Based on source-specific saccharide tracers, the characteristics
of biomass burning (BB) and biogenic emissions of saccharides were
investigated in three rural sites at Lincang, which is 65 % covered with forest in the southwest border of China. The total saccharides accounted for
8.4 ± 2.7 % of organic carbon (OC) and 1.6 ± 0.6 % of
PM2.5. The measured anhydrosugars accounted for 48.5 % of total
saccharides, among which levoglucosan was the most dominant species. The
high level of levoglucosan was both attributed to the local BB activities
and biomass combustion smoke transported from the neighboring regions of
Southeast Asia (Myanmar) and the northern Indian subcontinent. The measured
mono- or disaccharides and sugar alcohols accounted for 24.9 ± 8.3 %
and 26.6 ± 9.9 % of the total saccharides, respectively, and both
proved to be mostly emitted by direct biogenic volatilization from plant
material or surface soils rather than byproducts of polysaccharide breakdown
during BB processes. Five sources of saccharides were resolved by
non-negative matrix factorization (NMF) analysis, including BB, soil
microbiota, plant senescence, airborne pollen, and plant detritus with
contributions of 34.0 %, 16.0 %, 21.0 %, 23.7 %, and 5.3 %,
respectively. The results provide information on the magnitude of
levoglucosan and contributions of BB, as well as the characteristic of
biogenic saccharides, at the remote sites of southwest China, which can be
further applied to regional source apportionment models and global climate
models.
Introduction
Biomass burning (BB) and biogenic aerosols are thought to play important
roles in air quality, human health, and climate through direct or indirect
effects (Jacobson et al., 2000; Christner et al., 2008; Pöschl et al., 2010;
Després et al., 2012; Chen et al., 2017; Tang et al., 2019). Atmospheric
saccharide components have been extensively reported to originate from
natural or anthropogenic biomass burning (BB), suspended soil or dust, and
primary biological aerosol particles (PBAPs), e.g., fungal and fern spores,
pollens, algae, fungi, bacteria, plant debris, and biogenic secondary
organic aerosol (SOA) (e.g., Rogge et al., 1993; Graham et al., 2003;
Jaenicke, 2005; Medeiros et al., 2006; Elbert et al., 2007; Fu et al.,
2013). As one of the major classes of water-soluble organic compounds,
saccharides in atmospheric aerosols have been detected over urban areas,
forests, mountains, and remote marine regions (Pashynska et al., 2002; Yttri
et al., 2007; Fu et al., 2009; Burshtein et al., 2011; Jia and Fraser, 2011;
Chen et al., 2013; Pietrogrande et al., 2014; Li et al., 2016a, b). It has
been reported that saccharides account for 13–26 % of the total organic
compound mass identified in continental aerosols and up to 63 % of oceanic
aerosols (Simoneit et al., 2004).
Levoglucosan and related anhydrosugar isomers (mannosan and galactosan),
produced from pyrolysis of cellulose and hemicellulose, are considered to be
relatively stable in the atmosphere (Schkolnik et al., 2005; Puxbaum et al.,
2007); thus, have been recognized as specific molecular markers for BB
source emissions (Simoneit et al., 1999; Fraser and Lakshmanan, 2000;
Sullivan et al., 2014). However, some studies have
challenged this knowledge and proved that levoglucosan alone was unsuitable
to be a distinct marker for BB in various regions and periods. This is because there
is evidence that levoglucosan was also emitted from non-BB sources (Wu et
al., 2021), such as coal burning (Rybicki et al., 2020; Yan et al., 2018),
open waste burning (Kalogridis et al., 2018), incense burning (Tsai et al.,
2010), and food cooking (Reyes-Villegas et al., 2018). It was reported that
the levoglucosan emission contribution of BB sources ranged from 21.3 % to
95.9 % (Wu et al., 2021). Current studies in China have reported values of 2.6–289.1 and 11.6–1803.1 ng m-3, respectively,
over Beijing and Wangdu in summer (Yan et al., 2019), 2.4–1064.1 ng m-3 over Shanghai all year round (Xiao et al., 2018), 15.6–472.9 ng m-3 over Guangzhou (Zhang et al., 2010), 21.1–91.5 ng m-3 over
Hong Kong (Sang et al., 2011), 60.2–481.9 ng m-3 over Xi'an (Yang et
al., 2012), 36.0–1820.9 ng m-3 over Chengdu (Yang et al., 2012), and
10.1–383.4 ng g-1 dry weight in cryoconites over the Tibetan Plateau
(Li et al., 2019). In north China, the high concentration of levoglucosan
was a serious problem due to the drastic enhancement of coal and BB for
house heating in winter and autumn (Zhang et al., 2008; Zhu et al., 2016).
The BB pollution might be exacerbated under unfavorable meteorological
conditions, such as in the Chengdu Plain (Chen and Xie, 2014). In general,
BB with a notable contribution to organic carbon (OC) was an important source of fine
particulate matter in China (Zhang et al., 2008; Cheng et al., 2013; Chen et
al., 2017). Controls on BB could be an effective method to reduce pollutant
emissions. Recently, a study reported that total levoglucosan emissions in China
exhibited a clear decreasing trend from 2014 (145.7 Gg) to 2018 (80.9 Gg)
(Wu et al., 2021), suggesting BB activities might be reduced in China.
