Black Carbon

What it is

Black carbon (BC) is the sooty particle formed when fossil fuels, wood, and other carbon fuels burn incompletely. It is the strongly light-absorbing component of fine particulate matter and almost always falls within the PM2.5 size range, with a large fraction in the ultrafine range (under 0.1 micrometers). Black carbon is what makes diesel exhaust black and wood smoke gray.

Although it is a subset of PM2.5, BC has distinctive properties: it absorbs sunlight intensely, it carries other toxic chemicals on its surface (including polycyclic aromatic hydrocarbons), and it is a short-lived climate pollutant.

Where it comes from

In North America and Europe, roughly 70 percent of black carbon emissions come from diesel engines, including trucks, buses, ships, locomotives, construction and agricultural equipment, and backup generators. Residential wood burning is the second largest source, particularly during winter heating season. In much of Asia, Africa, and Latin America, residential solid fuels (wood, dung, coal) used for cooking and heating produce 60 to 80 percent of black carbon.

Other sources include open biomass burning (wildfires, agricultural burns), industrial coal use, and gas flaring. Near busy freeways, ports, and rail yards, black carbon concentrations can be 5 to 10 times higher than the regional background.

Health effects

Because BC particles are small and biologically active, they reach deep into the lungs and can pass into the bloodstream. Long-term exposure is associated with cardiovascular disease, lung cancer, hypertension, stroke, and reduced lung function. Children living near roads with heavy diesel traffic show measurable deficits in lung development.

WHO has concluded that targeting BC sources is one of the most effective ways to reduce the most toxic fraction of PM2.5. Several large studies suggest that, per microgram, BC may be more harmful than the average PM2.5 mass.

How it's measured

Black carbon is measured optically by drawing air through a filter and tracking how much light is absorbed by the deposit. Common instruments include the aethalometer and the Multi-Angle Absorption Photometer (MAAP). Researchers also use elemental carbon (EC) measured by thermal-optical analysis as a closely related marker, especially in occupational settings.

There is no U.S. NAAQS for black carbon specifically; it is regulated indirectly through the PM2.5 standard (9.0 micrograms per cubic meter annual, 35 micrograms per cubic meter 24-hour). Typical U.S. urban BC concentrations run 0.5 to 2 micrograms per cubic meter, with hot spots near diesel sources reaching 5 to 10 micrograms per cubic meter or more.

What you can do

Most of what reduces PM2.5 also reduces black carbon: HEPA air purifiers, sealing leaky windows and doors, and avoiding outdoor activity during high-pollution episodes. Tight-fitting N95 or KN95 masks filter BC effectively because it travels on PM2.5 particles.

If you live near a freight corridor, port, or busy intersection, BC indoors is often higher in rooms facing the source. Place purifiers and bedrooms on the quieter side of the building if possible. Inside vehicles in heavy traffic, set ventilation to recirculate to cut intake by 50 percent or more.

At a community level, replacing old diesel engines with Tier 4 or electric ones, retiring old wood stoves, and ending open burning produce fast measurable air quality improvements.

Sources

• Black Carbon Research | US EPA
• Mitigating Black Carbon | US EPA
• Black carbon | Climate & Clean Air Coalition
• Black Carbon and Its Implications for Climate Change and Public Health | EESI
• Hidden Dangers: Black Carbon and Urban Health | Clarity
• The Damaging Effects of Black Carbon | Columbia Climate School