Urban Pollution Monitoring with the AOTF-based NO2 Camera: Validation Campaign in Rome

The NO2 camera is a novel instrument, developed by the Royal Belgian Institute for Space Aeronomy. It consists in an AOTF-based hyperspectral imager and DOAS-based data processing algorithms enabling to measure the differential slant column densities (dSCD) of NO2 along each pixel’s line of sight.  

In the framework of IDEAS-QA4EO, a measurement campaign was organized in an urban environment at the BAQUNIN supersite in Rome, in March 2024. The NO2 camera was compared to two collocated reference NO2 remote sensing instruments: a PANDORA spectrometer from LuftBlick deployed at the BAQUNIN supersite, and a SkySpec-2D Max-DOAS spectrometer from Airyx, operated by CNR Isac. All instruments were synchronized to a common pointing schedule, measuring the dSCD at low elevation angles (-2° to +10°) and around 4 agreed azimuth angles with unobstructed view of the horizon. The PANDORA and SkySpec obtained their measurements by scanning in elevation and in azimuth, up to 10° left and right of each reference direction. To the best of our knowledge, this is the first time these two instruments have been compared in this manner. All dSCDs were computed using as fixed reference a zenith spectrum acquired by each instrument at the same specified time. Three days of synchronized data are available for comparison, with clear to partly cloudy weather conditions. 

A total of 156 hyperspectral cubes were obtained by the NO2 camera during the synchronized periods. Several local enhancements were captured by the NO2 camera, showing the inhomogeneity of the Roman NO2 field. In total, about 6000 dSCD values from the NO2 camera were compared to the SkySpec, and about 700 values were compared to the PANDORA. The tolerance margins for inclusion in the comparison were set to 5 minutes for the acquisition time, and to 0.1° for the azimuth and elevation. Overlaying the retrievals from all instruments showed how the additional visual context captured by the camera may help understanding the location of NO2 emission sources. This can be seen in the example image below, as well as the good qualitative agreement between the instruments. More formally for the whole campaign, the measured dSCDs ranged from -2e16 to +22e16 molecules/cm². The disagreement between the camera and the SkySpec (resp. PANDORA) has mean -0.7e16 (resp. -0.01e16) molecules/ cm² and standard deviation 1.6e16 (resp 2.5e16) molecules/cm² for all points above the horizon. 

We will present more details on the campaign setup and results.