Vegetation fires are a major source of NH₃ in the atmosphere. A key parameter for the calculation of their emissions, which are still uncertain, is the NH₃ enhancement ratio relative to carbon monoxide (CO), ER_NH3 / CO. Here we derive new ER_NH3 / CO ratios for large tropical regions from the measurements of IASI. We find important variability between and within the studied biomes, as well as interannual variability. This highlights the need for the development of dynamic ER_NH3 / CO ratios.