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WP1 & WP2

Improving NO2 column densities from APEX through an optimization of APEX operation and retrieval algorithms

WP1: Derivation of improved NO2 slant/vertical column densities from APEX using existing data acquired above Antwerp city – optimization of APEX operation and retrieval algorithms.
WP2: Acquisition of additional APEX data in spectrally unbinned mode above 3 Belgian cities.

In these work packages, the spectral retrieval algorithm is developed in order to retrieve NO2 slant and vertical columns from existing APEX data, acquired above Antwerp in 2012. At the start of the project, this was the only APEX dataset acquired above Belgium in fully spectral unbinned mode, necessary to yield a sufficient spectral resolution to retrieve NO2.

APEX (Airborne Prism EXperiment) is an airborne imaging spectrometer developed through ESA’s Prodex programme by a Belgian-Swiss consortium. It features up to 532 spectral bands in full spectral mode, while APEX spectral programmability enables achieving higher Signal-to-Noise-Ratios (SNR) by reducing the number of bands in a binned configuration. Data are acquired in 1000 pixels across track with an FOV of 28°, resulting in ground pixel sizes of 1.5-2.5 m at typical flight levels of 3000-5000 m above target. APEX has been formally accepted by ESA at the end of 2010 and is since then operated by VITO-TAP.


Flightlines over the three most NO2 polluted cities in Belgium

Building on previous experience, retrieval algorithms based on existing tools developed for satellite retrieval have been adapted to retrieve NO2 vertical column densities (VCD) from high resolution APEX observations. The atmospheric features taken into account around the spectral region of interest are the NO2 feature at 430 nm and the O2 feature at 687 nm. The VCD retrieval algoritm based on APEX spectra that was developed and implemented in the APEX processing chain during this WP1 consists of the following steps:

(1) Spatial aggregation of the imaging spectrometer data to increase the signal-to-noise ratio, resulting in a spatial resolution of 60 meters by 80 meters.

(2) Spectral shift correction to attenuate the unstable slit function

(3) Differential Optical Absorption spectroscopy (DOAS) analysis of the preprocessed spectra in the visible wavelength region, with reference spectra containing low NO2 absorption

(4) Air mass factor calculations accounting for albedo, aerosol and NO2 profile shapes and viewing and sun geometry, in order to convert differential slant column densities to vertical column densities (VCDs)

(5) Georeferencing and visualisation of the NO2 spatial distribution

Initial developments of this retrieval scheme were performed using an existing APEX dataset, flown above Antwerp in 2012 in fully spectrally unbinned mode. An improved APEX operation and sensor calibration methodology for monitoring NO2 pollution was implemented after acquisition of additional APEX data in spectrally unbinned mode above 3 Belgian cities. The campaigns flights over Antwerp, Brussels and Liège were performed on April 14-15 and June 30 in 2015. The flights were performed in unbinned spectral mode during which APEX acquired 512 continuous spectral bands.


The final deliverables of these packages are:

  • - The implementation of improved APEX operation and sensor calibration for monitoring NO2 pollution
    - The integration of columnar NO2 spectral retrieval algorithms in the VITO processing chain
  • - The demonstration of the methodology for APEX NO2 monitoring and NO2 column retrieval algorithms, developed in WP1
    - High-resolution maps of total NO2 column values above the cities of Antwerp, Brussels and Liege.

Results flightine

NO2 VCD retrieval from 1 flightline of APEX data