Long term record of atmospheric CO2 and

GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 19, GB3021, doi:10.1029/2004GB002430, 2005

Long-term record of atmospheric CO2 and stable isotopic ratios
at Waliguan Observatory:
Background features and possible drivers, 1991–2002
Lingxi Zhou,1 Thomas J. Conway,2 James W. C. White,3 Hitoshi Mukai,4
Xiaochun Zhang,1 Yupu Wen,1 Jinlon Li,5 and Kenneth MacClune3
Received 8 December 2004; revised 28 June 2005; accepted 10 July 2005; published 14 September 2005.

[1] This paper describes background characteristics of atmospheric CO2 and stable
isotopic ratios (d13C and d18O) as well as their possible drivers at Waliguan Baseline
Observatory (WLG) (36170N, 100540E, 3816 m above sea level) in the inland plateau
of western China. The study is based on observational CO2 data (NOAA Climate
Monitoring and Diagnostics Laboratory discrete and WLG continuous measurements)
obtained at WLG for the period from May 1991 to December 2002. Over this period the
change in monthly means is +16 ppm for CO2, 0.2% for d13C, and 0.5%
for d18O. The overall increase of CO2 and subsequent decline of d13C, with a
Dd13C/DCO2 ratio (0.011 ± 0.105) % ppm1 at WLG, reflect the persistent worldwide
influence of fossil fuel emissions. The negative secular trend of d18O at WLG is probably
due to vigorous 18O exchange with soils in the Northern Hemisphere (NH) and
conversion from C3 to C4 plants via land use change. The CO2, d13C, and d18O mean

annual cycles with peak-to-peak annual amplitudes of 10.5 ppm, 0.499 %, and
0.819%, respectively, at WLG show typical middle-to-high NH continental features
that correspond to the seasonal cycle of the terrestrial biosphere. The significant CO2 and
d13C interannual variability at WLG is very likely caused by worldwide climate
anomalies and associated regional fluctuation in biospheric CO2 uptake in the Asian
inland plateau as well as long-range air mass transport. The results of this study help to
provide a basic understanding of the individual sources and sinks of carbon in this
area and help us to better address the role of the Asian inland terrestrial biosphere in the
global carbon cycle.
Citation: Zhou, L., T. J. Conway, J. W. C. White, H. Mukai, X. Zhang, Y. Wen, J. Li, and K. MacClune (2005), Long-term record of
atmospheric CO2 and stable isotopic ratios at Waliguan Observatory: Background features and possible drivers, 1991 – 2002,
Global Biogeochem. Cycles, 19, GB3021, doi:10.1029/2004GB002430.

1. Introduction
[2] CO2 fluxes at the Earth’s surface include respiration
and photosynthesis of the terrestrial biosphere, exchange
with the oceans, and anthropogenic sources such as fossil
fuel combustion and land use changes. Since the eighteenth
century, increasing emissions of anthropogenic CO2 have
been distributed among the reservoirs of CO2. About half of

1
Key Laboratory for Atmospheric Chemistry, Centre for Atmosphere
Watch and Services, Chinese Academy of Meteorological Sciences, China
Meteorological Administration, Beijing, China.
2
Climate Monitoring and Diagnostics Laboratory, NOAA, Boulder,
Colorado, USA.
3
Institute for Arctic and Alpine Research, University of Colorado,
Boulder, Colorado, USA.
4
Center for Global Environmental Research, National Institute for
Environmental Studies, Tsukuba, Japan.
5
School for Environmental Sciences, Peking University, Beijing, China.

Copyright 2005 by the American Geophysical Union.
0886-6236/05/2004GB002430

the CO2 released into the atmosphere by human activity

remains there, with the other half absorbed either by the
land biosphere or the oceans [Andres et al., 1996; Pearman
and Hyson, 1986; World Meteorlogical Organization
(WMO), 2003]. Long-term observation of the atmospheric
CO2 mixing ratio and d13C from a globally distributed in
situ and discrete air sampling network can enable the
determination of source/sink variability and allow quantitative partitioning of fluxes into terrestrial and oceanic reservoirs because these exchange paths influence CO2 isotopes
in different ways [Ciais et al., 1995a, 1995b; Francey et al.,
1995; Nakazawa et al., 1993, 1997a, 1997b; Tans et al.,
1990; Trolier et al., 1996]. Additionally, d18O measurements
allow the separation of terrestrial net ecosystem production
(NEP) into its photosynthetic and respiratory components
on the basis of their contrasting effects on the d18O of
atmospheric CO2 [Ciais et al., 1997a, 1997b; Flanagan et
al., 1997; Ishizawa et al., 2002; Miller et al., 1999]. The
CO2 mixing ratio and isotope measurements can also be
used in studies of natural variability in the carbon cycle and

