Biomass burning (BB) aerosols have a significant effect on regional climate, and represent a significant uncertainty in our understanding of climate change. Using a combination of cavity ring-down spectroscopy and integrating nephelometry, the single scattering albedo (SSA) and Ångstrom absorption exponent (AAE) were measured for several North American biomass fuels. This was done for several particle diameters for the smoldering and flaming stage of white pine, red oak, and cedar combustion. Measurements were done over a wider wavelength range than any previous direct measurement of BB particles. While the offline sampling system used in this work shows promise, some changes in particle size distribution were observed, and a thorough evaluation of this method is required. The uncertainty of SSA was 6 %, with the truncation angle correction of the nephelometer being the largest contributor to error. While scattering and extinction did show wavelength dependence, SSA did not. SSA values ranged from 0.46 to 0.74, and were not uniformly greater for the smoldering stage than the flaming stage. SSA values changed with particle size, and not systematically so, suggesting the proportion of tar balls to fractal black carbon change with fuel type/state and particle size. SSA differences of 0.15–0.4 or greater can be attributed to fuel type or fuel state for fresh soot. AAE values were quite high (1.59–5.57), despite SSA being lower than is typically observed in wildfires. The SSA and AAE values in this work do not fit well with current schemes that relate these factors to the modified combustion efficiency of a burn. Combustion stage, particle size, fuel type, and fuel condition were found to have the most significant effects on the intrinsic optical properties of fresh soot, though additional factors influence aged soot.
Biomass burning (BB) is recognized as one of the largest sources of absorbing aerosols in the atmosphere (Bond et al., 2013; Jacobson, 2014; Ramanathan and Carmichael, 2008; Moosmüller et al., 2009). Smoke from BB is composed of gaseous and aerosol constituents, including black carbon (BC), brown carbon (BrC), organic carbon (OC), and mineral dust, all of which have critical climate and health impacts. Global climate impacts of BB result from its truly massive contributions to aerosol optical depth over large areas and from secondary processes, such as cloud and ice nucleation, which can increase the radiative impact of the emissions. BB aerosols have significant impacts, not only on local climate, but also on regional climate, air quality, and hydrological cycles (Alonso-Blanco et al., 2014; Haywood et al., 2003, 2008; Fu et al., 2012; Lin et al., 2013; Yen et al., 2013; Reid et al., 2005, 2013).
With an estimated total climate forcing of
In the atmosphere, aerosols dynamically change in complex ways. BC is initially produced during the combustion of carbon-based fuels when oxygen is insufficient for complete combustion during BB (Bond et al., 2013; Bond and Bergstrom, 2006). The chemical composition and physical properties of particles then evolve during their atmospheric lifetime due to condensation, oxidation reactions, etc. Soot is formed from organic precursors in high temperatures and insufficient oxygen environments where volatiles and primary tars react to form secondary tars to form polyaromatic hydrocarbons (PAH), which subsequently form soot particles by further agglomeration and release of hydrogen (Nussbaumer, 2010).
A theoretical BC aging model was developed to account for three major stages
of aging: aggregates of graphitic spheres and primary tars freshly emitted
from BB, aggregates becoming coated with condensable material, and BC
particles undergoing further hygroscopic growth (He et
al., 2015). BB aerosols are subject to extensive chemical processing in the
atmosphere as they are exposed to sunlight, other pollutants like biogenic
volatile organic compounds (VOCs), and oxidants such as ozone (O
As these physical and chemical changes take place, the optical properties of
these particles are also altered. Variations in optical properties of soot
particles due to internal mixing in the atmosphere and aging remain highly
uncertain, hindering efforts to assess their impact on climate.
