Turnbull et al., Niwot Ridge 14CO2
1
A new high precision 14CO2 time series for North
American continental air
Jocelyn C. Turnbull1, Scott J. Lehman1, John B. Miller2,3, Rodger J. Sparks4, John R.
Southon5, Pieter P. Tans2
1
INSTAAR and Dept. of Geological Sciences, University of Colorado, Boulder,
Colorado; 2NOAA/ESRL, Boulder, Colorado; 3CIRES, University of Colorado, Boulder,
Colorado; 4Rafter Radiocarbon Laboratory, Lower Hutt, New Zealand; 5Dept. of Earth
System Science, University of California, Irvine
Turnbull et al., Niwot Ridge 14CO2
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2
Abstract
We develop a high precision 14CO2 measurement capability in 2-5L samples of
3
whole air for implementation within existing greenhouse gas flask sampling networks.
4
The long-term repeatability of the measurement is 1.8‰ (1-sigma), as determined from
5
repeated analyses of quality control standards and replicate extraction and measurement
6
of authentic field samples. In a parallel effort, we have begun a 14CO2 measurement
7
series from NOAA/ESRL’s (formerly NOAA/CMDL) surface flask sampling site at
8
Niwot Ridge, Colorado, USA (40.05°N, 105.58°W, 3475 masl) in order to monitor the
9
isotopic composition of carbon dioxide in relatively clean air over the North American
10
continent. 14CO2 at Niwot Ridge decreased by 5.7‰/yr from 2004 to 2006, with a
11
seasonal amplitude of 3-5‰. A comparison with measurements from the free
12
troposphere above New England, USA (41°N, 72°W) indicates that the 14CO2 series at
13
the two sites are statistically similar at timescales longer than a few days to weeks (i.e.,
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those of synoptic scale variations in transport), suggesting that the Niwot Ridge
15
measurements can be used as a proxy for North American free tropospheric air in future
16
carbon cycle studies.
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Turnbull et al., Niwot Ridge 14CO2
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1. Introduction
The Northern Hemisphere tropospheric radiocarbon burden almost doubled from
3
natural levels in the early 1960s due to atmospheric nuclear weapons testing, but has
4
since decreased markedly as the atmospheric overburden of 14C has been absorbed into
5
the oceans and terrestrial biosphere [Levin et al., 1985]. Soon after above ground
6
weapons testing ceased, the radiocarbon content of atmospheric carbon dioxide (14CO2)
7
fell by up to 100 permil per year (‰/yr), reflecting the rapid uptake of 14C by surface
8
reservoirs. Most “bomb 14C” was initially injected into the stratosphere, leading to
9
seasonal variations of 14CO2 within the troposphere as large as 200‰ in response to
10
seasonal variation in cross-tropopause exchange [Levin and Kromer, 1997; Nydal and
11
Lövseth, 1996; Manning et al., 1990]. As the atmospheric 14C burden has decreased and
12
become more uniform, both the annual secular change and seasonal cycle amplitudes
13
have fallen to current values of ~5‰/yr and ~10‰, respectively, so that the detection of
14
annual and seasonal changes now demands high precision measurement. Conventional
15
gas counting can obtain precisions of up to 1.2‰, but requires relatively large samples
16
(of order 15m3), which are typically collected over a period of days to weeks [Levin and
17
Kromer, 2004; Levin et al., 2003; Manning et al., 1990; Tans et al., 1979]. Here we
18
describe improvements in 14C measurement by accelerator mass spectrometry (AMS)
19
yielding a long-term 14C measurement repeatability of as good as 1.8‰ (1-sigma) in 2-
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5L samples of air. The small sample size allows measurement in existing greenhouse gas
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flask sampling networks (such as operated by NOAA/ESRL,
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http://www.cmdl.noaa.gov/ccgg/) and provides the opportunity for direct comparison
Turnbull et al., Niwot Ridge 14CO2
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with measurements of other species from the same samples (see below). The increased
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precision permits resolution of annual and seasonal trends in 14CO2, and provides for a
3
fossil fuel CO2 detection capability of better than 1 ppm of CO2 [Turnbull et al., 2006].
4
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Long-term atmospheric 14CO2 records exist for several sites in Europe and in the
5
Southern Hemisphere [e.g. Levin and Kromer, 2004; Nydal and Lövseth, 1996, Manning
6
et al., 1990], but there are no such records for North America. The existing records have
7
been used to estimate the exchange rate of CO2 with the ocean [e.g. Peacock, 2004; Orr
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et al., 2001; Broecker et al., 1995; Hesshaimer et al., 1994; Oeschger et al., 1975], the
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turnover time of carbon in the terrestrial biosphere [e.g. Trumbore, 1997; Gaudinski et
10
al., 2000], and cross-tropopause exchange [Randerson et al., 2002; Nakamura et al.,
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1994]). In addition, because fossil fuel CO2 emissions are uniquely characterized by the
12
absence of 14C, 14CO2 measurements have been used to quantify fossil fuel CO2 mixing
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ratios and emissions at regional scales in Europe [Levin et al., 2003; Meijer et al., 1996;
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Zondervan and Meijer, 1996; Mook, 1980] and, most recently, in aircraft samples from
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New England [Turnbull et al., 2006].
