Reductions in NO ₂ burden over north equatorial Africa from decline in biomass burning in spite of growing fossil fuel use, 2005 to 2017

Abstract

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.

Changes in biomass burning, wetland extent, or agriculture drive atmospheric NH<sub>3</sub> trends in several African regions

“…1b, d), pointing to a biomass burning source, as is also the case in central Africa. Earlier studies have found substantial declines in annual burned area across the north equatorial African biomass burning region as detected by MODIS (Andela et al, 2017;Andela and van der Werf, 2014) and related declines in NO 2 VCDs across the region (Hickman et al, 2021a), which would seem to stand in contrast to the increasing CO and NH 3 trends observed here.…”

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

“…The importance of biomass burning as a source of NO x in African savannas and woodland is well known, representing over half of all biomass burning emissions of NO x globally (Jaeglé et al., 2005; van der Werf et al., 2017). Its central role in seasonality of NO 2 concentrations in biomass burning regions has been documented in emission inventories and studies using satellite observations (Hickman et al., 2021; Mebust & Cohen, 2013; van der Werf et al., 2017; Whitburn et al., 2015), but it is only in site‐level studies that its role in the seasonality of NO 2 concentrations in specific African ecoregions—dry savanna, wet savanna, and forest—has been investigated (Adon et al., 2010; Delon et al., 2010; Martins et al., 2007; Ossohou et al., 2019). Our results are broadly consistent with these earlier studies, though where we did not find an overall relationship between monthly burned area and monthly NO 2 in the dry savanna, one of the INDAAF dry savanna sites found that biomass burning contributed to NO 2 seasonality (Adon et al., 2010; Delon et al., 2010).…”

“…As noted in Section 1, fossil fuel emissions of NO 2 in sub‐Saharan Africa are low relative to seasonal sources of NO x such as biomass burning and nitrification in soils (Jaeglé et al., 2005). For example, across 80% of the mesic savannas in Africa, fossil fuel emissions are estimated to be equivalent to roughly 2% of dry season emissions from biomass burning (Hoesly et al., 2017; van der Werf et al., 2006), and analysis of data from OMI suggests that that estimate is likely a substantial overestimate (Hickman et al., 2021). In addition, on the basis of emissions inventories and previous analyses of satellite observations, we do not expect strong seasonal variations of fossil fuel NO x emissions (Hoesly et al., 2017; van der A et al., 2008).…”

“…And although precipitation and biomass burning are currently the most important drivers of NO 2 seasonality, projections suggest a rapid increase in fossil fuel combustion will occur in sub‐Saharan Africa, contributing to an equally rapid rise in NO x emissions (Liousse et al., 2014). At the same time as the fertilizer and fossil fuel use is increasing, biomass burning regions in Africa are in the midst of a decline in burned area (Andela & van der werf, 2014; Andela et al., 2017), which has led to a decline in related NO x emissions (Hickman et al., 2021). These countering trends could result in a reduction in the amplitude of NO 2 seasonality in the coming decades, particularly in biomass burning regions.…”

“…Globally, the majority of NO x emissions are from fossil fuel combustion, and although that is also the case in South Africa, in equatorial Africa it is a minor source (Jaeglé et al., 2005). Instead, biogenic soil sources dominate NO x emissions in northern hemisphere equatorial Africa with most of the remaining NO x emissions coming from biomass burning, with the reverse being true in southern hemisphere equatorial Africa (Jaeglé et al., 2005), and fossil fuel emissions are a growing but still relatively minor source over Africa and unlikely to contribute to seasonal changes in atmospheric composition (Hickman et al., 2021; Hoesly et al., 2017; Jaeglé et al., 2005; van der A et al., 2008). That may change in the future, as fossil fuel combustion emissions of NO x are expected to increase by up to a factor of 6 as a result of the rapid growth in the adoption of two‐stroke motors, conceivably becoming the largest source of NO x in Africa by 2030 (Liousse et al., 2014).…”