Understanding the effect of aging on composition and the commensurate
optical property changes remains a challenge. Theoretical calculations are
consistent with measurements in extinction and absorption cross sections for
fresh BC aggregates, but overestimate the scattering cross sections for BC
with mobility diameters below
These radiative balance calculations require knowledge of aerosol optical
properties, including single scattering albedo (SSA), scattering and
absorption cross sections and efficiencies, and Ångstrom coefficients. SSA,
in particular, is crucial for predicting the direct radiative forcing of an
aerosol. A number of experimental techniques have been used to measure the
optical properties of BB aerosols (Bond et al., 1999; Holben et al.,
1998; Arnott et al., 1999, 2003; Haywood et al., 2003; Clarke et
al., 2004; Petzold and Schonlinner, 2004; Schnaiter et al., 2005b; Lack et al.,
2006; Moosmüller et al., 2009). By combining photoacoustic spectroscopy
(PAS) with nephelometry, one can simultaneously measure both absorption
and scattering (Chakrabarty et al., 2014; Nakayama et al., 2013; Lewis et
al., 2008; Gyawali et al., 2012; Flowers et al., 2010; Wang et al., 2014).
Massoli et al. (2009) examined the uncertainty in the SSA of
absorbing particles, based on measurements that combine cavity ring-down
spectroscopy (CRDS) for extinction measurements with either nephelometry for
scattering or PAS for absorption. Uncertainties in SSA using nephelometer
data are larger and are most significant for SSA < 0.7
(Massoli et al., 2009). Massoli et al. (2009)
observed nephelometer scattering cross section errors using the Anderson and
Ogren correction method to be 3 % at SSA
A sensitive technique for measurement of SSA is the combination of CRDS to
measure extinction (scattering
In this article, we report extinction, scattering, absorption, and SSA measurement results of freshly emitted soot aerosols impinged in distilled water from burning white oak, red pine, and cedar wood. Our main goal is to obtain a base line (i.e., fresh soot) to compare these same properties measured as aerosols' age. Most current measurements are limited to a single or few discrete wavelengths. The accurate measurement of aerosol optical properties over the entire solar spectrum is currently a technological challenge (Ramanathan and Carmichael, 2008). Accurate and realistic interpretation of aerosol radiative properties obtained by remote sensing and space-based measurements requires accurate measurements of the optical properties of aerosols in the laboratory. Featured absorption cross sections need to be determined, instead of assuming a power law relationship, which requires more effort and advanced instrumentation than single wavelength measurements. We report measurements of optical properties at a wide range of wavelengths to determine absorption cross sections as a function of wavelength which does not rely on any power law relationship.
Details of the experimental method and derivation of key equations for
particle optical properties and CRDS analysis have been described
(Singh et al., 2014, and references therein). We only summarize
the main points, and we encourage the reader to see the reference cited for
details. The key equation for CRD measurement is the extinction coefficient
The laser and optical components of the CRDS instrument.
The laser components of the system, shown in Fig. 1, included a Continuum
Surelite I-20 Nd:YAG laser running at 20 Hz. The 532 nm beam pumped a
single grating ND6000 dye laser with a bandwidth of 0.08 cm
The CRDS system was controlled by a combination of commercial (Continuum)
and home-built software. The ring-down measurements were recorded and
analyzed in LabVIEW (National Instruments, version 8.6). The exponential
decay was plotted in a log format and a line was fit between two cursors to
determine the slope and, therefore,
The integrated aerosol optical property measurement system.
The aerosol processing and CRDS setup was similar to the one described by
Spindler et al. (2007), with the only difference being
the use of a single CPC and the use of both an OPO and a dye laser as light
sources. A coupled differential mobility analyzer (DMA)-CPC (i.e., a scanning
mobility particle sizer; SMPS) was used to determine the size distribution
of aerosols. The current experimental setup is described below and shown in
Fig. 2. A constant output nebulizer (TSI, model 3076, modified) in
recirculation mode was used to generate aerosols from an aqueous solution of
suspended particles. This nebulizer was operated by supplying 35 psi of
filtered N
This gas passed from the N
The soot generation setup, consisting of a burning drum, particle conditioning system, SMPS, and impinger and impactor samplers.
