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Chinese Science Bulletin 2004 Vol. 49 No. 15 1637ü1641
Analysis of carbon isotopes in
airborne carbonate and implications for aeolian sources
1
1
1
CAO Junji , WANG Yaqiang , ZHANG Xiaoye ,
2
2
1
Lee Shuncheng , Ho Kinfai , CAO Yunning
1
& LI Yang
1. SKLLQG, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China;
2. Department of Civil and Structural Engineering, The Hong Kong
Polytechnic University, Hong Kong, China
Correspondence should be addressed to Cao Junji
(e-mail: cao@loess. llqg. ac.cn)
Abstract Methods were developed to determine the mass
ratios of carbon isotopes in trace amounts of aerosol carbonate. A Finnigan MAT 252 mass spectrometer fitted with an
on-line Kiel device was to determine the 13C/12C ratio in CO2
produced from the carbonate. A study using these methods
was conducted to characterize the carbonate carbon isotopes
in aerosol samples collected in Xi’an on dusty and normal
days during March and April 2002. Results of the study
demonstrate that insights into the origin of the dust can be
deduced from its isotopic composition. That is, the δ 13C of
carbonate for dust storm samples ranged from −1.4‰ to
−4.2‰, and this is consistent with sandy materials in dust
source regions upwind. In contrast, for non-dusty days δ13C
ranged from −7.5% to −9.3‰, which is more similar to fine
particles emitted from local surface soils. Comparisons of
dust storm aerosols with surface soils from source regions
and with aerosol samples collected downwind indicate that
the δ 13C values did not change appreciably during longrange transport. Therefore, carbon isotopes have the potential for distinguishing among source materials, and this approach provides a powerful new tool for identifying dust
provenance.
Keywords: aerosol, carbonate, carbon isotopes, dust provenance.
prevailing winds of middle latitude and deposited in East
Asia, and is found in near-surface atmosphere and deepsea sediments in the remote Pacific. These dust increases
alkalinity of surface sea, changes acid-alkali balance, and
enhances seawater to absorb atmosphere CO2. This phenomenon was confirmed by paleo-environmental records
in Japan Sea sediments. Carbonate plays the roles of
“alkalinity pump” and “biological pump” in seawater and
rainwater, respectively, which have significant contributions to the reduction of the atmospheric CO2 during the
last glacial period[5]. Since carbonate plays diverse roles in
land, atmosphere and ocean systems, it has even been said
to induce “Carbonate Mysteries” in the global climatic
system[6].
Even the concentrations of carbonate are generally
low from nD100 ngCm−3[7] to 3 µgCm−3[8] in ambient
atmosphere, they can reach high levels in the atmosphere
of northern China, especially under dust storm atmosphere
as they can account for 15%[9] in TSP (absolute concentrations are not clear). The specific information about carbonates in the atmosphere is pitifully small because there
has not been a convenient method of carbonate analysis
for particles collected on the filters. The basic methodology for airborne carbonate can be divided into two types:
one is acidification method[7,10], i.e. with HCl release from
the carbonate in the filters; the other is optical method[11],
i.e. with X-ray diffraction phase analysis. However, few
researches focus on the measurement of airborne carbonate carbon isotopes and its implications. Carbonates from
different origins and sources have different carbon isoü
topes[12 14], so it is possible to use carbonate carbon isotope to estimate its potential source regions and characteristics. This paper provides a new method to determine
carbonate carbon isotope in aerosol samples for the first
time and discusses the implications of δ13C for dust origins.
