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Absorption of sunlight by black carbon (BC) warms theatmosphere, which may be important for Arctic climate. The measurement of BCis complicated by the lack of a simple definition of BC and the absence oftechniques that are uniquely sensitive to BC (e.g., Petzold et al., 2013). Atthe Global Atmosphere Watch baseline observatory in Alert, Nunavut(82.5° N), BC mass is estimated in three ways, none of which fullyrepresent BC: conversion of light absorption measured with an Aethalometer togive equivalent black carbon (EBC), thermal desorption of elemental carbon(EC) from weekly integrated filter samples to give EC, andmeasurement of incandescence from the refractory black carbon (rBC) componentof individual particles using a single particle soot photometer (SP2). Basedon measurements between March 2011 and December 2013, EBC and EC are 2.7 and3.1 times higher than rBC, respectively. The EBC and EC measurements areinfluenced by factors other than just BC, and higher estimates of BC areexpected from these techniques. Some bias in the rBC measurement may resultfrom calibration uncertainties that are difficult to estimate here.Considering a number of factors, our best estimate of BC mass in Alert, whichmay be useful for evaluation of chemical transport models, is an average ofthe rBC and EC measurements with a range bounded by the rBC and EC combinedwith the respective measurement uncertainties. Winter-, spring-, summer-, andfall-averaged (± atmospheric variability) estimates of BC mass in Alertfor this study period are 49 ± 28, 30 ± 26,22 ± 13, and 29 ± 9 ng m−3, respectively. Averagecoating thicknesses estimated from the SP2 are 25 to 40 % of the160–180 nm diameter rBC core sizes. For particles of approximately 200–400 nmoptical diameter, the fraction containing rBC cores is estimated to bebetween 10 and 16 %, but the possibility of smaller undetectable rBCcores in some of the particles cannot be excluded. Mass absorptioncoefficients (MACs) ± uncertainty at 550 nm wavelength, calculated fromlight absorption measurements divided by the best estimates of the BC massconcentrations, are 8.0 ± 4.0, 8.0 ± 4.0, 5.0 ± 2.5 and9.0 ± 4.5 m2 g−1, for winter, spring, summer, and fall,respectively. Adjusted to better estimate absorption by BC only, the winterand spring values of MACs are 7.6 ± 3.8 and 7.7 ± 3.8 m2 g−1. There is evidence that the MAC values increase with coatingthickness

An evaluation of three methods for measuring black carbon in Alert, Canada