PUBLICATIONS
Journal of Geophysical Research: Atmospheres
RESEARCH ARTICLE
10.1002/2014JD021546
Key Points:
• BB organic species play an imperative
role in the g(RH) of Tanzanian aerosols
• WSOC may partially responsible for
the growth factors
• Formation of less hygroscopic K2SO4
or K2C2O4 particles during
atmospheric aging
Supporting Information:
• Readme
• Figures S1 and S2
Correspondence to:
K. Kawamura,
[email protected]
Citation:
Boreddy, S. K. R., K. Kawamura, S.
Mkoma, and P. Fu (2014), Hygroscopic
behavior of water-soluble matter
extracted from biomass burning aerosols
collected at a rural site in Tanzania, East
Africa, J. Geophys. Res. Atmos., 119,
12,233–12,245, doi:10.1002/
2014JD021546.
Received 28 JAN 2014
Accepted 4 OCT 2014
Accepted article online 7 OCT 2014
Published online 7 NOV 2014
Hygroscopic behavior of water-soluble matter extracted
from biomass burning aerosols collected
at a rural site in Tanzania, East Africa
S. K. R. Boreddy1, Kimitaka Kawamura1, Stelyus Mkoma1,2, and Pingqing Fu1,3
1
Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan, 2Now at Department of Physical Sciences,
Faculty of Science, Sokoine University of Agriculture, Morogoro, Tanzania, 3Now at LAPC, Institute of Atmospheric Physics,
Chinese Academy of Sciences, Beijing, China
Abstract
In this study, we present the hygroscopic behavior of water-soluble matter (WSM) extracted
from biomass burning derived particulate matter 2.5 (PM2.5) aerosols collected at a rural background site
in Tanzania during June–August 2011. Hygroscopic growth factors, g(RH), of WSM were measured by
hygroscopic tandem differential mobility analyzer (H-TDMA) with an initial dry particle diameter of 100 nm.
We observed that the g(RH) of WSM at 90% relative humidity (RH), g(90%)WSM, ranged from 1.10 to 1.47 with
an average of 1.25 ± 0.12. The H-TDMA retrieved hygroscopicity parameter of WSM, κWSM, ranged from 0.04
to 0.24 with a mean of 0.11 ± 0.07. We found that the observed g(90%)WSM is positively correlated with PM2.5
mass fractions of K+ (R2 = 0.61), Cl (0.54), and organic carbon (0.58). Moreover, it well correlates with
levoglucosan (0.67) and total diacids (0.76), implying that although the inorganic fraction may be the most
important factor to control the hygroscopicity; biomass burning organics play a significant role in the
hygroscopicity of Tanzanian aerosols. The lower growth factors obtained over the sampling site are probably
due to the formation of less water-soluble potassium oxalate (K2C2O4) or less hygroscopic K2SO4 particles
during atmospheric aging. We observed a moderate correlation (R2 = 0.33) between PM2.5 mass fraction of
WSOC and g(90%)WSM. The retrieved g(90%)WSOM values ranged from 1.0 to 1.25 with a mean of 1.16 ± 0.05.
This study demonstrates that the hygroscopicity of Tanzanian aerosols is largely controlled by the emission of
biomass burning products and the subsequent chemical aging during atmospheric transport.
1. Introduction
Ambient aerosols are defined as suspended solid and/or liquid particles in the atmosphere. They represent
a small fraction of atmospheric mass but have a disproportionately large impact on climate and
biogeochemistry [Mahowald et al., 2011; Reddy et al., 2011]. Biomass burning can generate significant
amounts of aerosols (2–3 pg yr1) [Andreae et al., 2004; Reid et al., 2005], called smoke particles or biomass
burning aerosols, which contain a significant amount of water-soluble organic carbon (WSOC) [Simoneit
et al., 1999; Graham et al., 2002; Mkoma et al., 2013] and water-soluble organic nitrogen [Mace et al., 2003].
Therefore, biomass burning organic species, formed during pyrolysis of cellulose and emitted to the
atmosphere during wood combustion [Simoneit et al., 1999], can potentially alter the hygroscopicity, cloud
condensation activity, and the global climate change [Crutzen and Andreae, 1990; Novakov and Corrigan,
1996; Liu and Wang, 2010].
Previous studies have shown that understanding the hygroscopicity of WSOC in atmospheric aerosols is
crucial for radiative forcing and other climate model studies [Saxena et al., 1995; Randles et al., 2004; Chan
et al., 2005; Jung et al., 2011], and WSOC also have been postulated to play roles similar to that of inorganics in
visibility degradation and cloud condensation nuclei (CCN) [Malm and Kreidenweis, 1997; Cruz and Pandis,
1998]. Saxena et al. [1995] reported the importance of organic species that can significantly alter the
hygroscopic behavior of particles in the atmosphere. Mochida and Kawamura [2004] studied the hygroscopic
properties of levoglucosan and related organics; they considered that oxygenated organics emitted from
biomass burning could significantly enhance the hygroscopic growth of atmospheric aerosols. Chan et al.
