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

Abstract

Abstract. Absorption of sunlight by black carbon (BC) warms the atmosphere, which may be important for Arctic climate. The measurement of BC is complicated by the lack of a simple definition of BC and the absence of techniques that are uniquely sensitive to BC (e.g., Petzold et al., 2013). At the Global Atmosphere Watch baseline observatory in Alert, Nunavut (82.5 • N), BC mass is estimated in three ways, none of which fully represent BC: conversion of light absorption measured with an Aethalometer to give equivalent black carbon (EBC), thermal desorption of elemental carbon (EC) from weekly integrated filter samples to give EC, and measurement of incandescence from the refractory black carbon (rBC) component of individual particles using a single particle soot photometer (SP2). Based on measurements between March 2011 and December 2013, EBC and EC are 2.7 and 3.1 times higher than rBC, respectively. The EBC and EC measurements are influenced by factors other than just BC, and higher estimates of BC are expected from these techniques. Some bias in the rBC measurement may result from calibration uncertainties that are difficult to estimate here. Considering a number of factors, our best estimate of BC mass in Alert, which may be useful for evaluation of chemical transport models, is an average of the rBC and EC measurements with a range bounded by the rBC and EC combined with the respective measurement uncertainties. Winter-, spring-, summer-, and fall-averaged (± atmospheric variability) estimates of BC mass in Alert for this study period are 49 ± 28, 30 ± 26, 22 ± 13, and 29 ± 9 ng m −3 , respectively. Average coating thicknesses estimated from the SP2 are 25 to 40 % of the 160-180 nm diameter rBC core sizes. For particles of approximately 200-400 nm optical diameter, the fraction containing rBC cores is estimated to be between 10 and 16 %, but the possibility of smaller undetectable rBC cores in some of the particles cannot be excluded. Mass absorption coefficients (MACs) ± uncertainty at 550 nm wavelength, calculated from light absorption measurements divided by the best estimates of the BC mass concentrations, are 8.0 ± 4.0, 8.0 ± 4.0, 5.0 ± 2.5 and 9.0 ± 4.5 m 2 g −1 , for winter, spring, summer, and fall, respectively. Adjusted to better estimate absorption by BC only, the winter and spring values of MACs are 7.6 ± 3.8 and 7.7 ± 3.8 m 2 g −1 . There is evidence that the MAC values increase with coating thickness.

Size-resolved mixing state of black carbon in the Canadian high Arctic and implications for simulated direct radiative effect

“…Conversely, Wang et al (2014) found an absorption enhancement of 1.8 in China, similar to laboratory findings. In addition, Sharma et al (2017) found values of mass absorption cross section to increase with coating thickness with a steeper slope than theoretical calculations. Thus, accurate model estimates of BC absorption must rely on an understanding of sources of BC and all other aerosol components.…”

“…For biomass burning plumes, Kondo et al (2011) measured a ratio of 1.3-1.6 during aircraft flights in the spring and summer in the Canadian Arctic. Finally, Sharma et al (2017) found a ratio of 1.4-1.25 at the Alert ground station in spring of 2012.…”

“…However, in the atmosphere, BC is often not mixed entirely as an internal or external mixture. For instance, Massoli et al (2015) found that 35 % of the total non-refractory submicrometer mass was associated with BC-containing particles in coastal California during the CALNEX campaign, and recent studies of Arctic aerosol suggest that a smaller percentage of aerosol particles contain BC (e.g., Raatikainen et al, 2015;Sharma et al, 2017). Thus, the assumption in aerosol models of a full internal or external mixture may lead to overestimates or underestimates of BC absorption.…”

Organic functional groups in the submicron aerosol at 82.5° N, 62.5° W from 2012 to 2014

“…Details of the analytical methods and quality control remain the same as described by Li and Barrie (1993). The quartz filters are analyzed for OC and EC by thermal volatilization using three temperature steps, as discussed by Huang et al (2006), Chan et al (2010) and Sharma et al (2017). Here, OC from this thermal method (hereafter TM-OC) is based on the sum of OC at 550 • C and pyrolyzed OC (POC) at 870 • C. Rationale for including POC comes from isotopic analyses indicating little or no carbonate in the samples.…”

Estimation of black carbon emissions from Siberian fires using satellite observations of absorption and extinction optical depths

“…Furthermore, the eBC concentration can be different from the EC concentration in the same aerosol sample, simply because eBC and EC measurements represent fundamentally different physical properties of the aerosol (Andreae and Gelencsér, 2006;Bond et al, 2013). Parallel field measurements of eBC (with a standard aethalometer) and EC (with a thermal technique) at an Arctic station (Sharma et al, 2017) revealed that eBC was systematically larger (by about 30 %) than EC in winter and spring (when the aerosol was predominantly anthropogenic) but almost 50 % smaller in summer 20 (when the contribution of BB aerosol might be significant). Although, due to all these reasons, the eBC observations that are widely used for characterizing radiative properties of aerosol in remote regions cannot provide a strong constraint on BC emissions (especially when the BC emissions are interpreted as those of EC), the analysis of the eBC observations performed during the YAK-AEROSIB campaign is useful as it allows us to get an idea about the consistency of different types of BB BC measurements in Siberia.…”