Intercomparison of MAX-DOAS, FTIR and direct sun HCHO vertical columns at Xianghe, China

Abstract. MAX-DOAS (Multi-AXis Differential Optical Absorption Spectroscopy), direct sun DOAS (DS) and FTIR (Fourier Transform InfraRed) measurements are considered nowadays as reference data for the validation of HCHO satellite observations. Recognizing their strengths and limitations, as well as evaluating their consistency, is crucial for generating robust and reliable validation datasets. So far, only a handful of studies have explored the complementarity between MAX-DOAS and direct sun FTIR HCHO measurements and the question of the consistency of the ground-based retrievals within the different networks currently used for satellite validation is still relevant. Here we take advantage of the presence of a MAX-DOAS spectrometer, incorporating a direct sun viewing mode capability, and an FTIR instrument operating in parallel at the Xianghe site (39.75° N, 116.96° E, China), to compare the retrieved HCHO vertical columns and investigate the reasons for the observed differences. First, we compare the UV and IR HCHO vertical column densities (VCDs) in the direct sun geometry, for which the uncertainty due to the light path is negligible. We find an excellent agreement between the measurements obtained in both wavelength ranges, with a median difference of less than −0.5 × 1015 molec. cm−2 (−6 % ± 11 % for (FTIR − DS) / DS). Second, the MAX-DOAS data from different retrieval strategies implemented within the European Space Agency Fiducial Reference Measurement centralized processing facility for DOAS observations are compared to the DS and FTIR ones. The MAX-DOAS HCHO columns correlate well with the direct sun DOAS and FTIR data, but underestimate them by about 20 %. Focusing on the vertical profiles, we show that this bias cancels out when taking properly into account the different a priori profiles and the respective vertical sensitivities of the MAX-DOAS and FTIR measurements. The underestimation in the current MAX-DOAS VCDs is coming from the limited vertical sensitivity of the technique and from the choice of the a priori profile, which neglects the free-tropospheric contribution (above 4 km), where the MAX-DOAS has no sensitivity. We test and suggest possible improvements to the current centralized MAX-DOAS HCHO retrievals processing, like using more appropriate a priori profiles, based on the CAMS and TM5 chemical-transport models (CTMs) that better estimate the HCHO content above 4 km.