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(36170N, 100540E, 3816 m above sea level), situated in
remote western China, is one of the World Meteorological
Organization’s (WMO) 22 Global Atmosphere Watch
(GAW) baseline stations scattered around the globe. The
locations of the GAW baseline stations are mostly coastal
(http://www.wmo.ch/web/gcos/gif/gaw.gif) [WMO, 1993,
2001, 2003; World Data Center for Greenhouse Gases
(WDCGG), 2003]. Funded by the United Nations and the
Chinese Government, WLG was officially opened in 1994
by the China Meteorological Administration as China’s
first long-term research station for the continuous monitoring of greenhouse gases, ozone, aerosols, and meteorology [Wen et al., 1994; Zhou et al., 2003]. Because of its
unique location the measurements from WLG provide

essential information on sources and sinks from within
the Eurasian continent, and they have received attention
in recent years [Climate Monitoring and Diagnostic
Laboratory (CMDL), 2004; Masarie and Tans, 1995;
Miller et al., 2003; WMO, 2003; WDCGG, 2003] (see
also http://www.cmdl.noaa.gov/ccgg/globalview).
[4] In this study, the monthly mean time series, secular
trend, annual cycle, and interannual variability of atmospheric CO2 and stable isotopic ratios observed at WLG are
presented and characterized. The results from WLG show
specific features that will provide additional constraints for
atmospheric models to help improve the understanding of
atmospheric CO2 and the global carbon cycle, particularly
over the Asian inland plateau.

2. Site and Experiment

Figure 1. Geographical location and topographical map of
the area (100 km radius) surrounding Mount Waliguan
(WLG). The triangular markings in the topographical map
represent mountain peaks (meters above sea level). The

dotted area indicates desert regions.
in calibrating global carbon budget models [Heimann and
Maier-Reimer, 1996; Keeling et al., 1989a, 1989b, 1995;
Tans et al., 1989, 1996].
[3] The measurements from various global monitoring
networks together with modeling studies have identified
the northern midlatitude terrestrial biosphere as a major
component of the ‘‘missing sink.’’ However, spatial coverage of the existing networks is still too sparse to be able
to reach satisfactory conclusions with respect to specific
fluxes, especially in midcontinental regions where it is
likely that important carbon sources and sinks are located
[Bakwin et al., 1998; Francey et al., 1998; Houghton et
al., 1998; Levin et al., 1995; Miller et al., 2003; Morimoto
et al., 2000]. Waliguan Baseline Observatory (WLG)

2.1. Sampling Site
[5] Figure 1 shows the geographical location and topography of the area (100 km radius) around Mount Waliguan
(WLG). Located at the edge of the northeastern part of the
Tibetan Plateau, the area surrounding the WLG station is
essentially untouched, maintaining its natural environment

of sparse vegetation along with arid and semiarid grassland
and some desert regions. Yak and sheep grazing is the main
activity during summer (June, July, and August), with small
agricultural regions located in the lower valley area. The
population density is less than 6 people km2, and the
station is relatively isolated from industrial and populated
centers.
2.2. Discrete Measurements
[6] In 1991 the National Oceanic and Atmospheric
Administration Climate Monitoring and Diagnostic Laboratory (NOAA CMDL) began a weekly air sampling
program at WLG. Two samples are collected in series from
5 m above ground using glass flasks and a portable batterypowered sampling apparatus (flushing and then pressurizing
glass flasks with a pump). The discrete air samples
collected at WLG are measured for CO2 (and other trace
gas species) by a nondispersive infrared (NDIR) analyzer
at the NOAA CMDL Carbon Cycle Greenhouse Gases
group in Boulder, Colorado, USA. Measurement precision
for CO2 determined from repeated analysis of the same air
is 0.1 ppm. The isotopic measurements of the discrete air
samples are performed at the Stable Isotope Laboratory of

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the Institute for Arctic and Alpine Research of the University of Colorado. A Micromass Optima dual inlet
isotope ratio mass spectrometer achieves overall reproducibility of ±0.01% for d13C and ±0.03% for d18O. The
monthly means from the discrete samples are produced by
first averaging all valid measurement results with a unique
sample date and time, then extracting values at weekly
intervals from a smooth curve fitted to the averaged data
and averaging these values for each month to give the
monthly means (http://www.cmdl.noaa.gov/ccgg/iadv/).
Descriptions of the sampling, measurement, standards,
calibration procedures, curve fitting, analysis, and interpretation of the CO2 and isotopes (d13C and d18O) data are
given in other works [Conway et al., 1994; Thoning et al.,
1989; Trolier et al., 1996].

