Introduction
The budget of atmospheric CO2 is widely studied using the temporal and
spatial variations in concentration and conventional isotopic compositions
(δ13C and δ18O) of CO2 (Francey and Tans, 1987;
Francey et al., 1995; Yakir and Wang, 1996; Ciais et al., 1995a, b, 1997;
Peylin et al., 1999; Cuntz et al., 2003; Drake et al., 2011; Welp et al.,
2011; Affek and Yakir, 2014). δ13C is useful to differentiate the
exchange of CO2 with the ocean and land biospheres. This is due to the
fact that the photosynthetic discrimination against 13C during exchange
with land plants is higher than that associated with the chemical dissolution
of CO2 in the ocean (e.g., Tans et al., 1993; Ciais et al., 1995a;
Francey et al., 1995; Ito, 2003; Bowling et al., 2014). The major limitation
of δ13C is that it cannot distinguish CO2 produced by high-temperature combustion or low-temperature respiration (Affek and Eiler, 2006;
Laskar et al., 2016a). δ18O in atmospheric CO2 is mainly
controlled by various water reservoirs (ocean, leaf and soil). In urban
locations, a significant fraction of CO2 may have combustion origin
possessing δ18O signature of atmospheric O2 (Kroopnick and
Craig, 1972; Ciais et al., 1997; Yakir and Wang, 1996). δ18O is
used for partitioning net CO2 terrestrial fluxes between soil
respiration and that exchange with plant leaves, the exchange rate is
enhanced by the presence of carbonic anhydrase in plants and soils (Francey
and Tans, 1987; Farquhar and Lioyd, 1993; Yakir and Wang, 1996; Ciais et al.,
1997; Peylin et al., 1999; Murayama et al., 2010; Welp et al., 2011). This is
because δ18O of CO2 fluxes originated from soil respiration
is different from that exchanged with leaf water. δ18O in soil
water reflects the δ18O value of local meteoric water while leaf
water is relatively enriched due to transpiration. The δ18O values
from these processes and interactions are different and hence the tracer is
widely used for constraining the gross production of CO2 (Francey and
Tans, 1987; Ciais et al., 1997; Gillon and Yakir, 2001; Cuntz et al., 2003;
Welp et al., 2011). However, due to rapid exchange of oxygen isotopes between
CO2 and different water reservoirs with diverse δ18O and
processes such as evapotranspiration complicate its interpretation (Riley et
al., 2003).
The doubly substituted isotopologues or clumped isotopes such as
13C18O16O in CO2, denoted by Δ47, provides an
additional and independent constraint to study the atmospheric CO2
budget. Δ47 in air CO2 can help to identify the mechanisms for
CO2 production and consumption. Unlike conventional isotopes, clumped
isotope studies for the atmospheric CO2 are very limited mainly because
of challenges to apply it to the atmospheric study (Eiler and Schauble, 2004;
Affek et al., 2007; Yeung et al., 2009). The available data are not
sufficient to address some key issues such as quantification of CO2 from
different sources and to what extent the air CO2 is in thermodynamic
equilibrium with leaf and surface waters, especially in regions with strong
anthropogenic activities such as urban areas. Also the effect of
photosynthesis on the Δ47 of air CO2 has not been studied
rigorously. δ18O and Δ47 were reported to have similar
timescales for the isotope exchange between CO2 and water (Affek, 2013;
Clog et al., 2015), but no comparative study on their behavior in presence of
other processes such as photosynthesis and respiration was done. A combined
assessment from all the three aforementioned isotopic tracers can better
constrain the budget of CO2 and associated processes than δ13C
or δ18O alone.
Theoretically it is shown that in thermodynamic equilibrium, Δ47
values of CO2 are temperature dependent (Eiler and Schauble, 2004; Wang
et al., 2004), verified over a wide range from 10 to 1000 ∘C (Dennis
et al., 2011). Processes that involve CO2 and liquid water as medium,
such as isotopic exchange with ocean water, are expected to have
Δ47 values close to the thermodynamic equilibrium. Δ47
values in ambient air CO2 should reflect a balance of CO2 fluxes
between biosphere–atmosphere exchange, ocean–atmosphere exchange and
emissions from combustion sources. Photosynthesis involves gas-phase
diffusion of CO2 into leaves, fixes about
one-third of the CO2 and
returns the rest back to the atmosphere. CO2 molecules inside a leaf are
generally expected to be in thermodynamic equilibrium with leaf water because
of presence of enzymatic carbonic anhydrase that greatly enhances the
isotopic exchange (Cernusak et al., 2004). Δ47 values of soil
respired CO2 is also not well constrained, though it is believed to be
in thermodynamic equilibrium with the soil water (Eiler and Schauble, 2004).
Here, we present clumped and conventional isotope data in near-surface air
CO2, covering a wide variety of processes and interactions. Air
samplings were made in South China Sea, two coastal stations in northern
Taiwan, an urban traffic street, a suburban location, a forest site, a
greenhouse, the top of a high mountain and car exhaust. The study was designed
and aimed to show the extents of the deviations of near-surface atmospheric
CO2 from thermodynamic equilibrium with local surface water. Possible
influences from other processes such as anthropogenic emission, respiration
and photosynthesis on clumped isotopes were explored.
Materials and methods
Stable isotopic compositions of CO2 including mass 47 were measured
using a Finnigan MAT 253 stable gas source isotope ratio mass spectrometer
configured to measure ion beams corresponding to m/z 44 through 49. The
instrument registers the major ion beams (44, 45 and 46) through resistors
108, 3×1010 and 1011 Ohm, respectively, and minor ion
beams (47, 48 and 49) through 1012 Ohm. All the measurements were
carried out at Research Center for Environmental Changes, Academia Sinica,
Taiwan.
Air samples were collected in 2 L flasks and compressed to
2 bar atmospheric pressure using a membrane pump. The
flasks, equipped with two stopcocks, were first flushed with the ambient air
for ∼ 10 min before starting the sample collection. We then closed the
downstream end stopcock, allowed the pressure to build to
2 bar and then isolated by closing the other
stopcock. The air pumping for flushing and sampling was carried out through a
column packed with magnesium perchlorate to remove moisture. The moisture
content was reduced from the ambient value of 70–90 % to less than
1 % relative humidity, checked using a LI-COR infrared gas analyzer
(model 840A, LI-COR, USA). See, for example, Liang and Mahata (2015) for more
details of air sampling.
Left panel: map of Taiwan and South China Sea with the locations of
marine air sampling stations (a–e). The coastal stations 1 and 2
are Fugui Cape and Keelung and 3 is the high mountain station Hehuan
(∼ 3.2 km a.s.l.). Right panel: part of Taipei city with sampling
stations Roosevelt Road (RR), grassland in the National Taiwan University
(NTU) Campus, suburban site inside the campus of Academia Sinica (AS) and
forest site.
To show how photosynthesis and respiration affect the abundances of CO2
isotopologues and to demonstrate what different information the Δ47
can give from the other isotopologues, we performed analyses for CO2
collected in a controlled greenhouse with cemented floor located in the top
(third) floor of the Greenhouse Building, Academia Sinica. The size of the
greenhouse was about 8 m long, 5 m wide and 5 m high. It was closed at
least 1 day before each experiment and the ventilation was kept off. More
than 70 % of the ground area inside the greenhouse was occupied with
Cinnamomum cassia plants, each of ∼ 2 m height kept in pots.
Samples were collected at intervals of less than half an hour to a few hours
on 3 sunny days and 1 cloudy day to investigate the influence of
photosynthesis and respiration on the isotopologues of CO2. Inside the
room relative humidity was ∼ 50–70 % for the 3 sunny days and was
above 90 % for the cloudy day.
Forest air CO2 was collected from a dense natural forest at the west end
of the Academia Sinica Campus. The vegetation mainly consists of medium to
big size trees with canopy heights varying between 10 and 20 m. The samples
were collected ∼ 100 m inside the forest on a small plateau at a
height of ∼ 30 m from the ground in the slope of a hill; the dense
vegetation allowed little sunlight penetrating to the surface. The relative
humidity at the site was 80–90 % during the sampling days and wind speed
was nearly zero due to presence of hills on three sides of the sampling spot.
