use in atmospheric applications (in particular for polar latitudes). Within this study we apply these O4 absorption cross sections to zenith sky measurements using the same instruments (see sections 2.2 and 2.3).
2.5. Zenith-Scattered Light Under Cloudy Sky Conditions
2.5.1. Spectra
[24] Clouds affect the atmospheric light paths in several ways compared to clear-sky conditions. First, because of multiple Mie scattering inside the clouds the light path can be largely increased, leading to enhanced absorptions of trace gases that are present inside the cloud [Erle et al., 1995; Van Roozendael et al., 1994; Wagner et al., 1998a; Pfeilsticker et al., 1999]. In particular under heavy clouds the absorptions of O4 and other tropospheric trace gases can be strongly enhanced. It should be noted that for the observation of zenith-scattered light especially thin high clouds can also reduce the absorption paths below the cloud. For details of this effect, see Wagner et al. [1998a] and Pfeilsticker et al. [1999]. Here we consider a vertically extended cloud (from 200 m to 11 km) on March 5, 1994, when a warm front passed over the measurement site (see Figure 6, left panel). This case is described in more detail by Wagner et al. [1998a].
[25] Because of the large size of cloud particles Mie scattering in clouds is expected to show no wavelength dependence [Van de Hulst, 1957]. Thus the modifications of the light path due to thick clouds are expected to be similar over a large wavelength range. The observation of the O4 absorptions at different UV-vis wavelengths can be used to test this expectation. In addition, because of the large O4 absorptions derived under heavy clouds the shape and spectral position of the atmospheric O4 absorptions can be compared to that of laboratory measurements.
[26] In Figure 1b the DOAS fitting results for O4 are shown for two measurements under the heavy cloud cover during March 5, 1994. All reference spectra including O4 are simultaneously fitted to the logarithm of the ratio of two spectra measured during this day. The small average intensity observed in the first spectrum (7:40 UT) indicates a relatively thick cloud cover (see Figure 4); the much higher intensity of the second spectrum (measured about two and a half hours later at 10:14 UT) indicates a relative thin cloud cover. Correspondingly the light path enhancement and thus the O4 absorption differ strongly in both spectra, leading to a strong O4 absorption in the difference spectrum (Figure 1b). It should be noted that compared to the moonlight observations especially in the UV the signal to noise ratio is much better. It is obvious that (similar to the moonlight observations) the laboratory spectrum of Greenblatt et al. [1990] is well suited to describe the atmospheric observations under cloudy sky conditions over a large wavelength range. This confirms that light path modifications due to the clouds are essentially independent of wavelength as expected from the Mie-theory.
[27] It should be noted that (similar to the direct moon measurements) also here the relative strength of the different O4 bands in the atmosphere seem to be slightly different from that of the Greenblatt spectrum. The bands at 360.5 and 577.2 nm are by about 5–6% larger compared to those at 360.5 and 477.3 nm, respectively. The band at 630.0 nm is about 2% larger compared to that at 477.3 nm. From the observations for cloudy sky conditions we investigated the spectral calibration of the O4 cross section of Greenblatt et al. [1990] (see section 1.2). We found small changes for the different absorption bands (see Table 3).
2.5.2. Diurnal Variation
[28] Besides from meteorological observations (in particular satellite images) information about the cloud cover over the measurement site can be derived also from the measured spectra. From additional Mie scattering the observed intensities are usually enhanced by clouds for zenith scattering measurements. Moreover, because of the different wavelength dependence of Mie and Rayleigh scattering the scattered light is affected differently at different wavelengths; the ratio of these intensities, the so-called color index (CI), is thus a very sensitive indicator of the appearance of clouds. The diurnal variation of the average intensity, the O4 absorptions for 380.2, 477.3, 577.2 and 630.0 nm, as well as the CI (here defined as the ratio of the intensities at 670 and 388 nm; see, for example, Sarkissian et al. [1991] and Enell et al. [1999]) is displayed in Figure 4. In order to allow a direct comparison for all wavelengths here the fit coefficients of the O4 reference spectrum [Greenblatt et al., 1990] are displayed rather than the optical densities (which are different for several O4 absorption bands). It is obvious that the variation of the O4 fit coefficients for all selected bands is nearly identical during the whole cloudy day, which is another confirmation of the wavelength independence of Mie scattering in clouds.
