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Atmospheric Environment 180 (2018) 184-191 



ELSEVIER 


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Atmospheric Environment 

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Enhanced light absorption due to the mixing state of black carbon in fresh 0 I 
biomass burning emissions I 

Qiyuan Wang'*’*, Junji Cao'*’’*’**, Yongming Han'*’**, Jie Tian^*, Yue Zhang^, Siwatt Pongpiachan®, 
Yonggang Zhang'*, Li Li'*, Xinyi Niu**, Zhenxing Shen^*, Zhuzi Zhao'*, Danai Tipmanee^, 

Suratta BunsomboonsakuE, Yang Chen®, Jian Sun*^ 

^ Key Laboratory of Aerosol Chemistry and Physics, State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, 

Xi'an, 710061, China 

^ Institute of Global Environmental Change, Xi'an Jiaotong University, Xi'an, 710049, China 
School of Humcm Settlements and Civil Engineering, Xi'an Jiaotong University, Xi'an, 710049, China 

Department of Environmental Science and Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China 
^School of Social & Environmental Development, National Institute of Development Administration (NIDA), Bangkok, 10240, Thailand 
^Faculty of Technology and Environment, Prince of Songkla University, Phuket Campus, Kathu, Phuket, 83120, Thailand 
^ Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, 400714, China 


GRAPHICAL ABSTRACT 



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ARTICLE INFO 


ABSTRACT 


Keywords: 

Black carbon 
Mixing state 
Size distribution 
Light absorption 
Biomass burning 


A lack of information on the radiative effects of refractory black carbon (rBC) emitted from biomass burning is a 
significant gap in our understanding of climate change. A custom-made combustion chamber was used to si¬ 
mulate the open burning of crop residues and investigate the impacts of rBC size and mixing state on the 
particles' optical properties. Average rBC mass median diameters ranged from 141 to 162 nm for the rBC pro¬ 
duced from different types of crop residues. The number fraction of thickly-coated rBC varied from 53 to 64%, 
suggesting that a majority of the freshly emitted rBC were internally mixed. By comparing the result of observed 
mass absorption cross-section to that calculated with Mie theory, large light absorption enhancement factors 
(1.7-1.9) were found for coated particles relative to uncoated cores. These effects were strongly positively 
correlated with the percentage of coated particles but independent of rBC core size. We suggest that rBC from 
open biomass burning may have strong impact on air pollution and radiative forcing immediately after their 
production. 


* Corresponding author. 

** Corresponding author. Key Laboratory of Aerosol Chemistry and Physics, State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy 
of Sciences, Xi'an, 710061, China. 

E-mail addresses: wangqy@ieecas.cn (Q. Wang), cao@loess.llqg.ac.cn (J. Cao). 
https://doi.Org/10.1016/j.atmosenv.2018.02.049 

Received 29 December 2017; Received in revised form 21 February 2018; Accepted 26 February 2018 

Available online 01 March 2018 

1352-2310/ © 2018 Elsevier Ltd. All rights reserved. 



















































Q. Wang et al 


Atmospheric Environment 180 (2018) 184-191 


1. Introduction 

Black carbon (BC) is produced during the incomplete combustion of 
carbon-containing materials, and it is the dominant light-absorbing 
form of atmospheric particulate matter for visible and infrared wave¬ 
lengths of light (Bond et al, 2013). Light absorption by anthropogenic 
BC particles can perturb the Earth's radiative balance and in so doing 
cause warming aloft and surface dimming on both regional and global 
scales (Ramanathan and Carmichael, 2008; Booth and Bellouin, 2015). 
Climate modeling studies indicate that BC is the second largest con¬ 
tributor to current global warming after carbon dioxide (CO 2 ) 
(Jacobson, 2001; Bond et al., 2013). In addition, BC plays an important 
role in haze pollution through its impacts on the aerosol-planetary 
boundary layer (Ding et al., 2016). Further, BC is associated with ad¬ 
verse impacts on human health and crop yields (Tollefsen et al., 2009; 
Li et al., 2016), and it also has been linked to reductions in precipitation 
and negative influences on terrestrial and aquatic ecosystems (Forbes 
et al., 2006; Hodnebrog et al., 2016). 