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difference (±2s, discrete minus continuous) of (0.59 ±
0.23) ppm. One possible reason for this difference is the
different air sampling heights (5 m above ground for
discrete and 80 m for continuous measurement). The
CO2 monthly means at WLG over the entire period from
May 1991 to December 2002 (140 months in total) used in
this study are merged from all of the continuous and
discrete air sample measurements. The monthly means of
the d13C (120 months) and d18O (110 months) at WLG
over the entire period were derived from the discrete
samples. The CO2 and isotope data from the discrete
samples from November 1994 to June 1996 are rejected
because a sampling malfunction contaminated the samples.

3. Results and Discussion
2.3. In Situ Continuous Measurements
[7] The main building housing the WLG in situ CO2
measurement systems and an 89-m triangular steel tower for

ambient air sampling (80 m) is located at the top of Mount
Waliguan. The atmospheric CO2 mixing ratios were measured using a Licor6251 NDIR analyzer and a HP5890 gas
chromatograph (GC) equipped with a flame ionization
detector (FID). The NDIR system began in November
1994 with an ambient analysis frequency of one per minute.
The GC-FID system began in July 1994 with 64 ambient
injections per day (CO2 converted to CH4 by a nickel
catalyst tube heated to 350). The overall precision of the
NDIR and the GC-FID analyses is below 0.02% and 0.05%,
respectively, from repeated analysis [Wen et al., 1994; Zhou
et al., 1998]. The standard scale employed by the WLG in
situ CO2 measurements is tied to the NOAA CMDL CO2
measurement scale and compared through periodic intercomparison experiments (see http://www.cmdl.noaa.gov/
ccgg/globalview). The CO2 monthly means from the in situ
NDIR and GC-FID measurements are integrated (to reduce
data gaps) from the selected hourly data representative of
background atmospheric conditions. The system configurations and routines, CO2 standard gases and reference
scale, intercomparison experiments, calibration and quality
control, impact of local winds and long-range transport on
the continuous CO2 record, background data filtering and

merging methodology, etc., are described in previous
studies [Zhou, 2001; Zhou et al., 2003].
2.4. Measurement Scale and Observational Data
[8] In this paper, the CO2 mixing ratios are reported on
the NOAA CMDL measurement scale [CMDL, 2004] (see
also http://www.cmdl.noaa.gov/ccgg/globalview) in units
of mmol mol1 (106 mol CO2 per mol of dry air). The
isotopic ratios are expressed in per mil (%) relative to the
standard isotopic ratio Vienna Peedee belemnite CO2
[Trolier et al., 1996] for both d13C and d18O.
[9] A total of 118 monthly mean CO2 values (May 1991
to December 2002, NOAA CMDL network) were derived
from the discrete samples. A total of 84 monthly mean CO2
values (August 1994 to December 2002, WLG in situ
measurements) were extracted using continuous data. A
comparison of the overlapping (62 months) discrete and
continuous monthly mean CO2 data resulted in a mean