Marine air was collected during a cruise in the South China Sea (for the
cruise track see Fig. 1) at a height of ∼ 10 m a.s.l. and from two
coastal stations: Keelung (25∘09′6′′ N,
121∘46′22′′ E) and Fugui Cape (25∘18′ N,
121∘32′ E) (Fig. 1) at a height of ∼ 5 and
∼ 20 m a.s.l., respectively. Sea surface temperatures were measured at
the time of sampling. Urban air was collected at a bus stop on Roosevelt
Road, a busy street in Taipei. Suburban air was collected from an open roof
(∼ 30 m above ground) of Institute of Earth Science Building, Academia
Sinica (AS; 25∘2′41′′ N, 121∘36′52′′ E); grassland
air was collected from a grass field in front of the Department of
Atmospheric Science, National Taiwan University Campus (NTU;
25∘1′ N, 121∘30′ E), Taipei. In addition, we collected
air from the summit of the Hehuan mountain (24∘8′15′′ N,
121∘16′32′′ E; 3.2 km a.s.l.) (Fig. 1) on 9 October 2013. All
air samplings were made when there was no rain to avoid direct interaction
with the rainwater. Car exhaust was collected from a Mazda 3000 cc TRIBUTE
and a Mitsubishi 2400 cc New Outlander, using evacuated 2 L glass flasks
from ∼ 20 cm inside the exhaust pipes through a column of magnesium
perchlorate.
CO2 was extracted from air using a glass vacuum line connected to a
turbo molecular pump by cryogenic technique. The vacuum line as well as the
sample flask connection assembly including its head space was pumped to high
vacuum before starting the CO2 extraction. Air in the flask was pumped
through a series of five coiled traps, with the first two immersed in dry
ice–acetone slush (-77 ∘C) for trace moisture removal followed by
three in liquid nitrogen (-196 ∘C). CO2 was collected from
the traps immersed in liquid nitrogen by repeated freeze–thaw technique at
liquid nitrogen and dry ice temperatures for further removal of traces of
water (see Mahata et al., 2012; Liang and Mahata, 2015 for details). The air
was pumped for 40–45 min at a controlled rate of ∼ 90 mL min-1
using a mass flow controller; the pressure on the post-mass-flow controller side of the vacuum line was ∼ 10 mm of Hg. No measurable isotopic
fractionation caused by mass flow controller at this flow rate was observed
when checked using several aliquots of air from a high-volume compressed air
cylinder (∼ 40 L at 2000 psi). For car exhaust, an aliquot of exhaust
air was transferred to a 60 mL bottle and CO2 was fully extracted
cryogenically following the same protocol as discussed above (but with mass
flow controller step skipped).
Diurnal variation of (a) concentration,
(b) δ13C and (c) δ18O of CO2
sampled in the greenhouse. Keeling plots for
(d) δ13C and (e) δ18O and
(f) scatter plot of δ13C and δ18O to show their
covariance.
Diurnal variation of δ13C and δ18O and clumped
isotopes (Δ47) for greenhouse CO2. Temperatures estimated
using Δ47 values and actual air temperatures inside the greenhouse
at the time of sampling are also presented.
Date
Time
Conc.
δ13C (‰)
δ18O (‰)
δ47 (‰)
SE
Δ47 (‰)
SE
Δ48 (‰)
Estimated
Air temp.
(ppmv)
(VPDB)
(VSMOW)
(ARF)
temp. (∘C)
(∘C)
28 Jul 2015
04:50
481
-11.60
39.61
6.99
0.02
0.927
0.016
0.2
24
25.5
06:00
462
-10.90
39.92
8.16
0.02
0.936
0.018
0.6
21
26
07:06
435
-9.80
40.54
9.71
0.02
0.911
0.017
0.2
28
29
08:10
428
-9.60
40.92
10.38
0.02
0.883
0.014
-0.2
33
33.5
09:15
416
-9.06
41.36
11.30
0.01
0.908
0.011
0.2
24
39
10:15
422
-9.55
40.82
NA
NA
NA
NA
NA
NA
NA
12:40
407
-8.77
41.58
11.75
0.01
0.898
0.010
0.2
27
48
31 Jul 2015
05:00
522
-12.72
38.66
5.10
0.01
0.926
0.015
0.3
24
26
06:00
512
-12.37
38.95
5.94
0.01
0.926
0.014
0.5
25
26
07:00
451
-10.08
40.36
9.39
0.02
0.923
0.011
0.4
25
28
08:15
405
-8.82
40.98
11.25
0.02
0.912
0.020
0.4
28
33
09:10
412
-9.12
41.07
11.26
0.02
0.880
0.020
0.6
34
37.5
10:00
414
-9.35
40.83
11.52
0.01
0.906
0.010
0.6
23
43.5
11:20
411
-9.26
40.99
11.12
0.02
0.896
0.025
0.5
31
48
15:00
432
-9.90
40.36
9.55
0.02
0.877
0.015
0.5
34
41.5
17:25
423
-9.22
41.07
12.48
0.02
0.929
0.013
0.7
25
32
21:30
462
-10.92
39.99
7.90
0.01
0.911
0.012
0.4
28
27
4 Aug 2015
04:50
465
-11.03
40.37
8.41
0.01
0.936
0.012
0.27
23
24
05:50
455
-10.82
40.26
NA
NA
NA
NA
NA
NA
NA
06:28
448
-10.27
41.00
10.01
0.02
0.931
0.017
0.7
24
25.5
06:50
439
-9.90
41.32
10.10
0.02
0.942
0.009
0.6
22
26
07:15
420
-9.34
41.22
11.05
0.01
0.914
0.013
0.6
28
28.5
07:40
419
-9.18
41.22
11.05
0.01
0.927
0.011
0.3
25
30
08:10
405
-8.55
41.56
12.79
0.02
0.900
0.015
0.6
31
32.5
09:45
427
-9.75
40.73
10.81
0.02
0.870
0.023
0.3
36
40
14:00
414
-9.20
41.01
11.02
0.01
0.896
0.011
0.6
31
46
16:15
414
-9.09
41.11
11.11
0.01
0.944
0.014
0.7
22
36.5
19:15
413
-9.01
41.38
13.28
0.01
0.921
0.010
0.9
26
29.2
22:30
450
-10.58
40.61
9.34
0.02
0.924
0.022
0.4
25
26.5
12 Oct 2015
05:45
418
-9.30
40.87
10.80
0.01
0.934
0.013
0.5
23
22
07:00
413
-9.08
41.18
10.95
0.02
0.940
0.021
0.4
22
22
10:00
390
-7.78
41.66
13.00
0.02
0.918
0.014
0.6
26
25
11:50
388
-7.84
41.71
15.25
0.01
0.919
0.010
0.6
26
27
14:30
382
-7.82
42.24
14.27
0.02
0.891
0.017
0.4
31
28
20:10
418
-9.17
40.61
10.85
0.02
0.933
0.017
0.5
23
23
CO2 was further purified from other condensable species like N2O,
CH4 and hydrocarbons by means of gas chromatography (Agilent 6890N, with
a 3.0 m × 0.3 cm stainless steel column packed with PorapakQ
80/100 mesh, supplied by Supelco Analytical, Bellefonte, PA, USA) with the
column kept at -10 ∘C. High-purity helium (> 99.9999 %
supplied by Air Products and Chemicals, Inc.) at 20 mL min-1 was used
as carrier gas. CO2 was eluted first, followed forthwith by N2O,
and CH4, hydrocarbons and traces of water came out much later. To get an
optimized condition for CO2, we checked the separation of CO2 from
N2O with varying proportions and at various temperatures (25 to
-20 ∘C) and found a temperature of -10 ∘C at which
column separated CO2 from N2O perfectly (see Laskar et al., 2016b,
for details). The column was baked at 200 ∘C for more than 2 h
prior to use. The conditioned column is good for purifying three samples. At
the end of the day, long baking (8–10 h) was performed. At the initial
phase the working gas was taken from a high-purity commercial CO2 called
AS-2 (δ13C =-32.54 ‰ with respect to
VPDB and
δ18O =36.61 ‰ with respect to
VSMOW) procured from a local supplier (Air
Products and Chemicals, Inc.). As the difference between the isotopic
compositions of samples and AS-2 was high, we later changed the reference to
Oztech CO2 (δ13C =-3.59 and
δ18O = 24.96 ‰) (Oztech Trading Corporation, USA) from
December 2014 onward. No detectable difference in isotopic compositions
including Δ47 was observed between the analyses from different
working references. All δ13C values presented in this work are
expressed in VPDB scale and δ18O in VSMOW scale, unless specified
otherwise. Δ47 is calculated following Affek and Eiler (2006):
Δ47=R472R13R18+2R17R18+R13R172-R462R18+2R13R17+R172-R45R13+2R17+1×1000,
where R13 and R18 (ratios 13C /12C and
18O /16O) are obtained by measuring the conventional masses 44, 45
and 46 in the same CO2 sample and R17 is calculated assuming a
mass-dependent relation with R18 given by R17=RVSMOW17R18/RVSMOW18λ, where exponent λ=0.5164 is used for all Δ47 calculations (Affek and Eiler, 2006).