[2005] reported that water-soluble organic compounds such as amino acids (glycine, alanine, serine,
glutamine, threonine, arginine, and asparagine) and biomass burning organic species (levoglucosan,
mannosan, galactosan, and D-glucose) can potentially affect the hygroscopicity and cloud condensation
activities of atmospheric aerosols. However, the knowledge about the hygroscopicity of biomass burning
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aerosol particles is still limited [Mochida and Kawamura, 2004; Chan et al., 2005; Fors et al., 2010; Carrico et al.,
2010; Dusek et al., 2011; Martin et al., 2013].
More than 80% of the biomass burning on the globe occurs in tropical regions and further, more than 30% of the
tropical biomass burning is estimated to be derived from savanna fires in Africa [Andreae, 1991; Gao et al., 2003]. The
major types of biomass burning in Africa include forest and savanna fires [Cachier et al., 1991, 1995; Lacaux et al.,
1993; Liu et al., 2000]. Agricultural field burning is recognized as another source of biomass burning aerosols, which
may be a regionally very important source of atmospheric aerosols during the dry season [Lacaux et al., 1993].
In this study, we examined the hygroscopic properties of water-soluble matter (WSM) in particulate matter
2.5 (PM2.5) aerosol particles collected at Morogoro, Tanzania in eastern Africa in order to characterize the
physical and chemical properties of African biomass burning aerosols. Previous studies on PM10 and PM2.5
aerosol particles from Tanzania reported carbonaceous components, levoglucosan, mannosan and
water-soluble inorganic ions [Mkoma et al., 2013], molecular composition of dicarboxylic acids, ketocarboxylic
acids, α-dicarbonyls, and fatty acids [Mkoma and Kawamura, 2013]. In this study, we measured, using
a hygroscopic tandem differential mobility analyzer (H-TDMA), hygroscopic properties of the WSM, which
were extracted from biomass burning PM2.5 aerosol samples. Here we discuss the link between hygroscopicity
and the chemical composition of aerosols. We also present the calculated g(RH) of water-soluble organic
matter (WSOM) to compare with chemical compositions of the WSM.
2. Experimental
2.1. Aerosol Sampling
Aerosol samples (PM2.5) were collected on precombusted (450°C for 4 h) quartz filters (Pallflex 2500QATUP, 47 mm)
at 2.7 m above ground level using low volume sampler (Gent type, flow rate 17 L min1) during June–August 2011 at
a rural background site, Morogoro (06°47′40.8″S, 37°37′44.5″E; altitude 504 m above sea level), Tanzania, in East
Africa. The interval of sample collection was 24 h, and the filters were exchanged at 7:30 A.M. LT. After sampling,
the filter samples were kept in a clean glass vial with a Teflon-lined screw cap and stored in a freezer at 20°C.
In total nine quartz filters, collected during June–August 2011, were used in this study. The aerosol samples were
influenced by the biomass burning, determined based on backward trajectory analysis and fire information
(see Figures S1 and S2 in the supporting information). We chose the nine aerosol samples that originated from
almost same source regions but experienced a different amount of biomass burning plumes. We considered that
there is a strong link between the hygroscopicity and the intensity of biomass burning plumes over the sampling
site. The sampling site and method are described elsewhere [Mkoma and Kawamura, 2013; Mkoma et al., 2013].
Agriculture is the main industry in Morogoro, where important sources of local aerosols are field burning of
crop residue and agriculture waste. Emissions from livestock (dairy cattle or farm) and domestic as well as
forest fires are also considerable sources over Tanzania [Mkoma and Kawamura, 2013; Mkoma et al., 2013].
2.2. Analysis of Chemical Species
To determine water-soluble organic carbon (WSOC), a punch of 2.59 cm2 from each quartz fiber filter was
extracted with 15 mL organic-free ultrapure water (resistivity of >18.2 MΩ cm, Sartorius arium 611 UV). For
inorganic ions (NH4+, SO42, NO3, Na+, Cl, K+, Ca2+, and Mg2+), a punch of 2.59 cm2 of each quartz fiber
filter was extracted twice with 5 mL ultrapure water. These extracts were filtrated through a syringe filter
(Millex-GV, 0.22 μm pore size, Millipore) after ultrasonication (15 min) and analyzed using a total organic
carbon (TOC) analyzer (Model TOC-Vcsh, Shimadzu, Kyoto, Japan) [Miyazaki et al., 2011] and an ion
chromatograph (761 Compact IC, Metrohm, Switzerland) [Mkoma et al., 2013], respectively.
Major anions and cations were measured using a SI-90 4E Shodex column with 4 mmol an eluent of 1.8 mmol
Na2CO3 plus 1.7 mmol NaHCO3 (Showa Denko, Tokyo, Japan) and Metrosep C2-150 (Metrohm) column with 4 mmol
tartaric acid plus 1 mmol dipicolinic acid as the eluent, respectively. The volume of injection loop was 200 μL for
inorganic ions [Mkoma et al., 2013]. For WSOC measurements, the water extracts were acidified with 1.2 M HCl and
purged with pure air to remove dissolved inorganic carbon and volatile organics [Miyazaki et al., 2011].
