14CO and its application in studies of atmospheric chemistry and
transport: A brief history
Carl A. M. Brenninkmeijer
The primary origin of atmospheric
14CO is production by cosmic radiation.
High energy cosmic rays (mainly protons) induce large nucleonic
particle cascades in the atmosphere and produce atmospheric neutrons.
Most of them diffuse and thermalize before they are captured by nitrogen
nuclei forming 14C
The recoil 14C atom rapidly
oxidizes to 14CO,
with a yield that has been determined to be approximately 95%
[ Pandow et al., 1960;
MacKay et al., 1963].
In this way, a natural tracer is produced throughout the atmosphere,
almost equally partitioned between the stratosphere and the troposphere,
however with its maximum in polar regions caused by the influence of the
geomagnetic field on the primary cosmic ray particles. The average source
strength is 1.6 - 2 molecules per second and per square-centimeter of the
Earth's surface, corresponding to a total production of approximately
13-16 kg 14CO per year.
Since the cosmic ray flux reaching the atmosphere is modulated by the solar
wind intensity, the cosmogenic
14CO production rate oscillates with a phase
of 11 years (solar cycle) with higher production rates during times of low
The secondary (``biogenic'') contribution, comprising
20-25% of the total source, consists of recycled 14CO
from the biosphere, entering or evolving in the atmosphere by oxidation
of natural methane and higher hydrocarbons, and by biomass burning.
The use of fossil fuel does not contribute to atmospheric 14CO
as geological production times vastly exceed the
14C half life of about 5730 years.
The significance of 14CO is that it constitutes a natural
tracer that can be used to assess the hydroxyl radical (OH)
abundance, because 14 CO + OH is its main sink reaction, with an
average tropospheric lifetime of 14CO of about 2-3 months.
Already before the discovery of the important role of OH in the
estimated the residence time of CO, using
14CO measurements, or more precisely the specific activity of CO
MacKay et al. .
The implicit assumptions of the approach of
Weinstock  and the
implications for CO budget calculations have been discussed by
Junge et al. .
Volz et al.  applied the
14CO concept in a
systematic manner and concluded that the abundance and seasonality of
14CO is in accordance with that of OH used in a two-dimensional
(2-D) atmospheric chemistry model.
14CO measurements had been exclusively performed
using proportional gas counters requiring large amounts of air
(~ 200 m3) to be processed.
Routine measurements of 14CO in smaller air samples
with increased precision became possible with the arising
technique of the accelerator mass spectrometry (AMS).
Air sampling techniques suitable for isotopic analysis and
extraction procedures for isolating CO from the air samples are
described for instance by
Brenninkmeijer and Roberts ,
Mak and Brenninkmeijer , and
Aspects of the AMS measurements are discussed in
Rom et al. [2000b].
Brenninkmeijer et al. 
observed lower 14CO levels in the southern hemisphere,
which were attributed to higher southern hemisphere (SH) OH levels,
in contradiction to ideas about higher northern hemisphere (NH) OH
values due to the importance of NO in recycling OH
[Crutzen and Zimmermann, 1991].
Mak et al. ;
Mak et al. 
measured 14CO in air sampled in the
free troposphere, applied two different 2-D models
and concluded that apart from the NH-SH asymmetry, generally
atmospheric levels seemed lower than inferred by the models employed.
Quay et al. 
investigated various 14CO measurements with a 2-D
model and concluded that either a higher horizontal mixing or a higher
OH concentration in the SH is responsible for the observed inter-hemispheric
asymmetry of 14CO.
In the meantime, more and more 14CO measurements at surface level
and in the free troposphere have become available
[Mak and Southon, 1998;
Tyler et al., 1999;
Röckmann and Brenninkmeijer, 1997;
Röckmann et al., 1999;
Kato et al., 2000;
Rom et al., 2000a;
Quay et al., 2000].
The first 14CO analysis of lower stratospheric air samples
is reported by
Brenninkmeijer et al. .
Three independent estimates of the primary cosmogenic
14CO source distribution
O'Brien et al., 1991;
Masarik and Beer, 1999]
which differ mostly according to the vertical
gradient of the production rate. However,
Jöckel et al. 
(and Jöckel )
showed that this uncertainty is not a principle problem for the
Also the effect of the solar variation is well understood and can be
taken into account when
14CO observations of different epochs are to be compared
[Jöckel et al., 2000].
This sets the fundament for compiling a 14CO
climatology, i.e., a zonally averaged seasonal cycle at the surface
comprising 1088 14CO observations from 4 institutes
[Jöckel and Brenninkmeijer, 2002].
Jöckel et al. 
used this climatology for the evaluation of
two 3-dimensional atmospheric models and revisited
the observed inter-hemispheric asymmetry of atmospheric
14CO. Evidence for
a higher OH abundance in the SH is not longer supported, since the
asymmetry can consistently be explained by inter-hemispheric differences of
the exchange rate between the stratosphere and the troposphere.
Brenninkmeijer, C. A. M.
- Measurement of the abundance of 14CO in the atmosphere and
the 13C/12C and 18O/16O
ratio of atmospheric CO with applications in New Zealand and Antarctica.
