The team made measurements of black carbon (BC) with a high-sensitivity laser-induced incandescence (HS-LII) instrument and a single particle soot photometer (SP2); measurements were conducted upwind, downwind, and while driving on a highway dominated by gasoline vehicles. They used the results with concurrent CO₂ measurements to derive fuel-based BC emission factors for real-world average fleet and heavy-duty diesel vehicles separately.

Unlike the results for gasoline vehicles, the measured BC emission factor for heavy-duty diesel vehicles was in reasonable agreement with previous measurements. This suggests, the team concluded, that greater attention needs to be paid to black carbon from gasoline engines to obtain a full understanding of the impact of black carbon on air quality and climate and to devise appropriate mitigation strategies.

Black carbon—combustion particles consisting primarily of solid soot cores—can have a large impact on visibility, has been linked to adverse human health outcomes, and contributes to poor air quality, such as in higher PM_2.5 levels. BC also contributes to positive radiative forcing in the atmosphere through absorption of radiation, but can contribute to indirect negative radiative forcing through the seeding of clouds. Some have suggested that reducing BC emissions via reductions in BC number concentration will result in a decrease in global cloud radiative forcing. Other evidence also suggests that BC deposition in the Arctic is partly responsible for arctic climate change, the authors note.

To accurately assess the BC impacts on global atmospheric radiative energy balance, human health, and air quality, high-quality measurements of BC emissions, particularly from anthropogenic sources, have become critically important.

Much of the anthropogenic BC mass emissions have been attributed to diesel vehicles, particularly heavy-duty diesel vehicles (HDDV). Studies have shown that emissions of BC from diesel vehicles account for a large fraction of emitted BC in urban areas. In contrast, published BC emission factors for light-duty gasoline vehicles (LDGV), derived from near roadway/tunnel sites or chassis-dynamometer studies, indicate LDGV to be relatively minor sources of BC. However, the relative importance of BC emissions by these vehicle types can change, mainly due to changes in BC emission control technologies and fuels.

...As BC emissions from HDDVs and light-duty diesel vehicles (LDDV) are reduced through stricter emission controls, the relative importance of gasoline BC emissions can be expected to increase. Consequently, small uncertainties with respect to measured LDGV emission factors will have a greater potential to significantly impact total vehicular BC emission estimates. The advent of new technologies capable of definitive and ultra low level measurements of BC can help to further constrain BC emission factors from gasoline vehicles.

—Liggio et al.

The team measured BC and other pollutants at stationary ground sites and from a mobile laboratory between 16 August and 17 September 2010. Black carbon measurements were performed onboard the mobile laboratory with a modified high-sensitivity laser-induced incandescence (HS-LII) instrument and a single-particle soot photometer (SP2); both real-time instruments are based on the principle of laser-induced incandescence.

• In HS-LII, the sample volume is exposed to a pulsed laser beam resulting in rapid heating to just below the soot sublimation temperature (approximately 4000 K). The absolute incandescence intensity from the BC particles is measured using collection optics and photomultipliers. Using an appropriate calibration and analysis of the absolute incandescence signal, information on the soot volume fraction is obtained without the need for a source of soot particles of known concentration.

  To convert to a concentration, the required BC particle dependent parameters are the absorption function, E(m), and the particle material density. In contrast to the SP2, this instrument does not measure single BC particles, but rather determines the ensemble properties for all particles in the measurement volume at the time of the laser pulse. As a result, there are no BC particle size constraints, provided that sufficient total mass is present in the detection volume. This method has been used to measure laboratory generated BC particles smaller than 7 nm.

• Particles sampled by the SP2 are irradiated with a continuous laser (1064 nm) resulting in BC incandescence which was monitored in the visible band (λ = 300−550 nm). The BC mass for each particle was estimated from the incandescence peak intensity. The SP2 is able to detect single particles with effective diameter greater than about 90 nm with approximately 100% transmission efficiency. An experimentally determined transmission efficiency correction was applied to the data to account for non-unity transmission of particles between 70 and 90 nm.

The present results also have implications for BC measurements, modeling, and emission regulations...The gap between BC mass emissions of HDDV and LDGV is likely to shrink further as regulations for HDDV continue to take effect and alternate technologies for fuel delivery in gasoline vehicles (i.e., gasoline direct injection; GDI) become more popular. BC emissions from GDI engines have been observed to be significantly higher than those from conventional engines. The present results suggest that further dynamometer and on-road measurements of BC from gasoline vehicles are required in order to corroborate our findings and to improve emissions inventories in support of modeling, national and international policies, and estimates of impacts on health, the environment, and climate.

—Liggio et al.

Resources

• John Liggio, Mark Gordon, Gregory Smallwood, Shao-Meng Li, Craig Stroud, Ralf Staebler, Gang Lu, Patrick Lee, Brett Taylor, and Jeffrey R. Brook (2012) Are Emissions of Black Carbon from Gasoline Vehicles Underestimated? Insights from Near and On-Road Measurements. Environmental Science & Technology doi: 10.1021/es2033845