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In 2003 we began measurement of a new 14CO2 time series at NOAA/ESRL’s
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Niwot Ridge, Colorado, USA sampling site (site code NWR, 40.05°N, 105.58°W, 3475
18
masl), which we envision will be continued as a long-term time series, to complement the
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existing European and Southern Hemisphere records. CO2 mixing ratio measurements
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were begun at NWR in 1968 and measurements of other greenhouse gases (CH4, CO,
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SF6, N2O, H2) and the stable isotopes of CO2 and CH4 have been added over the years
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[Schnell et al., 2004; Trolier et al., 1996; Miller et al., 2002]. In addition, previous
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regional scale studies (in Wisconsin and Massachusetts) have made use of Niwot Ridge
Turnbull et al., Niwot Ridge 14CO2
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1
air as a proxy for North American free tropospheric “background” air against which
2
regional trace gas anomalies have been quantified [Helliker et al., 2004; Bakwin et al.,
3
2004]. Ideally, the new NWR 14CO2 series can be used analogously in order to provide
4
the background observation needed to quantify local enhancements of 14C-free fossil fuel
5
derived CO2 around North America. We therefore compare the new Niwot Ridge 14CO2
6
measurements with free tropospheric 14CO2 measurements from over New England
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(41°N, 72°W) and evaluate the skill of Niwot Ridge 14CO2 in quantifying the fossil fuel
8
CO2 contributions to boundary layer air over New England previously determined by
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Turnbull et al. [2006].
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2. Methods
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2.1. Methods: Sample collection
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Measurements are presented from both repeat analyses of a single tank of air and
13
from authentic atmospheric samples. The single tank (nominally 4200L at STP) was
14
collected at Niwot Ridge, Colorado, USA in November 2002 to provide replicate
15
extraction aliquots for evaluation of long term measurement repeatability and is
16
designated NWTstd. Authentic samples are collected each week at Niwot Ridge,
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Colorado, USA (site code NWR, 40.05°N, 105.58°W, 3475masl) as part of the National
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Oceanic and Atmospheric Administration Earth System Research Laboratory
19
(NOAA/ESRL) Global Co-operative Air Sampling Network [Schnell et al., 2004]. Two
20
flasks are filled in series, which are measured individually for several trace gas species
21
including CO2 and CO. Flasks are flushed with air for about nine minutes, followed by a
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fill time of about one minute. On alternate weeks, an additional pair of flasks is flushed
Turnbull et al., Niwot Ridge 14CO2
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and filled in a second ten minute period immediately following the filling of the first
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flask pair. The second flask pair is measured for CO2 mixing ratio only and provides
3
additional material for 14CO2 quality control purposes. On sampling dates when only
4
two flasks are collected, we combine the air from both flasks to provide a single sample,
5
filled at the same time. When four flasks are collected, we obtain two theoretically
6
identical samples by combining two flasks (one from each simultaneously filled pair) for
7
each sample. The resulting authentic replicate samples each represent the same time-
8
averaged filling period. Sample sizes for 14CO2 measurement are 2-3L of whole air for
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single samples, and 3-4L each for replicated samples.
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2.2. Methods: 14C preparation and measurement
Sample preparation is undertaken at the University of Colorado Laboratory for
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AMS Radiocarbon Preparation and Research. Extraction of CO2 is performed
13
cryogenically, using a method based on that of Zhao et al. [1997], with a scaled-up
14
system to allow faster extraction for the large sample size required for 14C measurement
15
(figure 1). Briefly, the sample air is allowed to flow through a vacuum system at 200
16
standard milliliters per minute (mL/min (STP)). Water is removed by an ethanol trap
17
cooled to -90°C with liquid nitrogen, and CO2 and N2O are frozen into a liquid nitrogen
18
trap (-196°C) while other gases pass through to the vacuum pump. The internal pressure
19
is controlled to stay below six Torr to ensure complete freezing of CO2 without freezing
20
O2 or CH4 (no attempt is made to separate non-interfering N2O).
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For NWTstd samples, extraction is continued for 20-60 minutes (depending on the
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final aliquot size required), and then the tank is closed off. For authentic flask samples,
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the extraction is continued until all the air has been extracted from the flask, with typical
Turnbull et al., Niwot Ridge 14CO2
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1
extraction times ranging from 20-40 minutes. In all cases, the CO2 yield is quantified in a
2
known volume manifold and then transferred to a flame-seal tube for graphitization on a
3
separate vacuum system. Sample size is 0.4–1.0 mg of carbon (mgC). For NWTstd, we
4
commonly extract twice this amount of CO2, and split it into two aliquots under
5
equilibrium conditions.
6
To test for variability associated with the CO2 extraction process, we measured
7
the 13C of CO2 aliquots extracted from our NWTstd tank. 13C measurements were made
8
at the INSTAAR Stable Isotope Laboratory, with a reported precision of 0.01‰
9
[http://instaar.colorado.edu/sil, Trolier et al., 1996]. Firstly, we tested for complete
10
extraction of CO2 by varying the flow rate during extraction from 40-200 mL/min
11
(STP). Secondly, we repeated extractions at 200 mL/min (STP) to estimate the
12
uncertainty due to the extraction process. Thirdly, we filled a series of sample flasks with
13
air from the NWTstd tank, and extracted these following the process used for authentic
14
samples to check for variability due to flask filling and extraction. 13C values for all
15
these tests differed by less than 0.05‰, which equates to less than 0.1‰ in 14C. These
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values suggest that extraction of CO2 is complete at the 200 mL/min (STP) flow rate,
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and that variability due to flask filling and extraction is not significant relative to our
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14C measurement precision.