Saccharide compounds including a variety of primary saccharides
(monosaccharides and disaccharides) and sugar alcohols (reduced sugars) have
been measured to estimate the contribution of biogenic aerosols, including
fungi, viruses, bacteria, pollen, and plant and animal debris
(Simoneit et al., 2004; Jaenicke et al., 2007). For instance, arabitol and
mannitol have been proposed as biomarkers for airborne fungal spores (Bauer
et al., 2008; Zhang et al., 2010; Holden et al., 2011; Liang et al., 2013a,
b), because both of them can function as storage or transport carbohydrates
to regulate intracellular osmotic pressure (Bauer et al., 2008). Glucose and
sucrose are thought to originate from natural biogenic detritus, including
numerous microorganisms, plants, and animals (Simoneit et al., 2004;
Tominaga et al., 2011). As the oxidation products of isoprene, methyltetrols
(including 2-methylthreitol and 2-methylerythritol) have been suggested as
tracers of isoprene-derived SOA (Claeys et al., 2004; Kleindienst et al.,
2007; Ding et al., 2016). In a previous study, the contributions of fungal
spores to OC were estimated to be 14.1 ± 10.5 % and 7.3 ± 3.3 %, respectively, at the rural and urban sites of Beijing (Liang et al.,
2013b). Airborne pollen and fungal spores contributed 12 %–22 % to the
total OC in ambient aerosols collected in Toronto (Womiloju et al., 2003).
Jaenicke (2005) found that PBAPs could account for 20 %–30 % of the
total atmospheric PM (>0.2 mm) from Lake Baikal (Russia) and
Mainz (Germany). However, studies on quantifying the abovementioned biogenic
aerosol contributions to ambient aerosol are inadequate.
Lincang, located on the southwest border of China, is a traditional
agricultural area of Yunnan Province, where planting large areas of tea,
sugar cane, rubber, macadamia nuts, etc. is common. It is the largest production base of
black tea and macadamia nuts in China. Referring to the official website of
Lincang Municipal People's Government, the forest coverage of Lincang
reaches 65 %. It has a wide variety of plant species and has six nature
reserves covering an area of ∼ 222 000 ha, accounting
for 8.56 % of the total area. As a residential area for ethnic minorities,
Lincang has unique culture, humanity and living habits. The proportion of
houses that use wood burning for cooking is very high in villages in the vicinity as well as in a large area of Southeast Asia, and forest fires frequently
occur in this area, especially in the dry season (March–April). These
facts imply that there are abundant biogenic aerosols in Lincang, and BB pollution
may be an essential potential source of air pollution. However, little
information on the magnitude of biogenic and BB tracers in this area is
available. The contributions of biogenic aerosol and BB, as well as BB
types, are poorly understood.
In this study, the sampling was conducted from 8 March to 9 April 2019 at
three mountaintop sites of Lincang, which is an ideal site for investigating
the BB emission characteristics. BB tracers, including anhydrosugars and
K+, as well as biogenic aerosol tracers (primary saccharides and sugar
alcohols), were measured to gain the information on source and contributions
of BB and biogenic emissions to PM2.5 over the rural Lincang. This
study would be useful and valuable for providing reliable information on
sources and magnitudes of saccharides involving rural BB and biological
emissions in China.
Experimental sectionAerosol sampling
PM2.5 samples were simultaneously collected on three mountaintop sites
in Lincang: Datian (24.11∘ N, 100.13∘ E; 1960 m a.s.l.), Dashu
(24.12∘ N, 100.11∘ E; 1840 m a.s.l.), and Yakoutian (24.12∘ N, 100.09∘ E; 1220 m a.s.l.), which are located ∼ 300 km west of Kunming (the capital
of Yunnan Province in China) and ∼ 120 km east from the Burma
border (shown in Fig. S1 in the Supplement). These sites are surrounded by massive mountains
and scattered villages without obvious nearby traffic or major industrial
emissions. Each sampling was performed over a 23.5 h period every day, and
was collected on quartz by high-volume air samplers (Thermo) equipped with a
size-selective inlet to sample PM2.5 at a flow rate of 1.13 m3 min-1. Altogether, 91 samples were collected.
Quartz filters (Whatman, 20.32 × 25.40 cm) were prebaked at 550 ∘C for 4 h in a muffle furnace to remove organic material, and
they were then stored in prebaked aluminum foils. The samples were stored at
about -20 ∘C in a refrigerator until analysis. Field blanks
were collected by mounting filters in the sampler without air flow to
replicate the environmental exposure. The data reported were corrected by
the blanks at the sampling sites.
Measurements
The concentrations of OC and elemental carbon (EC) were measured using a
multiwavelength carbon analyzer (DRI model 2015; Aerosol Inc., USA).
Typically, a 0.58 cm2 punch of the filter was placed on a boat inside
the thermal desorption chamber of the analyzer, and then stepwise heating
was applied. Carbon fractions were obtained following the Interagency
Monitoring of PROtected Visual Environments (IMPROVE-A) thermal/optical
reflectance (TOR) protocol (Chow et al., 2007). Replicate analyses were
conducted once every 10 samples. A blank sample was also analyzed and used to
correct the sample results.
A punch (4.7 cm2) of each quartz filter was ultrasonically extracted
with 10.0 mL deionized water (resistivity = 18.2 MΩ cm-1) for 40 min. The
aqueous extracts were filtrated through syringe filters (PTFE, 0.22 µm)
to remove insoluble materials. Ion chromatography (Metrohm, Switzerland)
coupled with Metrosep C6-150 and A6-150 columns was used to detect
water-soluble ions (Cl-, NO3-, PO4-,
SO42-, Na+, NH4+, K+, Mg2+, and
Ca2+) with a detection limit (DL) range of 0.001–0.002 µg m-3.
