IASI-derived NH3 enhancement ratios relative to CO for the tropical biomass burning regions
Abstract. Vegetation fires are a major source of ammonia (NH3) in the atmosphere. Their emissions are mainly estimated using bottom-up approaches that rely on uncertain emission factors. In this study, we derive new biome-specific NH3 enhancement ratios relative to carbon monoxide (CO), ERNH3 ∕ CO (directly related to the emission factors), from the measurements of the IASI sounder onboard the Metop-A satellite. This is achieved for large tropical regions and for an 8-year period (2008–2015). We find substantial differences in the ERNH3 ∕ CO ratios between the biomes studied, with calculated values ranging from 7 × 10−3 to 23 × 10−3. For evergreen broadleaf forest these are typically 50–75 % higher than for woody savanna and savanna biomes. This variability is attributed to differences in fuel types and size and is in line with previous studies. The analysis of the spatial and temporal distribution of the ERNH3 ∕ CO ratio also reveals a (sometimes large) within-biome variability. On a regional level, woody savanna shows, for example, a mean ERNH3 ∕ CO ratio for the region of Africa south of the Equator that is 40–75 % lower than in the other five regions studied, probably reflecting regional differences in fuel type and burning conditions. The same variability is also observed on a yearly basis, with a peak in the ERNH3 ∕ CO ratio observed for the year 2010 for all biomes. These results highlight the need for the development of dynamic emission factors that take into better account local variations in fuel type and fire conditions. We also compare the IASI-derived ERNH3 ∕ CO ratio with values reported in the literature, usually calculated from ground-based or airborne measurements. We find general good agreement in the referenced ERNH3 ∕ CO ratio except for cropland, for which the ERNH3 ∕ CO ratio shows an underestimation of about 2–2.5 times.