Level-2 Algorithms - Formaldehyde (HCHO) Vertical Column

Sentinel-5P Technical Guide - Level-2 Processing Overview - HCHO Vertical Column

The general method used for derivation of HCHO vertical column density is the DOAS method.

 

Fitting Window

The L2 HCHO retrieval algorithm uses a two-step fitting approach, based on two wavelength intervals, both in spectral band 3.

  1. 328.5-359 nm: this interval includes six BrO absorption bands and minimises the correlation with HCHO, allowing a significant reduction of the retrieved slant column noise. However, this interval includes part of a strong O4 absorption band around 360 nm, which is known to introduce geophysical artefacts of HCHO columns over arid soils.
  2. 328.5-346 nm: in a second step, HCHO columns are retrieved in a shorter interval, but using the BrO slant column values determined in the first step. This approach allows efficient decorrelation of BrO from HCHO absorption while, avoiding O4-related bias.

 

Reference Sector Correction

Residual biases in the columns, due to unresolved spectral interferences, are known to remain a limiting factor for the retrieval of weak absorbers such as HCHO. To reduce these biases, an absolute background normalization is applied on a daily basis using the Reference Sector Correction method. The reference sector is chosen in the central Pacific Ocean. In this region, the HCHO concentration is assumed to be at background levels, its only source being CH4 oxidation. The mean HCHO slant column in the reference sector is fitted using a polynomial Ns,0 which is subtracted from all the slant column densities of the day and replaced by the HCHO background taken from a 3D chemistry transport model in the same region (Nv,0ctm).

The end product of the HCHO vertical column retrieval procedure is a differential vertical column obtained using the following equation:

Nv = (Ns - Ns,0) / M + Nv,0ctm

  • Ns is the slant column density retrieved from the first step of the DOAS algorithm
  • Nv is the (differential) vertical column density
  • M is the air mass factor
  • Ns,0 and Nv,0ctm the values used for the reference sector correction.

It should be noted that this Air Mass Factor M implicitly includes the cloud contribution, i.e. the Independent Pixel Approximation (IPA) and Ghost column.