Soot was generated with a burning drum designed in our laboratory (Fig. 3),
and burns were conducted at an off-campus location. Burning stages were
differentiated visually. This burn drum was equipped with adjustable vents
and a lid that was attached to a support structure so that it could fit
tightly over the drum or only partially cover it, as needed. Smoke moving
through the lid exits a steel chimney pipe, which was sampled by a
0.5 in. inner diameter copper tube that acted as a passively cooled heat
exchanger. A Teflon tube connected the heat exchanger to a cross, where
particles are sampled by a cascade impactor, a liquid impinger (AGI-4; Grinshpun et al., 1997; Lin et al., 1997), and an SMPS. This resulted in a
suspension of black carbon in water, though some of it may have dissolved
(Miljevic et al., 2012). A sampling time of
The impinger, which contains water, is transferred to glass bottles with
Teflon-lined lids and brought to our lab. Samples were diluted and sonicated
prior to their introduction to CRDS and the nephelometer, and samples were agitated
using a magnetic stir bar throughout atomization. The TSI atomizer had been
modified to incorporate a stir plate and accept wide-mouth bottles, which
reduce the number of sample transfers and decrease the likelihood of sample
carryover. Samples were characterized for their particle size distribution
before and after nebulization using an SMPS, and several sizes of soot
particles were selected for measuring their optical properties. Baseline
measurements were taken with nebulized water without particles to take into
account any possible particles generated from residues in the water and to
minimize the change in water vapor concentration between the blank and
particle measurement experiments. The DMA was set to the same pass diameter
as the normal particle measurement, and
The calculation flow for determining average
The extinction and scattering cross section of fresh soot from white pine, red oak, and cedar were measured using the CRDS and nephelometer, which were used to calculate the absorption cross section and single scattering albedo. The measurements were made for two wavelength ranges, 500–580 and 580–660 nm. Earlier the measurements were done on different days for each wavelength range, since it involved changing mirrors and conducting realignment of the laser beam, though some later measurements were performed on the same day. Particle number density also varied somewhat over the experiment, but remained mostly consistent. Experiments where the number densities were found to fluctuate significantly were disregarded. The ring-down time was different in each run due to different high-reflectivity (HR) mirrors used for each wavelength range. The error was higher on the low end of the spectrum, due to decreased mirror reflectivity. We have extensively discussed the method used to calculate optical parameters and their associated errors in Singh et al. (2014), and the same method is applied to soot samples in this work.
The initial sample collection was done on 3 November 2014. The method of
soot collection in distilled water has not been previously reported. We
cannot account for any chemical modification of the soot during impingement
with our current instrumental capabilities. A comparison of the size
distributions of white pine during combustion and after renebulization
showed a change in the particle size distribution. The flaming sample had a
mode number-density diameter of 148 nm during the burn (interpolated from
peak edges due to detector overload) and 55 nm upon nebulization. The
smoldering stage sample similarly went from a mode diameter of 138 nm during
the burn to 50 nm after nebulization. In both cases, the mode diameter is
reduced by a factor of
Extinction, scattering, and absorption cross sections showed a decrease at
higher wavelengths for measurements done several weeks apart (i.e., a
different wavelength range at a different time). Even at overlapping
wavelengths, older samples had lower cross section values, resulting in an
abrupt discontinuity at 580 nm (the boundary between the ranges of the two
sets of mirrors used in this work). Even though the size distribution did
not change over the course of several weeks, we attempted to attribute the
decrease in optical values to either changes in chemical properties of the
soot or to an experimental artifact. When measurements were done on the same
day for both wavelength ranges, the abrupt change in the measured values was
reduced for most runs, showing this discontinuity. The discontinuity could
be a result of several factors, including extinction coefficient error,
which is about 1.3–1.7 % (1 s), and the run-to-run variability is similar
(
Figure 4 shows the steps followed in determining cross sections, SSA, and
their errors. For extinction, the coefficient is measured at a particular
size and wavelength multiple times (individually denoted by *). The error
(1 standard deviation, SD) is derived from this. The relative standard
deviation (RSD) of several factors is used to calculate the average cross
section and average error from the original extinction coefficient. A
similar process is shown for scattering with the inclusion of a correction
factor and its associated error. A broadband correction factor
The particle number density in the cavity (
As BB particles age, aerosol growth is not the only means in which they change. Often, dilutors are used in laboratory and field experiments on BB emissions to represent dilution due to diffusion in the atmosphere. These coatings can evaporate substantially during dilution of a smoke plume to ambient conditions. While the generation of volatile compounds cannot be ruled out in our work, we did not take into account the impact this may have on the optical properties of the soot samples collected in this work. In the sampling system used in this work, any coating on the soot could be lost (i.e., dissolved) after being impinged, and would make the measurement of the resuspended soot core drastically different from a core shell or more complex coating structure that might be generated. Alternatively, previously uncoated particles could be coated with water-soluble, but nonvolatile or semivolatile species. We aim to systematically address these issues in future work, when these measurements become available.