The value of λ varies between 0.516 and 0.523 (Hoag et al., 2005;
Barkan and Luz, 2012; Hofmann et al., 2012; Thiemens et al., 2014). The
variation in Δ47 was less than 0.01 ‰ at 25 ∘C
when the exponent was varied over the aforementioned range. This variation
was comparable to the measurement uncertainty and hence not considered here;
all the calculations were based on λ=0.5164. Δ47 is
obtained by measuring CO2 with respect to which the isotopes among
various CO2 isotopologues are distributed randomly
(Δ47∼ 0 ‰). Practically, this random distribution is
approached by heating CO2 at 1000 ∘C for more than 2 h (Eiler
and Schauble, 2004; Affek and Eiler, 2006). Measurements were made with a
stable ∼ 12 V signal at mass 44, with peak centering, background
scanning and pressure balancing before each acquisition started. Each sample
was analyzed for 10 acquisitions, 10 cycles each, at an integration time of
8 s; the total analysis time was approximately 2.5 h. Masses 48 and 49 were
monitored to check isobaric interferences due to contamination of
hydrocarbons (Ghosh et al., 2006). Details about the corrections due to
nonlinearity related to Δ47 measurements in the mass spectrometer
and
reference frame equation for
expressing the measured Δ47 values in absolute reference frame
(ARF) were discussed in Laskar et al. (2016b). To obtain the temperature from
the Δ47 values, we used the following relation (Dennis et al.,
2011):
Δ47=0.0031000T4-0.04381000T3+0.25531000T2-0.21951000T+0.0616.
The reproducibility (1 σ standard deviation) for air CO2
measurements was established from three aliquots of CO2 extracted from a
compressed air cylinder with CO2 concentration ([CO2]) of
∼ 388 ppmv. The 1 σ standard deviations were 0.07, 0.08 and
0.01 ‰ for δ13C, δ18O and Δ47,
respectively (Table S1 in the Supplement). The long-term reproducibility in
Δ47 measurements was found to be 0.014 ‰ (Laskar et al.,
2016b) and the accuracy in Δ47 values in terms of temperature,
based on CO2 equilibrated with water at known temperatures were better
than 3 ∘C (see Table S2 in Supplement).
Diurnal variation of the Δ47 and δ18O values in
the greenhouse for samples collected on 4 days of 2015:
(a) 28 July, (b) 31 July, (c) 4 August and
(d) 12 October. The first 3 days (a–c) were bright
sunny days and the last was (d) a cloudy day with covered rooftop
(see texts for details). The error bars are 1 standard error associated with
the measurements.
For [CO2] measurements, flasks of volume 350 cc were used. These small
flasks were connected in series with the larger flasks used for isotopic
measurements. [CO2] was measured using a LI-COR infrared gas analyzer
(model 840A, LI-COR, USA) at 4 Hz, smoothed with 20 s moving average. The
analyzer was calibrated against a working standard (air compressed in a
cylinder) with a nominal [CO2] of 387.7 ppmv and a CO2-free
N2 cylinder. The reproducibility of LI-COR was better than 1 ppmv. The
working standard was calibrated using a commercial Picarro analyzer (model
G1301, Picarro, USA) by a series of NOAA/GMD certified tertiary standards
with [CO2] of 369.9, 392.0, 409.2 and 516.3 ppmv, with a precision
(1 σ standard deviation) of 0.2 ppmv. The [CO2] in car exhaust
was estimated by gravimetric technique using an MKS Baratron gauge.
Ambient temperatures were taken from the nearest governmental weather
stations (operated by Central Weather Bureau, Taiwan): Nankang (for AS;
station code: C0A9G0; 25∘03′27 N, 121∘35′41 E;
42 m a.s.l.), Taipei (for NTU; station code: C1A730; 25∘00′58 N,
121∘31′53 E; 22 m a.s.l.), Hehuan mountain (station code:
C0H9C1; 24∘08′41 N, 121∘15′51 E; 3240 m a.s.l.) and
Keelung coast (for the two coastal sites; station code: 466940;
25∘08′05 N, 121∘43′56 E; 26.7 m a.s.l.).
Results
Greenhouse CO2
Diurnal variation in the concentration and isotopic compositions of CO2
inside the controlled greenhouse is shown in Fig. 2. The lowest CO2
concentration [CO2] and highest δ13C and δ18O values
were observed during late morning hours, while highest [CO2] and lowest
δ13C and δ18O values were observed during nighttime and
early morning before sunrise (Table 1 and Fig. 2a–c), indicating that
respiration and photosynthesis played the major role in controlling the
variations of the [CO2] and isotopic compositions. A Keeling plot, a
graphical approach plotted between isotopic composition and the inverse of
the concentration, is used to determine the isotopic composition of the source
(Pataki et al., 2003). It is valid for a mixing of two components; the
intercept of the plot gives the source isotopic composition. Respiration was
the main source of CO2 here added to the background CO2. Keeling
analysis for δ13C had an intercept of
-26.32 ± 0.40 ‰ (Fig. 2d), a value expected for C3
plant respired CO2. The Keeling plot for δ18O had an intercept
of 30.68 ± 0.73 ‰ (Fig. 2e), which could be explained by a
combined effect of respired CO2 equilibrated with soil water and kinetic
fractionation associated with the diffusion of CO2 from soil to the air.
A Keeling plot for δ13C with the early morning and nighttime
greenhouse data, when photosynthesis was absent, was found to have same
intercept as observed with all the data, only the correlation was better for
the latter (R2=0.999, not shown). The tight correlations among
[CO2], δ13C and δ18O (Fig. 2d–f) suggest that
photosynthesis and respiration were the dominant processes controlling their
variations, while mixing with ambient air and anthropogenic contribution of
CO2 were insignificant.
Correlation between the observed and thermodynamic equilibrium
Δ47 values for greenhouse CO2 samples collected when
(a) photosynthesis was weak and respiration was strong and
(b) photosynthesis was strong and respiration was weak.
Stable carbon and oxygen isotopic composition and clumped isotopes
(Δ47) for car exhaust CO2. Temperatures estimated using
Δ47 values and lowest possible combustion temperatures are given.
Car model
Conc.
δ13C (‰)
δ18O (‰)
δ47 (‰)
SE
Δ47 (‰)
SE
Δ48 (‰)
Estimated
Combustion
(ppm)
(VPDB)
(VSMOW)
(ARF)
temp. (∘C)
temp. (∘C)
Mazda 3000 cc
39 400
-27.73
25.43
-22.20
0.01
0.251
0.013
-0.4
300
800
TRIBUTE
Mitsubishi 2400 cc
39 300
-27.67
25.27
-23.08
0.02
0.294
0.007
-0.3
265
800
New Outlander
Average ± 1σ
39 350 ± 50
-27.70 ± 0.03
25.35 ± 0.07
-22.64 ± 0.44
0.273 ± 0.021
283 ± 18
In contrast, Δ47 shows different patterns of diurnal variability
due to the effect of photosynthesis and respiration. Figure 3a–d detail
diurnal variations in Δ47 in the greenhouse CO2 on 4 different
days. The first three were bright sunny days with photosynthesis as the
dominant process while the last one was a dark cloudy day affected more by
respiration. To further reduce photosynthetic activity on the last day, two
layers of black cloth that cut down the incident sunlight by ∼ 50 %
were deployed. The measured Δ47 values were also compared with the
thermodynamic equilibrium values. The maximum value of Δ47 was
observed in the morning before ∼ 08:00 and at night and the values were
similar to the thermodynamic equilibrium values at the ambient temperatures.
