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Study links decreased BC concentrations in California to diesel emission reductions

17 June 2013

A California study found that reductions in emissions of black carbon (BC) since the late 1980s—mostly from diesel engines as a result of air quality programs—have resulted in a measurable reduction of concentrations of global warming pollutants in the atmosphere.

The 3-year-study, titled “Black Carbon and Regional Climate of California,” was conducted by UC San Diego and the US Department of Energy’s Lawrence Berkeley National Laboratory and Pacific Northwest National Laboratory. It is the first comprehensive regional assessment of the climate impact of black carbon on California. In conducting the study, scientists used computer models and air pollution data collected by aircraft, satellite and ground monitors.

Black carbon has been suspected to be one of the major short-lived contributors to climate change. The major sources of black carbon in California are diesel-burning mobile sources, residential wood burning in fireplaces and heaters, agricultural burning and wildfires.

Black carbon levels have decreased by about 90% since 1967, mostly as a result of state regulations for diesel engine emissions. The reductions occurred during a time when diesel fuel consumption increased by about a factor of five, attesting to the effectiveness of California diesel emission regulations.

When all sources of BC emissions from diesel fuel combustion are considered, including diesel trucks and buses, farming and construction equipment, trains and ships, the equivalent reduction in CO2 emissions can be as high as 50 million metric tons per year over the past 20 years—roughly equal to a 13% reduction in the total annual CO2 emissions in California.

“If California’s efforts in reducing black carbon can be replicated globally, we can slow down global warming in the coming decades by about 15 percent, in addition to protecting people’s lives,” said the lead author of the study Dr. V. Ramanathan. “It is a win-win solution if we also mitigate carbon dioxide emissions simultaneously.”

Black carbon has the effect of warming the atmosphere because it is effective at absorbing sunlight. However, it is emitted together with a range of other particle pollutants, including organic carbon, sulfur and other chemicals, some of which have a cooling effect, typically by reflecting sunlight. Reducing diesel emissions can therefore lead to a reduction of both warming and cooling particles. The study found—based on both observations and computer modeling—that the warming effect of BC dominates, overwhelming any cooling effect of other pollutants. This confirms the positive impact reducing diesel emissions has on fighting climate change.

The report also found positive radiative forcing for brown carbon (the light absorbing part of organic carbon aerosols) linking brown carbon emissions to warming. Therefore, a view that organic particles from wildfires primarily reflect sunlight, and cause cooling, was not supported by the study.

The study represents the first attempt to estimate the radiative forcing of BC for one region (California in this case), both from a bottom-up approach (starting with emission inventory as input to aerosol transport models) and a top-down approach (adopting satellite data in conjunction with ground based column averaged aerosol optical properties). However, there was a large discrepancy between the top-down and the bottom-up approach of estimating radiative forcing. The study has suggested ways to close this gap.

Source: California ARB