Estimates from modeling studies indicate that the direct radiative 
forcing caused by BC is about + 0.71Wm“^, but the uncertainty of the 
estimates is large, ~ 90%, ranging from -I- 0.08 to +1.27 W m“^ (Bond 
et al., 2013). One of the difficulties in making reliable estimates of BC 
radiative effects is that the calculations are sensitive to whether the 
particles are treated as internally- or externally-mixed with non-BC 
materials (Bauer et al., 2010). Furthermore, there also are still un¬ 
certainties concerning the effects of BC mixing state on light absorption. 
Both laboratory studies and field measurements have shown that par¬ 
ticles' light absorption can be enhanced by the internally-mixed BC. For 
example, Liu et al. (2015) demonstrated clearly that coatings can sub¬ 
stantially enhance light absorption, with the magnitude strongly de¬ 
pending on extent of the BC coatings and their sources. Wang et al. 
(2014a) reported an absorption enhancement of 1.8 in a polluted urban 
city of China due to the large percentage of coated BC particles. Peng 
et al. (2016) found an absorption amplification factor of 2.4 for BC 
particles after they aged several hours. In contrast, however, Cappa 
et al. (2012) observed a small BC absorption enhancement of only 6 % at 
two sites in California, and the effect increased weakly with photo¬ 
chemical aging. Lan et al. (2013) similarly found that coated BC par¬ 
ticles only amplified light absorption by ~ 7% in an urban atmosphere 
of South China. These discrepancies may be attributed to several fac¬ 
tors, such as particle's size, shape, and coatings as well as the emission 
sources. Our understanding of how the BC mixing state affects the 
particles' light absorption is still limited. 

Biomass burning is one of the largest sources for BC in the global 
atmosphere (Bond et al., 2013). In China, open biomass burning is an 
especially important contributor to BC and estimated to be 137 Gg in 
2013 (Qiu et al., 2016). Of the biomass sources, the burning of crop 
residues (e.g., rice, wheat, and corn) has its most significant impact on 
BC emissions during the summer/autumn harvest seasons. The tradi¬ 
tional method of “slash and burn” agricultural is often used to clear 
fields of leftover plant residues and return nutrients to the soil. Al¬ 
though the Chinese government has taken measures to prohibit the 
open burning of agricultural crop residues, local enforcement of the 
regulations is still uneven. According to the agricultural fire map from 
Zha et al. (2013), the numbers of total agricultural fire sites in China 
were 5514 in 2009 and 4225 in 2010, and > 80% of them were dis¬ 
tributed in the agricultural regions. Moreover, recent studies have 
shown that crop field burning activities not only led to local air pol¬ 
lution but also had effects on regional air quality through the transport 
and dispersal of pollutants (Long et al., 2016). 

Studies on BC emissions from open burning of crop residues in 
China have been presented in previous publications (Chen et al., 2017, 
and references therein), but limited investigations have specifically 
focused on the effects of the BC size and mixing state on particles' op¬ 
tical properties. In this study, a custom-made combustion chamber was 
used to simulate the open burning of several representative types of 


crop residues. We demonstrate substantial light absorption enhance¬ 
ment of refractory BC (rBC) in fresh biomass-burning emissions relative 
to uncoated particle cores. Through detailed physicochemical analyses, 
we show that the absorption enhancement is strongly related to the 
amounts of coatings on the rBC particles. The results contribute to our 
understanding of the optical properties of rBC particles produced 
through biomass burning. 

2. Experimental methods 

2.1. Combmdon chamber experiment 

Test burns were conducted in a custom-made combustion chamber 
at the Institute of Earth Environment, Chinese Academy of Sciences 
(lEECAS) to simulate the open burning of crop residues. The combus¬ 
tion chamber is a ~ 8 m^ cavity container with a length, width, and 
height of 1.8, 1.8, and 2.2 m, respectively. The chamber has 3 mm thick 
passivated aluminum walls to withstand high combustion temperatures 
inside the chamber. The combustion chamber is equipped with a 
thermocouple, a thermoanemometer, and an air purification system. A 
dilution sampler (Model 18, Baldwin Environmental Inc., Reno, NV, 
USA) was installed downstream of the chamber to dilute the smoke 
before sampling. A schematic of the instrumental setups of the experi¬ 
ments is shown in Fig. 1. Tian et al. (2015) provided a detailed de¬ 
scription of the structure and evaluation of this combustion chamber. 