Five saccharide alcohols (glycerol, erythritol, inositol, arabitol, and
mannitol) and five primary saccharides (fructose, glucose, mannose, sucrose,
and trehalose), together with three anhydrosugars (levoglucosan, mannosan,
and galactosan), were quantified by an improved high-performance
anion-exchange chromatograph coupled with a pulsed amperometric detector
(Engling et al., 2006; Caseiro et al., 2007; Zhang et al., 2013). This
method developed by Engling et al. (2006) was validated to be a powerful
method for the detection of carbohydrates without derivatization techniques,
and it has been successfully applied to the atmospheric tracers (e.g., Zhang
et al., 2010; Holden et al., 2011; Liang et al., 2013a, b; Li et al., 2016a,
b; Kalogridis et al., 2018; Yan et al., 2018). The separation of the
saccharides was performed on an ion chromatograph (Metrohm, Switzerland)
equipped with a Metrosep Carb 4–250 analytical column and a guard column.
The aqueous eluent of sodium hydroxide and sodium acetate was pumped by a
dual-pump module at a flow rate of 0.4 mL min-1. The low concentration
of 50 mM sodium hydroxide and 10 mM sodium acetate (eluent A) was applied to
pump 1, while the high concentration of 250 mM sodium hydroxide and 50 mM
sodium acetate (eluent B) was applied to pump 2. The gradient generator was
set as 0–10 min, 100 % of eluent A; 10–20 min, 50 % of eluent A and
50 % of eluent B; 20–50 min, 100 % of eluent B; and 50–60 min, 100 % of
eluent A for equilibration. The extraction efficiency of this analytical
method was determined to be better than 90 % based on analysis of quartz
filters spiked with known amounts of mannitol. The method DL of the referred
carbohydrate compounds was 0.005–0.01 mg L-1. All carbohydrate
species were below detection limits in the field blanks.
Other data
The meteorological parameters, including temperature (T), relative humidity
(RH), solar irradiation (W m-2), and rainfall (mm) were obtained from
the Physical Sciences Laboratory of NOAA (https://psl.noaa.gov, last access: 8 September 2020). The temporal changes in meteorological variables over the observation sites
during the sampling periods are shown in Fig. S2.
In order to characterize the origin and transport pathway of the air masses
to the sampling sites, 72 h back-trajectories of the aerosol were calculated
using the HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT)
model developed by NOAA/ARL (Draxler and Hess, 1998) via the NOAA ARL READY
website (http://ready.arl.noaa.gov/HYSPLIT.php, last access: 6 September 2020) with an
endpoint height of 1500 m. To investigate the influence of BB emissions,
fire pixel counts were obtained from Moderate Resolution Imaging
Spectroradiometer (MODIS) observations on NASA satellites (https://earthdata.nasa.gov/, last access: 3 September 2020).
Statistical analysis
A Pearson's correlation test was performed using the statistical product and
service solutions software for the dataset containing ambient concentrations
of the measured saccharides, inorganic ions, and solar irradiation.
Non-negative matrix factorization (NMF) analysis was utilized to resolve
potential emission source and estimate their contributions to atmospheric
saccharides. NMF introduced by Lee and Seung (1999) was similar to positive
matrix factorization (PMF). Both methods find two matrices (termed the
contribution matrix of W and the source profile matrix of H) to reproduce
the input data matrix (V) using the factorization approach (V=WH) as a
positive constraint (W≥0 and H≥0). However, PMF forces the
negative factors to be positive, but the NMF method only retains non-negative
factors. NMF minimizes the conventional least-squares error and the
generalized Kullback–Leibler divergence (Shang et al., 2018). Therefore, the
results obtained from NMF are more responsive to the original
characteristics of input dataset and fewer factors will be
extracted (Zhang et al., 2019). Half of the DL was used for the value below
the detection limit. In this study, galactosan, mannose, and inositol were
excluded, because their concentration in most samples was below the DL.
Concentrations of the other 10 saccharide species for a total of 91 samples
were subjected to NMF analysis. The uncertainties in NMF analysis were
estimated as 0.3 plus the analytical detection limit according to the method
by Xie et al. (1999). The constant 0.3 corresponding to the log (geometric
standard deviation) was calculated from the normalized concentrations for
all measured species, and it was used to represent the variation of
measurements.
Results and discussionSaccharide concentration and composition
The temporal variations of PM2.5 mass, OC, EC, and various saccharides
measured in all samples are shown in Fig. 1. A statistical summary of all
the data is listed in Table S1. During the sampling periods, the PM2.5
mass concentrations ranged between 13.7 and 87.8 µg m-3 with an
average value of 41.8 µg m-3. The concentrations of OC and EC,
respectively, were in the ranges of 2.5–22.4 and 0.3–4.3 µg m-3
with average values of 8.4 and 1.7 µg m-3. OC accounted for 19.9±3.7 % of total PM2.5 mass. The ambient concentrations of the
total saccharides varied between 244.5 and 1291.6 ng m-3 with an
average value of 638.4 ng m-3. The total saccharides quantified in
PM2.5 accounted for 8.4 ± 2.7 % (range: 3.8 %–20.6 %) of
the OC and accounted for 1.6 ± 0.6 % (range: 0.6 %–3.0 %) of
the PM2.5. Figure 2 presents the average concentration levels of 12
measured saccharide compounds, categorized into anhydrosugar, mono- or disaccharide, and sugar alcohol, as well as the relative contribution of these
saccharides for all samples. The values for each site are shown in Fig. S3.
Temporal variations of OC, EC, PM2.5 and total sugars at the
three sites during the sampling periods.