This indicates that the respired CO2 was in close thermodynamic
equilibrium with the leaf and soil water. The daytime (from 09:00 to 17:00)
Δ47 values for the 3 sunny days were higher than the thermodynamic
equilibrium values. The Δ47 values were observed to decrease
steadily in the early morning before ∼ 09:00 and increase afterwards
(Fig. 3). By comparing the Δ47 values acquired on the sunny days
with that on the cloudy day, we noticed that when photosynthesis was weak,
the Δ47 value was close to the thermodynamic equilibrium with soil
and leaf water (Fig. 4). The correlation between Δ47 and
[CO2], δ13C or δ18O (Fig. 3d) was observed only when
the photosynthesis was weak. This suggests that Δ47 carries
information different from concentration and conventional isotopic
composition when photosynthesis occurs. See Sect. 4.1 for detailed
discussion.
Car exhaust
The [CO2], δ13C and δ18O values of car exhaust
CO2 were 39 350 ± 50 ppmv, -27.70 ± 0.03 and
25.35 ± 0.07 ‰, respectively (Table 2). δ13C value
was similar to that reported elsewhere (Newman et al., 2008; Popa et al.,
2014), the δ18O was slightly higher than the atmospheric O2
(∼ 23.5 ‰), and the source of O2 for combustion. The
average value of Δ47 for the exhaust from the two cars was
0.273 ± 0.021 ‰, which gave a temperature of
282 ± 17 ∘C (Table 2). This temperature is much lower than the
fuel combustion temperatures (> 800 ∘C). The possible reason for
higher values of δ18O and Δ47 in the exhaust CO2
than expected was post-combustion partial exchange with water and other
gaseous species, released during combustion, inside the catalytic converter
and the exhaust pipe (see discussion in Sect. 4.2).
Stable isotopic composition including Δ47 for atmospheric
CO2 collected over South China
Sea and two coastal stations (see Fig. 1 for sampling locations).
Temperatures estimated using Δ47 values and the sea surface
temperatures at the time of samplings are also presented.
Marine air CO2
Date
Time
Conc.
δ13C (‰)
δ18O (‰)
δ47 (‰)
SE
Δ47 (‰)
SE
Δ48 (‰)
Estimated
Sea surface
(ppm)
(VPDB)
(VSMOW)
(ARF)
temp. (∘C)
temp. (∘C)
South China Sea*
15 Oct 2013
08:15 (A)
403
-8.42
40.85
28.752
0.016
0.901
0.017
1.9
30
28.3
13:15 (B)
400
-8.46
40.80
28.441
0.012
0.919
0.011
2.6
26
28.3
18:00 (C)
406
-8.75
40.54
28.133
0.013
0.933
0.013
2.2
24
28.3
16 Oct 2013
07:00 (D)
391
-8.76
40.53
27.916
0.024
0.903
0.023
3.9
29
28.2
12:05 (E)
397
-8.44
40.86
28.535
0.015
0.910
0.015
3.3
28
28.2
14:00 (E)
391
-8.30
40.96
28.922
0.021
0.934
0.021
3.0
23
28.2
17:20 (E)
395
-8.31
41.02
28.944
0.017
0.908
0.016
1.9
29
28.1
20:20 (E)
388
-8.19
40.52
28.909
0.018
0.930
0.018
3.8
24
28.1
17 Oct 2013
08:40 (E)
383
-8.26
40.41
28.194
0.018
0.925
0.018
4.3
25
28.1
Average ± 1σ
395 ± 7
-8.43 ± 0.19
40.72 ± 0.20
28.52 ± 0.36
0.918 ± 0.012
27 ± 2
28.2 ± 0.1
Keelung
3 Oct 2013
11:30
380
-8.31
40.31
28.053
0.020
0.896
0.021
3
31
27.5
12:30
384
-8.40
40.92
29.089
0.017
0.917
0.016
1.9
27
27.5
13 Nov 2013
11:00
401
-8.45
40.62
29.645
0.015
0.946
0.016
4.0
21
27.5
21 Nov 2013
12:30
-8.47
40.78
29.866
0.017
0.890
0.010
1.1
32
27.5
28 Nov 2013
12:00
410
-8.60
40.21
28.992
0.011
0.908
0.010
2.2
28
27.5
Average ± 1σ
394 ± 12
-8.45 ± 0.09
40.57 ± 0.26
29.12 ± 0.63
0.911 ± 0.020
28 ± 4
27.5
Fugui Cape
13 Nov 2013
13:30
401
-8.47
40.76
29.56
0.02
0.916
0.016
1.1
27
27.5
21 Nov 2013
15:30
399
-8.41
40.89
29.37
0.01
0.880
0.012
2.5
34
27.5
28 Nov 2013
15:00
407
-8.70
41.16
30.11
0.01
0.886
0.010
3.1
33
27.5
Average ± 1σ
402 ± 3
-8.53 ± 0.12
40.94 ± 0.16
29.68 ± 0.29
0.894 ± 0.015
31 ± 3
27.5
* Sampling Stations (see Fig. 1 for
locations in South China Sea)
(a) Carbon Keeling plot for air CO2 collected over
South China Sea (blue symbol) and coastal
stations (pink symbol) (Keelung and Fugui Cape).
(b) Δ47 values observed over the South China Sea and
coastal stations. The error bars are the 1 standard error associated with the
measurements. Lines show Δ47 values for the CO2 in
thermodynamic equilibrium at ambient temperatures.
Atmospheric CO2 over ocean and coasts
Isotopic compositions including Δ47 values obtained for CO2
over ocean and coasts are presented in Table 3. The averaged [CO2] over
ocean between latitudes 18∘03′ and 21∘17′ N was
395 ± 7 ppmv, and the values of δ13C and δ18O were
-8.43 ± 0.19 and 40.72 ± 0.20 ‰, respectively
(Table 3). In the coastal stations, the averaged values of [CO2],
δ13C and δ18O were 397 ± 10 ppmv,
-8.48 ± 0.11 and 40.70 ± 0.29 ‰, respectively. Both
the [CO2] and δ13C values over the ocean and coasts were
similar to those observed at Mauna Loa during the sampling period, suggesting
little contribution from local/regional anthropogenic sources. The Keeling
analysis for δ13C gave an intercept of -13.61 ‰
(Fig. 5a) for the air CO2 collected over the ocean and coasts.
δ18O of air CO2 over the ocean was close to the isotopic
equilibrium values with the surface sea water at the sea surface temperatures
(see Sect. 4.3). The Δ47 values varied between 0.880 and
0.946 ‰ for the marine and coastal CO2 (Table 3, Fig. 5b),
similar to that predicted at thermodynamic equilibrium at sea surface
temperatures (obtained using Eq. 2). Therefore, both δ18O and
Δ47 values suggest that the air CO2 over the ocean was in
close thermodynamic equilibrium with the underlying sea water.
Stable isotopic composition including clumped isotopes
(Δ47) for air CO2 collected in urban and suburban stations,
grassland, forest and high mountain environments. Temperatures estimated
using Δ47 values and air temperatures are also
presented.
Date
Time
Conc.
