Fig 1.1.6.5.1: Convective uplift in thunderstorm clouds.
Image: Bernhard Mühr, Karlsruher Wolkenatlas
Tropical and sub-tropical Africa - especially the Congo basin - has a high level of thunderstorm activity, itself a source of NO2 from lightning.
The products of biomass burning are pumped into the upper troposphere by convective uplift from tropical storms.