Samples of rice, wheat, corn, cotton, and soybean straw and stalks 
were collected from seven major Chinese crop producing provinces 
(e.g., Shandong, Shaanxi, Hunan, Henan, Hebei, Jiangxi, and Anhui), 
which accounted for -40% of total mass of those crops in China in 
2015 (China Statistical Yearbook, 2016). Meanwhile, Ni et al. (2015) 
have pointed out that there are no significant differences in PM 2.5 
chemical source profiles for the same crop residues from different re¬ 
gions. The samples were stored at a stable temperature of - 20 °C and 
relative humidity of 35-45% for at least one month before burning. 
Aliquots of ~ 52 g were weighed, and the samples were burned on a 
platform inside the combustion chamber for -5-10 min. The smoke 
emitted from each test burn was first diluted with the dilution sampler 
and then sampled by several on-line instruments downstream. The di¬ 
lution ratio was - 20-25 for most burning cases. A total of 57 tests were 
conducted as follows: 9 for rice straw, 10 for wheat straw, 11 for corn 
stalks, 15 for cotton stalks, and 12 for soybean stalks. Detailed in¬ 
formation on each test burn is summarized in Table 1. 

2.2. Quantification of rBC mass, size and mixing state 

The mass, size, and mixing state of rBC particles were determined 
with a single-particle soot photometer (SP2, Droplet Measurement 
Technology, Boulder, CO, USA), which uses a laser-induced in¬ 
candescence for the measurements (Schwarz et al., 2006; Gao et al., 
2007). An rBC particle that enters the instrument is heated by an intra¬ 
cavity Nd: YAG laser (X = 1064 nm) to its vaporization temperature, 
and that causes the emission of thermal radiation, which is measured by 
two types of optical detectors. The peak incandescence signal is pro¬ 
portional to the rBC mass, and it is not affected by the particle mor¬ 
phology or mixing state (Slowik et al., 2007). In this study, the peak 
intensity of the incandescence signal was converted to rBC mass using a 
standard fullerene soot sample (Lot F12S011, Alfa Aesar, Inc., Ward 
Hill, MA, USA). An atomizer (Model 9302, TSI Inc., Shoreview, MN, 
USA) was used to generate BC particles from the fullerene soot. After 
the particles passed through a diffusion silica-gel dryer, they were size- 
selected with a differential mobility analyzer (Model 3080, TSI Inc.) 
before the instrumental analysis. The uncertainty of the SP2 measure¬ 
ments is - 20%. Detailed descriptions of the SP2 calibration procedures 
can be found in our previous publications (Wang et al., 2014a, 2014b). 

The mass-equivalent diameters of rBC cores were calculated from 
the measured rBC masses by assuming the rBC particles were solid 


185 


Q. Wang et al 


Atmospheric Environment 180 (2018) 184-191 


HEPA Activated 
Flowmeter Filter Charcoal 


Valve 

Venting Fan 

Thermoanemometer 



dilution air 
Air Compressor 


Dilution Sampler 


SP2 




1 ^ 


-►tx 


PAX870 


z: 




CO Analyzer 


Fig. 1. Schematic of the instrumental setups of the experiments. 


spheres with a density of 1.8gem (Bond and Bergstrom, 2006), and 
the values ranged from ~ 70 to 700 nm (see Fig. 2). It is important to 
note that the rBC core sizes measured in this way do not include the 
contributions of non-rBC materials to the particle diameter because 
those materials are vaporized as described above. The rBC mass fraction 
outside the lower and upper particle size limits for the SP2 (~ 10%) was 
estimated by fitting a log-normal distribution to the measured rBC 
mass-size distribution (Wang et al., 2016a). 

The rBC mixing state was characterized by the lag-time between the 
peaks of incandescence and scattering signals. The lag-time occurs be¬ 
cause coatings have to be removed from the rBC core before in¬ 
candescent temperatures of the cores are reached. Fig. 3 shows that the 
lag-times displayed a bimodal distribution with ~ 2 ps separating two 
distinct populations for all types of crop residues emissions. The rBC- 
containing particles with lag-times < 2 ps were classified as uncoated 
or thinly-coated while those with lag-times > 2 ps were considered to 
have significant amounts of coatings and therefore classified as thickly- 
coated particles (Wang et al., 2016b). The degree of rBC mixing is ex¬ 
pressed as the number fraction of thickly-coated rBC and calculated as 
the percentage of rBC-containing particles with lag-times > 2 ps. 


2.3. Light absorption measurements 

The light absorption coefficient (Babs) of particles was directly 
measured with a Photoacoustic Extinctiometer (PAX, Droplet 
Measurement Technologies, Boulder, CO) a.t X = 870 nm, which uses 
intracavity photoacoustic technology. A laser beam in the acoustic 
chamber of this instrument heats the sampled light-absorbing particles, 
and this heating produces a pressure wave that is detected with a 
sensitive microphone. Additionally, PAX also can simultaneously mea¬ 
sure light scattering coefficient (Bscat) with a wide-angle integrating 
reciprocal nephelometer in the scattering chamber. Before the biomass¬ 
burning experiments, ammonium sulfate and freshly-generated propane 
BC were used to calibrate the Bscat and Babs, respectively. The light 
extinction coefficient (Bext = Bscat + Babs) can be calculated from the 
laser power of the PAX; thus, a correction factor can be established from 
the relationship between the calculated Babs (= Bext - Bscat) and the 
measured Babs- The equation of Bext is given by: 

Bex, =- — X In— X 10« [Mm-1] 

0.354 lo (1) 


Table 1 

Summary experiments of open burning of crop residues. 