A filter punch of 1.54 cm2 from each filter was used to determine the concentrations of organic carbon (OC)
and elemental carbon using a field-type carbon analyzer (Sunset Laboratory Inc., USA) with thermal/optical
transmission method [Birch and Cary, 1996] following the Interagency Monitoring of Protected Visual
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Environments thermal evolution protocol. Detailed analytical procedures can be found elsewhere
[Wang et al., 2005; Mkoma et al., 2013].
Water-soluble dicarboxylic acids were determined by the method described elsewhere [Kawamura and
Ikushima, 1993]. Briefly, a 1.54 cm2 punch of each filter was extracted with organic-free ultrapure water
(10 mL) under ultrasonication (10 min × 3 times). These extracts were adjusted to pH = 8.5–9.0 with 0.1 M
KOH and concentrated using rotary evaporator under vacuum and then derivatized with BF3/n-butanol at
100°C for 1 h. After the concentration using a nitrogen blow down to near dryness, n-hexane (100 μL) was
added to the ester fraction and the derivatives were determined using a gas chromatographic (GC) flame
ionization detector. Peaks were identified by comparing GC retention time with an authentic standard and
confirmed by mass spectral examination using a gas chromatograph mass spectrometer (GC-MS). The results
are presented elsewhere [Mkoma and Kawamura, 2013].
For the determination of levoglucosan (LG), the filter samples were analyzed by the method described in
Fu and Kawamura [2011]. Briefly, a piece (1.54 cm2) of each filter sample was extracted 3 times with
dichloromethane/methanol (2:1; vol/vol) under ultrasonication for 10 min. Sugar compounds in the extracts
were derived to trimethylsilyl ethers with 50 μL of N,O-bis-(trimethylsilyl) trifluoroacetamide containing 1%
trimethylsilyl chloride and 10 μL of pyridine at 70°C for 3 h. The derivatives were diluted by addition of 140 μL
of n-hexane containing 1.43 ng μL1 (internal standard: C13 n-alkane) and then determined with a GC-MS.
Fragment ions of m/z 217 and 204 were used for the quantification of LG [Fu et al., 2012]. The results are
presented elsewhere [Mkoma et al., 2013].
2.3. Measurement of Particle Hygroscopicity
Hygroscopicity of WSM particles that were generated from water extracts of aerosol-filtered samples were
measured using a H-TDMA with an initial dry particle diameter of 100 nm. All H-TDMA experiments were
performed at 287–294 K with a mean of 291 K and atmospheric pressure of 1 atm. The instrumentation and
methodology used in this study were described in detail by Mochida and Kawamura [2004] and Boreddy et al.
[2014]. Briefly, a piece of the quartz filter with 7 mm in diameter was extracted with organic-free ultrapure
water (7 mL) under ultrasonic bath (5 min × 3 times). The water extracts were then filtrated (Millex-GV,
0.22 μm, Millipore disk filter) and used for H-TDMA measurements. The aerosol particles were generated by
nebulizer from WSM and were dried (5% RH) by two silica gel diffusion dryers connected in series and were
introduced into the H-TDMA.
The H-TDMA consists of two differential mobility analyzers (DMAs, TSI model 3081), an aerosol bipolar
charger (Am-241), humidifiers (optional prehumidifier and aerosol humidity conditioner composed of a
Nafion tube) for the aerosols and sheath flow, and a condensation particle counter (CPC, TSI model 3010).
The monodispersed particles with an initial diameter of 100 nm at RH <5% were classified by the first DMA
(DMA1), which was operated with a constant flow of 0.3 and 3 L/min for the polydispesed aerosol and
sheath air, respectively. Monodispersed dry particles were introduced to the RH-controlled (5%–95%)
humidity conditioner in hydration experiments or to the prehumidifier (where the RH is about 95%) in
dehydration experiments. The resulting particle size was measured using the second DMA (DMA2), and the
particle numbers were counted by a CPC. More details on RH control and size distribution are described in
Mochida and Kawamura [2004].
The hygroscopic growth factor, g(RH), of an aerosol particle can be defined as the ratio of the particle
diameter at elevated RH relative to that of the initial dry particles and is given by the following equation:
gðRHÞ ¼
DðRHÞ
D0
(1)
where D0 is the initial dry particle diameter at RH <5% and D(RH) is the diameter at an elevated RH.
A shape correction factor ( χ) was introduced to interpret the measured g(RH) [Kreidenweis et al., 2005] due to
the nonspherical shape of the particles or for restructuring. The restructuring can be defined as the reduction
of particle diameter at low RH during hydration experiment caused by the internal mixing of water-soluble
organic components with inorganic species such as (NH4)2SO4 [Sjogren et al., 2007] or NaCl [Krämer et al.,
2000]. The χ can be defined as follows:
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Journal of Geophysical Research: Atmospheres
fn ¼
De
1 C c ðDe Þ
¼
Dmob χ C c ðDmob Þ
10.1002/2014JD021546
(2)
where Cc is the Cunningham slip correction factor [Hinds, 1999] and fn is mobility correction factor, which is
defined as the ratio of minimum mobility diameter in our size spectra during the dehydration experiment to
the initial mobility dry particle diameter (100 nm); De is the volume equivalent diameter, and Dmob is the dry
mobility equivalent diameter (D0). The dynamic shape correction factor is χ, which converts Dmob into De
[Kasper, 1982]. It varies depending on the shape of particles. For compact spherical and cubic particles, χ is
equivalent to 1 and 1.08, respectively. It increases up to 2 or more for agglomerated and irregularly shaped
particles [Brockmann and Rader, 1990].