3.1. Atmospheric CO2, D13C, and D18O
Monthly Mean Time Series
[10] Monthly mean atmospheric CO2 mixing ratios, d13C,
and d18O at WLG from May 1991 to December 2002 are
shown in Figure 2. The CO2 and d13C monthly data vary
seasonally. The CO2 data showed typical spring maxima
and summer minima. The d13C data showed an opposite
annual cycle that is almost a mirror image to that of the
CO2. The d18O monthly data exhibited relatively large
scatter with annual maxima that occurred mostly in June
(early summer at WLG).
[11] The overall change of the mean mixing ratio is
+16 ppm for CO2, 0.2% for d13C, and 0.5% for
d18O. It is well known that the secular increase of atmospheric CO2 mixing ratio and decline of d13C are mainly
driven by fossil fuel combustion and terrestrial deforestation, which add carbon with low isotopic ratios to the
atmosphere. The CO2 and d13C trends are also affected by
uptake and release of CO2 from the oceans and terrestrial
biosphere. Though less addressed than d13C, the d18O of
atmospheric CO2 contains information about both plant and
soil respiration in addition to plant photosynthesis. Gillon
and Yakir [2001] suggested that the recent observed decline
of d18O in atmospheric CO2 (0.02% yr1) is partly due to
the conversion of C3 forest to C4 grasslands via land use
change. Ciais et al. [1997b] suggested that vigorous 18O
exchange with soils is primarily responsible for the persistent depletion in d18O over the high latitudes of the Northern
Hemisphere (NH), while leaf isotopic exchange opposes
this effect in the NH. Ishizawa et al. [2002] speculated that
the recent d18O downward trend (0.5% yr1 from
Northern Hemisphere to Southern Hemisphere for the
period from 1993 to 1997) is mainly caused by enhanced
photosynthetic activity in the NH. The observed decline of
d18O in atmospheric CO2 at WLG (0.5% from May
1991 to December 2002) is consistent with the above
mentioned viewpoints and is likely caused by active 18O
exchange with soils in this region of the NH.
3.2. Atmospheric CO2 and D13C Annual Means,
Growth Rates, and Interannual Variability
3.2.1. Annual Means and Growth Rates
[12] Annual means (derived from monthly means within
each independent year) with linear regressions and growth

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Figure 2. Monthly mean atmospheric CO2 mixing ratios, d13C, and d18O at WLG from May 1991 to
December 2002. The solid circles represent CO2, d13C, and d18O monthly means, respectively, from the
NOAA Climate Monitoring and Diagnostics Laboratory flask air-sampling program. The open circles are
CO2 monthly means from the WLG in situ measurements.

rates (estimated incremental value in each year) of the
atmospheric CO2 mixing ratio and d13C at WLG from
1992 to 2002 are shown in Figure 3. The d13C annual
means and growth rates in 1994, 1995, and 1996 are
missing because of large data gaps (no data available from
November 1994 to June 1996 due to sampling malfunction).
The d18O annual means (growth rates as well as interannual
fluctuations) have not been computed and discussed
because of frequent data gaps within most of the years
and the large scatter of the monthly data.
[13] The CO2 annual means vary from 356.65 ppm
(minimum in 1992) to 372.65 ppm (maximum in 2002)
and increase approximately linearly with a 10-year average
(±2 s) mean growth rate of 1.60 ± 0.38 ppm yr1 from 1992
to 2002. The d13C annual means vary from 7.867%
(heaviest in 1993) to 8.115% (lightest in 2000) and
decrease almost linearly with a 10-year average (±2s) mean
decrease rate of 0.017 ± 0.040% yr1. The 10-year
average (±2 s) mean ratio of the secular trends from
1992 to 2002 (Dd13C/DCO2) is calculated to be (0.011 ±

0.105)% ppm1. The ratio observed at WLG is much
higher than the Dd13C/DCO2 ratios of  0.030 to
0.020% ppm1 [Keeling et al., 1979; Mook et al.,
1983; Nakazawa et al., 1993, 1997a, 1997b] observed
from other background tropospheric measurements during
previous time spans. Additionally, the ratio at WLG is
higher than the global average ratio of 0.021% ppm1
estimated a decade ago by using a two-dimensional (2-D)
transport model [Pearman and Hyson, 1986] and is close
to ratios of 0.015 to 0.011% ppm1 at La Jolla,
Mauna Loa, Fanning-Christmas Islands, and the South
Pole deduced from a 3-D transport model [Keeling et
al., 1989a]. On the basis of other studies [Ciais et al.,
1995a, 1995b, 1999; Miller et al., 2003; Nakazawa et al.,
1993, 1997a, 1997b; Still et al., 2003] the average Dd13C/
DCO2 ratio at WLG is much higher than the rate of
0.05% ppm1 expected for the short-term fossil fuel
combustion sources or discrimination by C3 plants. It is,
however, lower than the 0.005% ppm1 expected
from short-term oceanic CO2 sources and close to the

