A Closer Look at U.S Black Carbon Emissions Reveals Some Surprises

Black Carbon

Diesel engines and burning of biomass for fuel are responsible for the lion's share of black carbon emissions in the United States, according to an EPA report to Congress on emissions of the carbon particulates. EPA's report on black carbon--the most effective form of particulate matter, by mass, at absorbing solar energy--holds some interesting facts in regard to the largest U.S. sources.

First, here’s the breakdown of total U.S. black carbon emissions that appears in the agency’s Report to Congress on Black Carbonreleased March 30:

  • 52 percent of black carbon emissions are the result of mobile sources (mostly diesel engines).
  • 39 percent are due to biomass burning (wildfires, agricultural and prescribed burning, residential heating and cooking, wood-fired boilers, charbroiling). 
  • 7  percent are emitted from the generation of energy and power. 
  • 2 percent are attributed to other industrial sources.

As detailed in a March 30 World Climate Change Reportarticle, EPA was charged by Congress to provide an inventory of black carbon emissions, which is the soot that results from inefficient combustion of fossil fuel and biomass, and its health effects.

Black carbon in the atmosphere, while short-lived (several days to weeks), absorbs large amounts of heat from the sun. And due to its dark color, when it falls on glaciers and polar ice caps it absorbs additional heat and contributes to the melting of those ice formations.

Mobile Sources: No Surprise Here

Not surprisingly, emissions from diesel engines (both on-road and nonroad) accounted for 93 percent of black carbon emissions from mobile sources.

The report said that since 1990, significant reductions have occurred from the mobile source category, and the decline is expected to continue due to existing EPA regulations that will be in effect through 2030.

What About Wildfires?

About, 60 percent of black carbon biomass emissions are attributable to wildfires, and of note, 33 percent are the result of Alaskan wildfires, directly impacting ice and snow in the Arctic, the report said.

EPA stipulated that emissions from U.S. wildfires can vary greatly from year to year, and that estimates are based on 2002 numbers, which is consistent with an average of wildfire activity in the United States over a 10-year period from 2001-2010.

However, the report found that emissions from biomass burning contain more light-absorbing organic carbon, also known as brown carbon. How much of the inventoried brown carbon emissions are light-absorbing is not yet known and more research is needed.

The report also said more research is needed to determine if black carbon’s effect on cloud formation causes either a cooling or warming effect (although most scientific findings indicate the latter).

A Surprise: Natural Gas a Big Carbon Black Emitter?

Surprisingly, the report list natural gas combustion as the largest source of black carbon from the energy/power sector.

But hold on, the agency said its numbers likely are due to “severe constraints on the data used to generate the estimates,” and it believes that additional source testing and research will indicate that natural gas combustion sources that follow good combustion practices are not a dominate source of black carbon emissions.

Other emissions of black carbon--from the generation of electricity from the burning of coal and other fuel types--represent relatively small contributions to the U.S. black carbon inventory due to control technologies currently in place, such as baghouses and scrubbers.

`Potential Interest’ Found in the `Other Source Category’

Finally, black carbon emissions in the “other” source category, which includes manufacturing, also is relatively small due to control technologies installed to comply with EPA regulations for particulate matter emissions, the report said.

However, one “potential interest” for additional controls is existing stationary diesel engines, such as generators and emergency equipment. EPA’s new source performance standards for new engines, which became effective in the mid-2000s, will continue to result in lower black carbon emissions, but existing sources remain uncontrolled.

In fact, as detailed in a March 2010, article, a slew of witnesses before the now defunct House Select Committee on Energy Independence and Global Warming testified that steps should be taken to reduce black carbon emissions from existing diesel engines, which garnered support by some Republicans.

Other categories in this sector with relatively high emissions include charbroiling and fugitive dust.

How to Decrease Emissions?

To further control stationary source emissions, the report said deployment of cleaner fuels, use of control technologies, such as diesel particulate filters, and improvements in diesel engines can be a cost-effective way to reduce black carbon.

Expanded efforts for the prevention and containment wildfires also would result in reduced black carbon emission, EPA said.