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Formaldehyde (HCHO) columns have been retrieved from ground-based Fourier transform infrared (FTIR) campaign measurements in 2004 and 2007 and from UV-Visible MAX-DOAS measurements in 2004-2005 at the NDACC site of Réunion Island (21°S, 55°E). The FTIR and MAX-DOAS daily mean formaldehyde total columns are intercompared in their common measurement period, from August to October 2004. The ground-based data are also compared to correlative SCIAMACHY data. The comparisons account for the vertical sensitivity differences of the data sets, by including their respective averaging kernels. Complete error budgets are also presented. 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 February-March. 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 prescribed OH fields) and/or an underestimated contribution of large-scale transport of HCHO precursors from Madagascar. The latter hypothesis is comforted by the large observed day-to-day 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. [ABSTRACT FROM AUTHOR]

Ground-based high spectral resolution Fourier-transform infrared (FTIR) solar absorption spectroscopy is a powerful remote sensing technique to obtain information on the total column abundances and on the vertical distribution of various constituents in the atmosphere. This work presents results from two short-term FTIR measurement campaigns in 2002 and 2004, held at the (sub)tropical site Ile de La Réunion (21°S, 55°E). These campaigns represent the first FTIR observations carried out at this site. The results include total column amounts from the surface up to 100 km of ozone (O3), methane (CH4), nitrous oxide (N2O), carbon monoxide (CO), ethane (C2H6), hydrogen chloride (HCl), hydrogen fluoride (HF) and nitric acid (HNO3), as well as some vertical profile information for the first four mentioned trace gases. The data are characterised in terms of the vertical information content and associated error budget. In the 2004 time series, the seasonal increase of the CO concentration was observed by the end of October, along with a sudden rise that has been attributed to biomass burning events in southern Africa and Madagascar. This attribution was based on trajectory modeling. In the same period, other biomass burning gases such as C2H6 also show an enhancement in their total column amounts which is highly correlated with the increase of the CO total columns. The observed total column values for CO are consistent with correlative data from MOPITT (Measurements Of Pollution In The Troposphere). Comparisons between our ground-based FTIR observations and space-borne observations from ACE-FTS (Atmospheric Chemistry Experiment - Fourier Transform Spectrometer) and HALOE (Halogen Occultation Experiment) confirm the feasibility of the FTIR measurements at Ile de La Réunion. [ABSTRACT FROM AUTHOR]