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 from "bottom-up" approaches which 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 on board the Metop-A satellite. This is achieved for large tropical regions and for a 8-year period (2008–2015). We find substantial differences in the ERNH3/CO between the studied biomes with calculated values ranging from 4.4 × 10−3 to 17 × 10−3. For Evergreen Broadleaf Forest these are typically 75–100 % 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 also reveals a (sometimes large) within-biome variability. On a regional level, Woody Savanna shows for example a mean ERNH3/CO for the region of Africa South of the Equator which is 50–100 % lower than in the other five studied regions, 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 observed for the year 2010 for all biomes. These results highlight the need for the development of dynamic emission factors that better take into account local variations in fuel type and fire conditions. We also compare the IASI-derived ERNH3/CO with values reported in the literature, usually calculated from ground-based or airborne measurements. We find a general underestimation over the referenced ERNH3/CO of about 40 % for Woody Savanna and Savanna and up to a factor 1.5–4 for Evergreen Broadleaf Forest and Cropland. Beyond a possible overestimation of the ERNH3/CO in the literature, the observed differences could also be related to various factors including instrumental limits, bias in the retrieval of the NH3 columns, parameterization in the calculation of the ERNH3/CO or accumulation of CO in the studied regions during the fire period.
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Simon Whitburn, Martin Van Damme, Lieven Clarisse, Daniel Hurtmans, Cathy Clerbaux, et al.. IASI-derived NH3 enhancement ratios relative to CO for the tropical biomass burning regions. Atmospheric Chemistry and Physics, European Geosciences Union, 2017, 17 (14), pp. 12239-12252. ⟨10.5194/acp-17-12239-2017⟩. ⟨insu-01518475⟩

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