Continental and Ecoregion‐Specific Drivers of Atmospheric NO2 and NH3 Seasonality Over Africa Revealed by Satellite Observations.

Ammonia (NH3) and nitrogen oxides (NOx: nitrogen dioxide [NO2] + 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 NO2 vertical column densities (VCDs) from the Ozone Monitoring Instrument from 2005 through 2017 and atmospheric NH3 VCDs from the Infrared Atmospheric Sounding Interferometer from 2008 through 2017 to evaluate seasonal variation of NO2 and NH3 VCDs across Africa and in seven African ecoregions. In regions where mean annual precipitation (MAP) is under 500 mm yr−1, we find that NO2 and NH3 VCDs are positively related to monthly precipitation, and where MAP is between 500 and 1,750 mm yr−1 or higher, NO2 VCDs are negatively related to monthly precipitation. In dry ecoregions, temperature and precipitation were important predictors of NH3 and NO2 VCDs, likely related to variation in soil emissions. In mesic ecoregions, monthly NO2 VCDs were strongly related to burned area, suggesting that biomass burning drives seasonality. NH3 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 NH3 seasonality. In northern mesic ecoregions, monthly temperature explained most of the variance in monthly NH3 VCDs, suggesting that soil sources, including animal excreta, determined NH3 seasonality. In southern mesic ecoregions, monthly CO VCDs explained more variation in NH3 VCDs than temperature, suggesting that biomass burning may have greater influence over NH3 seasonality. Plain Language Summary: Ammonia (NH3) and nitrogen oxides (NOx: nitrogen dioxide [NO2] + nitric oxide [NO]) are gases that are emitted naturally as well as through human activity and contribute to air pollution. Throughout most of Africa, emissions of these gases are primarily from vegetation fires and from microbial activity and chemical transformations in soils. Most ecoregions in Africa experience a distinct dry season and rainy season, with fires occurring during the dry season, and more microbial activity occurring in soils during the rainy season. We used satellite observations of NH3 and NO2 concentrations in the atmosphere to understand how and why concentrations of these gases vary seasonally across seven distinct African ecoregions. Overall, we find that in dry ecoregions, NO2 and NH3 concentrations increase during the rainy season, when increases in precipitation and temperature stimulate greater soil microbial activity and increase emissions of both gases from soils. In contrast, in wetter ecoregions, NO2 and NH3 concentrations increase during the dry season. The increase in NO2 concentrations is a result of the increase in vegetation fires during this time of the year, but NH3 concentrations increase due to seasonal changes in both soil and fire emissions. Key Points: For mean annual precipitation (MAP) below 500 mm yr−1, monthly gas concentrations were positively related to monthly precipitationFor MAP between 500 and 1,750 mm yr−1, monthly nitrogen dioxide (NO2) was negatively and monthly ammonia (NH3) was unrelated to monthly precipitationThe importance of soil and biomass burning for NO2 and NH3 seasonality varied across biomes depending on rainfall and other characteristics [ABSTRACT FROM AUTHOR]