Crop residues 

Crop producing province 

Test number 

Weight (g) 

Dilution ratio 

Combustion time (min) 

Rice straw 

Anhui, Hunan, Shandong, and Jiangxi 

9 

50.2-55.3 

20-26 

4-8 

Wheat straw 

Henan and Shaanxi 

10 

51.2-53.5 

20-25 

7-9 

Corn stalk 

Hebei, Henan, Hunan, Shandong, and Shaanxi 

11 

50.1-55.2 

22-25 

5-9 

Cotton stalk 

Anhui, Henan, Hunan, and Shandong 

15 

53.1-56.7 

26-38 

4-12 

Soybean stalk 

Anhui, Henan, Hunan, and Shaanxi 

12 

51.6-57.7 

15-35 

4-10 


186 



















































































Q. Wang et al 


Atmospheric Environment 180 (2018) 184-191 


(a) Rice straw (b) Wheat straw (c) Com stalk (d) Cotton stalk (e) Soybean stalk 



Volume equivalent diameter (nm) 


Fig. 2. Average mass size distributions of rBC in volume equivalent diameters for different crop residues emissions. The solid lines represent single mode lognormal fits. 



Lag-time (las) 

Fig. 3. Frequency distributions of the incandescence lag-times for —1.1 x 10^-1.5 x 10^ 
arbitrary-selected rBC particles from different types of crop residues emissions. The light 
grey and light yellow regions represent the uncoated or thinly-coated rBC particles and 
the thickly-coated ones, respectively. (For interpretation of the references to colour in this 
figure legend, the reader is referred to the Web version of this article.) 


where 0.354 is the path length of the laser beam through the cavity in 
meters; 10^ is a conversion factor to express Bgxt in Mm“^; Iq is the 
average laser power before and/or after calibration; and I is the laser 
power during calibration. Because the scattering produced by BC 
cannot be negligible, the Babs is calculated by subtracting Bscat froni Bext- 
The Bscat should be calibrated first following the same calibration steps 
of Babs- A linear relationship is then established between extinction- 
minus-scattering coefficient and measured Babs- The slope of the linear 
regression is used as the correction factor inputted into the PAX as the 
new absorption factor. In this study, the same steps of absorption ca¬ 
libration were repeated until the correction factor was stable within 
~5%. 


2.4. Calculation of modified combustion efficiency (MCE) 

The combustion conditions during each test burn were character¬ 
ized by calculating the MCE, which is a function of the relative amounts 
of carbon emitted as CO 2 and carbon monoxide (CO) (Kondo et al., 
2011 ): 


MCE = 


^[C02] 

zl[C02] +d[CO] 


( 2 ) 


values obtained during the test burn. Real-time CO 2 and CO mixing 
ratios were measured with a nondispersive infrared CO 2 analyzer 
(Model SBA-4, PP System, Amesbury, MA, USA) and a CO analyzer 
(Model 48i, Thermo Scientific Inc. Franklin, MA, USA), respectively. 


3. Results and discussion 

3.1. Size distributions of rBC cores 

The mass-equivalent diameters of the rBC cores of the burning re¬ 
sidues were well represented by mono-modal lognormal distributions 
(Fig. 2), and this finding is consistent with previous observations from 
both laboratory and field biomass-burning studies (Schwarz et al., 
2008; May et al., 2014; Taylor et al., 2014). Fig. 4a shows the dis¬ 
tributions of rBC mass median diameters (MMDs) of each test burn for 
the five types of crop residues emissions. The rBC MMDs were found in 
relatively narrow ranges, varying from 129-152, 136-159, 137-204, 
133-157, and 132-163 nm for rice, wheat, corn, cotton, and soybean 


s 

Q 



u 

CQ 

u, 

P-H 


w 



Crop residues 


where A[C02] and A [CO] are the excess mixing ratios of CO 2 and CO, 
respectively, which are calculated by subtracting the combustion 
chamber background, that is, the air measured before ignition, from the 


187 


Fig. 4. Distributions of (a) rBC mass median diameter (MMD), (b) number fraction of 
thickly-coated rBC (FrBc)j and (c) light absorption enhancement (Eabs) for different types 
of crop residues emissions. 








































































