| 06 Aug 2025
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. 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 in detail the reasons for the observed differences. First, we compare the UV and IR HCHO vertical columns 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 x1015 molec/cm2 (-6 %). Second, the MAX-DOAS data are compared to the DS and FTIR ones. The study addresses the impact of using different MAX-DOAS retrieval methods all implemented within the FRM4DOAS centralized processing facility, and discusses differences related to the vertical sensitivity of the measurements in each geometry.
The MAX-DOAS HCHO columns correlate well with the direct sun DOAS and FTIR data, but have a tendency to underestimate them by -22 % and -20.8 % respectively. This bias can be reduced to 1 % when taking properly into account the different a-priori profiles and the respective vertical sensitivities of the MAX-DOAS and FTIR measurements. If the comparison is restricted to the 0–4 km altitude range where MAX-DOAS measurements have their best sensitivity, differences are further reduced with a bias of about -4.6 % for the original comparison and of 2.5 % after taking into account the respective vertical sensitivities and a priori profiles. The underestimation in the retrieved MAX-DOAS vertical columns (VCDs) is shown to be due to the choice of the a priori profile, which neglects the free-tropospheric contribution (above 4 km), where the MAX-DOAS has no sensitivity. These results suggest that improvements to the current FRM4DOAS MAX-DOAS HCHO retrievals are possible.
We investigate whether the underestimation of the MAX-DOAS tropospheric VCDs can be reduced by using more appropriate a priori profiles, based on the CAMS and TM5 chemical-transport models (CTMs). We illustrate the bias reduction with respect to our reference direct sun data. The impact is different depending on the season. The use of CTM-based a priori profiles has a positive impact on the retrieved VCDs in all seasons except winter. When restricting the comparison to the 0–4 km altitude range, the impact of the a priori profile is only significant in the winter period, also leading to a degradation of the agreement with FTIR data. The improvement of the agreement between MAX-DOAS and FTIR data is thus mainly related to a better handling of the free-tropospheric part of the profile, smaller in winter than in other seasons.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Measurement Techniques.
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