The absolute concentration (bar chart) and the relative
contribution (pie chart) of various saccharide compounds during the sampling
periods.
Anhydrosugars
The average concentrations of levoglucosan and mannosan were 287.7 and 31.6 ng m-3, respectively, with respective ranges of 95.6–714.7 and
0–134.7 ng m-3 for all 91 samples. Galactosan was detected only in six
samples, with a range of 2.5–5.5 ng m-3. The anhydrosugars accounted
for 50.1 % of the total measured saccharides. Levoglucosan was the most
dominant species among all the saccharides. The average levoglucosan
concentration in this study was comparable to the value at urban Beijing
collected in spring of 2012 (above 200 ng m-3) (Liang et al., 2016) and
at urban Xi'an collected in winter of 2015 (268.5 ng m-3) (Wang et al.,
2018). It was higher than the value at the rural Tengchong mountain site (193.8 ng m-3) (Sang et al., 2013), at urban Shanghai collected in spring of
2012 (66.0 ng m-3) (Li et al., 2016a), and at urban Hong Kong collected
in spring of 2004 (36.0 ng m-3) (Sang et al., 2011), as well as at
urban Beijing collected in summer of 2013 (49.4 ng m-3) (Yan et al.,
2019), but it was lower than that at a rural site in Xi'an (0.93 mg m-3)
(Zhu et al., 2017) and at a rural site in eastern central India (2258 ng m-3) (Nirmalkar et al., 2015). During the observation period, several
instances of elevated levoglucosan occurred, peaking on 8, 16, 23 March and 1 April. It was thought that the ambient levoglucosan was primarily attributed
to domestic biomass fuel burning; the high levoglucosan emission on these
peak days might be from open BB events.
Regression analyses of levoglucosan and the other two anhydrosugars
(mannosan, galactosan) are shown in Fig. 3a. Levoglucosan was highly
correlated with mannosan and galactosan, with coefficients of determination
(R) of 0.81 (P<0.01) and 0.89 (P<0.01), respectively,
indicating similar combustion sources of them. The ratios of levoglucosan to
mannosan (L/M) and mannosan to galactosan (M/G) had been employed to
identify the specific types of BB, although these ratios were quite variable
(Fabbri et al., 2009). Previous studies suggested that
L/M ratios for the burning of softwood were 3–10, hardwood ratios were 15–25, and
those from crop residues were often above 40 (Cheng et al., 2013; Zhu et
al., 2015; Kang et al., 2018). The average L/M and M/G ratios were
statistically reported as 32.6 and 1.2 for crop residue combustion, 4.0 and
3.9 for softwood combustion, and 21.5 and 1.5 for hardwood combustion,
respectively (Sang et al., 2013; Shen et al., 2018). In this study, the
ratios of L/M and M/G ranged from 4.7 to 16.1 (average: 9.7, n=91) and
from 3.9 to 6.1 (average: 4.8, n=6), respectively, crudely indicating
major contribution from softwood burning. The samples collected from 31 March to 1 April and from 8 to 10 March, respectively, had considerably
lower and higher concentrations of mannosan than predicted by the
levoglucosan–mannosan regression model (Fig. 3a). The results suggested
that BB aerosols collected from 31 March to 1 April (L/M= 11.52 ± 1.34) and from 8 to 10 March (L/M= 6.57 ± 0.53) originated from
different types of BB compared with the remaining sampling periods (L/M= 9.34 ± 1.20). Therefore, the high levoglucosan emission from 31 March
to 1 April and from 8 to 10 March might be from different open BB events,
possibly an open agricultural waste burning event or a forest fire, while
the BB of most sampling days originated from biomass fuel for domestic
cooking and heating. It is worth noting that the peak days from 31 March to
1 April (L/M= 11.52 ± 1.34) neared the Qingming Festival. One
possibility for BB events is that people burned joss paper as a sacrifice to
their ancestors according to Chinese tradition.
(a) Regression analyses of levoglucosan versus the other two
anhydrosugars; (b) K+ versus the other three anhydrosugars; (c) levoglucosan
versus PM2.5, OC, and EC; and (d) K+ versus PM2.5, OC, and EC.
Both anhydrosugars and water-soluble potassium (K+) have been widely
utilized as source tracers of BB emissions (e.g., Puxbaum et al., 2007; Wang
et al., 2007; Zhang et al., 2008; Engling et al., 2011). The daily variations
of concentrations of levoglucosan and K+ are shown in Fig. S4; the
regression analysis of K+ and three anhydrosugars is shown in Fig. 3b. The K+ concentration was weakly correlated with levoglucosan, mannosan, and
galactosan, with R values of 0.33, 0.28, and 0.74, respectively. This could be
explained by the additional emissions of K+ from soil and seawater.
Since Lincang is far from the coast, sea salt could be negligible. Because of
the inhomogeneity of crustal K+ associated with soil types, it was
difficult to fully account for crustal K+ contributions from soil
(Harrison et al., 2012; Cheng et al., 2013). The ratio of levoglucosan to
K+ (L/K+) was also used to track possible sources of BB in the
previous studies. The ratios of L/K+ strongly depended on BB processes,
namely, smoldering and flaming. Studies suggested that relatively high
L/K+ ratios were obtained from smoldering combustion at low
temperatures compared with flaming combustion (Schkolnik et al., 2005; Lee
et al., 2010). Previous results showed the emissions from the combustion of
crop residuals such as rice straw, wheat straw, and corn straw exhibited
comparable L/K+ ratios, typically below 1.0. The average L/K+
ratio in this study was 0.48 ± 0.20, which is higher than the ratio
for wheat straw (0.10 ± 0.00) and corn straw (0.21 ± 0.08) but
is lower than the ratio for Asian rice straw (0.62 ± 0.32) (Cheng et
al., 2013). In this study, higher L/K+ ratios were observed during 8–10 March (1.20 ± 0.19) compared to those from 31 March to 1 April (0.40 ± 0.13), which suggested that the open fire event from 8 to 10 March was more
possibly due to smoldering combustion of residues at low temperatures.
