Air pollution and climate forcing of the charcoal industry in Africa

Abstract

Demand for charcoal in Africa is growing rapidly, driven by urbanization and lack of access to electricity and other reliable and clean off-grid energy. Charcoal production and use, including plastic burning to initiate combustion, release large quantities of trace gases and particles that impact air quality and climate. In this work, past (2000-2014) trends in charcoal production and use in Africa are quantified and the dominant drivers identified to forecast the future (2030) of the industry. An inventory of current (2014) and future (2030) emissions from the charcoal supply chain in Africa has also been developed and implemented in the GEOS-Chem chemical transport model to quantify the contribution of charcoal to surface concentrations of PM$_{2.5}$ and ozone, and direct radiative forcing due to aerosols and ozone. Charcoal production (and use) increased from 2000 to 2014 in Africa by a factor of 2. In 2014, the charcoal industry required 140-460 Tg fuelwood, depending on efficiency of combustion, to produce charcoal and 260 tonnes plastic for charcoal use to initiate combustion. The variability in wood required is due to variability in combustion efficiency of 9-30% that depends on kiln type and technology. The plastic use is an educated guess in the absence of statistics on its use and is most prevalent in low-income homes. A rough estimate of forest loss suggests that by 2030, 4.4-15 million hectares of forest will be lost in Africa due to charcoal production. By 2030, charcoal production will almost double which will drive a similar increase in emissions from the industry. Charcoal production makes a substantial contribution to CH$_4$ emissions in Africa. These may outcompete CH$_4$ emissions from open fires in West Africa by 2025. As expected, inefficient combustion emissions of CH$_4$, NMVOCs, and CO are predominantly from charcoal production, whereas BC and NO$_x$, signatures of efficient combustion, are predominantly from charcoal use. An exception is OC that in the inventory is mostly from charcoal use, but should be predominantly from charcoal production, due to the use in the inventory of emission factors that in recent field studies are shown to underestimate OC emissions from charcoal production. In 2014, the largest enhancements in PM$_{2.5}$ (0.5-1.4 $\mu$g m$^{-3})$ and ozone (0.4-0.7 ppbv) occur in densely populated cities in East and West Africa. Cooling due to aerosols (-100 to -300 mW m$^{-2}$) is concentrated over dense cities, whereas warming due to ozone is widespread, peaking at 4.2 mW m$^{-2}$ over the Atlantic Ocean. Population-weighted PM$_{2.5}$ exposure increases from 0.34 $\mu$g m$^{-3}$ in 2014 to 0.51 $\mu$g m$^{-3}$ in 2030. The air quality and climate impact of charcoal will only worsen with ongoing dependence on this energy source, as most of the future (2100) largest megacities will likely be in Africa