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Graphite is produced by reduction of CO2 with hydrogen over an iron catalyst,
20
shown schematically in figure 2 and based on the method of McNichol et al. [1992]. The
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iron catalyst is pre-baked at 400°C for 30 minutes, except for samples analyzed prior to
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December 2004, for which the pre-bake was done at 800°C. The sample CO2 is then
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introduced into a small reaction manifold (4-7mL volume) and hydrogen gas is added in a
Turnbull et al., Niwot Ridge 14CO2
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ratio of 2.5-3 times the number of moles of sample CO2 to provide a slight stoichiometric
2
excess and ensure complete reaction. The reduction is performed at 625°C, precipitating
3
graphite onto the iron catalyst. Water produced by the reaction is frozen using a thermo-
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electric cooler at -15 to -17°C, except for samples analyzed prior to July 2005, when
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water was frozen using an ethanol-liquid nitrogen slush bath at temperatures between -5
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and -20°C. Reaction progress is monitored by pressure; reactions typically take four to
7
five hours, but are allowed to continue for eight hours to ensure completion.
8
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Minimally different reaction conditions are used depending on the AMS
laboratory conducting the 14C measurement. Low lithium quartz glass reaction tubes
10
were used for measurements made on the Rafter Radiocarbon Laboratory (RRL) AMS
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system where di-lithium contamination interferes with the measurement, and for RRL the
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Fe catalyst to graphite mass ratio was 1.2 to 1.3. At the University of California, Irvine
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(UCI) AMS facility, we use 3-4mg of Fe catalyst regardless of sample size. This choice
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avoids an apparent mass-independent fractionation observed when sample targets are
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near exhaustion in the ion source, which we believe is related to a small loss at the edges
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of the 12C ion beam as it passes through apertures in the AMS; this effect is determined
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by the abundance of each isotope, rather than the isotopic mass, and substantially impacts
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only the large 12C ion beam. For both AMS systems, the resulting graphite/catalyst
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mixture is packed into an aluminum target holder for AMS 14C measurement.
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We use NBS Oxalic Acid I (Ox-I) as our primary 14C reference standard [Stuiver
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and Polach, 1977; Olsson, 1970]. Aliquots of Ox-I CO2 are split under equilibrium
22
conditions (whereby the CO2 is allowed to equilibrate at room temperature into a single
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large volume, then divided into two aliquots by closing a valve to separate the large
Turnbull et al., Niwot Ridge 14CO2
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volume into two smaller volumes) from a single large parent flask and graphitized in the
2
same manner as unknowns. We use NBS SRM 4990 Oxalic Acid II (Ox-II) [Stuiver,
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1983], prepared in the same manner as Ox-I as a secondary quality control check. The
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total processing and measurement blank is estimated by regular extraction and
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measurement of CO2 from a tank of synthetic 14C-free air. Preparation and measurement
6
follow the same protocols as for unknowns. Further quality control is provided by
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repeated measurements of NWTstd, and from replicate measurements of authentic
8
samples (see section 3).
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The 14C content of each graphite sample is measured by AMS at either UCI
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(analyses since June 2004) or RRL (prior to June 2004). A batch or “wheel” of samples
11
is measured with concurrent standards in a single run. At UCI, a wheel of up to 40
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targets is measured over a 24-hour period, including eight Ox-I primary standards, a
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blank, two Ox-II and three to five NWTstd quality control targets, and 24-26 unknowns.
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Each target is measured to 300,000-750,000 14C counts (for blanks, a time limit is used)
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in 6-15 separate exposures, with standards interspersed throughout the wheel. The 14C
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content measured in each exposure is calculated firstly as the 14C/13C ratio each sample
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exposure divided by the 14C/13C of the 6 closest Ox-I standard exposures. The mean of
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this ratio for all exposures for the sample is determined to obtain the “ratio to standard”.
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The fraction modern (ratio of sample to the absolute radiocarbon standard) is calculated
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from the “ratio to standard”, following calculations described in detail in Donahue et al.
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[1990] and Stuiver and Polach [1977]. This calculation includes a correction for the
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process blank, which is measured in the same wheel. The result is also corrected for
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isotopic fractionation and normalized to a 13C of -25‰ using the 13C measurement
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Turnbull et al., Niwot Ridge 14CO2
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obtained in the AMS system concurrently with the 14C measurement. This value reflects
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any fractionation during graphitization and AMS measurement, and may deviate from the
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environmental 13CO2 value by up to several permil.
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AMS measurement and data analysis procedures differed slightly for
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measurements at RRL. A “wheel” consisted of eight unknowns along with three to four
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Ox-I standards, and one or two NWTstd targets. We measured two or three such wheels
7
over a consecutive two or three day period. Each target was measured to ~300,000 14C
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counts in ten exposures, and the “ratio to standard” for each exposure was determined
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from a line of best fit to all Ox-I exposures in the wheel. The 13C value was obtained
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offline from a stable isotope mass spectrometer measurement of 13CO2 from the same
11
flask, performed at the INSTAAR Stable Isotope Laboratory
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[http://instaar.colorado.edu/sil, Trolier et al., 1996]. This measurement does not account
13
for any fractionation during graphitization and AMS measurement. Fraction modern was
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then determined using the same method as for UCI, but using the offline 13C value. Use
15
of the latter may contribute to the larger spread in measurements at RRL.
