Size distribution and coating thickness of black carbon from the Canadian oil sands operations

Abstract

Black carbon (BC) plays an important role in the Earth's climate system. However, parameterizations of BC size and mixing state have not been well addressed in aerosol-climate models, introducing substantial uncertainties into the estimation of radiative forcing by BC. In this study, we focused on BC emissions from the oil sands (OS) surface mining activities in northern Alberta, based on an aircraft campaign conducted over the Athabasca OS region in 2013. A total of 14 flights were made over the OS source area, in which the aircraft was typically flown in a four-or five-sided polygon pattern along flight tracks encircling an OS facility. Another 3 flights were performed downwind of the OS source area, each of which involved at least three intercepting locations where the well-mixed OS plume was measured along flight tracks perpendicular to the wind direction. Comparable size distributions were observed for refractory black carbon (rBC) over and downwind of the OS facilities, with rBC mass median diameters (MMDs) between ĝ1/4 135 and 145ĝ€-nm that were characteristic of fresh urban emissions. This MMD range corresponded to rBC number median diameters (NMDs) of ĝ1/4 60-70ĝ€-nm, approximately 100ĝ€-% higher than the NMD settings in some aerosol-climate models. The typical in-and out-of-plume segments of a flight, which had different rBC concentrations and photochemical ages, showed consistent rBC size distributions in terms of MMD, NMD and the corresponding distribution widths. Moreover, rBC size distributions remained unchanged at different downwind distances from the source area, suggesting that atmospheric aging would not necessarily change rBC size distribution. However, aging indeed influenced rBC mixing state. Coating thickness for rBC cores in the diameter range of 130-160ĝ€-nm was nearly doubled (from ĝ1/4ĝ€-20 to 40ĝ€-nm) within 3ĝ€-h when the OS plume was transported over a distance of 90ĝ€-km from the source area.