Figure 3c and d show the scatterplots and regression analyses of K+
versus PM2.5, OC, and EC, and levoglucosan versus PM2.5, OC, and
EC, respectively. Linear regression of K+ on PM2.5, OC, and EC
resulted in R values of 0.64, 0.63, and 0.62, respectively, which were
generally higher than those of levoglucosan on PM2.5, OC, and EC, with
R values of 0.40, 0.54, and 0.48, respectively. This shows that K+ is
more highly correlated with PM2.5, OC, and EC, which could be explained
by either the photo-oxidative decay of levoglucosan (Hennigan et al., 2010) or different types of BB processes (Schkolnik et al., 2005; Lee et al.,
2010). Even so, the results suggest that BB imposed a great impact on fine
aerosols. The ratio of levoglucosan to PM2.5 (L/PM2.5) was also
helpful in distinguishing the contributions of different levoglucosan
sources (Wu et al., 2021). The ratio of L/PM2.5 in this study was
0.0041–0.0162 (average: 0.0072), indicating that the levoglucosan emission
in the areas might mainly come from wood (0.01–0.09) and crop straw
(0.001–0.008), not excluding incense burning (0.001–0.007), ritual item
burning (0.004–0.086), and meat cooking (0.005–0.06). However, certainly,
it was not from coal burning (0.0001–0.001) or waste incineration
(0.0022).
An empirical ratio of levoglucosan to OC (8.2 %), calculated from main
types of Chinese cereal straw (rice, wheat, and corn) based on combustion
chamber experiments (Zhang et al., 2007), was used to estimate the
BB-derived OC. The average mass concentration of BB-derived OC was 3534.4 ng m-3, whilst the contributions of BB to OC was 41.3 %, with a large
range of 19.1 %–73.9 %. The contributions were higher than those
previously reported, such as 6.5 %–11 % in Hong Kong (Sang et al., 2011),
18 %–38 % in Beijing (Zhang et al., 2008), 18.9 %–45.4 % over
southeastern Tibetan Plateau (Sang et al., 2013), and 26.4 %–30.2 % in
Xi'an (Zhang et al., 2014). The large range of 19.1 %–73.9 % revealed
that the daily contribution of BB varied greatly, suggesting that an open BB event
or forest fire happened occasionally. The contribution apportionment of
primary BB might be underestimated due to the degradation of levoglucosan
during atmospheric aging of BB-influenced air mass after long-range
transport (Hennigan et al., 2010; Mochida et al., 2010; Lai et al., 2014).
Moreover, Wu et al. (2021) have reported that the total levoglucosan emission exhibited a clear decreasing trend in China. However, it was
noteworthy that the average concentration of levoglucosan (287.7 ng m-3) and the BB contributions to OC (41.3 %) at Lincang mountain site
were both higher than the values of 191.8 ng m-3 and 28.4 % at
Tengchong mountain site in 2004 in spring (Sang et al., 2013). This result
suggests that there is no significant reduction in BB emissions in southwest Yunnan
Province.
Mono- or disaccharides
The total concentrations of five mono- or disaccharides, including glucose,
fructose, mannose, sucrose, and trehalose, were in the range of 25.2–373.7 ng m-3 (average: 158.9 ng m-3), which contributed 24.9 ± 8.3 % of the total measured saccharides. The average values of glucose,
fructose, mannose, sucrose, and trehalose were 31.2, 24.6, 2.7, 86.4, and
13.8 ng m-3, respectively. Sucrose was the dominant species in the mono- or disaccharides group. The results agreed with those of previous studies (Yttri et
al., 2007; Jia et al., 2010; Fu et al., 2012), which found that sucrose
was one of the dominant species in fine aerosols in spring. Ruptured pollen
may be an important source of sucrose, especially in the spring blossom
season (Yttri et al., 2007; Fu et al., 2012; Miyazaki et al., 2012). In
spring and early summer, farmland tilling after wheat harvest causes an
enhanced exposure of soil containing wheat roots to the air, which is
beneficial for the release of sucrose stored in roots (Medeiros et al.,
2006), thus resulting in a sharply increased sucrose concentration.
It was reported that sugars, such as glucose, sucrose, and fructose, could be
emitted from developing leaves (Graham et al., 2003). Glucose could be
released from both soils and plant material (e.g., pollen, fruits, and their
fragments) (Graham et al., 2003; Simoneit et al., 2004; Fu et al., 2012).
Glucose and sucrose were rich in biologically active surface soils (Rogge et
al., 2007). In this study, positive correlations were found between
sucrose and glucose (R=0.52) (Table S2), suggesting a similar origin of
glucose and sucrose in this study. Glucose and fructose have also been
identified as a minor product of cellulose pyrolysis, because they were
found to be enriched in BB emissions (Nolte et al., 2001), and they correlated well
with K+ (Graham et al., 2002) and levoglucosan (Kang et al., 2018).
Herein, no significant correlation was found between K+, levoglucosan,
and these mono- or disaccharides. Therefore, the detected glucose, fructose,
and sucrose might mostly be emitted by direct volatilization from plant
material or surface soils rather than as products of polysaccharide
breakdown during BB processes. The high abundance of sucrose, as well as
glucose and fructose, was responsible for biogenic aerosols associated with
developing leaves and flowers, as well as surface soil suspension.