J. Geophys. Res., 98(D6), 10595-10614, 1993.
Brenninkmeijer, C. A. M., D. C. Lowe, M. R. Manning,
R. J. Sparks, and P. F. J. v. Velthoven.
- The 13C, 14C, and 18O isotopic
composition of CO, CH4 and CO2
in the higher southern latitudes lower stratosphere.
J. Geophys. Res., 100(D12), 26163-26172, 1995.
Brenninkmeijer, C. A. M., M. R. Manning, D. C. Lowe,
G. Wallace, R. J. Sparks, and A. Volz-Thomas.
- Interhemispheric asymmetry in OH abundance inferred from
measurements of atmospheric 14CO.
Nature, 356, 50-52, 1992.
Brenninkmeijer, C. A. M. and P. A. Roberts.
- An air-driven pressure booster pump for aircraft-based sampling.
J. Atm. Oc. Tech., 11(6), 1664-1671, 1994.
Crutzen, P. J. and P. H. Zimmermann.
- The changing photochemistry of the troposphere.
Tellus, 43AB(4), 136-151, 1991.
Jöckel, P., C. A. M. Brenninkmeijer, M. G. Lawrence,
A. B. M. Jeuken, and P. F.J. van Velthoven.
- Evaluation of stratosphere - troposphere exchange and the hydroxyl
radical distribution in 3-dimensional global atmospheric models using
observations of cosmogenic 14CO.
J. Geophys. Res., 107(D20), 4446,
Jöckel, P. and C. A. M. Brenninkmeijer.
- The seasonal cycle of cosmogenic 14CO at the surface level:
A solar cycle adjusted, zonal average climatology based on observations.
J. Geophys. Res., 107(D22), 4656,
Cosmogenic 14CO as tracer for atmosperic chemistry and
Ph.D. thesis, Combined Faculties for the Natural Sciences and for
Mathematics of the Rupertus Carola University of Heidelberg, Germany,
Jöckel, P., C. A. M. Brenninkmeijer, and M. G. Lawrence.
- Atmospheric response time of cosmogenic 14CO to
changes in solar activity.
J. Geophys. Res., 105(D5), 6737-6744, 2000.
Jöckel, P., M. G. Lawrence, and C. A. M. Brenninkmeijer.
- Simulations of cosmogenic 14CO using the three-dimensional
atmospheric model MATCH: Effects of 14C production distribution
and the solar cycle.
J. Geophys. Res., 104(D9), 11733-11743, 1999.
Junge, C., W. Seiler, and P. Warneck.
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Kato, S., Y. Kajii, H. Akimoto, M. Bräunlich,
T. Röckmann, and C. A. M. Brenninkmeijer.
- Observed and modeled seasonal variation of 13C,
18O, and 14C of atmospheric CO at Happo,
a remote site in Japan, and a comparison with other records.
J. Geophys. Res., 105(D7), 8891-8900, 2000.
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- On the chemistry of natural radiocarbon.
J. Geophys. Res., 68, 3929-3931, 1963.
Mak, J. E. and C. A. M. Brenninkmeijer.
- Compressed air sample technology for isotopic analysis of atmospheric
J. Atm. Oc. Tech., 11(2), 1994.
Mak, J. E., C. A. M. Brenninkmeijer, and M. R. Manning.
- Evidence for a missing carbon monoxide sink based on tropospheric
measurements of 14CO.
J. Geophys. Res., 19(14), 1467-1470, 1992.
Mak, J. E., C. A. M. Brenninkmeijer, and J. Tamaresis.
- Atmospheric 14CO observations and their use for estimating
carbon monoxide removal rates.
J. Geophys. Res., 99(D11), 22915-22922, 1994.
Mak, J. E. and J. R. Southon.
- Assessment of tropical OH seasonality using atmospheric 14CO
measurements from Barbados.
Geophys. Res. Lett., 25(15), 2801-2804, 1998.
Masarik, J. and J. Beer.
- Simulation of particle fluxes and cosmogenic nuclide production in
the Earth's atmosphere.
J. Geophys. Res., 104(D10), 12099-12111, 1999.
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and D. F. Smart.
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J. Geophys. Res., 105(D12), 15147-15166, 2000.
Röckmann, T. and C. A. Brenninkmeijer.
- CO and CO2 isotopic composition in Spitsbergen during
the 1995 ARCTOC campaign.
Tellus, 49B, 445-465, 1997.
Röckmann, T., C. A. M. Brenninkmeijer, M. Hahn,
and N. F. Elansky.
- CO mixing and isotope ratios across Russia; Trans-Siberian
railroad expedition TROICA 3, April 1997.
Chemosphere Glob. Change Sci., 1, 219-231, 1999.
Rom, W., C. Brenninkmeijer, M. Bräunlich, G. R.,
M. Mandl, A. Kaiser, W. Kutschera, A. Priller,
S. Puchegger, T. Röckmann, and P. Steier.
- A detailed 2-year record of atmospheric 14CO in the
temperate northern hemisphere.
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W. Kutschera, A. Priller, S. Puchegger,
T. Röckmann, and P. Steier.
- Methodological aspects of atmospheric 14CO measurements
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Manning, J. M. Conny, and A. J. T. Jull.
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composition of tropospheric carbon monoxide at Niwot Ridge, Colorado.
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