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A tank of synthetic 14C-free air (CO2-free air spiked with 14C-dead CO2) is used as
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a process blank. An aliquot is extracted and measured for 14C in every measured wheel
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(at RRL, one blank was included in each set of 2-3 wheels). The measured 14CO2
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ranges from -997 to -999‰ with an average of -998‰. The effect of this level of
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contamination on the 14C of modern air samples is insignificant, but we continue these
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measurements in each measured wheel as a quality control measure to monitor for
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contamination during sample preparation and AMS analysis.
Turnbull et al., Niwot Ridge 14CO2
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All results from either lab are corrected for radioactive decay since the date of
2
collection and reported as 14C, the permil deviation from the absolute radiocarbon
3
standard, such that
14C  FN e(1950x) 11000‰
4
5
(1)
where FN is the normalized fraction modern,  is the decay constant of 14C and x is the
6  date of collection for each sample [Stuiver and Polach, 1977].
7
The AMS single sample precision is determined from the statistical uncertainties
8
on both sample and associated standards, accounting for both the “internal” statistical
9
uncertainty determined from the number of 14C counts, and the “external” uncertainty
10
determined from the variability amongst the different exposures for that sample.
11
Occasionally, unstable operating conditions in the UCI AMS system occur which are
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readily identified by the larger scatter in the measured Ox-I values. For these wheels, we
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adjust the reported uncertainty to reflect the scatter in the Ox-I values. This has occurred
14
in six of 28 wheels measured at UCI since April 2004, and these are treated as a separate
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dataset for quality control purposes (designated “UCI Poorly Performing” or “UCIPP”).
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At RRL, the reported single sample precision was 2.1-2.4‰ in 14C. At UCI, where we
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are able to obtain more 14C counts, the single sample precision is 1.4-3.0‰, with the
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higher uncertainties reflecting samples where instability problems occurred. We assess
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how well these reported precisions reflect the true measurement repeatability below.
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3.
14C
Analysis Precision and Repeatability
21
The true repeatability of our 14C measurements is examined primarily by
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preparing and measuring replicate aliquots of NWTstd (figure 3). Individual samples are
Turnbull et al., Niwot Ridge 14CO2
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1
prepared by extracting sufficient air to obtain either: (a) 1.0–1.5mg C and then splitting
2
the extracted CO2 into two aliquots under equilibrium conditions; or (b) by extracting
3
0.4–1.0mg C for a single sample. Each sample is then graphitized individually and
4
measured by AMS at RRL or UCI. No significant inter-laboratory difference is observed
5
(mean values and standard errors are 73.5±0.5‰ for RRL and 73.3±0.2‰ for UCI).
6
Some wheel-to-wheel offsets can be seen in the UCI data, but there has been no long-
7
term drift in the mean value. The 1-sigma standard deviation of the NWTstd
8
measurements is 2.5‰ for RRL and 1.8‰ for typical (non-UCIPP) UCI wheels, with the
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larger spread in RRL measurements resulting mainly from the lower number of 14C
10
counts and the offline 13C measurement. The UCIPP wheels have a 1-sigma standard
11
deviation of 2.7‰. We therefore assign uncertainties to our authentic sample
12
measurements as the repeatability for the given dataset (RRL, UCI or UCIPP), or the
13
AMS reported single sample precision, whichever is larger.
14
As a secondary check, we also measure aliquots of Ox-II at UCI (figure 3)
15
prepared in the same manner as our primary Ox-I standard, and these have a mean value
16
of 339.0‰ (decay corrected to the 1950 value) and a one-sigma repeatability of 1.6‰.
17
The book value for Ox-II is 340.66‰ [Stuiver, 1983].
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The variability (but not absolute value) of Ox-I targets within each wheel provides
19
a third measure of the intra-wheel repeatability (figure 3), and we obtain 2.5‰ for RRL
20
and 1.8‰ for regular UCI wheels, consistent with the NWTstd estimates of repeatability.
21
For poorly performing UCI wheels the repeatability is 2.9‰. Because the Ox-I values
22
are always normalized to the book value of Ox-I (Fraction modern of 1.03290 in 2005),
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the repeatability of this measurement indicates the intra-wheel repeatability, rather than
Turnbull et al., Niwot Ridge 14CO2
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the inter-wheel variability reflected by the other quality control standards. There is no
2
significant difference between the intra-wheel and inter-wheel variances, suggesting that
3
we can legitimately compare results for different wheels.
4
Finally, we use authentic field sample replicates to confirm that we have correctly
5
assigned the uncertainties using the methods described above (figure 4). Two types of
6
authentic sample replicates are obtained: (1) two sets of flasks are filled simultaneously,
7
and then treated as completely different samples throughout sample preparation and
8
analysis; and (2) a single CO2 extraction is performed, and the resulting CO2 gas is split
9
(under equilibrium conditions) into two replicates for graphitization and measurement.
10
Sample replicates have come from a variety of locations, including 35 pairs from NWR.
11
We have measured 74 replicate sample pairs since 2004, and obtain a reduced chi-square
12
(2) of 1.07 for all replicate pairs. The 35 replicate pairs of NWR (all of which except
13
one were obtained using method 1) yield an 2 = 0.79. These values indicate that we
14
have accurately represented the uncertainties for these samples.
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4. Results and discussion
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4.1. NWR results
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We began biweekly 14CO2 measurements at Niwot Ridge, Colorado, USA
18
(NWR, 40.05°N, 105.58°W, 3475masl) in May of 2003 and present results up to January
19
2006 (table 1, figure 5a). Samples were collected over periods of 1-20 minutes as
20
described above, typically during the afternoon. The high altitude location on the
21
continental divide and generally westerly air stream suggest that NWR measurements
22
should be representative of relatively clean free tropospheric air over North America.