Trehalose as a stress protectant of various microorganisms and plants
(Medeiros et al., 2006; Jia and Fraser, 2011) was found to be abundant in
the fine-mode soil, and has it been proposed as a marker compound for fugitive
dust from biologically active surface soils (Simoneit et al., 2004; Medeiros
et al., 2006; Rogge et al., 2007; Fu et al., 2012). A previous study found a
positive correlation between trehalose and calcium (Nishikawa et al., 2000).
In this study, there was no significant correlation between trehalose and
calcium. Besides, mannose has been reported to be one of the major
monosaccharide components in phytoplankton, which originated from marine
biological fragments (Tanoue and Handa, 1987). Mannose was detected in only
a few samples and presented with low concentrations in this study.
Sugar alcohols
Five sugar alcohol compounds, including glycerol, threitol, mannitol,
arabitol, and inositol, were detected in PM2.5. These reduced sugars are
often reported to be related to plant senescence and decay by microorganisms
(Simoneit et al., 2004; Tsai et al., 2013); they are produced by fungi,
lichens, soil biota, and algae (Elbert et al., 2007; Bauer et al., 2008). The
average concentration of total sugar alcohols was 159.9 ng m-3 with a
range of 53.1–254.0 ng m-3, which accounted for 25.1 ± 9.9 %
of the total measured saccharides. Glycerol has been widely found in soil
biota (Simoneit et al., 2004). Previous studies suggested that the source of
glycerol was not specific to biological emissions, but the biomass combustion
might increase atmospheric glycerol concentrations (Jia et al., 2010; Graham
et al., 2002; Wang et al., 2011). Herein, glycerol was the second most
abundant saccharide, with an average concentration of 123.7 ng m-3,
accounting for 5.1 %–44.6 % (average: 22.6 %) of the total measured
saccharides.
Mannitol and arabitol have been proposed as tracers for airborne fungal
spores (Elbert et al., 2007; Bauer et al., 2008; Zhang et al., 2010;
Burshtein et al., 2011). Mannitol and arabitol were detected with a
concentration range of 0.0–38.6 ng m-3 (14.7 ng m-3) and
0.0–21.1 ng m-3 (5.8 ng m-3), respectively. The average
concentrations of mannitol and arabitol were comparable to those (average:
11.3 and 9.1 ng m-3) reported in the Beijing spring aerosols (Liang et
al., 2013b) but were lower than those (average: 21.9 and 8.43 ng m-3)
in the Mediterranean summer aerosols (Burshtein et al., 2011) and (30 and 24 ng m-3) at Hyytiälä, Finland, in summer (Yttri et al., 2011).
Poor correlations (r=0.38) were found among mannitol and arabitol in
this study. Nevertheless, a positive correlation was found between trehalose
and mannitol (r=0.79, P<0.05) (Table S2).
In the previous studies, the total measured mannitol has been measured and
used for estimating the contribution of fungal spores to organic carbon
(Elbert et al., 2007; Bauer et al., 2008; Zhang et al., 2010). A factor of
mannitol per spore (0.49 ± 0.20 pg) was used to calculate the number
concentrations of fungal spores (Liang et al., 2013a) and then the carbon
content of fungal spores could be calculated using a conversion factor of 13 pg C per spore obtained earlier as the average carbon content of spores from
nine airborne fungal species, with an uncertainty of 20 % (Bauer et al.,
2008). The diagnostic tracer ratio of mannitol to OC was calculated to be
0.0377 according to this research (Bauer et al., 2008; Liang et al.,
2013a) and then used to estimate the contribution of fungal spores to
OC. The contribution of fungal spores might be underestimated, because
previous results had indicated that mannitol and arabitol were mainly
associated with the coarse PM fraction (Samaké et al., 2019). The
average mannitol concentration was 14.7 ± 11.2 ng m-3 during
the observation period. The average spore-derived OC was calculated to be
390.3 ng C m-3, which contributed to 4.9 % of the total OC.
Claeys et al. (2004) firstly identified two diastereoisomeric
2-methyltetrols as oxidation products of isoprene in the Amazonian rain
forest aerosols. Henceforward, 2-methyltetrols have been used as tracers for
isoprene-derived SOA (Liang et al., 2012; Fu et al., 2016; Yan et al.,
2019). In the previous studies, erythritol was often quantified as a surrogate
of 2-methyltetrols (2-methylthreitol and 2-methylerythritol) due to a lack of
standards (Claeys et al., 2004; Ding et al., 2013, 2016). In
this study, the concentration range of erythritol was 0.4–19.8 ng m-3
(average: 11.1 ng m-3). The values of inositol ranged from 0.0 to 22.8 ng m-3 with average values of 5.8 ng m-3. Moreover, the sugar
alcohols not only originate from biological emissions but also from BB (Wan
and Yu, 2007; Jia et al., 2010). Different levels of glycerol, arabitol,
mannitol, erythritol, and inositol in fine particles have been found during
the burning of crop residues and fallen leaves as well as indoor biofuel
usage for heating and cooking (Graham et al., 2002; Burshtein et al., 2011;
Wang et al., 2011; Yang et al., 2012; Kang et al., 2018). In this study,
only inositol correlated with levoglucosan (R=0.42), suggesting that
inositol might be linked to biomass combustion sources. Hence, the primary
source of sugar alcohols associated with fine particles was biogenic
aerosols at the observation sites.