1
Aerosol sampling and chemical analysis
Atmospheric aerosol, as an important driven factor,
has become one of the hotspots in international global
change study[1]. Within the past decade there has been
increasing interest in airborne carbonate because of its
important role in atmospheric chemistry, global climate
and radiative forcing[2,3]. Carbonate can affect atmospheric
chemistry and aerosol characteristics because its alkalinity
favors uptake of SO2 and NOx and conversion to SO4 and
NO3 on the surface, as well as direct deposition of HNO3
and H2SO4 from the gas phase. Moreover, carbonate reacts
directly or indirectly with sulfuric, nitric and some organic
acid (such as acetic acid) through homogeneous and nonhomogeneous chemical reactions[4]. In addition, large
amount of dust emitted from Central Asia is carried by
Sampling station is located on a high dust storm occurrence city, Xi’an, which is situated at the south margin
of the Loess Plateau. Aerosol samples were collected in
this site in non-dust storm and dust storm periods from
March to April in 2002. PM2.5 (aerodynamic diameter less
than 2.5 µm) samples were collected using mini-volume
samplers (Airmetrics, USA) operating at flow rates of 5
LCmin−1. Sampling ranged from 8 to 24 h for dust storm
periods and normal days periods (Table 1). All the samples
were collected on 47 mm Whatman (Whatman, UK)
quartz microfibre filters (QM/A). The filters were preheated at 900 k for 3 h before use in order to remove
potential carbonate contaminations. Detailed procedures
refer to reference[15].
Aerosol mass concentrations were determined
gravimetrically using an electronic microbalance (Mettle
M3, Switzerland) with a 1 µg sensitivity at the Institute of
Chinese Science Bulletin Vol. 49 No. 15 August 2004
1637
DOI: 10.1360/03wd0375
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Earth Environment, CAS (IEECAS) (Table 1). The samples were analyzed for carbonate carbon using DRI Model
2001 (Thermal/Optical Carbon Analyzer) with a standard
operation process[7]. The concentrations of carbonate
(CO32−) were converted from carbonate carbon (C) directly by a factor of 5.
2
Carbon isotope measurement
( Determination of aerosol sample amounts.
Carbon isotope analysis on carbonate has been widely
applied in deep-sea sediment, soil, coral and stalagmite
studies[16]. In this paper, the classic McCrea method[17] for
carbonate analysis is utilized. The stable carbon and oxygen isotope measurements for trace amount of carbonate
on the filters were performed on a Finnigan MAT 252
mass spectrometer (Thermo Finnigan, Germany) fitted
with an on-line Finnigan automatic carbonate reaction
system (“Kiel device”) (Thermo Finnigan, Germany).
Carbonate (shown as CaCO3) on the filters were determined by 100% phosphoric acid. The following is the
equation of chemical reaction:
2H3PO4 + 3 CaCO3 = Ca3(PO4)2 + 3 H2O + 3CO2
(1)
The above reaction should be carried out in the condition of superfluous H3PO4, just to ensure the reaction to
be finished completely and also to avoid isotope fractionation. The most suitable CO2 pressure from the reaction
should be around 400800 µb on the MAT 252 isotope
ratio mass spectrometer, and the pressure being higher or
lower than this range will spoil the results. Therefore, via
experiments the appropriate filter area is 13 cm2 and the
proper quantity of H3PO4 is 1015 drops.
() CO2 preparation. The filters were cut into few
small pieces and put into the glass test tube, and then
baked about 1 h at 70. Once the vacuum arrived at the
standard (less than 150 µb), H3PO4 was dropped to react
with carbonate. The reaction was stabilized under the constant temperature (70) for 600 s, the impurity gases
were eliminated in another 90 s. Then CO2 was transferred to a liquid nitrogen dewar in −170 liquid itrogen.
Impurity gas was eliminated by different gas condensation
points, CO2 was purified, and then the dewar was heated
to release the CO2 to another dewar, and at last all CO2 in
the dewars was released and transferred to ion source for
carbon isotope analysis.
() Mass spectrum measurement. The purified
CO2 was measured using MAT-252 to determine carbon
isotope ratio. The difference between carbon isotopes was
denoted by δ:
δ sample = (Rsample/Rstandard − 1)1000‰,
(2)
13
where Rsample is the ratio of sample carbon isotope, i.e. C/
12
C, Rstandard is the ratio of standard carbon isotope. The
isotopic results are reported in δ13C as per mil deviation
from PDB standard. The standard deviation is ± 0.1‰ for
the analysis. All the above experiments were completed in
the stable isotope laboratory at IEECAS.