Q. Wang et al 


Atmospheric Environment 180 (2018) 184-191 


220 


-1-1- 

(a) Rice straw 
y = 84.79 + 65.54X 
r = 0.27,n = 9 
-p = 0.48 

-1-1-1- 

(b) Wheat straw 
y= 164.38- 17.12X 
r = -0.07,n= 10 
- p = 0.84 

-1-1- 

(c) Com straw 
y = %.62-402.16x 
r=-#55,n= 11 

-p = 0^ 

• ^ 

_1_1_ 

-1-1- 

(d) Cotton straw 
y = 385.61 -257.11X 
r=-0.39,n= 15 
-p = 0.15 

-1- 

(e) Soybean straw 
y= 110.73+ 42.02X 
r = 0.19,n=12 
-p = 0.56 

_1_1_ 

^ • 

_1_1_1_ 

* • • 

_1_1_ 

• 

_1_ 


180 


140 




100 
0.80 


0.84 


0.88 0.75 0.80 0.85 0.90 0.80 


0.85 


0.90 0.90 


0.93 


0.96 0.75 


0.85 


0.95 


Modified combustion efficiency 

Fig. 5. Relationship between rBC mass median diameter and modified combustion efficiency for five types of crop residues emissions. 


residues, respectively; with corresponding arithmetic mean values 
( ± standard deviation, SD), also in nm, of 141 ( ± 7), 150 ( ± 8), 162 
( ± 19), 147 ( ± 7), and 149 ( ± 9). The student's t-tests for the rBC 
MMDs from the different types of fuels showed that there was a sta¬ 
tistically significant difference at a probability for chance occurrence 
of < 5% (p = 0.002) between rice straw and corn stalk emissions while 
the differences for other types of crop residues emissions were not 
significant (p = 0.15 to 1.0). 

The type of combustion, that is, whether the fire is flaming or 
smoldering, can lead to distinct differences in the properties of the 
emitted particles (Ni et al., 2015). The MCE values for the different test 
burns, which are a measure of how efficiently the fuels are burned 
(Yokelson et al., 1996), ranged from ~ 0.79 to 0.95, and this reflects the 
amount of variability in completeness of combustion from burn-to- 
burn. A MCE >0.9 is characteristic of the flaming phase while a 
MCE < 0.9 represents the smoldering phase (Reid et al., 2005). Fig. 5 
shows that the MMDs of the emissions correlated either weakly or in¬ 
significantly with the MCEs (r = -0.55 to 0.27 and p = 0.08 to 0.84), 
suggesting that the smoldering or flaming conditions had limited effects 
on the rBC core sizes. May et al. (2014) similarly found no clear re¬ 
lationship between MMDs and MCEs for the burning of some individual 
plant species in a laboratory combustion study. It should be noted that 
there were no test burns occurred under the condition of MCE > 0.95 in 
this study. Liu et al. (2014) reported that the single scattering albedo 
(scattering/(absorption -I- scattering)) from biomass burning dramati¬ 
cally decreased with the increasing MCE when it larger than 0.95, 
implying that large fraction of rBC may be produced. More rBC particles 
favor rBC-rBC coagulation, and thereby leads to increases in rBC core 
size. Thus, the bad correlation between MMDs and MCEs in this study 
may be also related to the relative weak rBC-rBC coagulation under 
MCE < 0.95. 

Compared with previous biomass-burning observations made with 
an SP2, our average MMDs fall within the lower limits of ~ 140-190 nm 
from laboratory-based biomass-burning experiments reported by May 
et al. (2014). However, the average MMDs found in our study are 
considerably smaller than those for aircraft measurements (altitudes: 
~ 1.8-5.0 km) made in biomass-burning plumes in the ambient atmo¬ 
sphere (Kondo et al., 2011; Sahu et al., 2012; Taylor et al., 2014). For 
example, Kondo et al. (2011) found that MMDs were 177-197 nm in 
fresh biomass-burning plumes (age < 1 day) that originated from North 
America and 176-238 nm in aged biomass-burning plumes (age: 2-3 
days) from Asia. Taylor et al. (2014) reported MMDs of 194 nm (age: 
~1 day) and 196 nm (age: ~2 days) in two biomass-burning plumes 
from a Canadian boreal forest. Sahu et al. (2012) observed MMDs of 
172-210 nm for biomass-burning plumes encountered over different 
regions of California. In addition to the fact that different types of 
biomass (e.g., crop residues versus various forest vegetation) can pro¬ 
duce distinct MMDs, the larger MMDs in ambient biomass-burning 
studies may be also related to their higher MCEs compared with our 
laboratory study. In most cases, only an active flaming fire (e.g.. 