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Figure 3. Annual means (solid circles) and growth rates
(open circles and thin lines) of the atmospheric CO2 mixing
ratio and d13C at WLG, 1992– 2002. Linear regressions
(thick lines) were passed through all the annual data in order
to illustrate the secular trend more clearly.
0.01% ppm1 from short-term discrimination by terrestrial C4 plants. We postulate that the average changing
ratio at WLG is due to the initial CO2 released into the
atmosphere by fossil fuel combustion and deforestation
exchanging and equilibrating between the atmospherebiosphere and the atmosphere-oceans over a 10-year
timescale, during which the isotopic signal is diluted faster
than that of the original CO2 emitted into the atmosphere.
The fast equilibration during short-term CO2 exchange
along with isotopic disequilibrium could affect the final
isotopic value of the atmosphere that is measured and the
interannual variations.
3.2.2. Interannual Variability
[14] The CO2 growth rates from 1992 to 2002 vary from
0.77 ppm yr1 (minimum in 1992 and a notable low value
0.99 ppm yr1 in 2000) to 2.52 ppm yr1 (maximum in

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1998), showing significant interannual variability. The d13C
growth rates from 1992 to 2002 vary from 0.069% yr1
in 1998 to 0.044% yr1 in 2000, showing significant
interannual changes.
[15] On the basis of numerous other studies [Andres et al.,
1996; Ciais et al., 1999; Conway et al., 1994; Francey et
al., 1990, 1995, 1998; Keeling et al., 1989a, 1989b, 1995;
Morimoto et al., 2000; Nakazawa et al., 1993, 1997a,
1997b; Trolier et al., 1996; Watanabe et al., 2000; WMO,
2003; Zahn et al., 2000] we summarized that the atmospheric CO2 mixing ratio is highest in northern high and
midlatitudes reflecting strong net sources in these areas,
with a North Pole minus South Pole gradient of 3 ppm.
The high global growth rates in 1983, 1987/1988, 1994/
1995, and 1997/1998 are associated with warm El Nin˜o–
Southern Oscillation (ENSO) events. The anomalously
strong El Nin˜o event in 1997/1998 brought about a record
high growth rate in 1998. The exceptionally low growth rate
in 1992 was caused by the low global air temperatures
following the eruption of Mount Pinatubo in 1991. The
observed trend in globally averaged CO2 is 1.6 ppm yr1
during last the 2 decades. The North Pole minus South Pole
d13C gradient is 0.3 %, driven primarily by isotopically
light excess fossil fuel CO2 being preferentially released in
the Northern Hemisphere. The rate of decrease in the d13C is
enhanced in the years corresponding to larger increases in
CO2. The observed trend in d13C during the last 2 decades
averaged 0.025% yr1, and a more persistent flattening
was observed globally beginning in 1988 – 1992, most
likely as a result of increased net uptake by the terrestrial
biosphere.
[16] Biospheric model studies predict that plant respiration and soil decomposition are enhanced in ENSO years
by above-average temperatures and precipitation, which
leads to a reduction in net CO2 uptake by the terrestrial
biosphere [Kindermann et al., 1996; Ito and Oikawa,
2000]. A recent model calculation [Cao et al., 2003] on
the interannual variations and trends in terrestrial carbon
uptake caused by climate variability in China during the
period of 1981 –1998 indicated that both temperature and
precipitation reached the highest value in 1998, a record
for the twentieth century, and resulted in the largest soil
heterotrophic respiration (HR) in arid northwest China
where WLG is located. In 1992, however, both temperature and precipitation reached the lowest values. Cao et al.
[2003] also suggested that in arid northwest China, HR
varied with changes in temperature, but its correlation with
precipitation was not significant. For instance, in another
warm but dry year, 1997 the HR increased significantly in
northeast and southwest China, but it had no increase in
arid northwest China.
[17] It is well known that CO2 exchange between the
atmosphere and oceans or between the atmosphere and
terrestrial C4 plants produces only minor change in atmospheric d13C. The interannual variation of atmospheric CO2
and d13C trends observed at WLG is very likely caused by
worldwide climate variability (e.g., El Nin˜o) and associated
variability in biospheric uptake (including changing C3/C4
composition caused by land use activities). The maximum
CO2 increase along with a maximum d13C decrease