Sources and transport
Since the distinct concentration of the studied compounds was due to
different emission sources arising from different wind directions, the 72 h
backward trajectories for the samples at the Dashu site (24.12∘ N, 100.11∘ E) and the spatial distribution of the fire spots (8 March to 9 April 2019)
were calculated to understand the source of saccharides in aerosol (Fig. 4). The analysis of air-mass backward trajectories suggested that the air
mass over Lincang was almost from the westerlies during the sampling
periods and could be separated into two episodes of remote western source
above 2000 m and local western source below 2000 m, as shown in
wine red and green lines. Specifically, 51.6 % of air-mass backward trajectories were
generally above 2000 m, whereas 48.4 % of them were below 2000 m.
Spatial distribution of the fire spots observed by MODIS, as well
as the corresponding 72 h backward air-mass trajectory clusters arriving at
1500 m above ground level (a.g.l.) during the sampling periods for the
collected samples. The backward trajectories were separated into two
episodes of remote western source above 2000 m and local western source
below 2000 m, as shown in wine red and green lines.
The average concentrations of saccharide compounds, as well as the
contributions, for the episodes above and below 2000 m are shown
in Fig. 5. The average concentrations of levoglucosan and mannosan for the
above 2000 m samples (327.4 and 35.6 ng m-3) were higher than
those for the below 2000 m samples (250.3 and 27.3 ng m-3). The
anhydrosugars accounted for 49.2 % and 36.9 % of total saccharides,
respectively, for the above and below 2000 m samples. This implied that
the levoglucosan at the observation site was attributed to both the local BB
activities and BB smoke transported from the neighboring regions of
Southeast Asia (Myanmar) and the northern Indian subcontinent. The southwest
wind from the Indian Ocean prevailed at Lincang all year round. In spring,
the southwest wind was often affected by the low-temperature downhill wind
blowing from the snow-covered Hengduan Mountains. The weather frequently
alternated between hot and cold, with unstable air pressure and strong wind.
Therefore, the lower air could be diluted by the relatively clean, cold air
over the plateau. The upper air mainly came from the westerlies. These
results were in agreement with the fact that residents across Southeast Asia
use wood as an energy source to cook and generate heat.
Average concentrations and contributions of saccharide compounds
for the aerosol samples separated above and below 2000 m.
For glucose, fructose, and sucrose, concentrations were a little higher in the below
2000 m samples (averages: 33.5, 26.4, and 106.2 ng m-3) than those
in the above 2000 m samples (averages: 29.2, 22.9, and 67.8 ng m-3). This implied that biogenic aerosols (such as ruptured pollen)
carrying sugars could cover long distance, which was supported by previous
studies, which have observed long-range atmospheric transport of fine pollen
from the Asian continent to the remote island Chichijima under the
influence of the westerlies (Rousseau et al., 2008). Although the pollen grains are
usually coarse with various shapes and hard shells; this results in a
relatively short retention time in the atmosphere. Therefore, it could be
concluded that, in addition to the local pollen, the concentration of
sucrose in Lincang was also influenced by the transport of airborne pollen
derived from South Asia.
Source apportionment of saccharides
Based on the compositional data of saccharides and key representative
markers for difference sources, five factors associated with the emission
sources of saccharides were finally resolved by NMF. As shown in Fig. 6a,
factor 1 was characterized by high levels of levoglucosan (71.8 %) and
mannosan (78.7 %), suggesting the source of BB (Simoneit et al., 1999;
Nolte et al., 2001). Factor 2 was characterized by trehalose (99.9 %) and
mannitol (100.0 %) and was enriched in the other saccharide components,
i.e., arabitol (44.1 %), glucose (29.6 %), erythritol (18.2 %),
glycerol (17.8 %), levoglucosan (14.7 %), and sucrose (8.6 %). These
saccharide compounds had all been detected in the suspended soil particles
and associated microbiota (e.g., fungi, bacteria, and algae) (Simoneit et
al., 2004; Rogge et al., 2007). A recent study found that leaves were a
major source of saccharide-associated microbial taxa in a rural area of
France (Samaké et al., 2020). Hence, this factor was attributed to soil
and leaf microbiota. Factor 3 had high levels of glycerol (71.4 %) and
erythritol (58.2 %) and showed loadings of glucose (12.8 %) and fructose
(11.8 %). Kang et al. (2018) reported that glycerol and erythritol
presented large amounts in winter and autumn when vegetation is decomposed.
This factor was attributed to plant senescence and decay by microorganisms.
Factor 4 exhibited a predominance of sucrose (78.7 %) and showed loadings
of glucose (17.2 %) and arabitol (11.8 %). This factor was regarded as the
source of airborne pollen, because pollen was the reproductive unit of
plants and contains these saccharides and saccharide alcohols as nutritional
components (Bieleski, 1995; Miguel et al., 2006; Fu et al., 2012). Factor 5
characterized by the dominance of fructose (88.2 %) was resolved, and it was
enriched in glucose (38.2 %) and arabitol (21.2 %); thus, it could be
regarded as the source of plant detritus.
(a) Factor profile obtained by NMF analysis. (b) Source
contribution of the five factors to the total saccharides in PM2.5
samples.