[29] It should be noted that in addition to the O4 absorptions also the absorptions of O3 (in the Chappuis and Huggings bands), NO2 and H2O show a similar variation during that day, also caused by the light path modifications due to the clouds. The derived average light path enhancement (compared to clear-sky conditions) due to multiple Mie scattering was determined as up to 100 km for these conditions [Wagner et al., 1998a].
2.6. Zenith-Scattered Light Under Clear-Sky Conditions
2.6.1. Spectra
[30] Zenith-scattered light observations of O4 absorptions under clear-sky conditions allow us to test the understanding and modeling of the atmospheric radiative transport. Because of the strong wavelength dependence of Rayleigh scattering on air molecules the optical depth of the atmosphere varies strongly with wavelength. As a consequence for large solar zenith angles (SZA > 75°) the AMF for the various O4 absorption bands differs strongly. This finding can be clearly seen in Figure 1c, where the DOAS analysis for O4 is shown for a clear day (22.03.1994) at Kiruna. All reference spectra including O4 are simultaneously fitted to the logarithm of the ratio of two spectra measured at different SZA (89.5° and 75.3°) on that day. However, in contrast to the direct moon measurements or the observations under cloudy sky conditions, in this case it is not possible to analyze the whole spectral range simultaneously, because of the strong wavelength dependence of the clear-sky AMF for tropospheric species. For example, in Figure 1c the absorption of the O4 band at 630.0 nm is similar to that at 577.2 nm although the respective O4 absorption cross sections differ by nearly a factor of two. This finding can be attributed to the difference of the AMFs for both wavelengths. Also the absorption band at 477.3 nm is much weaker than expected from the laboratory cross sections, and the bands at 360.5 and 380.2 are even inverted indicating a larger O4 absorption in the spectrum measured at smaller SZA.
2.6.2. Diurnal Variation
[31] In Figure 5 the diurnal variation of the average intensity, the O4 absorptions for 380.2, 477.3, 577.2 and 630.0 nm, and the CI (similar to Figure 4) at Kiruna for a mostly clear day (22.03.1994) are shown. The Fraunhofer spectrum used for the analysis of the O4 absorption was taken at a SZA of 67°. From the measured CI and the average intensity as well as from satellite images of that day (Figure 6) we conclude that during the morning the sky above Kiruna was clear while during the afternoon thin clouds appeared. Correspondingly during the morning the difference between the O4 absorptions for different wavelengths is large, while during the evening the difference becomes smaller.
2.6.3. Modeling of the Radiative Transport Under Clear-Sky Conditions
[32] We selected the morning measurements of March 22, 1994 as a test case for the modeling of the atmospheric radiative transport under clear-sky conditions (our measurements during that winter were stopped a few days later and no totally clear day appeared within that period). In the upper panel of Figure 7 the morning O4 absorptions (as shown in Figure 5) are plotted, now as a function of the SZA. In order to allow a direct comparison with the model results we here express the measured O4 absorptions as AMF. For that purpose a value for the O4 VCD* (1.27 · 1043) was used which was calculated from a radiosonde launched at Sodankylä (about 300 km in the east of Kiruna), because on that day at Kiruna no radiosonde data were available.
[33] Also shown in Figure 7 are the modeled AMFs for a pure Rayleigh atmosphere as well as for different tropospheric aerosol loadings. Both the measured and modeled O4 AMFs are expressed as differences to the respective values for a SZA of 67°; at this SZA the Fraunhofer spectrum was measured.