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Figure 4. Atmospheric CO2, d13C, and d18O mean annual
cycles at WLG from May 1991 to December 2002. The
circles and thin lines are derived from the detrended CO2,
d13C, and d18O monthly data in this period. The thick lines
show the marine boundary layer (MBL) reference mean
annual cycles from NOAA Globalview for the same latitude
and the same period.
observed at WLG in 1998 represented the least amount of
carbon entering the biosphere during the period from 1992
to 2002. The lower CO2 and d13C trends at WLG since
1999 indicate that more carbon has been entering the
biosphere since then. The exceptional rises in the d13C
in atmospheric CO2 during the periods 1992 to 1993 and
2000 to 2001 observed at WLG indicate large land sinks
for carbon during those years.
3.3. Atmospheric CO2, D13C, and D18O Mean Annual
Cycles and Year-to-Year Changes
3.3.1. Constructing Detrended Monthly Data
[18] A monthly mean time series of the atmospheric CO2
mixing ratio or isotopes, which is often produced by
removing local effects with very short term variation, is
an integration of variation on different timescales, such as
annual cycle, secular trend, and interannual variation.
According to several studies [Denning et al., 1995; Levin
et al., 1995; Randerson et al., 1997, 2002; Trolier et al.,
1996; WMO, 2003], to investigate mean annual cycle
features or the relationship between seasonal changes of

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the atmospheric CO2 and isotope ratios, secular trends
should first be removed. In this paper, all the detrended
monthly data have been constructed by simply subtracting a
linear secular trend from each of the monthly mean time
series.
3.3.2. CO2, D13C, and D18O Mean Annual Cycles
[19] The atmospheric CO2, d13C, and d18O mean annual
cycles at WLG for the period from May 1991 to December
2002 are shown in Figure 4. Each of the mean annual
cycles was constructed by separately averaging all available data in the detrended monthly mean time series to get
a mean value for each month. Also shown are the annual
cycles for the marine boundary layer (MBL) reference (see
http://www.cmdl.noaa.gov/ccgg/globalview) for the same
latitude.
[20] The CO2 mean annual cycle at WLG has a maximum
in April and minimum in August, declining rapidly during
the May – July growing season and climbing slowly during
September – November. The peak-to-peak annual amplitude
is 10.5 ppm. The CO2 minimum in the mean annual cycle
occurred almost a month earlier than in the MBL reference
(MBL maximum in April and minimum in September,
peak-to-peak amplitude of 9.8 ppm). The phasing of the
d13C mean annual cycle is opposite to that of CO2. The
lowest d13C values occurred at the beginning of the growing
season in April, and the highest occurred at the end of the
growing season in August. This reflects the seasonality of
terrestrial vegetation growth in the middle to high latitudes
of the NH. The peak-to-peak annual amplitude is 0.499%.
The d13C maximum and minimum occurred in the same
months as in the MBL reference (MBL peak-to-peak
amplitude 0.570%). A similar relationship between the
phase of the seasonal cycles of d13C and CO2 has been
observed at other locations in the NH troposphere. These
studies [Nakazawa et al., 1993, 1997a, 1997b; Friedli et al.,
1987; Mook et al., 1983; Keeling et al., 1984, 1989a; Trolier
et al., 1996] indicated that the strong seasonality of atmospheric CO2 and d13C in the NH is mainly due to exchange
of CO2 between vegetation and the atmosphere, i.e., photosynthesis and respiration of the terrestrial biosphere
throughout the year, in which photosynthesis is dominant
in the warm season and respiration exceeds photosynthesis
in the cold season.
[21] The d18O mean annual cycle at WLG is maximum
in June, then decreases sharply during July – September to
reach a minimum in October. The peak-to-peak annual
amplitude is 0.819%. The d18O maximum in the mean
annual cycle at WLG occurred nearly a month later than
the MBL reference (MBL maximum in May and minimum
in October, peak-to-peak amplitude of 1.05%). The lag
of the d18O in the mean annual cycle versus CO2 maximum and minimum at WLG is 2 months, comparable to
the results obtained at Barrow, Alaska (BRW) (71N,
63W) from observations [Trolier et al., 1996] and model
simulation [Ciais et al., 1997b]. An interpretation of the
seasonal d18O variation is much more difficult than for the
d13C and the CO2 mixing ratio. This is due to complicated
combinations of different seasonally varying fluxes of
biospheric CO2 in the atmosphere and the various weatherdependent factors (e.g., solar radiation, temperature, pre-

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Figure 5. Year-to-year fluctuations of the annual cycle amplitudes for the atmospheric CO2 mixing ratio
(solid circles and thick lines) and d13C (open circles and thin lines) at WLG derived from detrended
monthly data in each year, 1992– 2002. The gray line is a linear fit to the annual cycle amplitudes for the
atmospheric CO2 mixing ratio.

