3 April 2012
The US Environmental Protection Agency (EPA) released Report to Congress on Black Carbon—a major study on domestic and international black carbon (BC) emissions. The report, requested by the US Congress in 2009, covers BC emission inventories, climate impacts of BC, approaches for quantifying the climate effects of BC (such as radiative forcing and warming effects), as well as identification and analysis of approaches to reduce BC emissions.
BC is emitted directly into the atmosphere in the form of fine particles (PM2.5). Important sources of BC emissions include open biomass burning including wildfires (35% of BC emissions globally and in the USA); transport (19% globally, 52% US); domestic/residential sources, such as wood stoves (25% globally, 3.6% US); and industry (19% globally, 1% US).
Climate Impact. BC emissions contribute to the adverse impacts on human health, ecosystems, and visibility associated with PM2.5, concludes the report. The overall climate impact of BC, on the other hand, has been deemed uncertain. BC influences climate by: (1) directly absorbing light, (2) reducing the reflectivity (“albedo”) of snow and ice through deposition, and (3) interacting with clouds. The direct and snow/ice albedo effects of BC are widely understood to lead to climate warming. However, the globally averaged net climate effect of BC also includes the effects associated with cloud interactions, which are not well quantified and may cause either warming or cooling. Therefore, though most estimates indicate that BC has a net warming influence, a net cooling effect cannot be ruled out, says the report.
Selected other conclusions of the study are:
- Based on recent emissions inventories, the majority of global BC emissions come from Asia, Latin America, and Africa.
- Sensitive regions such as the Arctic and the Himalayas are particularly vulnerable to the warming and melting effects of BC.
- BC is emitted with other particles and gases, many of which exert a cooling influence on climate. Therefore, estimates of the net effect of BC emissions sources on climate should include the offsetting effects of these co-emitted pollutants. This is particularly important for evaluating mitigation options.
- BC’s short atmospheric lifetime (days to weeks), combined with its strong warming potential, means that targeted strategies to reduce BC emissions can be expected to provide climate benefits within the next several decades.
- Achieving further BC reductions, both domestically and globally, will require adding a specific focus on reducing direct PM2.5 emissions to overarching fine particle control programs.
Mitigation. In the United States, total mobile source BC emissions are projected to decline by 86% by 2030 due to regulations already promulgated. BC emissions from mobile diesel engines (including on-road, non road, locomotive, and commercial marine engines) are being controlled through two primary mechanisms: (1) emission standards for new engines, including requirements resulting in use of diesel particulate filters (DPF); and (2) retrofit programs for in-use mobile diesel engines, such as EPA’s National Clean Diesel Campaign. Mitigation options are also evaluated to further reduce BC emissions from stationary and residential sources.
Globally, the most important BC emission reduction opportunities identified by the report include residential cookstoves in all regions, brick kilns and coke ovens in Asia, and mobile diesels in all regions. For mobile sources, both new engine standards and retrofits of existing engines/vehicles may help reduce BC emissions in the future. However, in developing countries, BC emissions from mobile sources are expected to continue to increase. Emission control requirements lag behind in some regions, as does on-the-ground deployment of DPFs and low sulfur fuels. Further reductions in BC will depend on accelerated deployment of clean engines and fuels.
Despite some remaining uncertainties about BC that require further research—concludes the report—currently available scientific and technical information provides a strong foundation for making mitigation decisions to achieve lasting benefits for public health, the environment, and climate.
Source: US EPA