Intercomparison of HCHO column and profile retrievals from ground-based MAX-DOAS, FTIR and Pandora instruments

Ground-based MAX-DOAS, FTIR and Pandora remote sensing techniques provide complementary information on formaldehyde (HCHO) vertical columns and profiles. However, differences in vertical sensitivity and retrieval strategies can lead to systematic inconsistencies that complicate intercomparisons and satellite validation. The centralized FRM4DOAS processing framework offers a harmonized approach for MAX-DOAS retrievals currently providing non-official HCHO datasets. However recent results at the Xianghe site (China) revealed a systematic underestimation of about 20 % relative to HCHO direct-sun measurements in the UV and IR, which themselves show excellent mutual agreement. This discrepancy largely disappears when accounting for differences in a priori profile assumptions and vertical sensitivities between MAX-DOAS and FTIR measurements. It is primarily attributed to the limited sensitivity of MAX-DOAS measurements above 2–4 km altitude combined to the use of a priori profiles that neglect the free-tropospheric HCHO contribution. Based on these findings, the use of model-based a priori profiles was recommended as an input for (MMF) optimal estimation retrievals.

In this work, we propose to test this approach at sites hosting co-located MAX-DOAS, FTIR and Pandora instruments. Target candidates are the Bremen, Toronto, Lauder and Ny-Ålesund stations. Where possible, selected Pandora data sets will be processed using the FRM4DOAS system and results will be compared with operationally produced column and profile data from the Pandonia Global Network (PGN). The aim is to report on the consistency between MAX-DOAS and PGN retrievals and investigate possible differences. Such investigations are crucial for robust satellite bias assessment and network interoperability in the context of current and upcoming satellite missions such as TROPOMI, TEMPO, GEMS, Sentinel-4 and Sentinel-5.