To validate proper H-TDMA sizing and operation, pure (NH4)2SO4 particles were generated and introduced to
H-TDMA system to measure the growth factor at 85% RH. The g(85%) of (NH4)2SO4 was 1.57 ± 0.01 (n = 3),
which agrees well with the theoretical growth factor (g(85%) = 1.56) using the thermodynamic aerosol
inorganic model [Clegg et al., 1998]. Moreover, we performed reproducibility test, which showed that the
analytical errors of g(RH) were ~3% at 90% RH.
2.4. Hygroscopicity Parameter (κ)
The hygroscopicity parameter, κ, introduced by Petters and Kreidenweiss [2007], was used to describe the
hygroscopicity of the aerosol particles; κ is defined according to
gðRHÞ3 1 ð1 aw Þ
Kappa; κ ¼
(3)
aw
1
4σ s=a MW
aw ¼ RH exp
(4)
RTρW gðRHÞD0
where aw is the water activity, ρW is the density of water, Mw is the molar mass of water, σ s/a is the
surface tension of the solution-air interface, R is the universal gas constant, T is the Kelvin temperature,
and D0 is the droplet diameter. In this model, the number of soluble entities is assumed to be
independent of water activity.
3. Result and Discussion
3.1. Hygroscopic Growth Curves of WSM
Figure 1 shows changes in the g(RH) of the water-soluble matter (WSM) of aerosol particles nebulized from
water extracts of the ambient aerosol samples collected at Morogoro, Tanzania, as a function of RH under
hydration and dehydration experiments. The g(RH) of WSMs below 30% RH was removed for all samples
during hydration experiment due to the restructuring of aerosol particles. It is also noteworthy that the
g(RH) are corrected using the dynamic shape correction factor ( χ), which indicates a slight restructuring of
particles. Although the theoretical growth factors are calculated with a volume equivalent diameter (De), the
H-TDMA measures a mobility diameter (Dmob), which is only equal to the De for a spherical particle. The Dmob of
a nonspherical particle must be shape corrected to obtain the De. The calculated dynamic shape correction
factors ( χ) of all laboratory-generated WSM particles in this study were ranged from 1.04 to 1.09 with a mean
of 1.06 ± 0.02 as shown in Table 1. We also found a moderate correlation between χ and WSOM (R2 = 0.46),
indicating that the presence of water-soluble organic components with low deliquescence RH might induce
this microstructural rearrangement of WSM particles. Jung et al. [2011] reported that high abundance of WSOM
partially contributes to microstructural rearrangement of Ulaanbaatar PM2.5 aerosols. Similar results were
observed by Gysel et al. [2004].
The measured growth factors at 90% RH of all WSM particles, g(90%)WSM, during hydration experiment ranged
from 1.1 to 1.47 with a mean of 1.25 ± 0.12 (see Table 1 and Figure 2). The measured g(90%) of Tanzanian
aerosols are comparable to that of biomass burning organic species (1.26) such as levoglucosan and mannosan
[Chan et al., 2005; Mochida and Kawamura, 2004] and Amazonian biomass burning aerosols (1.05–1.24) [Zhou
et al., 2002; Swietlicki et al., 2008] as well as continental aerosols (1.27 ± 0.03) collected at Storm Peak Laboratory
(SPL) in the Park Range of northwestern Colorado [Hallar et al., 2013]. The H-TDMA derived kappa of WSM
(κWSM) ranged from 0.04 to 0.24 with a mean of 0.11 ± 0.07 during the study period. Several studies have
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Figure 1. Changes in hygroscopic growth factor of the water-soluble matter (g(RH)WSM) of aerosol particles nebulized from
water extracts of aerosol samples collected at Morogoro, Tanzania, as a function of RH under hydration and dehydration
experiments. The g(RH) of WSM can be calculated from the ratio of the particle diameter at elevated RH relative to that of
the initial dry particle RH. The g(RH) of WSMs below 30% RH were removed for all samples during hydration experiment due
to the restructuring of aerosol particles.
focused on the biomass burning particles emitted from open vegetation fires [Kaufman et al., 1998; Lee et al.,
2006; Vestin et al., 2007; Petters et al., 2009]. Andreae and Rosenfeld [2008] reported that biomass burning
particles have wide κ values ranging from 0.01 to 0.55 for grass burning. Dusek et al. [2011] reported an average
κ of 0.08 for 100 nm particles for freshly emitted particles from the burning of different hard and soft woods
under controlled laboratory conditions. DeMott et al. [2009] derived κ values (0.02–0.56) from H-TDMA data
from biomass combustion particles. Interestingly, the mean of κWSM (0.11) in this study is comparable to that of
smoke aerosols dominated by carbonaceous species, which typically had a unimodal growth factor with mean
κ of 0.1 (range: 0–0.4) [Carrico et al., 2010].