Turnbull et al., Niwot Ridge 14CO2
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However, upslope easterly wind events occasionally bring locally polluted air from the
2
Denver metropolitan region 40 miles to the east. To obtain a “clean troposphere” dataset,
3
we use measurements of carbon monoxide (CO) from the same sample flasks to identify
4
polluted air (the CO observations are available digitally at
5
ftp.cmdl.noaa.gov/ccg/co/flask/event). CO is produced by incomplete fossil fuel
6
combustion, so high CO values are a qualitative indicator of polluted air. Additional CO
7
measurements from samples collected at NWR, but not measured for 14CO2, are
8
included in the CO dataset. A curve is fitted to the CO data [Thoning et al., 1989] to
9
define the (seasonally varying) background and samples where the CO value is more than
10
15 parts per billion (ppb) above the fitted curve are flagged as polluted (figure 5b). Using
11
these criteria, 11 of the 79 14CO2 sampling dates are excluded from the original dataset.
12
Alternately, we flagged all points where the wind direction was between 0° and 180° (i.e.
13
from the east). This excluded 22 of the 79 data points, including all the points that were
14
flagged using the CO method. We chose the CO flagging method, as it appeared to best
15
identify the polluted samples.
16
The cleaned dataset is shown in figure 5a, and the curve shown is fitted following
17
Thoning et al. [1989], using one linear and two harmonic terms, with fit residuals being
18
added back using a low-pass cutoff filter with a 180 day cutoff in the frequency domain,
19
equivalent to an averaging filter in the time domain with a full width half-maximum of
20
106 days. Adding additional polynomial or harmonic components did not improve the
21
fit. The residual scatter about the fitted curve (of 2.6‰ at one standard deviation) likely
22
results from the individual measurement uncertainties and authentic, short-term
23
environmental variability. A possible example of the latter effect is the high 14C value
Turnbull et al., Niwot Ridge 14CO2
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1
observed at NWR on January 7th, 2004 (77.7±2.6‰), which may reflect influence of air
2
originating in the upper troposphere where substantial production of 14CO2 occurs
3
(consistent with analysis of the back trajectory for this sample (data not shown)).
4
There is a consistent decrease in 14CO2 through the record, with 14CO2
5
changing by -5.7‰/yr between the beginning of 2004 and 2006 according to the fitted
6
curve shown in figure 5a. The simple linear trend component of the fitting process is
7
similar at -5.2±0.1‰/yr, although the short length of the record may result in bias in this
8
fit. Given current fossil fuel carbon emission rates of about 7.5GtC/yr [Marland et al.,
9
2003] and the current atmospheric loadings of about 380ppm CO2 and 60‰ 14CO2,
10
global emissions of 14C-free fossil fuel CO2 are expected to reduce 14CO2 by 9.7‰/yr.
11
Therefore, the effect of all other sources and sinks must be to increase the atmospheric
12
14CO2 by 4-4.5‰/yr. This is consistent with only a small remaining 14C disequilibrium
13
between the atmosphere and surface reservoirs, and a change in sign (from negative to
14
positive) of the biospheric disequilibrium flux as bomb 14C taken up by the biosphere in
15
the last few decades returns to the atmosphere [Naegler and Levin, 2006; Randerson et
16
al., 2002]. These changes imply that fossil fuel emissions are now a dominant control on
17
the temporal trend in 14CO2.
18
An analysis of the harmonic component of the fit reveals an apparent seasonal
19
cycle, with a maximum in August, a minimum in February-March, and peak-to-trough
20
amplitudes of 3-5‰, very similar to the average behavior at Jungfraujoch, Switzerland
21
(46.55°N, 7.7°E, 3450 masl) between 1986 and 2003 [Levin and Kromer, 2004]. Fossil
22
fuel CO2 emissions have a maximum in the Northern Hemisphere winter, with an
23
estimated seasonal amplitude of 20% of the total emissions for the United States and
Turnbull et al., Niwot Ridge 14CO2
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1
likely a similar or slightly larger seasonal amplitude for other regions [Blasing et al.,
2
2005]. This seasonal change has been modeled to produce a 3-4‰ seasonal amplitude in
3
14CO2 in the Northern Hemisphere mid-latitudes [Randerson et al., 2002] with a 14CO2
4
maximum in the Northern Hemisphere summer. This is consistent with the NWR 14CO2
5
observations for 2004 and 2005, both in the phase of the seasonal cycle and in its
6
amplitude. The 2003 results (which do not contribute significantly to the harmonic fit)
7
appear to oppose this seasonal trend, showing a minimum in August of 2003, but the time
8
series is too short to determine whether this is a real anomalous seasonal signal. The
9
seasonal amplitude is larger in 2005 than in 2004, and this year-to-year variability in
10
seasonal amplitudes is consistent with past variability in other 14CO2 records [Levin and
11
Kromer, 2004], suggesting that this difference is not simply due to the small number of
12
samples measured. It is also unlikely that fossil fuel emissions vary sufficiently from
13
year to year to explain this. The observed changes in amplitude of the seasonal signal are
14
more likely due to differences in atmospheric transport or to other seasonally varying
15
sources, such as the magnitude of net cross-tropopause exchange or terrestrial biospheric
16
respiration.
