Black carbon particles coated with secondary organic areosol.
Black carbon particles coated with secondary organic areosol.

This winter, research colleagues from around the country came to the Atmospheric Measurement Laboratory on the Pacific Northwest National Laboratory (PNNL) campus to conduct the Soot Aerosol Aging Study (SAAS). This laboratory campaign is designed to examine the interactions between soot or black carbon particles and secondary organic aerosol (SOA)—a complex mixture of water and organic molecules—in a controlled laboratory environment. It is a follow-on field campaign, to the Carbonaceous Aerosol and Radiative Effects Study (CARES), which took place in 2010 in California.

Rahul Zaveri, a scientist at PNNL and the principal investigator for the both studies, spoke about the potential impact coated black carbon could have on the atmosphere and climate models.

“The theory is that a black carbon sphere, coated with a transparent material like organic aerosol, will enhance the absorption of the black carbon core by up to a factor of two or more,” says Zaveri. “If the absorption is enhanced by a factor of two, then that would enhance the warming potential of those particles.”

When black carbon is emitted into the atmosphere, SOA and other materials coat the particles and change its size, shape, and structure—or morphology. Upon being emitted, the black carbon particles have a fractal structure. Condensation of SOA on black carbon can cause the fractal structure to collapse into a sphere, enhancing the ability of the particles to absorb sunlight, and therefore, heat the surrounding air. On the other hand, coagulation of black carbon particles with pre-existing SOA may not significantly enhance the light absorption of the mixture. SAAS is the first study to systematically investigate the relative roles of condensation and coagulation in modifying the climate-related properties of black carbon.

As shown in the video campaign overview, black carbon particles are produced by a diesel engine outside of the laboratory. The soot is pumped into a sealed chamber surrounded by ultraviolet (UV) lights, mimicking natural atmospheric conditions. Organic gases are injected into the chamber and SOA is then generated when the lights are turned on, coating the black carbon particles. The particles are analyzed with a variety of instruments that measure the shape, size, chemical composition, and the optical properties of these particles.

Like most campaigns, SAAS is a collaboration between a variety of national laboratories and universities. Noopur Sharma from Michigan Technological University, enjoys learning from her colleagues.

“I get to learn how they [her colleagues] generate samples here and about different instruments, which are not available at my institution,” says Sharma.

The lab experiments took place for several weeks in both November 2013 and January 2014. They involved researchers from DOE’s Brookhaven National Laboratory, Los Alamos National Laboratory, Lawrence Berkeley National Laboratory, and PNNL, as well as university researchers from Michigan Technological University, Carnegie Mellon University, and the University of the Pacific. The study is sponsored by the Atmospheric System Research Program, with supplemental instrument support by the Atmospheric Radiation Measurement Climate Research Facility.