Table 1. Dynamic Shape Correction Factor ( χ), Water-Soluble and -Insoluble Fractions of PM2.5, Hygroscopic Growth Factor (g(90%)), and Hygroscopicity Parameter
(κ) of Particles Nebulized From Water-Soluble Matter (WSM) Extracted From PM2.5 Aerosol Samples Collected at Tanzania
Hygroscopic Growth of Water-Soluble Matter
Fraction of PM2.5
Sample ID
TN06 JUN11
TN07 JUN11
TN09JUN11
TN28JUL11
TN29JUL11
TN02AUG11
TN05AUG11
TN06AUG11
TN07AUG11
Average
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Dynamic Shape Correction Factor ( χ)
Water-Soluble
Water-Insoluble
g(90%)WSM
κWSM
1.07
1.04
1.05
1.04
1.07
1.07
1.05
1.05
1.05
1.05
0.33
0.28
0.29
0.35
0.53
0.42
0.24
0.24
0.25
0.32
0.67
0.72
0.71
0.64
0.47
0.58
0.76
0.76
0.75
0.68
1.30
1.14
1.20
1.10
1.41
1.47
1.28
1.16
1.22
1.25
0.13
0.05
0.08
0.04
0.20
0.24
0.12
0.06
0.09
0.11
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As seen in Table 1, water-soluble fraction
ranged from 0.24 to 0.53 with a mean of 0.32
for PM2.5 aerosols, whereas the waterinsoluble fraction varied from 0.47 to 0.76
with a mean of 0.68. We found a negative
correlation between measured g(90%) and
water-insoluble fraction with a relatively
strong correlation coefficient (R2 = 0.56) (not
shown as a figure). These results suggest that
water-insoluble fractions probably depress
the hygroscopicity of ambient aerosols. It is
important to emphasize that the reported g
(90%) and kappa values here are derived
Figure 2. Temporal variations of g(90%)WSM and hygroscopicity paraonly for the water-soluble fraction, not for
meter (κWSM) during the study period at Morogoro, Tanzania.
bulk PM2.5 aerosols over the sampling site.
3.2. A Link Between the Hygroscopicity and Chemical Composition
In order to assess the link between the hygroscopicity and chemical composition, we performed
regression analysis between g(90%) WSM and PM 2.5 mass fractions of chemical species as shown in
Figure 3. Figure 3a clearly demonstrates that the growth factors are largely affected by biomass
burning products due to a good correlation between g(90%) WSM and levoglucosan (LG), a tracer of
biomass burning. Although it is also clear that the two data points on the top right of the Figures 3g
and 3h are isolated from others (e.g., NH4 + and K+), they are characterized by high growth factors and
high abundances of levoglucosan as seen by the superimposed concentrations, indicating that those
two data points are strongly influenced by biomass burning. Therefore, Figure 3 clearly demonstrates
an importance of biomass burning on the hygroscopicity of aerosols, although there is a difference in
g(90%) WSM between the cluster on the bottom left and other two points on the top right as stated
above. Further, we performed t test between g(90%) WSM and PM2.5 mass fractions of chemical species.
The results (two-tailed p value) are reported for each fraction in Figure 3, confirming that the overall
difference is considered to be statistically significant (p ≤ 0.05) for all the mass fractions except for NH4+,
which is related to biomass mass burning products.
We found relatively strong correlations between g(90%)WSM and biomass burning tracers such as K+ (R2 = 0.61)
and Cl (R2 = 0.54), implying that water-soluble inorganics from biomass burning are important for the growth
factor of total water-soluble matter over Tanzania during the study period. Moreover, the g(90%)WSM strongly
correlates with PM2.5 mass fractions of total diacids (R2 = 0.76, see Figure 3b) and LG (R2 = 0.67, see
Figure 3a); the latter compound is produced by the pyrolysis of cellulose and hemicellulose and has been
used as a unique molecular tracer of biomass burning aerosols [Simoneit et al., 1999; Fraser and Lakshmanan,
2000]. These results suggest that although inorganic fractions may be the most important factor to control
the hygroscopicity, it is noteworthy that biomass burning organic species can significantly contribute to the
hygroscopicity over Tanzania aerosols based on the strong linkage between g(90%)WSM and mass fractions of
total diacids and LG in PM2.5.
SO42 showed no correlation with g(90%)WSM, but NH4+ showed the positive correlation. We found that NH4+ is
significantly correlated with organic acids (R2 = 0.80, not shown as a figure) but not with SO42 (R2 = 0.02),
suggesting that ammonium is involved with the formation of organic acid salts by heterogeneous reactions.