3
Results and discussion
Two dust storm events occurred in Xi’an on 20
March, 2002 and 14 April, 2002, respectively. Five day
back-trajectories (NOAA HYSPLIT model) showed that
dust storm on 20 March originated from mid-Asia, passed
Xinjiang and arrived at Xi’an, then spread over 18 provinces in the downwind regions to the east (Fig. 1). Sugimoto[18] and Chung[19] also observed the influence of this
dust storm over Beijing and Chongwon-Chongju (Korea),
respectively. Another dust storm originated from Taklimakan Desert and dust cloud arrived at Xi’an along the
north margin of Qinghai-Tibet Plateau. This dust storm
event was also detected by Yulin Aerosol Observation
Station 600 km north of Xi’an[20].
The mass concentrations of PM2.5 and carbonate
were very high during dust storm period (Table 1). The
concentration of PM2.5 in Xi’an was as high as 350
µgm−3 on 20 March while TSP in Beijing reached peak
Table 1 The comparison of PM2.5 in dust storm and normal atmospheres
PM2.5 concentration Carbonate * concentraSampling date and duration
Carbonate/PM2.5 (%)
/µgCm−3
tion /µgCm−3 a)
δ13C (‰)
No.
Sample type
1
dust storm
March 20, 8 h
350.4
52.5
15.0
−1.4
2
dust storm
March 20, 16 h
268.4
27.6
10.3
−2.6
3
dust storm
March 21, 24h
352.8
44.4
12.6
−2.8
4
dust storm
April 14, 8 h
847.8
131.7
15.5
−3.4
5
dust storm
April 14, 14.5 h
686.9
96.9
14.1
−4.2
6
normal days
March 23, 24 h
307.5
24.0
7.8
−7.5
7
normal days
April 9, 24 h
212.4
10.0
4.7
−8.3
8
normal days
April 11, 24 h
204.0
27.4
13.4
−6.9
9
normal days
April 12, 24 h
213.9
31.4
14.7
−9.3
10
normal days
April 13, 24 h
208.8
21.2
10.2
−9.2
a) CaCO3 shows as calcite.
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Chinese Science Bulletin Vol. 49 No. 15 August 2004
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Fig. 1. Isentropic trajectory back in time from Xi’an (1.5 km altitude)
on 20 March and 14 April, 2002, at 0000 UTC.
values to 11000 µgCm−3 in the same day[18]. PM2.5 in Korea also reached 331 µgCm−3 on 23 March[19]. In addition,
the concentration of PM2.5 of 8 h sampling was as high as
847.8 µgCm−3 on 14 April and its average daily concentration reached the highest value, 767.4 µgCm−3, which is
11 times as higher as the PM2.5 average daily concentration
of 65 µgCm−3 according to USA National Ambient Air
Quality Standards (NAAQS) (China has no PM2.5 national
standard currently). This implied dust storm exerted a severely negative influence on atmosphere environment. On
the same day, TSP concentration in Yulin was 4650
µgCm−3[20].
With high dust loading, carbonate in aerosol was
rather high and its average daily concentration ranged from
40.1 to 114.3 µgCm−3. The percentages of carbonate in
PM2.5 in the two dust storms were 10.3% and 15.5%, respectively, with an average of 13.5%. This value is close
to that in Luochuan loess profile (11.1%)[12], showing that
the carbonate from northwest source regions was high
before it transported to the Loess Plateau by wind, which
leads to high primary carbonate in loess. This result is
consistent with Wen’s speculation[12]. The percentage of
airborne carbonate under dust storm atmosphere was also
close to the percentage of carbonate (11.8%) in surface
samples on Taklimakan desert[14], which were collected
from desert source regions in northwest China. However,
measurement for airborne carbonate was less, especially
in dust storm. Fortunately, carbonate concentration in TSP
was found to be 6.3 µgCm−3, which was measured by a
Japanese scientist in Kosan, Korea during the dust storm on
20 March[21]. This result corresponds to 53.5 µgCm−3
carbonate (calculated with CaCO 3 ) which also
corresponds to our observed result.
The comparison of carbonate percentages between
non-dust storm and normal atmospheres is summarized in
Chinese Science Bulletin Vol. 49 No. 15 August 2004
Table 1. Although carbonate percentages in normal days
samples varies, the difference of two kinds of samples is
not notable. Some researchers have put forward that the
increase of carbonate carbon concentration (the content of
carbonate carbon larger than 1%, i.e. 8.3% carbonate) in
aerosol can be used as an indicator of dust storm event[9].
