Carbonyl sulphide (COS) is considered to be a major source of the stratospheric sulphate aerosol during periods of volcanic quiescence. We have measured the COS concentrations at the tropical tropopause and find mixing ratios 20-50% larger than assumed in models. The enhanced COS levels are correlated with high concentrations of biomass burning pollutants like CO and HCN. The analysis of backward trajectories and global maps of fire statistics suggest that biomass burning emissions transported upwards by deep convection are the source of the enhanced COS in the upper tropical troposphere.