This is further supported by the study of Reid et al. [1998], who found that NH4+ was highly correlated with
oxalate and other organic species rather than SO42 in the regional hazes dominated by biomass burning
smoke in Brazil. Similar results were reported by Reid et al. [2005]. We also calculated ionic mass balance; the
results showed that aerosols over Tanzania are alkaline. The ratio of cation to anion equivalents ranged 9–24
with a mean of 10.9 ± 4.9. The t test results showed that the overall difference is considered to be statistically
significant. The two-tailed p value is <0.0001. Therefore, it is very likely that the relation between SO42 and g
(90%)WSM obtained in this study may be due to the formation of less hygroscopic K2SO4 particles during
atmospheric aging [Magi and Hobbs, 2003], which decreases the hygroscopic growth of aged aerosols. It is
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Journal of Geophysical Research: Atmospheres
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Figure 3. Scatter plots between g(90%)WSM versus PM2.5 mass fractions of different chemical species in the PM2.5 aerosols
collected at Morogoro, Tanzania. The color scale indicates intensity of biomass burning, represented by the concentrations
of levoglucosan (LG) (a tracer of biomass burning).
further supported by a moderate correlation (R2 = 0.33) between mass fractions of WSOC and g(90%)WSM. These
points will be discussed thoroughly as below.
Rural sites are generally characterized by aged air masses that show an internally mixed aerosol, because
atmospheric processes tend to gradually transform the aerosols toward a state of internal mixing. This mixing
state can vary because rural areas may experience both aged and freshly formed aerosols. It is traditionally
documented that atmospheric processes (aging) tend to make atmospheric aerosols more hygroscopic.
However, it is not clear in the case of biomass burning aerosols. Field studies have shown that aged biomass
burning particles are less hygroscopic than fresh biomass burning particles [Chan et al., 2005] due to the
conversion of more hygroscopic KCl in a fresh biomass burning particles to less hygroscopic K2SO4 or KNO3 in
aged biomass burning particles [Posfai et al., 2003; Li et al., 2003]. Recently, Engelhart et al. [2012] quantified the
hygroscopic properties of particles freshly emitted from biomass burning and after several hours of photochemical
aging in smog chamber for 12 biomass fuels commonly burned in North American wildfires. They found that
photochemical aging reduces the variability of biomass burning hygroscopicity parameter (CCNκ).
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The conversion of freshly emitted organic species such as
LG (g(90%) = 1.38, Mochida and Kawamura [2004]) into
simple organic acids (C2-C5) is also possible during
photochemical aging [Gao et al., 2003]. It is known that
oxalic acid, whose growth factor g(85%) is 1.03 [Peng
et al., 2001], is an oxidation product of many organic
compounds with a relatively low vapor pressure and
therefore quite common in the organic fraction of
aerosols [Buchholz and Mentel, 2008]. Oxalic acid is the
most abundant dicarboxylic acid detected in biomass
burning particles [Graham et al., 2002]. Therefore, it is
reasonable to assume that the formation of potassium
oxalate (K2C2O4) through heterogeneous reactions may
depress the hygroscopicity in Tanzania because K2C2O4 is
a less water-soluble salt [Buchholz and Mentel, 2008].
+
Figure 4. Scatter plots of (a) potassium (K ) versus oxalic
+
acid (C2di) and (b) potassium (K ) versus sulfate (SO4 )
during the study period. The data point in the parentheses was not included in the regression analysis, because
this data point is not statistically significant.
In order to better interpret the importance of the above
mentioned evidences (formation of K2SO4 and K2C2O4)
in our samples, we performed a regression analysis
between the concentration of potassium and oxalic acid
(C2di), and potassium and sulfate. The results are plotted
in Figure 4. We found an excellent correlation between
potassium versus C2di (R2 = 0.96) and potassium versus
sulfate (R2 = 0.90). These results support that potassium
oxalate and potassium sulfate are formed by chemical
aging during atmospheric transport of aerosols [Posfai
et al., 2003; Chan et al., 2005].
Further, Martin et al. [2013] have recently studied the
hygroscopic properties of fresh and aged biomass
burning particles from beach wood under controlled
laboratory conditions in several smog chamber
experiments. They found that κ of fresh wood
combustion generally increases with aging, whereas in smoldering phase experiment, κ decreased with
increasing time. Therefore, it is important to note that biomass burning fuel characteristics, as well as the
combustion conditions such as flaming or smoldering, are of great importance to the hygroscopicity of
biomass burning particles [Carrico et al., 2010].
Figure 5. Temporal variation of g(90%)WSM along with LG/PM2.5 (%),
WSOC/OC, and LG/OC (%) for the Tanzanian aerosols (PM2.5).
BOREDDY ET AL.
©2014. American Geophysical Union. All Rights Reserved.
Figure 5 shows the temporal variation of
g(90%)WSM along with LG/PM2.5 mass
ratio and organic carbon mass ratios in
Tanzania during the sampling period.
