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The FTIR and MAX-DOAS daily mean total columns agree very well: no significant bias is observed and the standard deviation of the comparisons is only 8%. Both FTIR and MAX-DOAS HCHO total columns are in good agreement with SCIAMACHY values in the 2004-2005 period, with standard deviations of 21% and 31%, respectively. The same seasonal cycle is observed by the different instruments, with a minimum in austral winter and a maximum in FebruaryMarch.The FTIR and MAX-DOAS data are confronted with HCHO columns calculated by a global CTM, the IMAGES model. The model underestimates the HCHO columns by 23-29% in comparison with FTIR, and by 15% in comparison with DOAS. This bias might have multiple causes, including an underestimation of OH concentrations in the model (as indicated by a sensitivity study using prescribedCorrespondence to: C. Vigouroux (corinne.vigouroux@aeronomie.be) OH fields) and/or an underestimated contribution of largescale transport of HCHO precursors from Madagascar. The latter hypothesis is comforted by the large observed day-today variability of HCHO columns, and by the observation that the peak values of FTIR columns can often be associated with free tropospheric transport patterns from source regions over Madagascar to Réunion Island, according to simulations performed with the Lagrangian particle dispersion model FLEXPART.

Absorption cross sections of SO2 have been recorded at 295 K at the resolutions of 2 and 16 cm−1. The 27000‐ to 40000‐cm−1 spectral region has been investigated. The comparison with data available from the literature shows a good agreement between the different data sets (less than 5%). However, local discrepancies, for example at the peaks of absorption, can reach 20%.