MCE > 0.95) can produce enough heat to convect the plume to higher 
altitudes, and the high MCE is favor to rBC-rBC coagulation leading to 
relative large MMDs. Moreover, another possible reason for the larger 
MMDs in the studies of the ambient atmosphere compared with our 
laboratory study is that atmospheric aging/coagulation processes may 
cause growth in rBC cores in the field as the particles in most of the 
ambient studies were sampled a day or more after their production. 

3.2. Mixing state of rBC 

Freshly emitted rBC particles are typically externally mixed with 
other aerosol components, but they become internally mixed through 
physicochemical aging processes in the atmosphere (China et al., 2015). 
In biomass-burning plumes, rBC particles are thought to become coated 
with other materials in the first few hours after emission (Akagi et al., 
2012). A more efficient burning phase (flaming; MCE > 0.9) will favor 
production of rBC relative to organic aerosol, while less efficient 
burning condition (smoldering; MCE < 0.9) will tend to produce more 
organic aerosol compared with rBC, leading to large formation of 
thickly-coated rBC particles (Kondo et al., 2011; Collier et al., 2016). As 
shown in Fig. 4b, the average number fraction of thickly-coated rBC is 
comparable among different types of biomass-burning emissions, with 
arithmetic means ± SD (in %) of 64 ± 2, 62 ± 2, 63 ± 3, 53 ± 7, 
and 58 ± 6 for burning straw or stalks of rice, wheat, corn, cotton, and 
soybean, respectively; and this shows that the rBC particles were coated 
even though they were freshly emitted. 

To investigate the potential influence of the MCE on rBC mixing 
state, the number fraction of thickly-coated rBC is plotted against MCEs 
in Fig. 6. Except for the emissions from rice straw burning, the thickly- 
coated rBC number fraction was found to be significantly anti-corre¬ 
lated (r = -0.73 to -0.65, p = 0.002 to 0.03) with the MCEs. This 
implies that when crop residues burn in smoldering fires, more coated 
rBC particles are produced compared with the particles produced by 
flaming fires. The larger implication of this finding is that differences in 
the types of both fuels and fires may affect the optical properties of the 
particles that are produced, and this in turn could influence their im¬ 
pact on radiative fluxes and hence climate. 

3.3. Light absorption enhancement 

The mass absorption cross-section (MAC, expressed in m^ g“^) re¬ 
lates rBC mass concentrations to light absorption, and it is one of the 
key variables used in radiative transfer models (Bond et al., 2013). In 
our study, a MAC of rBC at X. = 870 nm (MACsyo) was calculated by 
dividing the absorption coefficient measured with the PAX by the rBC 
mass concentration detected with the SP2 (MACsyo = absorption/rBC). 
Fig. 7 shows that -90% of the MACsyo values for all burning cases 
mainly fell within a relatively narrow range of 6.5-8.5 m^ g“^, which 
are comparable with the values of 5.7-8.3m^g“^ that are influenced 
by biomass-burning emissions in previous studies (Kondo et al., 2009; 


188 










Q. Wang et al 


Atmospheric Environment 180 (2018) 184-191 


-1-1- 

(a) Rice straw 

-1-1-1- 

(b) Wheat straw 

- 1 - 1 - 

(c) Com straw 

- 1 - 1 - 

(d) Cotton straw 

- 1 - 

(e) Swbean straw 




. • 

. 




1 


- y = 78.85 - 17.45X 

- y = 93.32-36.77X 

- y= 133.34-81.19x 

- y = 47'M5^6.67x 

-y= 160.71-113.14x - 

r = -0.26,n = 9, 

r = -0.69,n= 10 

r = -0.65,n= 11 

r = -0.73,n= 15, 

r = -0.73,n= 12 

p = 0.49, 

P = ^_ 

p = 0.03 

p = 0.002 • , 

p = 0.007 , 


35 I_I__ jiir _I_I_ lj: _ iitj _ I _ lj: __i_ lj: - - -i_ 

0.80 0.84 0.88 0.75 0.80 0.85 0.90 0.80 0.85 0.90 0.90 0.93 0.96 0.75 0.85 0.95 


Modified combustion efficiency 

Fig. 6. Relationship between number fraction of thickly-coated rBC (FrBc) and modified combustion efficiency for five types of crop residues emissions. 



6 7 8 9 10 

MAC (m^ g'^ 


Fig. 7. Frequency distribution of rBC mass absorption cross section (MAC) for five types 
of crop residues emissions. 