Mean SSA values, their error (1
It has been shown that the presence of large, multiply charged particles
passed by the DMA can artificially increase measured cross sections, even if
their number density is relatively small (Uin et al., 2011). An
inline impactor with a 1
SSA of 300 nm particles from white pine, red oak, and cedar sampled during the flaming stage.
SSA of 400 nm particles from white pine, red oak, and cedar sampled during the flaming stage.
The SSA as a function of wavelength for fresh soot produced from cedar,
red oak, and white pine had a slope close to zero over the wavelength range
of 500–660 nm, with values ranging from 0.46 to 0.74. While our measured
optical properties of fresh soot are within the range of values measured by
other groups, reflecting both the dynamic nature of fires, these variations
may be due to significant differences in smoke aging processes, burning
conditions, sample handling and processing, and measurement techniques used
(Schnaiter et al., 2005a; Lewis et al., 2008; Mack et al., 2010; Liu et al.,
2014). The SSA for cedar, red oak, and white pine were plotted for 300, 400,
and 500 nm particles during the flaming stage (Figs. 5, 6 and 7) and during
the smoldering stage (Figs. 8, 9 and 10). The solid lines represent the mean
values of SSA, and the dotted lines represent error about the mean. The mean
values and their errors are shown in Table 1. Cedar had an SSA that was
significantly greater for the flaming stage than the smoldering stage for
300 nm particles. The smoldering stage was greater than flaming for larger
particles, but not significantly for 400 nm particles. It has been shown
that the smoldering phase emits larger, higher SSA particles
(Reid et al., 2005). It is likely that, at small particle
diameters, such as 300 nm, elemental carbon (EC) has a greater contribution to particle mass
than OC, giving rise to lower SSA values for the smoldering stage of this
fuel at this size. To make up for this, larger particles could have a
greater contribution of OC, resulting in its greater abundance in particulate
matter with a diameter of less than 2.5
Previous SSA measurements of fresh BB aerosols.
The SSA values of fresh soot from the smoldering stage for 300, 400, and 500 nm
particle sizes, are slightly dependent on size parameter (
SSA of 500 nm particles from white pine, red oak, and cedar sampled during the flaming stage.
SSA of 300 nm particles from white pine, red oak, and cedar sampled during the smoldering stage.
A number of ambient field studies on optical properties of BB aerosols have
been done, several of which are reported in Table 2. These were mainly
measured at a single wavelength, but not all were done at the same
wavelength. In general, soot particles generated by burning propane or
ethylene in the laboratory or emitted from diesel engines have a much lower
SSA than BB soot (Wei et al., 2013; Khalizov et al., 2009a; Schnaiter et
al., 2005a; Schnaiter et al., 2006; Radney et al., 2014). Liu
et al. (2014) measured SSA and AAE of fresh BB aerosols produced from 92
controlled laboratory combustion experiments of 20 different woods
(ponderosa pine (PP), red oak, wheat straw, rice straw, and others) and a
relatively fresh plume during a field-based measurement of the Las Conchas
wildfire in 2011. They demonstrated that an SSA of BB aerosol spans a large
range (
SSA of 400 nm particles from white pine, red oak, and cedar sampled during the smoldering stage.
SSA of 500 nm particles from white pine, red oak, and cedar sampled during the smoldering stage.
Bergstrom et al. (2003) performed broadband SSA estimates of the total
aerosol column using solar radiative flux and optical depth measurements
over 2 days during the SAFARI 2000 field experiment in southern Africa. A
detailed radiative transfer model resulted in SSA values from 0.85 to 0.90
at 350 nm, decreasing to 0.6 in the near-infrared (Bergstrom et
al., 2003). Observations with small optical depth over the ocean showed a
slightly decreasing SSA with wavelength, 0.84
Lewis et al. (2008) found SSA values at 405 nm ranging from 0.37 to 0.95
for flowering shrubs and pine needle litter during the Fire Laboratory at
Missoula Experiment (FLAME). Chemical and physical properties determined from X-ray
and electron microscopy methods found that the combustion products of pine
needles, wood, and litter (duff) are chemically similar, and their particles
consist of liquid oily OC with BC inclusions (Hopkins et
al., 2007). PP needles/twigs and duff were found to have a fire-integrated
SSA of 0.91 and 0.97, respectively, which is significantly larger than any
of our white pine measurements (Table 1), despite having similar burning
conditions. BB aerosols of southern longleaf pine needles were also
significantly greater, having an SSA of 0.89. While some of this variability
can be attributed to sp
SSA as a function of size parameter for all samples in the smoldering stage.