17
4.2. Comparison with other North American sites
18
Previous studies [Bakwin et al., 2004; Helliker et al., 2004] have used NWR as a
19
proxy for well-mixed free tropospheric air across much of the North American continent,
20
which was a primary motivation for establishing an initial 14CO2 monitoring effort at
21
this location. In order to begin to evaluate how well NWR 14CO2 represents North
22
American free tropospheric 14CO2, we compare the NWR results with a second free
Turnbull et al., Niwot Ridge 14CO2
17
1
tropospheric dataset collected over New England beginning in 2004 [Turnbull et al.,
2
2006]. The New England samples were collected via aircraft at varying altitudes between
3
3000 and 5000 m attitude at two locations: over Harvard Forest, Massachusetts (HFM,
4
42°32’N, 72°10’W) and above Portsmouth, New Hampshire (NHA, 42°57’N, 70°37’W).
5
Sampling altitudes for each date were chosen to obtain the “cleanest” air in the vertical
6
profile (the selection criteria are described in more detail in Turnbull et al. [2006]).
7
The NWR and New England datasets agree well, both in terms of absolute value
8
and in temporal trends (figure 6). The dip in 14CO2 values from 67‰ to 62‰ in May
9
2004 is present in both records, as well as the strong downward trend beginning in
10
August 2004. The absolute values are also similar, with six-monthly mean 14CO2
11
values overlapping at one standard deviation (table 2). Higher frequency variability in
12
the two datasets does not appear to be shared, but most of this difference may be
13
attributed to the snapshot sampling method and to measurement uncertainty, as the scatter
14
in the residuals from the fitted data approaches the measurement uncertainty for both
15
datasets.
16
In order to demonstrate the usefulness of NWR as a proxy for background air we
17
re-calculate the fossil fuel CO2 contribution in New England boundary layer air samples,
18
which were previously determined in Turnbull et al. [2006]. The fossil fuel CO2
19
contribution (Cff) is related to the observed 14CO2 (obs) and the background 14CO2
20
(bg) value, such that
21
C ff 

Cobs obs   bg  Cr  r   bg 

 ff   bg
 ff   bg
(2).
Turnbull et al., Niwot Ridge 14CO2
18
1
14CO2 of fossil fuels (ff) is, by definition, -1000‰. There is also a small correction
2
term for heterotrophic respiration (indicated as Cr and r), which is typically smaller than
3
the uncertainty in Cff. In the calculation performed by Turnbull et al. [2006], bg was
4
obtained from free tropospheric samples collected in the same vertical profile as the
5
continental boundary layer samples for which Cff was calculated. Here we recalculate the
6
fossil fuel CO2 contribution using the monthly mean 14CO2 values from NWR as bg
7
(figure 7). There is no significant or systematic difference between the estimates of Cff
8
obtained using the local, real-time value of bg vs. that based on NWR monthly means.
9
While not conclusive, because the New England samples examined here are
10
deliberately biased to clean air sampling, the observed agreement in figure 6 suggests that
11
the free troposphere is well mixed over North America, and that venting of 14C-free
12
emissions through the top of the continental boundary layer is small relative to the free
13
tropospheric mixing.
14
5. Conclusions
15
Improved precision in 14CO2 measurements allows us to resolve 14CO2
16
differences of a few permil. The ability to measure samples as small as two liters of
17
whole air allows direct comparison with other trace gases measured in the same flasks,
18
and provides a complementary method to the larger, time-averaged samples traditionally
19
used for atmospheric 14CO2 studies.
20
Our high-precision measurements from Niwot Ridge, Colorado, USA provide
21
(when screened for occasional, easily identified, local pollution events) a new
22
background 14CO2 record for North America for application to carbon cycle studies.
Turnbull et al., Niwot Ridge 14CO2
19
1
Changes in 14CO2 during the two-year record appear to be dominated by the impacts of
2
fossil fuel combustion, both in the secular decrease (of 5.7‰/yr), and in the seasonal
3
cycle. Comparison of the results from NWR and the New England free troposphere
4
suggest that the free troposphere is well mixed zonally across North America with respect
5
to 14CO2 on annual and seasonal time scales, suggesting that the NWR record may be
6
useful as a proxy for background 14CO2 for North America.
7
Acknowledgements
8
Funding for this research was provided by NOAA Office of Global Programs
9
(OAR4310098), the ISAT Linkages Fund (03-CSP-20-SPAR) and the Colorado
10
Mountain Club Foundation. We thank Chad Wolak for assistance with sample
11
preparation, Valerie Claymore for assistance with stable isotopic measurements, and
12
Duane Kitzis and Mark Losleben for collection of additional samples and standards at
13
Niwot Ridge.
