The incomplete combustion of fossil fuels, biofuels, and biomass produces pure carbon particles called black carbon. When those black carbon particles become airborne, they absorb sunlight and contribute to atmospheric heating. If the particles settle on the surface of snow or ice in the Arctic, they can melt faster and affect the Arctic climate.
During a research cruise in the summer of 2014, Taketani et al. measured atmospheric black carbon levels above the Arctic Ocean north of Alaska and Japan. Compared with black carbon concentrations farther south, Arctic concentrations were relatively low.
Measurement began in the Pacific Ocean on 31 August 2014, when the scientists departed Dutch Harbor, Alaska, aboard the research ship R/V Mirai. They sailed northward into the Arctic Ocean and stopped at a point north of Alaska from 6 to 25 September. Then they turned south and sailed back into the Pacific, arriving in Yokohama, Japan, on 10 October 2014.
To gauge the amount of black carbon suspended in the atmosphere, the scientists used an onboard instrument to detect refractory black carbon—the carbon mass obtained by heating atmospheric particles with a laser beam and measuring the particles’ subsequent thermal emission. (The scientists made sure to halt measurements whenever ship exhaust approached the instrument.)
They detected average black carbon concentrations of 1.0 ± 1.2 nanograms per cubic meter north of 70°N. The authors suggest that on the basis of weather patterns at the time, it is likely that the rest of the Arctic Ocean had similarly low concentrations. Black carbon levels above the Bering Sea and the North Pacific were higher than those above the Arctic Ocean, probably because of closer proximity to the Eurasian continent.
The team also sampled black carbon particles to analyze their structure. They used transmission electron microscopy to obtain images of individual black carbon particles. They also probed the samples with energy dispersive X-ray spectroscopy, which uses X-rays to analyze the elemental composition of the particle.
About 20% of the refractory black carbon over the Arctic Ocean had a particular structure normally associated with air affected by biomass burning or urban plumes. Spectroscopy showed that these structures likely formed when black carbon coagulated with sea salt. The scientists rarely observed these structures farther south.
The team’s measurements match well with the only other black carbon data for the summertime atmosphere over the Arctic Ocean. They also agree with measurements made by aircrafts at higher altitudes. The team’s measurements are lower than concentrations observed over the ground in Barrow, Alaska. Further studies are necessary to clarify how Arctic black carbon levels vary in different regions and during different seasons. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2015JD023648, 2016)
—Sarah Stanley, Freelance Writer
Citation: Stanley, S. (2016), Detecting black carbon in the Arctic atmosphere, Eos, 97, doi:10.1029/2016EO046135. Published on 17 February 2016.