SSA as a function of size parameter for all samples in the flaming stage.
Results of this work compared to FLAME-4 results (Liu et al,
2013; Pokhrel et al., 2016). A power law fit was performed in the form of
AAE
AAE values determined in this work are presented in Table 1. By making a
log
For each fuel and particle size, a larger AAE was found for smoldering
combustion, compared to the flaming combustion stage, which is consistent
with a significant absorption by BrC in the visible region (Chen et al.,
2006; Chang and Thompson, 2010). AAE values in this work are generally larger
than those observed in relatively fresh plumes from the Las Conchas wildfire
(2.1
The optical properties of aerosols are dominated by their chemical composition and physical characteristics, such as size and morphology, which lead to large uncertainties in quantifying how they directly alter the climate system. Despite the care taken in measuring SSA and AAE in this work, several of these effects require additional measurements to fully characterize their effects on aerosol optical properties. Freshly emitted BC particles are mostly hydrophobic and externally mixed with other aerosol constituents (Zhang et al., 2008). There is evidence that fires produce BC particles coated with organic matter in a manner that enhances some of their optical properties, specifically short-wavelength absorption by “lensing” (Lack et al., 2012), which alters the results of climate models (Bond and Bergstrom, 2006). We have observed the same tar-ball-like particles in SEM images, but we did not perform further analysis (Tumolva et al., 2010). Field measurements indicate that, during transport, fresh soot becomes internally mixed with sulfates and organics, leading to an enhancement of light absorption by about 30 % (Schwarz et al., 2008). Backman et al. (2010) measured the effect of heating on light scattering and absorption by aerosols at an urban background station in Helsinki. Heating mixed aerosols would volatilize scattering and low molecular weight organic constituents, producing an increase in light absorption, with SSA reduced to 0.4 after thermodenuding (Backman et al., 2010). Many of the SSA observations in this work, particularly at 300 and 400 nm diameters, are within this range. The aging process can also affect the morphology of soot by collapsing dendritic structures into more compact or near-spherical morphologies. Particles' ability to act as cloud condensation nuclei (CCN) is largely controlled by aerosol size rather than composition (Dusek et al., 2006). Field measurements suggest that in mixed aerosol populations, particle size is a good predictor of CCN ability. Aerosols particles can take up water, become larger in size than their dry equivalents, and hence, scatter more light. Wet particles also have different angular scattering properties and refractive indices than their dry counterparts, even at 50 % relative humidity (RH). An internal mixture of soot with other aerosol components is significantly more absorptive than the external mixture (Jacobson, 2000). The optical properties of fresh (uncoated) soot are practically independent of RH, whereas soot internally mixed with sulfuric acid exhibits significant enhancement in light absorption and scattering, increasing with the mass fraction of sulfuric acid coating and RH (Khalizov et al., 2009b). While these factors are recognized as important in affecting the optical properties of particles, they are not currently well constrained in this work.
