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. Black carbon (BC) plays an important role in the Earth's climate system.However, parameterizations of BC size and mixing state have not been welladdressed in aerosol–climate models, introducing substantial uncertaintiesinto the estimation of radiative forcing by BC. In this study, we focused onBC emissions from the oil sands (OS) surface mining activities in northernAlberta, based on an aircraft campaign conducted over the Athabasca OS regionin 2013. A total of 14 flights were made over the OS source area, in whichthe aircraft was typically flown in a four- or five-sided polygonpattern along flight tracks encircling an OS facility. Another 3 flights wereperformed downwind of the OS source area, each of which involved at leastthree intercepting locations where the well-mixed OS plume was measured alongflight tracks perpendicular to the wind direction. Comparable sizedistributions were observed for refractory black carbon (rBC) over anddownwind of the OS facilities, with rBC mass median diameters (MMDs) between ∼  135 and 145 nm that were characteristic of fresh urban emissions.This MMD range corresponded to rBC number median diameters (NMDs) of ∼  60–70 nm, approximately 100 % higher than the NMD settings insome aerosol–climate models. The typical in- and out-of-plume segments of aflight, which had different rBC concentrations and photochemical ages, showedconsistent rBC size distributions in terms of MMD, NMD and the correspondingdistribution widths. Moreover, rBC size distributions remained unchanged atdifferent downwind distances from the source area, suggesting thatatmospheric aging would not necessarily change rBC size distribution.However, aging indeed influenced rBC mixing state. Coating thickness for rBCcores in the diameter range of 130–160 nm was nearly doubled (from ∼  20 to 40 nm) within 3 h when the OS plume was transported over adistance of 90 km from the source area.

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