The work presented in this manuscript is devoted to the retrieval and the interpretation of the mixing ratio vertical profiles of hydrogen chloride (HCl) and formaldehyde (H2CO), derived from the measurements performed by the SPIRALE balloon-borne instrument (SPectroscopie InfraRouge par Absorption de Lasers Embarqués) over Teresina (Brazil, 5.1°S-42.9°S), on 22 June 2005 and 9 June 2008. The scientific questions associated with the study of these compounds, the characteristics of the tropical upper troposphere and lower stratosphere and those of the SPIRALE spectrometer are described. The feasibility study of the H2CO measurement has allowed to define the position and the intensity of the most suitable absorption line for the stratospheric measurement of this compound (at 2912.1 and 1701.5 cm-1). The mixing ratio vertical profiles of H2CO obtained during the SPIRALE flights are presented and compared. The HCl measurements presented are the first to be made in situ in the lower and mid-stratosphere. In the tropical transition layer, they have been combined with recent very short-lived species (VSLS) ones in order to estimate the contribution of these compounds to the chlorine budget of the stratosphere. In the midstratosphere, the signatures present on the vertical profiles of HCl and ozone acquired simultaneously are studied in connection with the quasi-biennal oscillation.
