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Biomass burning (BB) is a major source of aerosols that remain the most uncertain components of the global radiative forcing. Current global models have great difficulty matching observed aerosol optical depth (AOD) over BB regions. A common solution to address modelled AOD biases is scaling BB emissions. Using the relationship from an ensemble of aerosol models and satellite observations, we show that the bias in aerosol modelling results primarily from incorrect lifetimes and underestimated mass extinction coefficients. In turn, these biases seem to be related to incorrect precipitation and underestimated particle sizes. We further show that boosting BB emissions to correct AOD biases over the source region causes an overestimation of AOD in the outflow from Africa by 48%, leading to a double warming effect compared with when biases are simultaneously addressed for both aforementioned factors. Such deviations are particularly concerning in a warming future with increasing emissions from fires. Error attribution based on modelled relationships and satellite observations suggests that errors in global models are more important and require more concerns than emission errors in creating the overall uncertainties for biomass burning aerosols. [ABSTRACT FROM AUTHOR]

Atmospheric ammonia (NH 3) is a precursor to fine particulate matter and a source of nitrogen (N) deposition that can adversely affect ecosystem health. The main sources of NH 3 – agriculture and biomass burning – are undergoing are or expected to undergo substantial changes in Africa. Although evidence of increasing NH 3 over parts of Africa has been observed, the mechanisms behind these trends are not well understood. Here we use observations of atmospheric NH 3 vertical column densities (VCDs) from the Infrared Atmospheric Sounding Interferometer (IASI) along with other satellite observations of the land surface and atmosphere to evaluate how NH 3 concentrations have changed over Africa from 2008 through 2018, and what has caused those changes. In West Africa NH 3 VCDs are observed to increase during the late dry season, with increases of over 6 % yr -1 in Nigeria during February and March (p<0.01). These positive trends are associated with increasing burned area and CO trends during these months, likely related to agricultural preparation. Increases are also observed in the Lake Victoria basin region, where they are associated with expanding agricultural area. In contrast, NH 3 VCDs declined over the Sudd wetlands in South Sudan by over 1.5 % yr -1 , though not significantly (p=0.28). Annual maxima in NH 3 VCDs in South Sudan occur during February through May and are associated with the drying of temporarily flooded wetland soils, which favor emissions of NH 3. The change in mean NH 3 VCDs over the Sudd is strongly correlated with variation in wetland extent in the Sudd: in years when more area remained flooded during the dry season, NH 3 VCDs were lower (r=0.64 , p<0.05). Relationships between biomass burning and NH 3 may be observed when evaluating national-scale statistics: countries with the highest rates of increasing NH 3 VCDs also had high rates of growth in CO VCDs; burned area displayed a similar pattern, though not significantly. Livestock numbers were also higher in countries with intermediate or high rates of NH 3 VCD growth. Fertilizer use in Africa is currently low but growing; implementing practices that can limit NH 3 losses from fertilizer as agriculture is intensified may help mitigate impacts on health and ecosystems. [ABSTRACT FROM AUTHOR]

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]

Socioeconomic development in low- and middle-income countries has been accompanied by increased emissions of air pollutants, such as nitrogen oxides [NOx: nitrogen dioxide (NO2) + nitric oxide (NO)], which affect human health. In sub-Saharan Africa, fossil fuel combustion has nearly doubled since 2000. At the same time, landscape biomass burning--another important NOx source--has declined in north equatorial Africa, attributed to changes in climate and anthropogenic fire management. Here, we use satellite observations of tropospheric NO2 vertical column densities (VCDs) and burned area to identify NO2 trends and drivers over Africa. Across the northern ecosystems where biomass burning occurs--home to hundreds of millions of people--mean annual tropospheric NO2 VCDs decreased by 4.5% from 2005 through 2017 during the dry season of November through February. Reductions in burned area explained the majority of variation in NO2 VCDs, though changes in fossil fuel emissions also explained some variation. Over Africa's biomass burning regions, raising mean GDP density (USD·km-2) above its lowest levels is associated with lower NO2 VCDs during the dry season, suggesting that economic development mitigates net NO2 emissions during these highly polluted months. In contrast to the traditional notion that socioeconomic development increases air pollutant concentrations in low- and middle-income nations, our results suggest that countries in Africa's northern biomass-burning region are following a different pathway during the fire season, resulting in potential air quality benefits. However, these benefits may be lost with increasing fossil fuel use and are absent during the rainy season. [ABSTRACT FROM AUTHOR]