First results from the ESA Precursors for aerosols and ozone ECVs project: user requirements and intercomparison of tropospheric retrievals of NO2, HCHO, SO2, CO, NH3, and glyoxal

Satellite-based observations of tropospheric trace gas concentrations are used extensively to test models, infer emissions and their trends. These observations are also needed to develop emission-based scenarios for radiative forcing by tropospheric ozone and secondary aerosols, both from anthropogenic and natural sources. Although the detection of trace gases from space is feasible and operational data streams are in place for most relevant sensors, limited effort has been devoted to the generation and quality assessment of consistent multi-decadal climate data records. The ESA Climate Change Initiative (CCI) program was established for the systematic generation of Essential Climate Variables (ECVs). It builds on long-term global satellite observational datasets. Regarding atmospheric composition, the ECVs covered have recently been extended with trace gases that lead to the formation of ozone and aerosols.

Here we present first results from the ‘Precursors for aerosols and ozone ECVs’ project, funded by ESA, and the first steps in developing long-term climate data records on the precursor gases nitrogen dioxide (NO₂), formaldehyde (HCHO), carbon monoxide (CO), sulfur dioxide (SO₂), ammonia (NH₃), and glyoxal (CHOCHO). The project’s main goal is to build consistent and harmonized long-term data records from satellite instruments including GOME, SCIAMACHY, GOME-2, OMI, TROPOMI, IASI and MOPITT. A second goal is to demonstrate the fitness-for-purpose of the data records for various users including the European Copernicus Atmosphere Monitoring Service, currently operated by ECMWF. In this presentation, we report on what users need from such data records, and on round robin activities to test and evaluate the different retrieval approaches used in the scientific community. One key motivation will be to reconcile retrieval approaches so far independently developed in the EU QA4ECV and EUMETSAT AC-SAF projects, with the aim to harmonize products from all sensors.