MAXDOAS technique

The MultiAxis DOAS (MAXDOAS) technique has been recently developed as a new remote sensing tool for the monitoring of tropospheric pollutants by means of the differential optical absorption spectroscopy (DOAS) method (Heckel et al., 2005, Honninger et al., 2004). In contrast to zenith-sky DOAS instruments which have been commonly used over the last decade for stratospheric monitoring and satellite validation, e.g. as part of the Network for the Detection of Atmospheric Composition Change (NDACC), MAXDOAS instruments are designed to allow the quasi simultaneous observation of the scattered sun light in a range of different line-of-sight (LOS) directions from the horizon to the zenith, which leads to increased sensitivity towards atmospheric absorbers present close to the surface. Through adequate retrieval process, the near-surface concentration of atmospheric pollutants like NO2 can be determined, as well as their integrated tropospheric and stratospheric column abundances.

DOAS technique

The Differential Optical Absorption Spectroscopy (DOAS) technique is based on a modified Beer-Lamber law:

with:

• Io and I : radiances before and after the absorption
• sigma i : cross-sections of the "i" absorber [cm²/molec]
• SCi: concentrations integrated along an effective light path [molec/cm²]
• P: a polynomial, taking into account the broad-band contribution, e.g. of Rayleigh and Mie scattering

The DOAS method makes use of:

• the highly structured spectra to identify the absorbers
• the difference in amplitude of the spectral structures at different wavelength, to quantify the concentration of the absorber along the effective light path (the so-called "slant columns densities" (SCD)

An example of spectral fit for NO2 in the 425-450 nm region, is given below: This region has been chosen because of the large structures in the NO2 cross-section, and of the limited interferences with other absorbers present in the atmosphere.

MultiAxis DOAS instruments

The MAXDOAS instruments are mainly

• grating spectrometers covering the UV-Vis region,
• that collect scattered light from different directions (MultiAxis),
• and that analyze the spectra with the DOAS technique

The MAXDOAS geometry is characterized by:

• a long light path through the lower troposphere, especially for the low elevation angles
• a constant light path through the stratosphere, within a scan

By substracting the zenith contribution form the off-axis contribution, the tropopsheric SCD can be obtained during the day. By dividing the tropopsheric SCD by the so-called "Air Mass Factor (AMF)", the tropopsheric vertical columns (VCD) can be retrieved:

Vertical columns