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 O 4 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 O 4 absorption cross sections for atmospheric conditions. We found that the relative shape of the observed absorption bands is similar to those of the O 4 spectrum measured by Greenblatt et al. [1990] in the laboratory. However, in general (except for the absorption band at 380.2 nm), the O 4 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 O 4 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.

NO<sub>2</sub> seasonal evolution in the north subtropical free troposphere

“…It has been reported that paths obtained from O 4 are even larger than that RT computed for a Rayleigh atmosphere (Wagner et al, 2002) when using the generally accepted cross sections reported in the literature ( Greenblatt, 1990;Hermans, 2011), suggesting that cross sections are underestimated. There is, however, no agreement in the magnitude of the correct values.…”

Determination of aerosol properties from MAX-DOAS observations of the Ring effect

“…One reason for this discrepancy is probably the temperature dependence of the O 4 absorption at 360 nm (see e.g. Wagner et al, 2002). The atmospheric temperature at the measurement site weighted with the O 4 profile was about 267 K compared to 296 K for the laboratory measurement of the O 4 cross section.…”

“…from ground based, airborne or satellite based observations), the accurate correction of the Ring effect is an important prerequisite for the precise retrieval of atmospheric trace gas absorptions (e.g. Noxon et al, 1979;Solomon et al, 1987;Chance and Spurr, 1997;Vountas et al, 1998;Sioris and Evans, 2000;Aben et al, 2001;de Beek et al, 2001;Wagner et al, 2002Wagner et al, , 2004Platt and Stutz, 2008). Usually, for that purpose a socalled Ring spectrum is calculated and included in the spectral fitting process; it can be obtained from observations using polarisation filters or can be computed from measured solar spectra (Solomon et al, 1987;Bussemer, 1993;Vountas, 1998;de Beek et al, 2001).…”

A fast H<sub>2</sub>O total column density product from GOME – Validation with in-situ aircraft measurements

“…For the conversion of the observed O 4 absorption into the respective column density we applied an O 4 cross section of 9.61·10 46 cm 5 /molec. 2 which was determined from atmospheric observations (Wagner et al, 2002). In some cases the relative fitting error of O 4 can become significantly larger than that of H 2 O.…”

“…In Table 1 the magnitude of the underestimation of the GOME H 2 O VCD is summarised. (Newnham and Ballard, 1998;Wagner et al, 2002) Cloud top height Cloud fraction 20% Cloud fraction 50% Cloud fraction 70%…”