Continental and Ecoregion‐Specific Drivers of Atmospheric NO<sub>2</sub> and NH<sub>3</sub> Seasonality Over Africa Revealed by Satellite Observations
Jonathan E. Hickman, Niels Andela, Kostas Tsigaridis, Corinne Galy‐Lacaux, Money Ossohou, Enrico Dammers, Martin Van Damme, Lieven Clarisse, Susanne E. Bauer
Abstract
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.