[34] While the relative SZA dependence for the O4 absorptions at different wavelengths is already qualitatively well described by the modeled AMFs for a pure Rayleigh atmosphere, especially for 477.3, 577.2, and 630.0 nm the absolute values are by about a factor of up to four larger than the measured ones. This discrepancy can be attributed to the influence of aerosols (or sub visible clouds) on the measurements. In order to investigate the influence of aerosol on the radiative transport in more detail we calculated O4 AMFs for several cases including the effects of aerosols by varying both the altitude range and the optical depth of the assumed aerosol extinction. It should be noted that taking into account only the influence of background aerosols (various profiles with different total optical depths) it was possible to reduce the absolute values of the O4 AMFs but it was impossible to simulate the correct SZA dependence for large SZA (unfortunately, no data on the aerosol extinction profiles were available for the day of our measurements). The respective O4-AMFs are very similar to those for assumed layer heights of 0–1 km, see Figure 7.
[35] The resulting AMF (-differences) are presented in the lower part of Figure 7. Several general findings can be obtained: (1) In general, an increase of the optical depth of the aerosol extinction at a given altitude reduces the corresponding O4 AMFs (as an exception an aerosol layer near the surface can also slightly enhance the AMF for small SZA). (2) Increasing the altitude of the assumed aerosol layer shifts the maximum of the modeled AMFs towards higher SZA. Finding 1 can be explained by the increase of the scattering altitude of the observed light (from the zenith) for an increased aerosol extinction (see also section 2.2.1). Finding 2 can be explained by the fact that at large SZA (when the sun is below the horizon) the observed light can have already penetrated near to the surface before it is scattered in the zenith. This is, however, only possible if the light extinction by aerosols near the ground is small. For a large aerosol absorption near the ground, such light paths are strongly attenuated and thus can hardly contribute to the measured signal. Assuming in contrast that an aerosol absorption of similar optical depth occurs near to the tropopause, the near surface light paths are only weakly attenuated. In that case near surface light paths can contribute significantly to the measured signal leading to large O4 AMFs at large SZA.
[36] From the results displayed in Figure 7 it becomes evident that the O4 absorptions under clear-sky conditions are very sensitive to variations of the tropospheric aerosol extinction. This sensitivity in turn can also be utilized to derive aerosol properties from O4 measurements under clear-sky conditions.
[37] To illustrate this possibility we tried to determine a set of O4 AMFs (according to a specific aerosol extinction profile), which best describes the selected O4 measurements for 380.2, 477.3, 577.2 and 630.0 nm. For that purpose we varied the tropospheric aerosol profiles over a wide range including various altitude distributions and aerosol extinctions. We varied the height of the assumed aerosol layer from the surface to 14 km and the width from 1 km to 14 km. The aerosol extinction was varied between 0.01/km and 0.5/km. For all AMF calculations we assumed a background aerosol extinction according to a clear remote atmosphere (taken from the LOWTRAN database [Kneizys et al., 1988]), and at specific altitudes we added the layers of a stronger aerosol extinction. The best agreement between the model results and our measurements was obtained for an assumed additional aerosol layer between 11 and 12 km with an extinction of 0.1/km. This finding might indicate that during the observations a thin (sub visible) high cirrus cloud layer might have been present; some hints of such a cloud layer could also be concluded from the satellite images (Figure 6, right panel).
[38] Langley-plots using the above-described AMFs for the measurements during the morning of March 22 are shown in Figure 8. As in Figure 7 the measured O4 are expressed as AMF. The measurements for 630.0 nm fall closest to the expected straight line. However, for the observations at 477.3 and 577.2 nm still significant deviations from this line appear. In addition, the values of the slope deviate from the expected value of 1 by ±17%. We attribute these discrepancies to two major causes: First, the aerosol conditions might have varied throughout the observations. Second, the aerosol properties are only poorly captured by the parameters (aerosol extinction and phase function) in our radiative transfer model. In general, these parameters depend, for example, on wavelength, which is not considered in our model calculation. Also for high clouds Mie scattering might not be well suited, because the cloud particles are not spherical. A part of the discrepancies of the modeled AMF and the measured O4 absorptions might also be caused by differences of the actual and the assumed value (80%) for the ground albedo. From our modeling results it follows that reducing the ground albedo from 80% (for a snow surface) to about 5% (for a snow free surface) reduces the AMFs for all investigated wavelengths by about a factor of two (for SZA < 90°).
