An evaluation of three methods for measuring black carbon in Alert, Canada
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 A...
Main Authors: | , , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2017-12-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/17/15225/2017/acp-17-15225-2017.pdf |
Summary: | 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<sup>−3</sup>, 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<sup>2</sup> g<sup>−1</sup>, 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<sup>2</sup> g<sup>−1</sup>. There is evidence that the MAC values increase with coating
thickness. |
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ISSN: | 1680-7316 1680-7324 |