Testing the CO2M NO2 Retrieval Algorithm using TROPOMI Spatial Zoom Data

The future Copernicus Anthropogenic CO₂ Monitoring Mission (CO2M) will provide CO₂ data at a high resolution of 2x2 km² with unprecedented accuracy and precision. In addition to bands in the near-infrared and shortwave infrared, the CO2I spectrometer on board of CO2M also contains a visible band for the retrieval of NO₂ tropospheric columns. NO₂ observations will be performed to aid the detection and identification of CO₂ emission plumes, as NO₂ is co-emitted during combustion processes and acts as a tracer of CO₂. Due to its relatively low tropospheric background value and biospheric influence, local enhancements of tropospheric NO₂ are better detectable than those of CO₂ and allow for more accurate CO₂ emission estimates. Furthermore, this NO₂ product will be very valuable for air quality applications, especially emission source quantification.

To fully exploit the high resolution of the NO₂ measurements, five times higher than the operational resolution of the TROPOMI instrument onboard Sentinel 5P, improvements on multiple aspects of the air-mass correction in the tropospheric NO₂ retrieval algorithm are considered: (1) anisotropic surface treatment at high spatial resolution using MODIS-like products, (2) effective cloud/aerosol parameter retrieval as a scattering layer using co-registered cloud information provided by the on-board cloud imager instrument (CLIM) and the measured O₂-O₂ and O₂-A absorption bands and (3) high resolution NO₂ a-priori profiles from the CAMS global/regional model forecasts.

The TROPOMI instrument produced a limited data set during the commissioning phase, where measurements were performed with increased spatial sampling of 2.4x1.8 km². This data set captures typical scenes showing NO₂ emission plumes at a similar spatial resolution as the planned CO2M measurements, allowing for the CO2M NO₂ retrieval algorithm to be tested on real data. These TROPOMI zoom data are processed with the new prototype algorithm for CO2M and impacts of the improvements are quantified.