Subramanian et al., 2010; Laborde et al., 2013; Wang et al., 2015). 
Average MACsyo ( ± SD) for the rBC particles from rice, wheat, corn, 
cotton, and soybean burning were 7.6 ± 0.5, 7.5 ± 0.6, 7.2 ± 0.6, 
7.0 ± 0.3, and 7.4 ± 1.3 g“^, respectively. The t-tests showed that 

the differences in MACsyo among the various crop types were not sta¬ 
tistically significant at a probability of 5% (p = 0.06), suggesting that 
the absorption capacity of the rBC normalized by mass was independent 
on the type of plant matter burned. 

Based on the assumption of spherical for uncoated rBC particles, 
Mie theory was used to calculate the MACsyo of uncoated rBC particles 
(MAC 87 o,uncoated) using the core sizes of rBC measured with the SP2. 
More details regarding the Mie algorithms can be found in Bohren and 
Huffman (2008). For uncoated rBC, we used a refractive index of 
1.85-0.71i at X. = 550 nm, which is in the middle of the range sug¬ 
gested by Bond and Bergstrom (2006). Mie theory was first applied to 
estimate the MAC values of uncoated rBC at X. = 550 nm, and then 
those values were converted to MAC 87 o,uncoated based on an rBC ab¬ 
sorption Angstrom exponent of 1.0 (Lack and Langridge, 2013). The 
average absorption enhancement was calculated by comparing MACsyo 
for rBC with and without coatings (Enhancement = MACsyo/MA- 

C870,uncoated)* 

Large absorption enhancements were found in the fresh biomass¬ 
burning emissions, with average values of 1.9 ± 0.1, 1.8 ± 0.1, 
1.7 ± 0.2, 1.7 ± 0.1, and 1.8 ± 0.3 for straw or stalks of rice, wheat, 
corn, cotton, and soybean emissions, respectively (Fig. 4c). These ob¬ 
servations suggest that light absorption for relatively fresh rBC is en¬ 
hanced compared with that for uncoated particles. The refractive index 


of rBC is a key input parameter in the Mie model, and we bounded our 
calculations using the lowest (1.75-0.63i) and highest (1.95-0.79i) 
refractive index values suggested by Bond and Bergstrom (2006). This 
was done to evaluate the sensitivity of absorption enhancement calcu¬ 
lations to the parameterization of the refractive index, and the results 
show that the difference between the two extreme cases was within 
-15%. 

To further investigate the potential impacts of rBC morphology and 
mixing state on light absorption, we plotted the absorption enhance¬ 
ment values against the number fraction of thickly-coated rBC and 
against the MMDs. As shown in Fig. 8(a-e), except for the rice straw 
case, absorption enhancement was positively correlated (r = 0.72 to 
0.79, and p = 0.003 to 0.009) with the number fraction of thickly- 
coated rBC, suggesting that the magnitude of the light absorption en¬ 
hancement was strongly affected by the amounts of coatings on the 
particles. There is a good explanation for this; that is, light absorption 
caused by coated rBC is “enhanced” because the coatings act as a lens 
that refracts more light to the particle's core, which is called “leasing 
effects” (Lack and Cappa, 2010). Previous studies have shown that even 
if for the same amount of coatings, BC embedded within a particle of 
non-BC compounds can cause larger enhancement for MAC than the 
one attached to the surface of a non-BC particle (Fuller et al., 1999; 
Scarnato et al., 2013). The poor correlation for rice straw emissions 
here may be due to the different internal morphology of rBC compared 
with other crop residues emissions. However, this speculation needs 
further evidence in the future work. In addition to the coating amount, 
the rBC core size may also affect the absorption enhancement, because 
it provides a surface area to receive the incident light. Fig. 8(f-j) shows 
that there was no clear relationship between absorption enhancement 
and MMDs, suggesting that the absorption enhancement of coated rBC 
particles is independent of the rBC core size at the range of 
-129-204 nm. Thus, light absorption enhancement of rBC-containing 
particles is apparently affected by the ‘Tensing effects” of the coatings. 

