Subtropical southern Africa fire emissions of nitrogen oxides and ammonia obtained with satellite observations and GEOS-Chem.

Landscape fires in subtropical southern Africa (2-20°S) are a prominent regional source of nitrogen oxides (NO x ) and ammonia (NH3), affecting climate and air quality as precursors of tropospheric ozone and aerosols. Here we evaluate GEOS-Chem model skill at reproducing satellite observations of vertical column densities of NO2 from TROPOMI and NH3 from IASI driven with three distinct and widely used biomass burning inventories (FINNv2.5, GFEDv4s, GFASv1.2). We identify that GFASv1.2 use of fire radiative power and a NO x emission factor that is almost half that used by the other two inventories is most consistent with TROPOMI and that FINNv2.5 use of active fires and landscape-specific fuel loads and biomass consumed is most consistent with IASI. We use a simple mass-balance inversion to calculate top-down NO x emissions of 1.9 ± 0.6 Tg NO for June-October and NH3 emissions of 1.2 ± 0.4 Tg for July-October. All inventories collocate NO x and NH3 emissions, whereas most of the pronounced emissions of NO x and NH3 are separate and have distinct seasonality in the top-down estimate. We infer with GEOS-Chem more efficient ozone production (13 Tg ozone per Tg NO) with the top-down informed NO x emissions than the inventory emissions, as GFASv1.2 NO x is almost 20% less than top-down NO x and the 2.3- to 2.5-times greater FINNv2.5 and GFEDv4s NO x reduces sensitivity of ozone formation to NO x . Both NO x and NH3 top-down emissions are unaffected by use of plume injection heights, limited to GFASv1.2 in GEOS-Chem, and NH3 is insensitive to acidic sulfate and nitrate aerosol emissions absent in all inventories. The top-down emissions estimates and comparison to satellite observations suggest a hybrid bottom-up approach could be adopted to discern byproducts of smouldering and flaming fires.