One issue that can be addressed is the influence of large, multiply charged
particles (with the same electrical mobility as smaller,
Though there were differences in the size distribution between sampling and
nebulization, and chemical analysis was not available for this work, samples
appeared to be stable over the course of 2–4 weeks. A systematic study is
planned to determine the suitability of this sampling technique for storing
soot samples. A direct comparison of cooled and diluted soot with suspended
and re-aerosolized soot, examined as a function of wavelength and particle
size, would be required. Efforts are currently underway in our laboratory to
perform such a study. It is not currently known whether the optical properties of
size-selected particles are altered by this sampling process. Changes in
mixing state and particle morphology are possible, and not currently
constrained in this work. While previous work suggested that the effect of large, multiply charged particles was
not likely significant for particle diameters
When samples were stored for more than a few weeks, differences in the
extinction and scattering cross sections were observed. A statistical
framework, previously developed by our group for analyzing polystyrene
spheres (Singh et al., 2014), was applied to soot, and the
error in SSA was found to be 3–6 %. This error was dominated by the
truncation angle correction factor
SSA was determined for fresh BB soot using the extinction-minus-scattering method for a range of particle sizes (300–500 nm) and a wide range of wavelengths (500–660 nm), which is wider than previous direct measurements of BB aerosols. This is important, since the accurate measurement of aerosol optical properties over the entire solar spectrum is a technological challenge that must be addressed to quantify the impact of aerosols on climate. The optical properties (extinction, scattering, and absorption cross sections; Ångstrom absorption exponent; and SSA) were measured for fresh particles produced from burning white pine, red oak, and cedar. The extinction, scattering, and absorption cross sections decreased slightly toward higher wavelengths, producing a nearly uniform value of SSA for each particle size and fuel source. SSA values ranged from 0.46 to 0.74. Results show that SSA is not uniformly greater for the smoldering stage than the flaming stage. This was especially true for 300 nm particles, but even for larger particles where the mean SSA for the smoldering stage was greater than the flaming stage, half did not exhibit statistically significant differences.
While SSA exhibited no wavelength dependence in this work, there was particle size dependence. SSA increased with particle diameter for smoldering fires, whereas flaming fires did not exhibit any trend as a function of particle size. This is likely due to changes in the contribution of tar-ball-like spheres and fractal BC as a function of particle size. For radiative transfer models, it is inappropriate to assign a uniform SSA to all particle diameters, which are typically measured for the entire size distribution and integrated over both combustion stages.
In a comparison with literature values, white pine had an SSA that was
Despite the low SSA values observed in this work, AAE values were quite high
(1.59–5.57). AAE was larger for the smoldering stage than for the flaming
stage, which is consistent with the effects of a greater contribution of BrC
in smoldering flames. For white pine and cedar, such large values of AAE are
only observed when SSA is > 0.85 at 532 nm, which is inconsistent
with our SSA measurements. When also considering issues with low SSA and a
lack of SSA spectral dependence, this suggests that there are issues with the
MCE-based framework of Liu et al. and the EC/(EC
Biomass burning is a major global phenomenon with an unusually large number of degrees of freedom, which include morphology, size distribution, mixing state, age, composition, concentration, location, flaming condition, fuel type, fuel state, humidity, and chemical oxidants. It is practically impossible to account for all sources of uncertainty, but not all degrees of freedom are equally important. The most significant effects on the intrinsic optical properties of fresh BB particles (i.e., morphology and composition) seem to be burning stage, particle size, fuel type, and fuel condition (green, brown, mixed, litter, etc.). While this work investigates key parameters affecting fresh soot, the optical properties of aged particles are also significantly influenced by mixing state, humidity, and chemical processes.
Future work involves a plan to design and build an indoor chamber that will
be connected directly to the output of a furnace, where additional gases of
relevant organic compounds (or proxies of semivolatile species) and
nitrogen oxides can be added to simulate atmospheric aging of the BB
aerosols. This includes isoprene and many monoterpenes (like
The data used to generate Figs. 5–10 for SSA as a function of wavelength for cedar, red oak, and white pine for 300, 400, and 500 nm particles during the flaming stage and during the smoldering stage, and the data used to generate Figs. 11–12 for SSA as a function of size parameter are provided as an Excel file in the Supplement. The error analysis used in generating the data is described briefly in this paper, and more details are available in Singh et al. (2014).
Sujeeta Singh ran all the experiments and analyzed the data. Marc N. Fiddler developed the data analysis software and designed the experiments. Solomon Bililign is the principal investigator of the project and supervised the work.
This work is supported by the Department of Defense under grant no. W911NF-11-1-0188. We acknowledge the support from the Joint School of Nanoscience and Nanoengineering at NCA&T for the use of the imaging facilities. The authors also acknowledge the contribution of Damon Smith in collecting field samples. Edited by: P. Formenti Reviewed by: two anonymous referees