Intercomparison of MAX-DOAS, FTIR and direct-sun DOAS HCHO retrievals in Xianghe (China)

MAX-DOAS, direct-sun DOAS and FTIR measurements are increasingly used as fiducial reference for the validation of HCHO satellite observations. Understanding their strengths and limitations, and assessing their consistency is therefore crucial to produce robust and consolidated validation results. So far, only a few studies have explored the complementarity between MAX-DOAS and FTIR HCHO measurements. In the present study, we take benefit of MAX-DOAS and FTIR instruments being simultaneously operated at the Xianghe station (39.75° N, 116.96° E, approximately 55 km southeast of Beijing) to compare HCHO vertical columns retrieved from both instruments during one full year in a site under the influence of strong VOC emissions from biogenic and anthropogenic origins. In addition to its standard MAX-DOAS geometry, the IAP/BIRA instrument also provides regular direct-sun measurements suitable for comparison with FTIR solar absorption data. HCHO total columns from FTIR and direct-sun DOAS measurements are found to be in excellent agreement demonstrating the high level of consistency of spectroscopic parameters in the UV and infrared spectral ranges. We also investigate results obtained using different MAX-DOAS algorithms (namely bePRO, MMF and MAPA), assessing the agreement reached with respect to reference FTIR and direct-sun data. Various retrieval assumptions (e.g., a-priori profiles, covariance matrices, …) that can be tuned in the FRM4DOAS centralized-facility are tested. The study takes into account both MAXDOAS and FTIR sensitivities and a-priori profile information, and also links the obtained results to reference model data.