Turnbull et al., Niwot Ridge 14CO2
1
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3
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4
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Turnbull et al., Niwot Ridge 14CO2
25
Tables and figures
CURL lab code
7222
7083
7224
7084
7086
7087
7092
7093
7095
7100
7101
7143
7145
7147
7148
7245
7150
7225
7154
7227
7228
7159
7233
7237
7239
7240
7242
7243
7246
7312
7836
7313
7315
7318
7319
7324
7327
7321
7322
7331
7333
7328
1
date collected
5/27/03
7/29/03
8/5/03
8/12/03
8/26/03
9/9/03
9/23/03
9/23/03
10/7/03
11/4/03
11/4/03
11/25/03
12/9/03
12/23/03
12/23/03
12/30/03
1/6/04
1/20/04
1/20/04
2/3/04
2/3/04
2/17/04
3/2/04
3/2/04
3/16/04
3/16/04
3/30/04
3/30/04
4/13/04
4/27/04
4/27/04
5/4/04
5/11/04
5/25/04
5/25/04
6/8/04
6/8/04
6/29/04
6/29/04
7/13/04
7/13/04
7/27/04
2
time
colleected2
19:00
16:30
17:00
16:20
17:45
20:10
20:45
20:45
18:00
21:30
21:30
21:45
19:55
19:10
19:10
19:20
19:00
20:40
20:40
21:20
21:20
20:30
21:30
21:30
21:00
21:00
17:00
17:00
20:00
20:40
20:40
20:45
15:45
16:45
16:45
16:15
16:15
18:00
18:00
19:35
19:35
15:35
AMS lab3
UCI
RRL
UCI
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
UCI
RRL
UCI
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
UCI
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
RRL
14CO2
72.3
62.7
65.2
64.6
64.1
65.2
70.0
68.3
67.8
64.9
64.3
70.8
68.6
66.9
67.0
65.0
77.7
49.8
44.7
67.9
66.4
66.7
59.1
54.7
66.8
64.8
668
68.5
67.1
66.7
68.1
62.3
63.7
60.7
60.2
66.4
62.4
68.5
64.3
71.4
67.9
64.2
unc.4
2.82
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.7
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
Flag5
F
F
F
F
F
F
Turnbull et al., Niwot Ridge 14CO2
7330
7334
7336
7337
7339
7837
7876
7838
7877
7839
7878
7840
7879
7841
7880
7842
7881
7843
7882
7847
7883
7848
7849
7886
7851
7887
7852
7888
7853
7889
7854
7891
7855
7892
7856
7893
7857
7894
7859
7896
7897
7860
7861
7898
7899
8329
7900
8328
8331
7/27/04
8/10/04
8/10/04
8/24/04
8/24/04
9/7/04
9/7/04
9/28/04
9/28/04
10/12/04
10/12/04
10/26/04
10/26/04
11/9/04
11/9/04
11/23/04
11/23/04
12/7/04
12/7/04
12/21/04
12/21/04
1/4/05
1/17/05
1/17/05
2/8/05
2/8/05
2/22/05
2/22/05
3/8/05
3/8/05
3/22/05
3/22/05
4/5/05
4/5/05
4/19/05
4/19/05
5/3/05
5/3/05
5/31/05
5/31/05
6/14/05
6/14/05
6/28/05
6/28/05
7/5/05
7/12/05
7/19/05
7/26/05
8/2/05
26
15:35
17:35
17:35
19:00
19:00
19:00
19:00
20:45
20:45
17:15
17:15
19:45
19:45
22:00
22:00
22:00
22:00
22:30
22:30
21:50
21:50
21:30
21:10
21:10
22:00
22:00
22:00
22:00
21:30
21:30
22:30
22:30
19:30
19:30
20:45
20:45
20:25
20:25
18:00
18:00
18:50
18:50
20:00
20:00
14:30
17:00
15:00
15:15
14:30
RRL
RRL
RRL
RRL
RRL
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
61.1
63.2
61.6
70.3
64.7
69.7
66.5
62.6
63.1
63.4
63.2
63.5
66.7
64.0
60.9
62.4
59.8
61.3
62.6
61.5
59.0
53.1
60.0
63.9
57.8
55.6
54.8
51.1
58.3
64.4
58.4
59.4
55.0
57.1
57.6
58.1
45.2
45.1
60.2
58.2
59.3
53.6
53.4
56.7
59.0
56.7
60.2
60.7
60.8
2.6
2.6
2.6
2.6
2.6
2.7
1.8
2.7
1.8
2.7
1.8
2.7
1.8
2.7
1.8
2.7
1.8
2.9
1.8
2.7
1.8
2.7
2.7
1.8
2.7
1.8
2.8
1.8
2.7
1.8
2.7
1.8
2.7
1.8
2.7
1.8
2.7
1.8
2.7
1.8
1.8
2.7
2.8
1.8
1.8
1.8
1.8
1.8
1.8
F
F
F
F
F
F
F
F
F
Turnbull et al., Niwot Ridge 14CO2
8330
7901
8332
8333
8334
8335
8345
8337
8347
8338
8341
8346
8355
8339
8344
8340
8348
8349
8353
8350
8354
8352
8351
8/9/05
8/16/05
8/23/05
8/30/05
9/6/05
9/13/05
9/27/05
10/4/05
10/12/05
10/18/05
10/25/05
11/1/05
11/8/05
11/17/05
11/22/05
11/29/05
12/6/05
12/13/05
12/20/05
12/29/05
1/3/06
1/10/06
1/17/06
27
15:03
17:10
15:00
16:25
16:10
16:45
17:45
15:10
14:37
15:10
15:45
18:20
19:00
21:00
20:30
18:30
21:30
21:15
21:00
19:00
19:45
19:30
18:40
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
UCI
57.0
65.1
57.5
58.1
59.1
57.7
58.7
58.5
57.0
60.3
60.3
58.2
55.5
46.0
56.8
59.4
55.6
57.3
53.2
53.9
52.8
52.2
55.6
1.8
2.2
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
Table 1. 14CO2 measurements from Niwot Ridge, Colorado, USA.
1
CURL is the laboratory code for each analysis.
2
Date and time collected are in UTC.
3
AMS indicates which AMS facility was used for the AMS measurement; all sample
preparation was performed at the University of Colorado.
4
Uncertainty is the one-sigma standard deviation as described in the text.
5
An F in the flag column indicates samples where local fossil fuel pollution was
identified (the CO mixing ratio exceeded 15 ppb above the baseline level); these data
points are excluded from the background dataset.
