Continental and ecoregion-specific drivers of atmospheric NO2 and NH3 seasonality over Africa revealed by satellite observations

Abstract:

Ammonia (NH₃) and nitrogen oxides (NO_x: 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. The seasonality of these emissions varies across ecoregions that differ in vegetation and in the seasonality of temperature, precipitation, and biomass burning. Here we use satellite observations of tropospheric NO₂ vertical column densities (VCDs) from the Ozone Monitoring Instrument (OMI) from 2005 through 2017 and atmospheric NH₃from the Infrared Atmospheric Sounding Interferometer (IASI) from 2008 through 2017 along with other datasets to evaluate seasonal variation of NO₂ and NH₃ VCDs across Africa and in seven key African ecoregions. Thresholds in mean annual precipitation (MAP) separate the continent into dry regions (MAP <500 mm yr^-1) where NO₂ and NH₃VCDs are positively related to monthly precipitation, and mesic regions (MAP >500 mm yr^-1) where NO₂ VCDs are negatively related to monthly precipitation. In dry ecoregions, both temperature and precipitation were important predictors of NH₃ and NO₂ VCDs, pointing to variation in soil emissions as the cause. In mesic ecoregions with distinct rainy and dry seasons, monthly NO₂ VCDs were strongly related to burned area. Increases in NH₃ VCDs in mesic ecoregions lagged behind NO₂ increases by 1 to 2 months but were positively related to both monthly temperature and monthly CO VCDs, suggesting that a mixture of soil and biomass burning emissions influenced NH₃ seasonality. In northern mesic ecoregions—where livestock densities are high—monthly temperature explained most of the variance in monthly NH₃ VCDs, suggesting that soil and animal sources 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. In South Sudan, annual changes in temperature and the extent of the Sudd wetland exert strong influence on NH₃ seasonality, with VCDs increasing as wetland soils dry out and temperatures rise. We conclude that soil and biomass burning sources currently drive seasonality in NO₂ and NH₃VCDs, but increased fertilizer and fossil fuel use may change these patterns.