δ13C (‰)
δ18O (‰)
δ47 (‰)
SE
Δ47 (‰)
SE
Δ48 (‰)
Estimated
Air
(ppm)
(VPDB)
(VSMOW)
(ARF)
temp. (∘C)
temp. (∘C)
Urban CO2: Roosevelt Road, Taipei City
30 Dec 2015
12:30
510
-10.41
40.00
25.26
0.014
0.823
0.010
2.3
46
20
15:00
478
-11.50
38.49
22.63
0.012
0.754
0.008
0.9
62
19.5
17:00
461
-9.69
40.70
26.74
0.017
0.833
0.013
0.9
44
17
18:00
594
-12.30
38.14
21.56
0.014
0.819
0.015
1.5
47
16
20:00
457
-11.34
39.24
23.61
0.022
0.806
0.022
3.1
50
15
Average ± 1σ
500 ± 50
-11.05 ± 0.90
39.31 ± 0.94
23.96 ± 1.84
0.807 ± 0.028
50 ± 6
17 ± 2
Suburban air CO2
Academia Sinica Campus
17 Oct 2013
10:00
400
-7.83
40.44
28.47
0.015
0.899
0.008
3.7
30
25
14:30
402
-8.05
40.25
28.07
0.017
0.889
0.008
2.2
32
25
17:20
409
-8.44
39.90
27.26
0.019
0.877
0.020
2.3
34
22
30 Oct 2013
10:00
395
-8.48
40.57
28.47
0.012
0.876
0.010
2.8
35
25.2
14:30
400
-8.25
41.08
29.03
0.016
0.893
0.016
3.9
31
27.4
4 Nov 2013
10:30
411
-8.78
40.51
28.67
0.011
0.902
0.009
2.7
29
22.5
14:30
406
-8.64
40.62
28.97
0.017
0.895
0.016
2.2
31
22
18:30
415
-9.02
40.38
28.33
0.013
0.907
0.009
2.8
28
22.5
9 Nov 2013
10:30
405
-8.34
41.09
29.79
0.019
0.917
0.015
1.9
27
28.5
14:00
407
-8.25
41.25
30.63
0.015
0.919
0.009
1.6
26
30.6
18:30
425
-9.43
40.32
27.49
0.020
0.923
0.019
2.1
25
28
19 Nov 2013
10:00
419
-8.74
40.60
29.27
0.012
0.927
0.011
3.7
25
19.5
14:00
418
-8.71
40.52
29.59
0.019
0.881
0.012
1.2
33
19.6
18:00
414
-8.91
40.56
28.58
0.012
0.872
0.006
1.1
35
18.5
27 Jan 2014
10:30
403
-8.52
41.32
30.13
0.008
0.897
0.010
2.9
30
19.2
15:20
400
-8.68
41.23
30.03
0.011
0.914
0.010
0.7
27
19.6
18:00
404
-8.64
41.32
29.29
0.017
0.923
0.010
4.6
25
18.5
3 Feb 2014
11:00
408
-8.80
41.20
29.67
0.015
0.957
0.017
1.7
19
24.5
14:30
409
-8.86
41.39
NA
NA
NA
19:30
409
-8.95
41.41
30.57
0.011
0.972
0.010
3.0
16
19.3
17 Feb 2014
10:30
445
-10.30
40.40
27.60
0.016
0.878
0.010
3.0
34
22.4
14:30
408
-8.74
41.53
30.58
0.014
0.895
0.011
0.6
31
25
18:30
437
-9.92
41.07
28.49
0.012
0.893
0.008
1.3
31
22
19 Feb 2014
10:00
418
-9.12
40.61
29.12
0.020
0.895
0.018
0.9
31
13.3
18:00
424
-9.38
40.40
28.49
0.020
0.895
0.013
2.4
31
12.4
20 Feb 2014
14:30
410
-8.81
40.96
29.68
0.023
0.866
0.010
1.9
37
12.9
18:00
417
-9.02
40.66
29.59
0.018
0.863
0.014
1.6
37
12.5
22 Feb 2014
12:15
401
-8.44
41.49
30.63
0.013
0.872
0.013
0.6
35
17.5
17:00
402
-8.36
41.51
30.63
0.013
0.853
0.012
4.2
40
17.1
24 Feb 2014
17:30
406
-8.63
41.57
30.70
0.014
0.863
0.013
3.8
37
22
Average ± 1σ
411 ± 11
-8.78 ± 0.50
40.87 ± 0.46
29.23 ± 1.00
0.897 ± 0.027
30 ± 5
21 ± 5
Grassland: NTU Campus
14 Nov 2013
10:10
353
-7.95
40.96
30.18
0.02
0.885
0.013
0.4
33
23
14:05
366
-8.02
41.31
30.79
0.01
0.906
0.014
0.4
29
26
19:20
462
-9.94
38.33
25.64
0.02
0.907
0.019
0.2
29
24
15 Nov 2013
10:40
416
-9.12
39.42
29.51
0.01
0.954
0.013
0.6
20
22
14:10
421
-9.19
39.36
29.78
0.02
0.942
0.018
0.3
22
21
19:12
438
-9.92
38.28
28.08
0.04
0.989
0.009
0.0
13
20
16 Nov 2013
10:50
412
-8.78
40.03
28.54
0.02
0.948
0.018
1.8
21
21
17:10
408
-8.70
40.26
26.06
0.02
0.969
0.021
1.6
17
20
Average ± 1σ
409 ± 33
-8.95 ± 0.70
39.74 ± 1.00
28.57 ± 1.77
0.937 ± 0.030
23 ± 6
22 ± 2
Continued.
Date
Time
Conc.
δ13C (‰)
δ18O (‰)
δ47 (‰)
SE
Δ47 (‰)
SE
Δ48 (‰)
Estimated
Air
(ppm)
(VPDB)
(VSMOW)
(ARF)
temp. (∘C)
temp. (∘C)
Forest site near Academia Sinica Campus
7 July 2015
10:30
411
-9.07
41.43
11.54
0.01
0.890
0.017
0.3
32
32
14 Jul 2015
10:30
458
-10.43
39.74
9.01
0.02
0.890
0.017
0.4
32
31
28 Jul 2015
10:40
441
-9.99
40.86
10.07
0.02
0.887
0.015
0.2
32
30
11 Aug 2015
10:40
448
-10.46
40.09
9.50
0.01
0.920
0.009
0.5
26
30
18 Aug 2015
10:30
433
-9.99
39.80
8.99
0.02
0.888
0.016
0.4
32
30
Average ± 1σ
438 ± 16
-9.99 ± 0.50
40.39 ± 0.66
9.82 ± 0.94
0.895 ± 0.012
31 ± 2
31 ± 1
High mountain: Hehuan
9 Oct 2013
13:20
364
-8.21
40.89
28.79
0.02
0.895
0.016
3.2
31
10
17:00
NA
-8.25
40.28
28.41
0.01
0.914
0.014
2.9
27
10
Average ± 1σ
364
-8.23 ± 0.02
40.59 ± 0.30
28.60 ± 0.19
0.904 ± 0.009
30 ± 2
10
Carbon Keeling plots for air CO2 collected over
(a) suburban Academia Sinica Campus and (b) grassland at
National Taiwan University Campus.
Atmospheric CO2 over land
To show how anthropogenic emission affects the isotopic composition
especially the Δ47 values, we analyzed atmospheric CO2 samples
collected near Roosevelt Road, a busy street in downtown Taipei. The averaged
values of [CO2], δ13C and δ18O obtained were
500 ± 50 ppmv, -11.05 ± 0.90 and
39.32 ± 0.94 ‰, respectively (Table 4). A significantly higher
[CO2] and lower δ13C and δ18O values compared to
the marine CO2 showed signatures of a significant contribution from
vehicular emissions. Δ47 values near Roosevelt Road were found to
be in the range of 0.754 to 0.833 ‰, with an average of
0.807 ± 0.028 ‰ (Table 4). The values were lower by
∼ 0.15 ‰ compared to the thermodynamic equilibrium value at
20 ∘C, the ambient temperature around the sampling time, indicating
a significant fraction of CO2 produced at higher temperatures, i.e., of
combustion origin.
In the suburban location (Academia Sinica Campus), [CO2] averaged over
4 months was 410 ± 10 ppmv (Table 4), which was ∼ 15 ppmv
higher than that observed over the South China Sea and that at Mauna Loa
Observatory during the time of sampling. The higher [CO2] suggests
contribution from local anthropogenic emissions. δ13C values varied
between -7.83 and -10.30 ‰, with an average of
-8.78 ± 0.50 ‰. Keeling analysis for δ13C gave an
intercept of -26.16 ± 1.58 ‰ (Fig. 6), indicating source of
CO2 from C3 plant respiration and/or combustion. Δ47
values here varied between 0.853 and 0.972 ‰ (Table 4) with an
average of 0.897 ± 0.027 ‰, which were significantly less than
the thermodynamic equilibrium values (assuming water bodies had the same
temperature as the ambient) (Fig. 7).
Δ47 values in the near-surface atmospheric CO2 from
(a) urban site near Roosevelt Road on 30 December 2015,
(b) suburban station (Academia Sinica Campus),
(c) grassland in the National Taiwan University Campus and
(d) forest site near the Academia Sinica Campus. The error bars are
the 1 standard error associated with the measurements. Lines show
Δ47 values for the CO2 at thermodynamic equilibrium at ambient
temperatures.