Highest g(90%)WSM were observed on
TN02AUG11 followed by TN29JUL11 and
lowest on TN28JUL11. Moderate
Resolution Imaging Spectroradiometer
(MODIS) satellite image shows that severe
fire plumes were observed during 29 July
to 7 August 2011 [Mkoma and Kawamura,
2013; Mkoma et al., 2013] and is shown in
Figure 6a as a typical result. We
performed the Hybrid Single-Particle
Lagrangian Integrated Trajectory
(HYSPLIT) model and the FIRMS (fire
information for resource management
12,240
Journal of Geophysical Research: Atmospheres
10.1002/2014JD021546
system) for each sample during the study
period in order to check how far or close
biomass burning fire plumes were to
the sampling site and how much each sample
was affected. To make the point clearer, we
also embedded the g(90%), WSOC, and LG
concentrations for each sample on the fire
map. Please see Figures S1 and S2 in the
supporting information. As clearly seen in
Figure S1, biomass burning fire plumes are
very intensive during 6 June and 29 July to 5
August 2011 and are very close to the
sampling site. The mass ratios of LG/PM2.5 and
LG/OC showed similar variation with g(90%)
WSM but not with WSOC/OC, indicating that a
strong link should exist between the g(90%)
WSM and the amount of biomass burning
organics in Tanzanian aerosols. Figure 5 also
reveals that, based upon mass ratios, g(90%)
WSM over Tanzania significantly depend on the
amounts of biomass burning organic species
and partially on the amounts of WSOC
(Figure 3c). This is further supported by the
backward air mass trajectory analysis.
(a)
(b)
In order to check the influence of biomass
burning fire plumes on the hygroscopicity, we
computed 5 day backward air mass
trajectories at an altitude of 200 m for every
24 h for the samples TN09JUN11 (background
sample) and TN29JUL11 (intensive biomass
burning sample) [Draxler and Rolph, 2012],
whose results are shown in Figures 6b and
6c, respectively. The air mass trajectory
sectors for both the samples showed similar
transport pathways and source regions, i.e.,
originated from the Indian Ocean via the
passage over the lands of Madagascar,
Tanzania, and finally arrived over the
sampling site (see Figure S2 in the
supporting information for the all samples
during sampling study period). Therefore, it
should be noted from Figures 6b and 6c
that the observed hygroscopicity over
(c)
Figure 6. (a) Ten day MODIS fire map detected by
MODIS satellite from NASA website over southern
and eastern Africa and Madagascar during 30 July to
7 August 2011. A typical 5day backward air mass
trajectories at an altitude of 200 m for every 24 h for
the samples (b) TN09JUN11 (background sample)
and (c) TN29JUL11 (intensive biomass burning
sample) using the HYSPLIT model from NOAA/Air
Resources Laboratory.
BOREDDY ET AL.
©2014. American Geophysical Union. All Rights Reserved.
12,241
Journal of Geophysical Research: Atmospheres
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Table 2. Calculated Volume Fractions of Water-Soluble Inorganic Matter (WSIM), Water-Soluble Organic Matter
(WSOM), Growth Factor of Water-Soluble Organic Matter (g(90%)WSOM), and Hygroscopicity Parameter (κWSOM) for
the Tanzanian Aerosols
Volume Fractions
Sample ID
TN06 JUN11
TN07 JUN11
TN09JUN11
TN28JUL11
TN29JUL11
TN02AUG11
TN05AUG11
TN06AUG11
TN07AUG11
Average
a
Water-Soluble Organic Matter
WSIM
WSOM
g(90%)WSOM
κWSOM
0.17
0.27
0.41
0.30
0.35
0.39
0.39
0.26
0.33
0.32
0.83
0.73
0.59
0.70
0.65
0.61
0.61
0.74
0.67
0.68
1.22
1.03
a
≤1.0
a
≤1.0
1.17
1.25
1.10
a
≤1.0
a
1.00
1.16
0.10
0.01
a
0.00
a
0.00
0.07
0.11
0.04
a
0.00
a
0.00
0.06
Not included in average.
the sampling site is mainly controlled by the enhanced biomass burning or local field burning, but not
on the source regions. We also conclude that local biomass burning plays an important role in
controlling the hygroscopicity of atmospheric particles and the air quality over Tanzania, East Africa.
3.3. Hygroscopic Growth of WSOM
Generally, hygroscopic measurements of aerosol particles are conducted for the mixture of water-soluble
inorganic ions and organic components. Therefore, it is very difficult to directly determine the hygroscopicity
of WSOM in atmospheric aerosols. The additional water, which cannot be explained by the water-soluble
inorganic matter (WSIM), can be then attributed to the WSOM. The g(RH) of WSOM can be estimated as
follows [Seinfeld and Pandis, 1998; Cruz and Pandis, 2000; Jung et al., 2011]:
!1
3 3
gðRHÞWSM 3 ðεWSIM gðRHÞWSIM Þ
gðRHÞWSOM ¼
(5)
εWSOM
where εWSIM and εWSOM represent the volume fractions of WSIM and WSOM in the WSM, respectively (Table 2).
The g(RH)WSM and g(RH)WSIM represent the growth factors of water-soluble matter (measured by H-TDMA) and
water-soluble inorganic matter (calculated using the ISORROPIA II model), respectively.
BOREDDY ET AL.
Figure 7. Temporal variations of g(90%)WSOM during the study period
at Morogoro, Tanzania. We also showed g(90%)WSM for comparison.
“ISORROPIA II [Nenes et al., 1998, 1999;
Fountoukis and Nenes, 2007] is a
computationally efficient and rigorous
thermodynamic equilibrium model, which
exhibits robust and rapid convergence under all
aerosol types with high computational speed.
