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

Abstract

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.