The pie chart in Fig. 6b shows the contribution of each source to total
saccharides. BB of factor 1 (34.0 %) was found as the dominant contributor
to total saccharides. Factors 2–5 could all be associated with a biogenic source
accounting for a total contribution of 66.0 %. The sources of soil
microbiota (factor 2), plant senescence (factor 3), airborne pollen (factor 4), and plant detritus (factor 5), respectively, contributed 16.0 %,
21.0 %, 23.7 %, and 5.3 % to total saccharides. During the sampling
periods, daily variations in the proportion of the five factors are shown in
Fig. S5. Factor 2 soil microbiota emissions could be associated with soil
reclamation and cultivation of farming periods, whereas factor 3 (plant
senescence) and factor 5 (plant detritus) could be associated with the harvesting
of vegetation or crops. During the observation period of a month, along with
the weather warming as sunshine enhanced, humans left two obvious traces of
cultivated soil from 9 to 17 March and from 27 March to 8 April and a trace
of vegetation or crop harvest from 17 to 30 March. The stronger pollen
discharge occurred in March, probably due to the flowering of certain
plants. The BB emissions peaked on 9 and 16 March and 1 April were more prone
to be open waste burnings.
Since there is still some uncertainty in the factor apportionment, the
proportion of sources are only relative and uncertain. It is still difficult
to distinguish the contributions of microorganisms and plants to biogenic
aerosols. Furthermore, all the above speculations about farming and
harvesting periods are based on only 1 month of observation, and long-term
observations are needed to obtain more accurate and effective information.
Conclusion
With the help of various atmospheric saccharides, this study presents the
characteristics of BB and various biogenic emissions to ambient aerosol in
the rural sites of southwest China. Levoglucosan was the most dominant
species among all the saccharides, with a concentration of 287.7 ng m-3.
The ratios of levoglucosan / OC were 1.9 %–8.9 % (average: 3.7 %). BB
contributed to 19.1 %–73.9 % of OC (average: 41.3 %). The results
indicated that domestic biomass fuel burning, open BB events, possibly open
agricultural waste burning, forest fire, or sacrificial activity are
significant during the spring in this area. The total concentrations of five
mono- or disaccharides and five sugar alcohols respectively contributed 24.9 ± 8.3 % and 26.6 ± 9.9 % to the total measured saccharides.
Based on the regression analysis, these mono- or disaccharides and sugar
alcohols were mostly emitted by direct biogenic volatilization from plant
material or surface soils rather than BB processes. The sampling sites
suffered from both local emissions and BB via long-range transport from
Southeast Asia (Myanmar, Bangladesh) and the northern Indian subcontinent. Five
sources of saccharides were resolved by NMF analysis, including BB (34 %),
soil microbiota (16.0 %), plant senescence (21.0 %), airborne pollen
(23.7 %), and plant detritus (5.3 %) at rural Lincang in spring.
The data indicated that biofuel and open BB activities in rural southwest
China and neighboring regions could have a significant impact on ambient
aerosol levels. In addition to the residential biofuel usage, field burning
of agricultural residues, fallen leaves, and forest fire were
non-negligible. Some new technical measures of biomass resource utilization
are urgently needed to improve the open-burning emission scenario in rural
areas, along with strict prohibition policy of BB. Meanwhile, the
characteristic analysis of saccharides in the region can serve as a
valuable reference for future studies to evaluate temporal variations of
biomass combustion and biogenic emission during modeling predictions and
policy making.
Code availability
At present, the URLs of the database in our lab are still being established. The code
used in the non-negative matrix factorization (NMF) analysis is available upon request
from the corresponding authors.
Data availability
The meteorological parameters were obtained from the Physical Sciences
Laboratory of NOAA (https://psl.noaa.gov/data/, last access: 8 September 2020, NOAA, 2020). 72 h
backtrajectories of the aerosol were calculated via the NOAA ARL READY website
(http://ready. arl.noaa.gov/HYSPLIT.php, last access: 6 September 2020, ARL, 2020). Fire pixel
counts were obtained from Moderate Resolution Imaging Spectroradiometer (MODIS)
observations on NASA satellites (https://earthdata.nasa.gov/, last access: 3 September
2020, NASA, 2020).
The three websites have open-access data. At present, the URLs of the database in our lab are still being established. All data
described in this study are available upon request from the corresponding authors.
The locations of the sampling sites are shown in Fig. S1. Temporal
variations of RH, temperature, solar irradiation, and rainfall are shown in
Fig. S2. Average concentrations of saccharide compounds and the
contribution of them for the Datian, Dashu, and Yakoutian samples are shown
in Fig. S3. Daily variation on average concentrations of levoglucosan and
K+ (panel a), arabitol and mannitol (panel b), PM2.5, Ca2+ and
trehalose (panel c) at the three sites throughout the sampling period are shown in
Fig. S4. Figure S5 shows daily variations on the proportion of the five
factors to the total saccharides in PM2.5 sampled at the three sites during
the sampling periods. Table S1 lists the concentrations of the carbonaceous
components and soluble inorganic ions in PM2.5 during the sampling
periods of spring 2019. Correlation matrix for the dataset of the determined
saccharide compounds in PM2.5 samples is shown in Table S2. The supplement related to this article is available online at: https://doi.org/10.5194/acp-21-12227-2021-supplement.
Author contributions
ZZW, JMC and QL designed the research; DW, ZZW and RJC conducted the sampling activities; ZZW and ZYL did the measurements; ZZW and XNS analyzed the results; JMC, HDK, XL and HLO made suggestions for this paper; and ZZW wrote the article with contributions from all co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “The role of fire in the Earth system: understanding interactions with the land, atmosphere, and society (ESD/ACP/BG/GMD/NHESS inter-journal SI)”. It is a result of the EGU General Assembly 2020, 3–8 May 2020.
Financial support
This research has been supported by the National Natural Science Foundation of China (grant nos. 91843301, 91743202, 91843302).
Review statement
This paper was edited by Ivan Kourtchev and reviewed by three anonymous referees.
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