cipitation, etc.) governing the d18O composition in CO2
[Nakazawa et al., 1997b]. Ciais et al. [1997b] indicated
that in the NH the influence of soil respiration explains the
observed phase lag of d18O versus CO2. The October –
November minimum in d18O at the high-latitude NH site
BRW is due to isotopic exchange with soils as the
dominant component of the seasonal cycle. However,
canopy exchange contributes proportionally more to the
seasonality of d18O at the lower-latitude NH site Mauna
Loa, Hawaii (MLO) (20N, 155W) than at BRW because
leaf isotopic exchange causes an increase in d18O during
July – August at MLO, so that d18O at MLO reaches
maximum in June – July and minimum in September –
October. The lag of the d18O versus CO2 minimum at
MLO is less than a month. On the basis of the above
statements the measured d18O seasonal cycle in atmospheric CO2 at the middle- to high-latitude NH site WLG
was mainly caused by the latitudinal-dependent d18O of
precipitation and thus soil water.
3.3.3. CO2 and D13C Annual Cycle Amplitude
Variations
[22] Interannual variations in the seasonal cycle of atmospheric CO2 and d13C at WLG for the period of 1992 – 2002
are shown in Figure 5. The CO2 and d13C annual cycle
amplitudes are derived from detrended monthly data points
within each year (maximum minus minimum, i.e., peak to
peak). The d18O annual cycle amplitude interannual variation has not been computed and discussed because of large
data gaps and scatter within most of the years.
[23] During the period from 1992 to 2002 the CO2 annual
cycle amplitude reached the highest value, 11.2 ppm, in
1999 and the lowest value, 9.3 ppm, in 1994. The 11-year
average (±2s) mean annual amplitude is calculated to be
(10.4 ± 0.4) ppm. Except for the year 2000 the amplitudes
for 1995 –2002 are all larger than for 1992 –1994. The d13C
annual cycle amplitude reached a maximum of 0.562% in
1998 and a minimum of 0.479% in 2000. There are no
annual amplitude data for the years 1995 and 1996. The

interannual variations in d13C tend to track those of CO2.
The period of 1998 – 1999 is an exception, with an
amplitude increase in CO2 and a decrease in d13C. The
period of 1993 – 1994 is another exception, with an
amplitude decrease in CO2 and an increase in d13C. The
9-year average (±2s, 1992 – 1994 and 1997 – 2002) annual
amplitude is (0.517 ± 0.016)%.
[24] The interannual variability in the CO2 and d13C
seasonal cycles is due to variations in the seasonal balance
between photosynthesis and respiration, as well as seasonal
oceanic fluxes and atmospheric transport. In the NH middle
and high latitudes, interannual variations in the seasonal
cycles are at least an order of magnitude smaller than their
seasonal variations [Randerson et al., 1997; Zahn et al.,
2000]. A study of the secular trend of the CO2 seasonal
amplitude from longer atmospheric CO2 monitoring records
found that the seasonal amplitude in the NH had increased
[Bacastow et al., 1985]. Other studies have shown that at
stations north of 55N the atmospheric CO2 seasonal
amplitude increased at a mean rate of 0.66% yr1 from
1981 to 1995 [Randerson et al., 1997] and that the CO2
amplitude increased 20% at MLO (20N) from 1958 to
1994 and 40% at BRW (71N) from 1961 to 1994
[Keeling et al., 1996]. A similar trend was observed at
Alert, Nunavut, Canada (ALT) (82N) during the 1980s
[Conway et al., 1994]. Previous studies [Keeling et al.,
1989a; Randerson et al., 1997] also speculated that the
seasonal cycle of atmospheric CO2 at surface sites in
the NH is driven primarily by NEP, so the increase in
the seasonal cycle amplitude suggests that NH terrestrial
ecosystems are experiencing greater CO2 uptake during
the growing season and greater CO2 release during
periods outside the growing season. An increasing trend
in the seasonal cycle amplitude is observed in the WLG
atmospheric CO2 record for the period of 1992– 2002
(0.1 ppm yr1 by a linear fit to the data shown in
Figure 5) in agreement with results from NH sites. The
year 2000 is an exception. The observed lower CO2 and

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ZHOU ET AL.: WALIGUAN CO2 AND ISOTOPES

d13C annual cycle amplitudes are probably due to climate
perturbations and associated variability in biospheric
uptake.