This model treats the thermodynamics of K+–
Ca2+–Mg2+–NH4+–Na+–SO42–NO3–Cl–H2O
aerosol system, but organic species are not
taken into account. It applies the ZdanovskiiStokes-Robinson (ZSR) equation to estimate
the aerosol liquid water content with the
input of aerosol chemical composition
measured by IC [Mkoma et al., 2013]. The
temperature was set to 294 K. The growth
factors are derived from the cubic root of the
wet particle volume (Vdry + Vw) over the cubic
root of the dry particle volume (Vdry).
The ISORROPIA II model assumes that the
©2014. American Geophysical Union. All Rights Reserved.
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Journal of Geophysical Research: Atmospheres
10.1002/2014JD021546
aerosol curvature effect can be ignored and water uptake of aerosols has no effect on the ambient vapor
pressure.”
The retrieved g(90%)WSOM for all the samples during the study period is shown in Figure 7. We clearly found
that the obtained g(90%)WSOM >1 for the biomass burning influenced samples. The rest of the samples
showed g(90%)WSOM ≤1, indicating that biomass burning is an important factor to control the hygroscopic
growth over the sampling site. The H-TDMA derived hygroscopicity parameters of WSOM, that is, κWSOM, ranged
from 0.01 to 0.11 with a mean of 0.06 ± 0.04 (Table 2). The g(90%)WSOM varied between 1.03 and 1.25 with a mean
value of 1.16 ± 0.05 (Table 2). The mean value (1.16) is slightly higher than of the HULIS (Humic-Like Substances) of
K-Pushza, Hungary, (1.08–1.17) reported by Gysel et al. [2004] and lower than biomass burning organics such as
levoglucosan (1.29), mannosan (1.28), and galactosan (1.27) [Mochida and Kawamura, 2004; Chan et al., 2005]. Jung
et al. [2011] reported high growth factors of WSOM (g(85%) = 1.11–1.35; mean = 1.22 ± 0.08) for Ulaanbaatar
aerosols from Mongolia during cold winter in 2007. However, the present results are comparable with those of
background continental PM2.5 aerosol particles generated from isolated WSOC (1.10 ± 0.03) collected at Storm
Peak Laboratory (SPL) in the Park Range of northwestern Colorado [Hallar et al., 2013].
Based on the observed g(90%)WSM and significant link between the PM2.5 fraction of chemical species and
the growth factor of WSM as well as WSOM, we emphasize that Tanzania PM2.5 aerosols contained high
abundance of biomass burning products that significantly contribute to the increased hygroscopicity during
the intensive biomass burning season. Moreover, these results are supported by the studies of Mkoma et al.
[2013a, 2013b], which demonstrated that emissions from mixed biomass and biofuel burning activities
largely influence the air quality in Tanzania, East Africa.
4. Conclusions
In this study, we investigated the hygroscopic properties of water-soluble fraction from biomass burning
derived PM2.5 aerosols collected at a rural background site in Tanzania, East Africa, using H-TDMA.
The measured g(90%)WSM and κWSM at Tanzania ranged from 1.10 to 1.47 and 0.04 to 0.24 with mean of 1.25
± 0.12 and 1.01 ± 0.06, respectively. These results are comparable to those of previously reported biomass
burning aerosols. The link between g(90%)WSM and PM2.5 mass fractions of chemical compositions reveals
that although the inorganic fractions appear to be important factor in the hygroscopicity, biomass burning
organic species play an important role in the hygroscopicity of Tanzanian PM2.5 aerosols. WSOC may be
partially responsible for the growth factors. We suggest that the formation of potassium oxalate (K2C2O4)
through heterogeneous reaction between organic and inorganic species could reduce the hygroscopic
growth of aged aerosol particles. The present study also suggests the formation of less hygroscopic K2SO4
particles during atmospheric aging of biomass burning aerosols.
The mean value of g(90%)WSOM was 1.16 ± 0.12 ranging between 1.03 and 1.25. These values are well correlated
with total diacids/PM2.5 mass ratios with a correlation coefficient of R2 = 0.65 (not shown in the figure). The
present study demonstrates that based on the strong link between g(90%)WSM and PM2.5 mass fractions of
chemical species, freshly emitted biomass burning species can significantly contribute to the hygroscopicity of
Tanzanian aerosols during the sampling period. The obtained g(90%)WSOM in this study would be very important
for radiative forcing and climate model studies because there are no studies on growth factors of organic
aerosols in East Africa where biomass burning is very common but air pollution information is very limited.
Acknowledgments
This study was in part supported by the
Japan Society for the Promotion of Science
(grants-in-aid 1920405 and 24221001). The
authors gratefully acknowledge the NOAA
Air Resources Laboratory for the provision
of the HYSPLIT transport dispersion model
(http://www.arl.noaa.gov/ready/hysplit4.
html) and the Fire Information for Resource
Management System (FIRMS) (http://firms.
modaps.eosdis.nasa.gov/firemap/).
BOREDDY ET AL.
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