[39] The remaining differences between our measurements and modeling results might be resolved in future model calculations, which will use a more detailed aerosol parameter input. Further clear-sky O4 observations, especially for low ground albedo during summer might also be helpful. Nevertheless, from our studies we already conclude that the comparison of measured and modeled clear-sky O4 absorptions is a very sensitive tool to derive aerosol parameters from zenith sky ground-based measurements. It should be noted that this method can be expected to be even more sensitive for off-axis-viewing geometry (the viewing direction is slightly above the surface) or the novel multi-axis-viewing geometry (scattered light from several viewing directions is observed). For such observation geometries the tropospheric light paths and thus the O4 absorptions are significantly larger. Our method can thus provide independent, additional information to already existing aerosol measurement techniques.
3. Conclusions
[40] We investigated the radiative transfer through the atmosphere for different viewing geometries (direct light observations and zenith-scattered light observations) and different atmospheric conditions (clear and cloudy sky). For that purpose we observed the O4 absorption bands in the atmosphere at 344, 360.5, 380.2, 477.3, 532.2, 577.2 and 630.0 nm. From the direct (moon) light observations it was possible to determine the absolute O4 absorption cross sections for these bands for the atmospheric temperature and pressure. It should be noted that our O4 absorption cross sections are best suited for the conditions at polar latitudes; at mid and low latitudes the atmospheric temperatures might differ significantly from those of our measurements. We found that the relative shape of the observed absorption bands is similar to those of the O4 spectrum measured by Greenblatt et al. [1990] in the laboratory. However, small deviations of the absolute values appear with respect to several previous measurements. In general (except for the absorption band at 380.2 nm), the O4 absorption cross sections derived in this study are larger by several percent compared to those of other observations. Using the O4 absorption cross sections derived from the moonlight observations we investigated the radiative transport through the atmosphere for zenith-scattered light observations performed with the same instruments under clear-sky and cloudy sky conditions. These observations confirm the strong enhancement of the atmospheric absorption path for tropospheric absorbers due to multiple Mie scattering in heavy clouds. We found a nearly identical relative absorption enhancement for the different investigated wavelengths (380.2, 477.3, 577.2, and 630.0 nm). This finding confirms the wavelength independence of Mie scattering on cloud particles for the observed wavelength range. Finally we investigated the atmospheric radiative transport for clear-sky conditions. We compared the measured clear-sky O4 absorptions with modeled O4 absorptions for (1) a pure Rayleigh atmosphere and (2) including also additional Mie scattering by aerosols. We found that (while no obvious clouds were present during the observations) it was not possible to describe the selected clear-sky measurements with radiative transfer modeling including only Rayleigh scattering. However, taking into account also scattering on aerosols brings the modeled and measured absorptions in much better agreement. From the model results for different assumed aerosol extinctions we found that the atmospheric O4 absorptions are very sensitive to the (absolute) atmospheric aerosol extinction and on the (relative) aerosol extinction profile. The best agreement between our measurements and model results was achieved for an assumed aerosol layer between 11 and 12 km with an extinction of 0.1/km. The comparison of modeled and measured O4 absorptions can be used as a new, very sensitive tool to derive aerosol parameters from ground-based UV-vis measurements. Potential applications of this technique might include also the retrieval of cloud properties from UV-vis satellite instruments like GOME (on ERS-2) and SCIAMACHY (on ENVISAT) as well as the determination of effective AMFs for the interpretation of scattered light DOAS observations in off-axis or multiple axis geometry, which are highly sensitive to tropospheric trace gases.
Acknowledgments
[41] The authors would like to thank the staff of ESRANGE for their great hospitality. In particular, we thank O. Widell for his great support. The work was supported by the BMFT under grant 01 VOZ 12/3. Additional financial support from the EU is highly appreciated (grants EV5V-CT91-020 and EV5V-CT93-0341). We are grateful to the Dundee Satellite Receiving Station, Dundee University, Scotland (http://www.sat.dundee.ac.uk/), for providing us the satellite images.