4. Conclusions
[25] The WLG continuous in situ and NOAA CMDL
discrete air sample measurements of CO2 mixing ratios at
WLG are in good agreement with a mean difference (0.59 ±
0.23) ppm for the overlapping monthly means. The in situ
CO2 annual means vary from 356.65 to 372.65 ppm and
increase approximately linearly with a mean growth rate of
(1.60 ± 0.38) ppm yr1 from 1992 to 2002. The d13C annual
means vary from 7.867 to 8.115% and decrease almost
linearly with a mean decline rate of (0.017 ± 0.040)%
yr1 in the same period. The (Dd13C/DCO2) ratio in the
secular trend from 1992 to 2002 is (0.011 ± 0.105)%
ppm1. The CO2 and d13C growth rates both show significant interannual variability that is very likely caused by
worldwide climate anomalies and associated changes in
biospheric uptake. The maximum CO2 increase along with
a maximum d13C decrease at WLG in 1998 reflects the least
amount of carbon entering the biosphere during this period.
The minimum CO2 increase and an abnormal d13C increase
at WLG in 2000 suggest that more carbon is entering the
biosphere.
[26] The mean CO2 seasonal cycle at WLG has a
maximum in April and a minimum in August. The
CO2 minimum in the mean annual cycle occurred almost
1 month earlier than in the MBL reference at the same
latitude. The peak-to-peak annual amplitudes vary irregularly from year to year but appear to have been increasing
since 1995, except for the year 2000. The 11-year average
mean annual amplitude is (10.4 ± 0.4) ppm. The phasing
of the d13C mean annual cycle is opposite to that of CO2.
The d13C maximum and minimum occurred in the same
months as in the MBL reference. The d13C peak-to-peak
amplitudes also vary from year to year. The 9-year average
annual amplitude is (0.517 ± 0.016)%. The d18O mean
annual cycle has a maximum in June and a minimum in
October with peak-to-peak amplitude of 0.819%. The
d18O maximum in the mean annual cycle at WLG occurred
nearly a month later than the MBL reference. The study
presents an 11-year record of atmospheric CO2 and stable
isotopes observed in this particular region. The results will
contribute to a better understanding of the global carbon
cycle, especially in the inland plateau of the Eurasian
continent.

[27] Acknowledgments. This work is supported by a Key Project
sponsored by the Scientific Research Foundation for the Returned Overseas
Chinese Scholars (State Personnel Ministry [2004]99), a Climate Change
Research Foundation (China Meteorological Administration CCSF2005-3DH04), a Japan Society for Promotion of Science Post-doctoral Fellowship
(PB01736), and a United Nations GEF Fund (GLO/91/G32). We thank the
staff of Waliguan Station for their efforts in operating the continuous CO2
observing systems and collecting the flask air samples. We appreciate
NOAA CMDL and CU-INSTAAR for cooperation on the Waliguan NDIR
and flask air-sampling programs. MSC Canada is appreciated for the
cooperation on the Waliguan GC-FID program. We also appreciate the
WMO AREP Environment Division for the coordination of the GAW
program. Helpful comments and suggestions of two anonymous reviewers
are gratefully acknowledged. The authors would like to especially thank
one of the reviewers: The annotated manuscript contributed to a significant

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improvement in the resubmission and the further revised version of this
paper.

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T. J. Conway, Climate Monitoring and Diagnostics Laboratory, NOAA,
325 Broadway, Boulder, CO 80305, USA.
J. Li, School for Environmental Sciences, Peking University, Beijing
100871, China.
K. MacClune and J. W. C. White, Institute for Arctic and Alpine
Research, University of Colorado, Boulder, CO 80309, USA.
H. Mukai, Center for Global Environmental Research, National Institute
for Environmental Studies, 16-2 Onogawa Isulcuba Ibaraki 305, Tsukuba,
Ibaraki 305-8506, Japan.
Y. Wen, X. Zhang, and L. Zhou, Key Laboratory for Atmospheric
Chemistry (LAC), Centre for Atmosphere Watch and Services (CAWAS),
Chinese Academy of Meteorological Sciences (CAMS), China Meteorological Administration (CMA), 46 Zhong-guan-cun South Street, Beijing
100081, China. (zhoulx@cams.cma.gov.cn)

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