AMAX-DOAS: principle and data description

The Airborne Multi-AXis-Differential Optical Absorption Spectroscopy instrument collects intensity spectra of scattered UV-visible sun light from various viewing direction. Light is collected by telescopes in the canister and transmitted via glass fiber cables to the spectrograph-detector units in the cabin. Trace gases are identified by means of their individual absorption structures using the DOAS fitting routine.

The direct product of a stray light DOAS analysis is the differential Slant Column Density (dSCD) in molecules/cm². A dSCD is the weighted average over the integrated concentrations along all possible light paths with respect to a measured reference spectrum. Therefore a slant column is always dependent on the light path distribution.  By utilizing different viewing directions (multi-axis), the instrument becomes sensitive to specific atmospheric layers.

The AMAX-DOAS data files contain date and time in UTC, trace gas dSCDs of glyoxal (CHOCHO) and iodine oxide (IO) and information on the significance of the individual data points (dSCDsig). The significance is expressed as dSCDsig = dSCD - SCD(detection limit). SCD(detection limit) is derived by: optical density of trace gas absorption = optical density of 1 sigma of the DOAS fit residual.