Continental and Ecoregion‐Specific Drivers of Atmospheric NO 2 and NH 3 Seasonality Over Africa Revealed by Satellite Observations

Ammonia (NH 3 ) and nitrogen oxides (NO x : nitrogen dioxide [NO 2 ] + nitric oxide [NO]) play important roles in atmospheric chemistry. Throughout most of Africa, emissions of these gases are predominantly from soils and biomass burning. Here we use observations of tropospheric NO 2 vertical column densities (VCDs) from the Ozone Monitoring Instrument from 2005 through 2017 and atmospheric NH 3 VCDs from the Infrared Atmospheric Sounding Interferometer from 2008 through 2017 to evaluate seasonal variation of NO 2 and NH 3 VCDs across Africa and in seven African ecoregions. In regions where mean annual precipitation (MAP) is under 500 mm yr −1 , we find that NO 2 and NH 3 VCDs are positively related to monthly precipitation, and where MAP is between 500 and 1,750 mm yr −1 or higher, NO 2 VCDs are negatively related to monthly precipitation. In dry ecoregions, temperature and precipitation were important predictors of NH 3 and NO 2 VCDs, likely related to variation in soil emissions. In mesic ecoregions, monthly NO 2 VCDs were strongly related to burned area, suggesting that biomass burning drives seasonality. NH 3 VCDs in mesic ecoregions were positively related to both monthly temperature and monthly carbon monoxide (CO) VCDs, suggesting that a mixture of soil and biomass burning emissions influenced NH 3 seasonality. In northern mesic ecoregions, monthly temperature explained most of the variance in monthly NH 3 VCDs, suggesting that soil sources, including animal excreta, determined NH 3 seasonality. In southern mesic ecoregions, monthly CO VCDs explained more variation in NH 3 VCDs than temperature, suggesting that biomass burning may have greater influence over NH 3 seasonality.