Socioeconomic development in low- and middle-income countries has been accompanied by increased emissions of air pollutants, such as nitrogen oxides [NO
x: nitrogen dioxide (NO
2) + 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 NO
x source—has declined in north equatorial Africa, attributed to changes in climate and anthropogenic fire management. Here, we use satellite observations of tropospheric NO
2 vertical column densities (VCDs) and burned area to identify NO
2 trends and drivers over Africa. Across the northern ecosystems where biomass burning occurs—home to hundreds of millions of people—mean annual tropospheric NO
2 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 NO
2 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 NO
2 VCDs during the dry season, suggesting that economic development mitigates net NO
2 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.Socioeconomic development and population growth in low- and middle-income countries have been widely associated with increased environmental degradation, including rapid increases in emissions of air pollutants (
1–
3). In contrast, in countries with a high per capita gross domestic product (GDP), various socioeconomic, institutional, and regulatory factors often cause economic growth to be accompanied by reductions of some pollutant emissions, though these emissions may simply be outsourced to lower income countries (
4). The relationship between income level and environmental pressure—known as the Environmental Kuznets Curve—has often been conceptualized as an inverted U-shaped curve, but a wide array of functional relationships is possible (
3). For emissions of air pollutants, the relationship has generally been described as an inverted U-shaped curve, though carbon dioxide generally does not follow such a curve (
3,
5). Some researchers argue that low- and middle-income countries can mitigate or shorten the period of rapid emissions growth that tends to accompany socioeconomic development for at least some pollutants (
4). Africa, and sub-Saharan Africa in particular, is characterized by countries with low but growing per capita GDP and rapid population growth, which have been linked to increases in emissions of carbon dioxide and particulate matter (
6). As these countries continue their trajectories of economic development, emissions of air pollutants from fossil fuel and biofuel combustion are expected to experience explosive growth (
7).Nitrogen dioxide (NO
2) is a reactive gas and air pollutant with a lifetime in the atmosphere on the order of hours (
8). In the atmosphere, NO
2 interconverts rapidly with nitric oxide (NO), and the two species are collectively referred to as NO
x. NO
2 itself is toxic, is regulated by the US Environmental Protection Agency, and has been associated with premature mortality and asthma [though its direct effects on health are not clear (
9) and it may instead function as a proxy for other pollutants, such as ozone and aerosols that have direct health and mortality impacts (
10)]. NO
x is also a key precursor to the formation of tropospheric ozone (O
3), which is damaging to both crop productivity and human health; anthropogenic O
3 contributes to roughly half a million premature deaths annually, of which nearly 20,000 are in Africa (
11). In addition, NO
x is involved in reactions with atmospheric ammonia (NH
3) to form nitrate aerosols, which contribute to particulate matter pollution (
12) as well as in reactions with volatile organic compounds (VOCs), which form organic nitrates (
13). Because of the short lifetime of NO
2, and because it can function as an indicator for other pollutants, it can serve as an indicator of overall changes in air quality.NO and NO
2 are emitted from a variety of natural and anthropogenic sources. Fossil fuel combustion and anthropogenic alterations to soils through fertilization or livestock management are the primary sources of NO
x in many parts of the world. In sub-Saharan Africa (excluding South Africa), fossil fuel combustion and fertilizer use has been considerably lower than elsewhere, and natural soils and biomass burning have historically been more important sources (
14). This is true even in Nigeria (
15), which experiences substantial emissions of VOCs from the oil and gas industry (
16). NO
x emissions from Lagos have been shown to be either lower than (
15) or comparable to other megacities (
17), and NO
2 concentrations are generally low during the rainy season, but air quality can become heavily degraded during the biomass burning season (
15,
18). However, fossil fuel combustion in the region nearly doubled between 2000 and 2016 (
19) and associated emissions of NO
x are projected to increase sixfold by 2030 in the absence of regulation, as compared to 2005 levels (
7).This increase in fossil fuel combustion is occurring against the backdrop of Africa’s unique, fire-prone savanna ecosystems, home to 70% of the global area burned each year (
20). Biomass burning in Africa is estimated to be responsible for NO
x emissions of roughly 4 Tg N⋅yr
−1, equivalent to about half of all NO
x emissions for the continent (
21), and one third to half of NO
x emissions from biomass burning globally (
21–
23). The majority of biomass burning in Africa occurs in northern and southern bands of savanna, savanna-forest mosaic, and woodland ecoregions, with a seasonality that follows the migration of the intertropical convergence zone.The early part of the 21st century has been accompanied by a global decline in burned area, with some of the largest declines occurring in Africa’s northern fire band (
24). Some of the burned area decline in the northern fire band can be attributed to changes in precipitation that, in turn, affect the quantity and moisture content of available fuels (
24–
26). However, active anthropogenic suppression of fire has also played an important role (
24,
25). Burning is thought to be used as a management strategy—among other uses, humans ignite fires to mineralize nutrients, improve grazing, and reduce fuel loads and the potential for large, uncontrolled fires (
27). Increased population density and the introduction of agricultural land into African savanna landscapes—reflecting socioeconomic transitions from traditional nomadic pastoralist lifestyles (
28)—have been associated with a sharp decrease in burned area as people either reduce ignition or suppress fires to protect villages and farms, with a reduction in the amount of pasture area to be maintained (
25).Unfortunately, sub-Saharan Africa remains a severely understudied region—for example, agricultural soil NO fluxes have only been measured directly for two sites (
29,
30), and surface air quality monitoring is extremely limited compared to other parts of the world (
31). Remote sensing products provide an important tool for filling some of these data gaps. The short NO
2 lifetime in the planetary boundary layer makes it possible to use satellite observations to directly evaluate emissions sources, especially in regions with high temperatures, which tend to shorten the NO
2 lifetime, and in relatively polluted regions, where total column densities and surface emissions are highly correlated (ref.
8 and references therein). Although recent remote sensing work has evaluated long-term trends in NO
2 concentrations around the world, recent trends in the biomass burning region of northern Africa have not been explicitly evaluated, and the relative impacts of socioeconomic development—the possibility of reduced NO
x emissions because of anthropogenic fire suppression and of increasing NO
x emissions from growing fossil fuel use—remain unknown. In general, studies on global trends in NO
2 tend not to focus on Africa, likely because the regions with the highest NO
2 concentrations are in China, Europe, and the United States (e.g., refs.
1,
21). Some earlier studies observed a decline in NO
2 VCDs over north equatorial Africa (
32,
33), but others did not (
34). These and other large-scale studies (e.g., refs.
8,
34,
35) did not identify mechanisms for the observed NO
2 dynamics, but rather focused on understanding anthropogenic influences on trends in other regions.Indoor air pollution from biomass combustion for fuel is an important health concern (
36). We do not focus on this source. Biofuel combustion is responsible for emissions of 0.6 Tg NO annually across all of Africa (
37), which is less than 10% of the magnitude of landscape biomass burning emissions estimated by the Global Fire Emissions Database version 4s [GFED4s (
38)] and represents a much smaller proportion of NO
x emissions from landscape biomass burning regions during the dry season.Here, we use observations of NO
2 by the Ozone Monitoring Instrument [OMI (
39)] and burned area from the Moderate Resolution Imaging Spectroradiometer [MODIS (
40)] to demonstrate that the recent decline in burned area in the productive savannas of north equatorial Africa—home to over 275 million people—is associated with large declines in tropospheric NO
2 VCDs during the biomass burning season from 2005 through 2017, though positive trends explained in part by increasing fossil fuel combustion were observed in other seasons, especially over Nigeria.
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