The DOAS technique and instruments

The DOAS technique

Differential Optical Absorption Spectroscopy (DOAS) is a widely used inversion method for the retrieval of atmospheric trace gas abundances from multi-wavelength light measurements. It uses the structured absorption of many trace gases in the UV, visible and near-infrared spectral ranges. The DOAS method was originally developed for ground-based measurements (Platt 1994; Platt and Stutz 2008). It relies on the application of the Beer-Lambert law to the whole atmosphere in a limited range of wavelengths.

The Beer-Lambert law states that the radiant intensity traversing a homogeneous medium decreases exponentially with the product of the extinction coefficient and the path length. Applying this law to the atmosphere, we obtain 

The key idea of the DOAS method is to separate broad and narrow band spectral structures of the absorption spectra in order to isolate the narrow trace gas absorption features. In order to do this, some approximations are made: 

1. In the case where the photon path is not defined (scattered light measurements), the mean path followed by the photons through the atmosphere up to the instrument is considered;

2. The absorption cross sections are supposed to be independent of temperature and pressure, which allows us to introduce the concept of Slant Column Densities (SCDs);

3. Broadband variations, such as loss and gain from scattering and reflections by clouds and/or at the earth surface, are approximated by a common low order polynomial.

Molecular absorption cross sections are fitted to the logarithm of the ratio of the measured spectrum and the reference spectrum (i.e. an extraterrestrial irradiance spectrum for satellite measurements, or a spectrum measured around the local noon when the light path is minimum for ground-based measurements). The resulting fit coefficients are the integrated number of molecules per unit area along the atmospheric light path for each trace gas, the differential SCD. The slant column depends on the observation geometry, the position of the sun and also on parameters such as the presence of clouds, aerosol load and surface reflectance. 

Ground-based instruments using the DOAS technique 

Zenith-sky DOAS instruments measure scattered sunlight in the zenith direction. In this geometry, the solar radiation travels a short path through the troposphere and a longer path through the  stratosphere  -  this  effect  being  extreme  at  twilight  under  low  sun  conditions.  Twilight measurements are thus mainly sensitive to the stratosphere, and provide accurate stratospheric trace-gas column measurements at dusk and dawn.

Multi-axis DOAS instruments (MAX-DOAS) measure scattered sunlight under different viewing elevations from the horizon to the zenith. The observed light travels a long path in the lower troposphere (the lower the elevation angle, the longer the path) and the different elevations of one scan have the same path in the stratosphere. The stratospheric contribution can thus be removed by taking the difference in SCD between an off-axis elevation and a zenith reference. Tropospheric absorbers are measured all day long generally up to 85° of solar zenith angle (SZA). In addition, MAX-DOAS instruments can provide vertical profiles of trace gases and aerosols in the lowermost troposphere (e.g. Frieβ et al., 2006; Clémer et al., 2010; Hendrick et al. 2014).

Direct-sun instruments measure direct sun (ir)radiance during  daytime. The light travels through the whole atmosphere and the measurement is equally sensitive  to both troposphere and stratosphere. These instruments therefore provide accurate total-column measurements with a minimum of a-priori assumptions.

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