The averaged [CO2], δ13C and δ18O over the
grassland (inside National Taiwan University Campus) were
410 ± 33 ppmv, -8.95 ± 0.70 and 39.74 ± 1.00 ‰,
respectively. The Keeling plot for δ13C gave an intercept of
-16.98 ± 1.02 ‰ (Fig. 6), indicating that a significant
fraction of CO2 originated from C4 vegetation. This is not
surprising as the CO2 was sampled over a C4-dominated grassland
(area: ∼ 50 m × 50 m). Unlike greenhouse CO2, no
statistically significant correlation between δ18O and
1/ [CO2] in air CO2 in these sites was observed (not shown),
probably due to the influence of multiple sources and processes on oxygen
isotopes of atmospheric CO2. Figure 7c shows the Δ47 values in
air CO2 over the grassland at National Taiwan University Campus. A large
variation in Δ47 was observed (0.885–0.989 ‰) with an
average of 0.937 ± 0.030 ‰. Some of the values were close to
the thermodynamic equilibrium while the others deviated significantly.
In a small and dense forest near Academia Sinica Campus (Fig. 1), average
values of [CO2], δ13C and δ18O in air CO2 were
438 ± 16 ppmv, -9.99 ± 0.50 and 40.39 ± 0.63 ‰,
respectively (Table 4) during summer (July–August) of 2015. A significantly
higher [CO2] and lower δ13C values than the background
indicate strong contribution of CO2 from local respiration.
Δ47 values fall in the range of 0.887 to 0.920 ‰, with an
average of 0.895 ± 0.012 ‰ (Table 4). The values were similar
to that expected at thermodynamic equilibrium (Fig. 7d) except on 11 August,
when a significant increase in Δ47 was observed. The deviation was
probably due to the influence of a super typhoon, which passed over the region on
previous days, mixing and transporting air masses regionally.
Over the top of the Hehuan mountain (∼ 3.2 km a.s.l), [CO2],
δ13C and δ18O values in air CO2 samples collected
on 9 October 2013 were 364 ppmv, -8.23 ± 0.02 and
40.59 ± 0.30 ‰, respectively (Table 4). The lower [CO2]
and higher δ13C than Mauna Loa suggest photosynthetic uptake,
which was also seen at grassland site and inside greenhouse on a few
occasions. Here the averaged value of Δ47 was
0.904 ± 0.009 ‰, slightly less than that expected at the
ambient temperature (Table 4).
Discussion
A detailed discussion of the results obtained from different locations is
presented below.
Greenhouse air CO2
To minimize anthropogenic alteration and air mixing/transport and to maximize
the variations of CO2 isotopologues by biological processes, a
controlled greenhouse provides an ideal environment. Diurnal variation was
observed in [CO2], δ13C, δ18O (Fig. 2) and
Δ47 (Fig. 3) in the greenhouse. Good correlations between
[CO2], δ13C and δ18O suggest common processes
affecting all of them, and they were photosynthesis and respiration. Giving
31 July as an example, we estimated the rates of nighttime respiration and
daytime photosynthetic uptake using the conventional isotopic compositions
(analysis of Δ47 is discussed separately below). The dimension of
the greenhouse room was 8×5×5 m (length, width and height).
The nighttime respiration rate was then estimated to be about
∼ 10 ppmv h-1 (considering change of [CO2] from 17:30
to 21:30; Fig. 2a), or ∼ 4×1013 molecules cm-2 s-1. Using simple isotopic mass balance,
this increase of [CO2] could be satisfactorily explained assuming
C3 respiration as the main source of CO2 (δ13C ≈ -26 ‰; intercept in Fig. 2d) added to the background
(-8.5 ‰). Similarly, the same conclusion could be derived by
analyzing δ18O considering δ18O of respired and
background CO2 of 30.68 ‰ (intercept in Fig. 2e) and
∼ 40 ‰, respectively. Thus, we conclude that the main factor
that affected the changes in concentration as well as the isotopic
compositions in nighttime was respiration.
The daytime net uptake rate can be estimated by taking the changes from early
morning to noontime; the [CO2] reduced by 110 ppmv, δ13C
increased by 3.46 ‰ and δ18O by 2.23 ‰ in about
6 h. We calculated the number of molecules and their changes inside the
greenhouse assuming simple gas laws. The estimated net photosynthetic uptake
was ∼ 1×1014 molecules cm-2 s-1 assuming
constant respiration rate that was observed in the night. The photosynthetic
discrimination can be calculated using the Rayleigh distillation model
R=Rofα-1
where Ro and R are the initial and modified 13C /12C
or 18O /16O ratios (due to photosynthetic activity), respectively,
f is the fraction of the material left, and α is the fractionation
factor. The estimated discrimination in 13C defined by (α-1),
following Eq. (3), was -16.5 ‰, which was slightly higher than
that expected for C3 type vegetation (∼ -20 ‰)
(Farquhar et al., 1989). For 18O, in addition to photosynthetic uptake,
one has to consider an additional effect due to temperature-dependent
water–CO2 equilibrium fractionation. That is, the process decreases
δ18O by ∼ 0.2 ‰ for an increase of 1 ∘C in
temperature (Brenninkmeijer et al., 1983); from morning to noontime, the
temperature effect reduced δ18O by 4.4 ‰. Adding this
factor to the observed change in δ18O yielded a discrimination
factor of -12.0 ‰; the value becomes -7.0 ‰, if this
additional temperature-dependence is ignored. The value (-12.0 ‰)
observed considering the additional exchange with the soil water was slightly
higher than that observed previously (-14.4 ‰) (Flanagan et al.,
1997). Here the δ13C and δ18O values of the respired
components were assumed to be -26 and 30 ‰, respectively (see
Sect. 3.1).
We assume that ca. one-third of the CO2 molecules in stomata are fixed
photosynthetically and the remaining retro-diffuse back to the atmosphere
(Farquhar and Lloid, 1993) implying that the CO2–water isotopic exchange
rate was ∼ 2×1014 molecules cm-2 s-1. Also we
assume that the CO2 molecules that enter into the leaf stomata get
isotopically equilibrated with the leaf water before diffusing back to the
atmosphere. This implies an approximately 8 h of oxygen isotope exchange
time for CO2 in the greenhouse room. As a result, we do not expect that
CO2 reached to complete isotopic equilibrium with the substrate water in
a few hours inside the room. Δ47 values in the leftover CO2
could be used to check the disequilibrium. The respired CO2 was found
to be in thermodynamic equilibrium at the ambient temperature, shown by the
Δ47 values of CO2 in the early morning and nighttime
(Fig. 3a–c) and that collected on a cloudy day with suppressed
photosynthetic activity (Fig. 3d). The close thermodynamic equilibrium at
reduced photosynthetic condition is also shown in Fig. 4a that deviation in
Δ47 from that expected at ambient temperature is small. On sunny
days, the [CO2], δ13C and δ18O values change by
50–115 ppm, 2–4 and 1.1–2.2 ‰, respectively, in a time period
of ∼ 5 h in the morning (Fig. 2). Figure 3 shows that the
Δ47 values retained the thermodynamic equilibrium values in the
morning hours (until 09:00) and then deviate from the thermodynamic
equilibrium later of the day. The maximal reduction in the Δ47
values during these morning hours was ∼ 0.05 ‰ (Fig. 3a–c)
which is significant, as this value is much higher than the uncertainty of
the measurements. An increase in Δ47 values after
∼ 09:00 was observed. We attribute these changes in the
Δ47 values of the residual CO2 to photosynthesis as it is seen
when photosynthesis is strong. Also we note that there was no significant
correlation/anti-correlation between δ18O and Δ47 when
photosynthesis was strong (Fig. 3a–c), but it became significant when the
photosynthesis was weak (Fig. 3d). Therefore, the plant photosynthesis
decouples Δ47 and δ18O, in contrast to pure
water–CO2 isotopic exchange where the two behave similarly as far as
isotopic equilibration is concerned (Affek, 2013; Clog et al., 2015).
(a) CO2 concentration inside greenhouse on 31 August
2015: observed concentration (star) and decrease in concentration by
photosynthesis after subtracting the respiration (solid circle) are also
shown. Comparison of observed (b) δ13C,
(c) δ18O and (d) Δ47 values with that
modeled using discrimination factors of -16.5, -12.0 and 0.065 ‰
for δ13C, δ18O and Δ47, respectively.