F
F
F
Turnbull et al., Niwot Ridge 14CO2
NWR
New England
Jan-Jun 2004
65.9 ± 0.6
65.3 ± 0.6
Jul-Dec 2004
64.3 ± 0.5
64.5 ± 0.5
Jan-Jun 2005
58.6 ± 0.6
59.9 ± 0.8
Table 2. Six monthly mean values for NWR and New England. Uncertainties are the
standard error for the averaged values.
28
Turnbull et al., Niwot Ridge 14CO2
29
Figure 1. Vacuum system for extraction of CO2 from air. Sample flasks containing
whole air are attached to the left side of the line. Sample air is released through the flow
controller at 200 mL/min (STP) and while continuously pumping with a rotary vane
pump. Water is frozen out using ethanol at -90°C in the left hand cold trap. CO2 is
quantitatively recovered by freezing in liquid nitrogen in the second trap. Two trap loops
are included to ensure quantitative extraction of CO2. No attempt is made to remove
N2O, which freezes out along with CO2, but does not interfere with the 14C measurement.
The pressure in the CO2 trap is kept below six Torr, to ensure that methane is not also
collected. Extraction is continued either for a set time, or until all the sample air has
passed through the extraction system. The CO2 is sublimed and quantified in the known
volume cold finger, then flame-sealed into a Pyrex ampoule.
Turnbull et al., Niwot Ridge 14CO2
30
Figure 2. Graphitization system. The quartz side arm is loaded with iron powder as a
catalyst. Sample CO2 is introduced from the right hand side, and frozen with liquid
nitrogen as hydrogen gas is introduced. A furnace at 625°C is placed over the quartz side
arm, and a thermoelectric cooling system is attached to the vertical Pyrex tube. Graphite
is precipitated onto the iron catalyst, and water is frozen into the Pyrex tube, allowing the
reaction to go to completion. Reaction progress is monitored via the attached pressure
transducer.
Turnbull et al., Niwot Ridge 14CO2
31
Figure 3. Measurements of Ox-I, OX-II and NWTstd for measurements made between
November 2003 and March 2006. Measurements are shown in order of date of
measurement, and the y-axis is the same for all panels, but is not linear. Squares indicate
Turnbull et al., Niwot Ridge 14CO2
32
measurements at RRL (only NWTstd measurements are shown for RRL); diamonds
indicate measurements at UCI. Poorly performing wheels at UCI are shown as open
diamonds. Error bars are the one-sigma AMS single sample precision as described in the
text. The solid line indicates the expected or mean value for each sample type. For the
primary standard Ox-I, the mean for each wheel is set at the book value [Stuiver and
Polach, 1977], and the scatter represents the repeatability of the measurements within a
given wheel. For Ox-II, the expected value is 340.66‰ [Stuiver, 1983]. For NWTstd, the
mean value is 73.4‰, with values of 73.5 ± 0.5‰ for RRL and 73.3 ± 0.2‰ for UCI.
Note that while NWTstd and Ox-II are measured in every wheel containing air samples,
we also include additional Ox-I measurements from wheels containing other sample
types.
Turnbull et al., Niwot Ridge 14CO2
33
10
8
Difference from pair mean (‰)
6
4
2
0
-2
-4
-6
-8
-10
Figure 4. Pair differences for replicate analyses of authentic samples. Each pair 14CO2
is normalized to a mean of 0 ‰, and the symbols indicate the deviation of each
measurement from the pair mean. Error bars are the 1-sigma uncertainty on each
individual measurement as described in the text. Squares are measurements made at
RRL; circles are measurements made at UCI. Closed symbols indicate the first sample
from each pair; open symbols are the second sample from the pair. 2=1.07 for the 74
replicate pairs.
Turnbull et al., Niwot Ridge 14CO2
34
Figure 5. (a) Measured 14CO2 values for NWR. Open symbols indicate samples that
were flagged as polluted, as indicated by high CO values shown in panel b and described
in the text. Error bars are one sigma uncertainties as described in the text. The curve is
fitted to the remaining NWR 14CO2 background time series (solid symbols), using a
smoothed trend obtained from a linear fit with two harmonics and smoothed residuals
from a low-pass filter with a 180-day cutoff in the frequency domain [Thoning et al.,
Turnbull et al., Niwot Ridge 14CO2
35
1989]. (b) CO values measured at NWR, with a fitted curve [Thoning et al., 1989]. Open
symbols indicate CO values more than 15 ppb higher than the fitted curve.
Turnbull et al., Niwot Ridge 14CO2
36
Figure 6. Comparison of NWR and New England 14CO2 observations. Diamonds are
the NWR 14CO2 background time series as shown in figure 5, and upper troposphere
measurements from over New England are shown as open squares. In some cases, two
measurements were made at different altitudes in the New England free troposphere, or
measurements were made at both New England sites on the same day; in either case, both
results are shown. The solid line is a best fit to the NWR data, using the same method as
for figure 5, but using a shorter 90-day cutoff filter. The dashed line is a fit to the New
England data using the same fitting procedure.
Turnbull et al., Niwot Ridge 14CO2
37
Figure 7. Estimates of the recently added fossil fuel CO2 mixing ratio (Cff) in the
boundary layer over New England. Open symbols are the values from Turnbull et al.
[2006], where background 14CO2 values were obtained from free tropospheric
observations made on the same day in the same vertical profile. Closed symbols are Cff
estimates using the same method, except that background 14CO2 values were from the
NWR monthly mean.