From the above comparisons, it is not correct to only use
the increase of carbonate carbon concentration as an indicator [6] of dust storm. Actually even though there is no
dust storm in winter, Asian dust never stops ground-based
transporting from the source regions to the east and south
of China. Because it is almost stable for carbonate percentage in Asian dust from source regions, in deposition
regions, no matter what dust mass concentration is, there
is no large change existing in carbonate percentage, unless
obvious chemical reactions with some acidic aerosol occurs during dust transport.
Figure 2 shows the distribution of carbon and oxygen
isotopes for 10 dust storm samples. The isotopic composition of carbonate for surface soils, loess and paleosol in
the Loess Plateau as well as surface soils in the modern
dust source regions have been shown in the figure for
comparison in order to differentiate the potential sources
of carbonate. The previous studies demonstrated that δ13C
of carbonate in different sizes was similar for surface soils,
which showed that wind erosion had no impact on the
change of isotopic composition for surface soils in dust
source regions. In addition, there were little differences in
carbon isotope of carbonate for size-separated loess (paleodust) samples at Luochuan loess profile and Huanxian
loess profile, the differences of δ13C in coarse (>45 µm)
and fine (<2 µm) particles were less than −2‰ [22]. Moreover, three sets of size-separated samples had been collected in Xi’an on 18 March and 16 April, 1998 and 13
April, 2000, respectively. All the isotope differences of
carbonate carbon in PM2.5 and TSP are less than −2‰
(unpublished data), which fluctuate in the natural range.
Therefore, isotope data in PM2.5 can be well comparable to
those in surface soils and loess samples.
The isotopic compositions of dust storm samples and
normal samples with high carbonate contents are presented in Fig. 2. Distinct differences of carbon isotope can
be seen from Fig. 2 for normal and dust storm samples, i.e.
carbon isotope ranged from −1.4% to −4.2‰ for dust
storm samples, while ranging from −7.5% to −9.3‰ for
normal days samples. According to Fig. 1, dust storm on
20 March transported through Badain Juran Desert, Hexi
Corridor, Tengger Desert and Ulan Buh Desert. Carbonate
carbon isotopes of surface soils in these dust source regions are −0.2‰, −1.7‰, −2.6‰ and −2.7‰, respectively
(Fig. 2), which are close to the values of this dust storm
(Table 1). Moreover, Japanese scientists collected during
dust storm in Kosan, Korea on 20 March and they obtained that one of δ13C data for carbonate was −1.4‰[21],
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Fig. 2. Comparison of carbon isotopes in dust storm samples, normal days samples, loess, paleosol in the Loess Plateau, and surface soils in modern
dust source regions. DS refers to dust storm samples, Normal to normal days samples, Loess to loess sample[13], Paleosol to paleosol sample in the Loess
Plateau[13], Soil to the surface soils in Xi’an [12], TKLMG to Taklimakan Desert sample, KMTG to Kumutage Desert sample, BDJL to Badain Juran
Desert sample[14], HXZL to Hexi Corridor samples[14], WLBH to Ulan Buh Desert sample [14], TGL to Tengger Desert sample [14], MWS to Mu Us Desert
sample[14], NM to surface soil in the middle of the Inner Mongolia Plain[14] .
which was very close to our observed data (No. 1 sample
in Table 1). This implies that carbonate carbon isotope in
the long range transport is not fractionated, so δ13C can be
used as a tracer of source materials.