Strong influence of photosynthesis on Δ47 was also reported by
Eiler and Schauble (2004). They observed a decrease in the Δ47 values
of the residual CO2 due to photosynthetic assimilation though the effect
observed was different for different species. Here we observed a decrease in
Δ47 value of the residual CO2 initially (first 2 h) due to
photosynthesis similar to that observed by Eiler and Schauble (2004),
although
it later started increasing in response to the photosynthesis. Photosynthesis
as a source of disequilibrium was also shown recently by analyzing the
clumped isotopes of O2 (Yeung et al., 2015). Though enzymatic carbonic
anhydrase catalyzes the water–CO2 isotopic exchange toward equilibrium
(Peltier et al., 1995; Cernusak et al., 2004) its activity varies. A large
variation in the activity of carbonic anhydrase in different vegetation types
(C3, C4) or within the same type was noted previously (see Gillon
and Yakir, 2001, and references therein). Therefore, the reaction may be
incomplete, which is limited by the enzymatic activity inside leaves.
Furthermore, a box modeling by Eiler and Schauble (2004) demonstrated that
gas diffusion through leaf stomata during photosynthesis fractionates the
remaining air CO2Δ47 value, deviating it from the thermodynamic
equilibrium set by leaf water. Mixing of more than one component can also
cause change in Δ47 when δ13C and δ18O of the
components are different (Affek and Eiler, 2006; Laskar et al., 2016a), but
this can easily be ruled out as it was not observed when photosynthesis was
not very strong (Fig. 3d). More rigorous investigations with controlled
experiments using different plants with diverse carbonic anhydrase activities
are needed to resolve the issue.
Considering the discrimination for δ13C, δ18O and
variation in the concentration it is possible to model the observed isotopic
profile. The Rayleigh model (Eq. 3) in terms of δ notation can
approximately be written as δ=δo+ε×ln(f), where δo is the initial δ value, f is the
fraction of material left and ε is the enrichment factor.
Figure 8a shows the concentration profiles for 31 July 2015 inside the
greenhouse. With the calculated discrimination factors (ε) of
-16.5 and -12.0 ‰ for δ13C and δ18O, the
modeled isotopic profiles along with actual data are shown in Fig. 8b and c.
The model data are generated using Rayleigh fractionation relation. Assuming
this relation valid for Δ47, a discrimination factor of
0.065 ‰ due to photosynthesis was observed in the morning hours of
31 July 2015. Figure 8d shows the Δ47 profile for the same day
along with the actual observed values. The observed data match well with the
model plots. Unlike δs, Δ47 is not a linear quantity as
discussed later, the discrimination factor calculated may slightly change
when nonlinearity is taken into account. More data, probably at leaf
level, will allow us to estimate the photosynthetic
discrimination for Δ47.
Car exhaust CO2
Ideally, the Δ47 value of car exhaust CO2 should reflect the
temperature of fuel combustion inside the combustion chamber, which is
> 800 ∘C. However, the temperature estimated from Δ47
was found to be 283 ± 18 ∘C. It is likely that interaction of
the sample CO2 with the exhaust gases and water inside the catalytic
converter and exhaust pipe modified the Δ47 values. The catalytic
converter, which oxidizes CO and hydrocarbons to CO2, probably reset the
clumped signatures at relatively lower temperature. During combustion
water vapor is also released. We observed that the exhaust gas contained a
large amount of water vapor, part of which was condensed on the exhaust pipe
and the front part of the magnesium perchlorate column. Partial equilibration
with the stream of the exhaust gas and water inside catalytic converter and
the exhaust pipe was the likely cause for higher Δ47 values than
that expected. This was also supported by the higher δ18O values
than atmospheric O2, the source of O2 for water and CO2 here.
Normally isotopes in CO2 do not exchange with water vapor, but exchange
may take place at higher temperature in presence of catalyst. Inside the
catalytic converter, exchange could take place on the surface of the catalyst
at elevated temperatures of 200–400 ∘C (Farrauto and Heck, 1999;
Kašpar et al., 2003; Klingstedt et al., 2006). Affek and Eiler (2007)
also observed elevated Δ47 values for car exhaust and estimated a
temperature of CO2 production to be ∼ 200 ∘C. The
temperature estimated here (283 ∘C) is significantly higher than
that observed by Affek and Eiler (2007). Difference could be due to different
car models and the variations in the temperatures of the catalytic converters
from car to car.
Marine and coastal air CO2
Carbon Keeling plot for marine and coastal air CO2 gave an intercept of
-13.61 ± 1.14 ‰ (Fig. 5a), the source signature. The South
China Sea is net source of CO2 to the atmosphere (Zhai et al., 2005).
The CO2 released over ocean is mainly originated from the
remineralization of organic matter in the deeper ocean (Francois et al.,
1993; Goericke and Fry, 1994). The δ13C value of such organic
matter ranges between -20 and -30 ‰ in the tropical to
subtropical oceans, the intercept observed here (-13.6 ‰) is much
higher than this range, though the associated uncertainty is high due to a
small span of isotopic values of the samples. A possibility is that the
remineralized CO2 gets equilibrated with the dissolved inorganic carbon
before releasing to the atmosphere. Again, a complete equilibration of the
CO2 with the dissolved inorganic carbon would lead to a δ13C
value of released CO2 to be -9 to -10 ‰ (Mook, 1986;
Boutton, 1991; Zhang et al., 1995; Affek and Yakir, 2014); the observed value
of the intercept (-13.6 ‰) was significantly less than this.
Therefore, we infer that the
CO2 produced in the deeper ocean is partially equilibrated with the
dissolved inorganic carbon before releasing to the atmosphere.
The δ18O values of the surface sea water in the South China Sea
region in summer (July–September) and winter (December–February) were about
-1.7 and -0.6 ‰ (Ye et al., 2014). The sea surface temperatures
in the summer and winter are about 28 and 24 ∘C, and the
equilibrated δ18O values of the atmospheric CO2 should be 38.9
and 40.7 ‰, respectively, assuming fractionation factors at the
respective temperatures (Brenninkmeijer et al., 1983). Our observed values
lie in the range of 40.4–41.0 ‰ (Table 3), consistent with the
isotopic equilibrium values with the surface water. Therefore, we conclude
that oxygen isotopes in near-surface air CO2 over ocean are close to the
isotopic equilibrium with the surface sea water. This conclusion was further
supported by the observed Δ47 values which were found to be close
to thermodynamic equilibrium with the underlying sea surface water at the sea
surface temperature (Fig. 5b). This is due to the same water–CO2
exchange time for the two species (Affek, 2013; Clog et al., 2015). Comparing
this observation with the greenhouse data above, we conclude that
δ18O and Δ47 behave similarly when equilibrium is
achieved by simple water–CO2 exchange but respond differently when
photosynthesis is the main governing factor. Though carbonic anhydrase is
also present in the surface ocean and marine phytoplankton does
photosynthesize, δ18O and Δ47
in air CO2 over the ocean show the values at thermodynamic equilibrium
unlike the case of greenhouse CO2. The degree of deviation from
thermodynamic equilibrium probably increases with the increase in
photosynthetic activity. Normally photosynthesis by oceanic plants is much
less compared to their terrestrial counterparts; the deviation from
thermodynamic equilibrium by the oceanic photosynthesis, if present, is
probably not detectable with the present measurement precision. Compared to
δ18O, Δ47 is process sensitive and is not affected by the
isotopic composition of substrate water. Given that the surface air
temperature is better measured, we believe the clumped isotopes potentially
provide good tracers for global carbon flux study involving CO2,
complementing the commonly used species like [CO2], δ13C and
δ18O.
In the coastal stations, Δ47 values were similar to the
thermodynamic equilibrium with the sea surface water at the temperature of
∼ 27 ∘C (Fig. 5b). The recoded air temperature during the
sampling period over the coasts varied between 14 and 24 ∘C and was
not reflected in the Δ47 values. We note that the samples were
collected from two open spaces in the coasts where strong north and
northeasterly winds overwhelmed, carrying air masses from the oceans towards
the sampling locations (see Table S3 in Supplement). Therefore, we expect the
major contribution was marine air with little influence from local processes,
which could occasionally cause deviation from the thermodynamic equilibrium
values.