4. Conclusions and atmospheric implications 

We investigated the physicochemical properties of rBC particles 
produced in laboratory studies of open biomass burning, and one main 
focus of this work was on the optical properties of the particles and how 
they were affected by coatings on the particles. Our results showed that 
average rBC core size ranged from 141 to 162 nm for different types of 
crop residues emissions regardless of whether the fires were in the 
smoldering or flaming phase. Large number fractions of thickly-coated 
rBC (53-64%) were found in the freshly emitted particles. Smoldering 
crop residues tended to produce more coated rBC than flaming fires. 
The average rBC MAC 870 for different kinds of crop residues varied from 
7.0 to 7.6m^g“^. The t-tests showed that light absorption capacity of 
the rBC particles was independent of the types of crop residues that 
were burned. By comparing the result of observed MAC 870 with SP2 and 
PAX to that calculated with the Mie theory, it indicated that freshly 


189 






















Q. Wang et al 


Atmospheric Environment 180 (2018) 184-191 


e 

(D 

O 

a 

cd 

<L) 

c 

o 


< 


2.5 


2.0 - 


B 1.5 

& 

o 


1.0 


1 1 I 

(a) Rice straw 

1 - 1 - 

(b) Wheat straw 

- 1 - 1 - 1 — 

(c) Com straw 

- 1 - 1 - 1 — 

(d) Cotton Straw 

(e) Soybean strav^ 

mJL- 

• 

• 








-y= 1.58 + 0.004x 

-y = -2.08 + 0.06x 

" y = -#62 + 0.04x 

1.29 + 0.008X 

-y=*0.48 + ().04x 

r = 0.06, n = 9 

r = 0.77,n=10 

r = 0.79,n=ll 

r=0.72,n= 15 

r=0.77,n=12 

p = 0.88 

1 1 1 

p = 0.009 

1 _ 1 

p = 0.004 

1 _ 1 _ 1 

p = 0.003 

1 _ 1 _ 1 

p = 0.004 

1 _ 1 


55 


60 


65 70 55 


60 


65 55 


60 


65 70 35 


45 


55 65 45 


55 


65 


75 


a 

(D 

e 

(D 

O 

c 

cd 

C 

(D 

c 

o 


2.5 


Number fraction of thickly-coated rBC (%) 


- 1 - 1 

(f) Rice Straw 

- 1 1 1 

(g) Wheat straw 

1 1 - 

(h) Com Straw 

1 1 1 
(i) Cotton straw 

- 1 

(j) Soybean straw 

• • 

• 

• 





• • 

J 


-y = 2.78-0.007x 

-y = 2.28-0.003x 

"y = ]#6 - 0.002X 

-T= 1.39 + 0.002x 

-y = 1.77T0.0b003x 

r = -0.34,n = 9 

r = -0.17,n=10 

r=0.29,n=ll 

r = 0.20,n=15 

r = 0.001, n= 12 

p==0.37 

_ 1 _ 1 _ 

p = 0.64 

_ 1 _ 1 _ 1 _ 

p = 0.39 

_ 1 _ 1 _ 

p = 0.48 

_ 1 _ 1 _ 1 _ 

p = 0.99 

_ 1 _ 1 _ 


2 . 0 - 


.2 1.5- 


& 

s 

< 


1.0 
120 


140 


160 130 140 150 160 130 


160 


190 130 140 150 160 120 


140 


160 


Mass median diameter (nm) 

Fig. 8. Scatterplot of absorption enhancement versus (a-e) number fraction of thickly-coated rBC and (f-j) rBC mass median diameter for different types of crop residues emissions. The 
solid line fits were calculated by orthogonal regression. 


emitted biomass-burning rBC particles had large light absorption en¬ 
hancements compared with uncoated particles, with values of 1.7-1.9. 
The absorption enhancements were positively correlation with the 
number fraction of thickly-coated rBC, but there was no clear re¬ 
lationship with the rBC core size. This implies that absorption en¬ 
hancement of internally-mixed rBC is the result of ‘Tensing effects” 
caused by the coatings. 

For this study, there are at least three key implications for our 
findings (1) the open burning of crop residues may cause strong positive 
direct radiative forcing immediately after their production because a 
large fraction of the freshly emitted rBC particles have substantial 
coatings that cause increased light absorption; (2) the enhanced optical 
properties of rBC could contribute in significant ways to stabilization 
atmosphere through heating in the planetary boundary layer and in so 
doing depress the development of the planetary boundary layer which 
could increase the likelihood and severity of haze events; and (3) the 
presence of coatings and large absorption enhancement in rBC from 
fresh biomass-burning emissions implies that atmospheric aging may 
have limited effects on rBC light absorption although changes in the 
chemical composition of coatings with time could still affect how the 
particles interact with light. Each of these topics will be important for 
further research on the effects of biomass-burning emissions on the 
Earth's radiative balance and climate. 

Acknowledgments 

This study was supported by the National Natural Science 
Foundation of China (41230641, 41503118, 41661144020, and 
41625015) and China Postdoctoral Science Foundation 
(2015M580890). 

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