DOI: 10.25338/b8jp4v
title: Measurement and modeling of the multi-wavelength optical properties of uncoated flame-generated soot: Data from the BC2, BC3, BC3+ and BC4 studies

description: Optical properties of flame-generated black carbon (BC) containing soot
 particles were quantified at multiple wavelengths during the BC2, BC3,
 BC3+ and BC4 campaigns. The particles were produced using two different
 flames, either an ethylene premixed flame (sampled at multiple heights
 above the flame front) or a methane diffusion flame. Measurements were
 made both for size-selected nascent soot particles and for particles that
 were coated (with either dioctyl sebacate, sulfuric acid or secondary
 organic aerosol) and then thermally denuded to remove the coating. The
 coating + denuding process led to a collapse of the soot particles
 relative to their originally lacy, fractal-like morphology. Observed
 optical properties for methane diffusion flame-derived particles were
 similar to those from the ethylene premixed flame-derived particles when
 sampled 20 cm above the burner. In contrast, the optical properties of
 particles from the ethylene premixed flame sampled at only 5 cm above the
 surface differed from both the ethylene premixed flame sampled at 20 cm
 and the methane diffusion flame. For both the ethylene premixed flame at
 20 cm and the methane diffusion flame, the measured wavelength-specific
 mass absorption cross-sections (MAC) increased somewhat with particle size
 up to a volume equivalent diameter (dp,VED) of ~160 nm (corresponding to a
 size parameter x ~ 0.90 at λ = 532 nm), above which the MAC was
 approximately constant. The measured single scatter albedo (SSA) values
 exhibited similar behavior. Particle collapse generally led to an increase
 in SSA, but to negligible changes in the MAC and absorption Ångström
 exponent (AAE). The observations have been used to derive effective
 complex refractive index (RI) values for the particles and to test the
 ability of these theories to represent BC optical properties. Two optical
 theories were used: (i) spherical particle Lorenz-Mie theory (hereafter
 referred to as Mie theory), and (ii) Rayleigh-Debye Gans theory, which
 assumes the particles are aggregates of individual small spheres. Use of
 the derived RI values from Mie theory with larger particles (with x
 &gt; 0.9) led to a systematic under-prediction of the absorption
 cross-sections. In contrast, it was possible to determine complex RI
 values for use with Rayleigh-Debye-Gans approximation that reproduced the
 observations for particles with x &gt; 0.9, but over-predicted
 absorption cross-sections somewhat for lower x values. Although Mie theory
 reasonably reproduced the observed MACs for particles x &lt; 0.9, it
 was likely that the increasing MAC with size in this range was due to
 changes in soot maturity. Overall, these results indicate that, from the
 point of view of light absorption, these flame-generated particles behave
 more like agglomerations of individual BC spherules, even after particle
 collapse, than like large absorbing spheres. Importantly, this implies
 that the use of Mie theory within air quality and climate models, as is
 common, may lead to under-predictions in the absorption by BC, with the
 extent of under-prediction depending on the assumed BC size distribution.
 We suggest that it is more appropriate to assume a constant,
 size-independent (but wavelength-specific) MAC to represent absorption by
 uncoated BC particles within models.
description: All data were collected as part of the BC2, BC3, BC3+ and BC4
 campaigns, that took place between 2008 and 2016 at Boston College and
 Aerodyne Research. The data provided include: (i) the processed, raw data
 measured during each campaign and used in the associated manuscript and
 (ii) the fully processed data associated with each figure of the
 associated manuscript.  The processed, raw optical
 property data (absorption and extinction) have not been corrected for the
 influence of multiple charging. The fully processed data have been
 corrected for the influence of multiple charging. See the read me files
 for fuller descriptions of the data provided. The
 associated manuscript is Forestieri et al, "Measurement and modeling
 of the multi-wavelength optical properties of uncoated flame-generated
 soot," and the results are also documented in the dissertation by
 Foresteri, S.D. from the Department of Civil & Environmental
 Engineering, University of California, Davis, 2017.
description: Please see the readme files: "Read Me - data.txt" and
 "Read Me - figures.txt" for descriptions of the data provided.
 Data provided are either the run-by-run measurements (see Read Me-data) or
 the data associated with the figures shown in the manuscript by Forestieri
 et al, "Measurement and modeling of the multi-wavelength optical
 properties of uncoated flame-generated soot." The associated figures,
 full list of contributors and manuscript abstract are shown in the pdf
 file "Figures.pdf." Please contact the authors of this dataset
 if you use it, and consider offering co-authorship for any publications
 that arise from this dataset.