Urban and suburban air CO2
A significant fraction of anthropogenic CO2 was present in the air
CO2 over the urban site, indicated by the [CO2] as well as isotopic
compositions including Δ47. Anthropogenic contribution can be
estimated following two-component mixing: δ=fanth×δanth+(1-fanth)×δbgd, where δ
can be δ13C, δ18O or Δ47, f is the
corresponding weighting factor, and subscripts “anth” and “bgd” refer to
anthropogenic and background, respectively. We take the “anthropogenic” end
member as the isotopic compositions of the car exhaust values (Table 2) and
“background” end member as values observed over the ocean (for
δ13C and δ18O, Table 3) and thermodynamic equilibrium
value at the mean ambient temperature of ∼ 20 ∘C (0.95 ‰ for
Δ47) at the site during sampling. Assuming that the excess in [CO2] above the background was
originated from vehicular emission, the values of δ13C,
δ18O and Δ47 in the urban site obtained using the mixing
equation were -12.26, 37.68 and 0.809 ‰, respectively, which were
similar to those observed (Table 4). Δ47 is not a conserved
quantity and a linear mixing is not valid when the δ13C and
δ18O of the components are widely different (Affek and Eiler, 2006;
Laskar et al., 2016a). In the present case, the isotopic compositions of the
two components were not drastically different and fraction of anthropogenic
CO2 was much less (<1/4) than the background CO2, and hence the
error due to linear approximation was small (comparable to the uncertainty of
measurement). Anthropogenic CO2 can also be quantified using radiocarbon
(14C) as fossil fuels are highly depleted in 14C (Miller et al.,
2012); however, it cannot distinguish difference between CO2 from two
sources with modern carbon.
No systematic diurnal or temporal trend was observed in the Δ47
values in the suburban CO2 during the sampling period (Fig. 7b).
However, a weak trend was seen in δ13C and δ18O (not
shown) in response to the seasonal variation of the carbon assimilation and
oxygen isotopes in the rainwater (Peng et al., 2010; Laskar et al., 2014).
This furthermore demonstrates that Δ47 behaves differently from
[CO2], δ13C and δ18O. Almost all measured
Δ47 values were lower than that expected at the ambient temperature
except 2 days: 9 November 2013 and 3 February 2014. δ13C values
were also slightly lower than the background values. The reduced values of
Δ47 could be due to contribution of CO2 from combustion
processes which produced CO2 with low Δ47 values as discussed
in Sect. 4.2. We estimated the contribution of local anthropogenic
emission in δ13C and
Δ47 using the two-component mixing discussed above. The components
were the background air CO2 and car exhaust. The expected δ13C
and Δ47 values of the mixture were -9.1 and 0.92 ‰,
respectively. The observed Δ47 value was significantly different
from that estimated from simple two component mixing, though it was not
different for δ13C. After subtracting the local anthropogenic
contribution from the observed Δ47 values, a difference of
∼ 0.026 ‰ between the observed and estimated remains for
suburban station and it disappeared for urban station (see Table S4 in the
Supplement). This was not obvious in δ13C probably due to larger
variation. The lower Δ47 values in suburban station could possibly
be due to kinetic effect during photosynthetic assimilation, partial
contribution of marine air or a combination of them. It could also be due to
underestimation of the anthropogenic CO2 at the sampling spot. The
regional background [CO2] here could be lower than that assumed and the
actual anthropogenic fraction of CO2 could be higher. The marine air in
the vicinity of Taiwan, which was at thermodynamic equilibrium with the
surface sea water as discussed earlier, might have contributed partially to
the air CO2 at the sampling site. Varying contribution of marine air
could explain the lower Δ47 values to some extent. The most
plausible cause for observed deviation in the Δ47 values that
cannot be accounted for by anthropogenic and marine alterations was
photosynthesis, as discussed earlier for greenhouse CO2. This is not
unreasonable, as the Academia Sinica Campus is surrounded by thick
greeneries.
On 9 November 2013 and 3 February 2014, the Δ47 values were close
to that expected at thermodynamic equilibrium (Fig. 7b). The Δ47
values on 9 November were not very different from the values reported for the
previous or next days. However, the calculated thermodynamic equilibrium
values on that day were relatively low due to high ambient temperatures; air
CO2 probably did not get enough time to equilibrate. On 3 February
2014, the Δ47 values were higher and comparable to the
thermodynamic equilibrium values expected at ambient temperatures. A likely
explanation is that the air on that day was a mixture of two components at
the sampling region. A relatively strong wind from the southern land
(Table S3 in the Supplement) contributed the air CO2 and the higher
Δ47 values were probably due to mixing of the local air with that
transported from the south of Taipei.
Grassland, forest and high mountain air CO2
In the grassland station in Taipei city, the Keeling plot intercept for
δ13C (-17.0 ± 1.0 ‰) (Fig. 5d) indicated some
sources of CO2 with higher δ13C values compared to the most
expected sources, namely C3 vegetation and vehicle emission with a
δ13C value of ∼ -27 ‰. Though the sampling station
was located in an urban region, the sampling spot was at least ∼ 150 m
away from traffic streets, such as Keelung road, along with ∼ 60 m
wide, ∼ 10 m high C3 trees in between. As a result, anthropogenic
signals were not very prominent. The samples were collected just above the
surface of the grasses. Tropical warm grasses are mainly C4 type with
δ13C in the range of -9 to -19 ‰ and a global average
of -13 ‰ (Deines, 1980). We measured δ13C values of a
few grass samples and found values in the range of -15 to -17 ‰.
The soil and grass respired CO2 with higher δ13C contributed
significantly to the near-surface CO2, resulting in a higher value of
intercept (-17 ‰). The concentration was observed to be less than
the background level sometimes, probably due to strong CO2 uptake by
plants. The temperature gradually decreased from 26 to 20 ∘C during
the consecutive 3 days and clumped isotope followed similar trend,
reflecting the influence of temperature on CO2Δ47 and rapid
equilibration with the leaf and surface waters. One low value observed on the
second day was probably due to plumes of vehicle exhaust, also supported by
the elevated level of [CO2] and depletion in δ13C and
δ18O (Table 4). Effect of photosynthesis on the CO2 was also
expected specifically due the collection of samples at the grass level.
However, in an open system, it is difficult to assess this with limited data
points.
An elevated CO2 concentration and low δ13C and δ18O
values indicated significant contribution of respiration and/or anthropogenic
CO2 in the forest station (Table 4) near the Academia Sinica Campus.
Though the samples were collected at 10:00–11:00 under bright sunlight, the
vegetation was so dense that little sunlight reached the ground. Probably
photosynthetic activity was not very strong at the ground level in the
morning hours and the dominant process was respiration. Also, poor
circulation of air due to presence of high heels on the three sides of the
sampling spot made the site nearly isolated from the surroundings. As a
result the Δ47 values were observed to be similar to the
thermodynamic equilibrium expected at the ambient temperatures except on 11
August 2015 (Fig. 7f). This also supports our hypothesis, made in the case of
greenhouse CO2, that respired CO2 is in close thermodynamic equilibrium with the substrate
water. On 11 August 2015 a significantly higher Δ47 value was
observed. The higher value was likely due to the influence of the super
Typhoon “Soudelor” which passed over Taipei during 8–10 August 2015,
causing a decrease in temperature by 3–4 ∘C and air masses mixing
in a larger spatial scale.
A summary of Δ47 values in near-surface air CO2
obtained at different environments and compared with the thermodynamic
equilibrium values. Combustion temperature for car exhaust is assumed to be
800 ∘C (minimum value). Greenhouse CO2 are divided into two
categories: photosynthesis dominated (green open circle) and respiration
dominated (green open triangle).
For high mountain CO2, the observed Δ47 values (Table 4) were
lower than that expected at ∼ 10 ∘C, the ambient temperature
at the top of the mountain site during sampling. The Δ47 values
were similar to that observed in the plain and over the ocean. We note that
during the sampling period, the site was affected significantly by winter
monsoons. HYSPLIT 24 h back trajectory showed marine origin of air (not
shown) during the sampling time. The air CO2 on the mountain probably
did not get sufficient time to isotopically equilibrate with the local
surface and leaf water but showed the signature of the marine CO2.
The deviations in Δ47 from the thermodynamic equilibrium values in
different atmospheric environments and processes are summarized in Fig. 9. It
is obvious that the urban and suburban CO2 deviate the most towards
lower Δ47 values, mainly contributed by CO2 originated from
high-temperature combustions, i.e., vehicular emissions. The respired
CO2 are in close
thermodynamic equilibrium at the ambient temperature. In contrast, CO2
affected by strong photosynthesis shows significant increase in the
Δ47 values compared to the thermodynamic equilibrium values.