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

Abstract

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.Plain Language Summary Ammonia (NH 3 ) and nitrogen oxides (NO x : nitrogen dioxide [NO 2 ] + 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 NH 3 and NO 2 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, NO 2 and NH 3 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, NO 2 and NH 3 concentrations increase during the dry season. The increase in NO 2 concentrations is a result of the increase in vegetation fires during this time of the year, but NH 3 concentrations increase due to seasonal changes in both soil and fire emissions. HICKMAN ET AL.

County-level ammonia emissions monitored worldwide

“…It is possible that there are emissions from smaller scale fires, which sometimes can be missed in the Fire Radiative Power (FRP) products from instruments such as VIIRS and MODIS 35 and consequently might increase the uncertainty of our emission estimate in this location. However, another potential explanation is that the local agricultural emissions are underestimated in the current inventories 36,37 . The emission patterns, and especially the higher emissions found for Nigeria, match closely to distribution of livestock animals 33,38 .…”