Compared with loess-paleosol samples, δ13C in dust
storm samples are close to that in loess sample (−4.2‰),
while δ13C in normal days is close to those in paleosol
(−7.9‰) and in Xi’an surface soil (−7.6‰). The principle
of isotope evolution for carbonate carbon in desert-loess-paleosol[9] is as follows: Primary carbonates in
loess are original carbonates from underlie rock and their
δ13C values are greater than or close to zero. The primary
carbonates are dissolved in HCO3− solutions in the
course of loess formation and then they change into second carbonates. During the formation of second carbonates, the carbon isotope composition of these secondary
carbonates is determined by δ13C of CO2 in the loess
sediments. The loess sediment CO2 is a mixture of atmospheric and biogenic CO2 with lighter carbon isotope
(around −15‰). The dissolution-precipitation processes
might happen many times during the depositing of loess
sediments. It causes δ13C to be rich in secondary carbonates and δ13C of secondary carbonates becomes lighter and
lighter along with the increase of dissolution-precipitation
processes. So δ13C in the loess is −4.2‰ while in the paleosol is −7.9‰ due to more dissolution processes. On the
basis of paleosol, the carbonate in modern surface soils in
Xi’an has been influenced by anthropogenic activity, and
its δ13C is also depleted (−7.6‰). The δ13C of carbonate in
loess profile has been used as a paleoclimate index, i.e. the
1640
more negative of the δ 13C, the more warm and humid the
corresponding climate, the more biogenetically blooming
the paleoenvironment, and vice versa[12]. Therefore, δ13C
of carbonate in modern desert arid regions ranges from
−3.6‰ to 2.1‰ (Fig. 2). While in the Loess Plateau, secondary carbonates were in favor of forming under warm
and humid climate, which leads to their δ13C (−7‰ to
−8‰) lower than those of desert source regions. This
phenomenon has also been observed in the distribution of
carbonate carbon of surface soils on northern China. Previous study showed that δ 13C of carbonate in surface soils
in dust source regions was more negative with the increasing of precipitation (Fig. 2)[14]. Take Inner Mongolia
(northern and northeast China) for example, δ13C of carbonate in surface soil reached to −8.3‰ (Fig. 2).
Because carbonate carbon isotope in the long range
transport is not fractionated evidently, δ13C of carbonate in
dust storm samples is very close to those in modern sand
in dust source regions. While under normal days, surface
soils naturally emit fine particles into atmosphere, which
leads to depleted δ13C (−9.7‰) in carbonate for normal
days samples. Thus, their isotopic composition has notable
difference from those of dust storm samples. Therefore,
major dust sources can be inferred from their δ 13C values in
carbonate under ambient air. If δ 13C is lighter (such as
−9‰), it maybe comes from sandy soil, degraded grassland
and local dust, having abundant precipitations and frequent
anthropogenic activities. If δ 13C is heavier (such as −3‰),
northwest desert regions can be considered as dominant
contributions to δ13C. Therefore, δ13C of carbonate may be
Chinese Science Bulletin Vol. 49 No. 15 August 2004
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an effective tool to trace the sources for the currently
complex research of source regions determination.
4
Conclusion
Aforementioned analyses suggest: that a Finnigan
MAT 252 mass spectrometer fitted with an on-line Kiel
device III can provide an effective measurement for the
isotopic composition of airborne carbonate. Contrary to
carbonate content, δ13C in carbonate has distinct differences under different conditions, i.e. δ13C (−2.9‰ ± 1.0‰)
in carbonate for dust storm samples was higher evidently
than the normal days samples (−8.2‰ ± 1.0‰). This indicates that the isotopic composition of airborne carbonate
can provide a potential power tool to trace dust origins.
Meanwhile, it will benefit to the further research on the
dust, loess accumulation and paleoclimate evolutions.
toring in China (in Chinese), 2002, 18(2): 11ü15.
10. Clarke, A. G., Karani, G. N., Characterization of the carbonate content of atmospheric aerosols, Journal of Atmospheric Chemistry,
1992, 14: 119ü128.
11. Esteve, V., Rius, J., Ochando, L. E. et al., Quantitative X-ray diffraction phase analysis of coarse airborne particulate collected by
cascade impactor sampling, Atmospheric Environment, 1997, 31:
3963ü3967. [DOI]
12. Wen Qizhong, Geng Ansong Chinese Loess Geochemistry (in Chinese), Peking: Science Press, 1989, 115ü158.
13. Li Chunyuan, Wang Xianbin, Wen Qibin et al., The relationship
between carbon and oxygen isotopic composition characteristics of
carbonates in loess sediments and paleoclimate, Science in China,
Series B, 1995, 38(8): 979ü986.
Acknowledgements The authors wish to thank Dr. Arimoto R. in New
Mexico State University, USA for polishing the manuscript. This work
was supported by the National Natural Science Foundation of China
(Grant No. 40205018) and the Ministry of Science and Technology
(Grant No. 2001CCB00100).
14. Wang, Y. Q., Cao, J. J., Zhang, X. Y. et al., The carbonate content
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