Physical and chemical properties of black carbon and organic matter from different combustion and photochemical sources using aerodynamic aerosol classification
Authors: Hu Dawei , Alfarra M. Rami , Szpek Kate , Langridge Justin M. , Cotterell Michael I. , Belcher Claire , Rule Ian , Liu Zixia , Yu Chenjie , Shao Yunqi , Voliotis Aristeidis , Du Mao , Smith Brett , Smallwood Greg , Lobo Prem , Liu Dantong , Haywood Jim M. , Coe Hugh , Allan James D. 
Published: November 2021

Abstract. The physical and chemical properties of black carbon (BC) and organic
aerosols are important for predicting their radiative forcing in the
atmosphere. During the Soot Aerodynamic Size Selection for Optical
properties (SASSO) project and a EUROCHAMP-2020 transnational access
project, different types of light-absorbing carbon were studied, including
BC from catalytically stripped diesel exhaust, an inverted flame burner, a
colloidal graphite standard (Aquadag) and controlled flaming wood
combustion. Brown carbon (BrC) was also investigated in the form of organic
aerosol emissions from wood burning (pyrolysis and smouldering) and from the
nitration of secondary organic aerosol (SOA) proxies produced in a
photochemical reaction chamber. Here we present insights into the physical
and chemical properties of the aerosols, with optical properties presented
in subsequent publications. The dynamic shape factor (χ) of BC
particles and material density (ρm) of organic aerosols was
investigated by coupling a charging-free Aerodynamic Aerosol Classifier
(AAC) with a Centrifugal Particle Mass Analyzer (CPMA) and a Scanning Mobility
Particle Sizer (SMPS). The morphology of BC particles was captured by
transmission electron microscopy (TEM). For BC particles from the diesel
engine and flame burner emissions, the primary spherule sizes were similar,
around 20 nm. With increasing particle size, BC particles adopted more
collapsed/compacted morphologies for the former source but tended to show
more aggregated morphologies for the latter source. For particles emitted
from the combustion of dry wood samples, the χ of BC particles and the
ρm of organic aerosols were observed in the ranges 1.8–2.17 and
1.22–1.32 g cm−3, respectively. Similarly, for wet wood samples,
the χ and ρm ranges were 1.2–1.85 and 1.44–1.60 g cm−3,
respectively. Aerosol mass spectrometry measurements show no clear
difference in mass spectra of the organic aerosols in individual burn phases
(pyrolysis or smouldering phase) with the moisture content of the wood
samples. This suggests that the effect moisture has on the organic chemical
profile of wood burning emissions is through changing the durations of the
different phases of the burn cycle, not through the chemical modification of
the individual phases. In this study, the incandescence signal of a Single Particle Soot Photometer (SP2) was calibrated with three different types of
BC particles and compared with that from an Aquadag standard that is
commonly used to calibrate SP2 incandescence to a BC mass. A correction
factor is defined as the ratio of the incandescence signal from an
alternative BC source to that from the Aquadag standard and took values of
0.821 ± 0.002 (or 0.794 ± 0.005), 0.879 ± 0.003 and 0.843 ± 0.028 to 0.913 ± 0.009 for the BC particles emitted from the
diesel engine running under hot (or cold idle) conditions, the flame burner
and wood combustion, respectively. These correction factors account for
differences in instrument response to BC from different sources compared to
the standardised Aquadag calibration and are more appropriate than the
common value of 0.75 recommended by Laborde et al. (2012b) when
deriving the mass concentration of BC emitted from diesel engines.
Quantifying the correction factor for many types of BC particles found
commonly in the atmosphere may enable better constraints to be placed on
this factor depending on the BC source being sampled and thus improve the
accuracy of future SP2 measurements of BC mass concentrations.