# Focus on tropospheric NO2: the DOAS method

Extracted from Chimot, J., Global mapping of atmospheric composition from space – Retrieving aerosol height and tropospheric NO2 from OMI, PhD book, Delft University of Technology (TU Delft), The Royal Netherlands Meteorological Institute (KNMI), July 2018.

The Differential Optical Absorption Spectroscopy (DOAS) method is a specific atmospheric retrieval approach employed for UV and visible absorbing trace gases. The various DOAS techniques rely on the same key concept: a simultaneous fit of several trace gas slant column densities from the fine spectral features due to their absorption (i.e. the high frequency part) present in passive UV–visible spectral measurements of atmospheric radiation (Platt and Stutz, 2008). The Beer-Lambert law is used as forward model. It is commonly employed for absorption spectroscopy analyses of NO2 – Nitrogen dioxide, SO2 – Sulfur dioxide, HCHO – Formaldehyde and O3 – Ozone from the OMI, TROPOMI, GOME, GOME-2 and SCIAMACHY sensors (Boersma et al., 2011).

To take into account the simultaneous presence of several absorbers together with Rayleigh and Mie scattering, the Beer-Lambert law, that describes the light attenuation as a function of the travelled distance, gas concentration and its spectral absorption intensity, is commonly re-written as follow:

The spectral fit is achieved within a predefined spectral window (e.g. 405–465 nm for OMI NO2) on the spectral reflectance R. Usually, R is divided into two parts: 1) a spectrally smooth part, modelled by a polynomial, accounting for Rayleigh and Mie scattering, surface reflection, and the low frequency of the absorption cross sections, 2) a spectrally differential part from which trace gas information is retrieved. The slant column density Ns gives then the column density along the average light path traveled by the detected photons from the Sun through the atmosphere, surface and back to the satellite sensor.

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In the case of OMI tropospheric NO2 It is obtained through a least-squares fit approach applied to the logarithm of R over 405–465 nm:

Special attention is generally paid, during this fit, to the Ring effect: i.e. a so called Ring spectrum is included like a cross section at the right side of equation 1.25 (Chance and Spurr, 1997; Boersma et al., 2007, 2011). Furthermore, the temperature sensitivity of the NO2 absorption cross section is accounted for by computing an a posteriori temperature correction term between an effective atmospheric temperature along the average photon path and the 220 K cross section spectrum used in the fitting procedure (Vandaele et al., 1998). The stratospheric NO2 contribution must be separated from the tropospheric one in the slant column. In the case of OMI DOMINO product, this is done by assimilating slant columns into a chemistry transport model (Boersma et al., 2007, 2011)