The Isotopic Composition of Carbon Dioxide in the Middle Atmosphere
Mao-Chang Liang1, Geoffrey A. Blake1, Brenton R. Lewis2, and Yuk L. Yung1
1Division
Abstract
of Geological and Planetary Sciences, California Institute of Technology, Pasadena, USA
2Research School of Physical Sciences and Engineering, The Australian National University, Canberra, Australia
The isotopic composition of long-lived trace gases provides a window into
atmospheric transport and chemistry. Carbon dioxide is a particularly powerful tracer,
because its abundance remains >100 ppmv in the mesosphere. For the first time, we
successfully reproduce the isotopic composition of CO2 in the middle atmosphere.
The mass-independent fractionation of oxygen in CO2 can be satisfactorily explained
by the exchange reaction with O(1D). In the stratosphere, the major source of O(1D) is
O3 photolysis. Higher in the mesosphere, we discover that the photolysis of 16O17O
and 16O18O by solar Lyman- radiation yields O(1D) 10-100 times more enriched in
17O and 18O than that from ozone photodissociation. New laboratory and atmospheric
measurements are proposed to test our model and validate the use of CO2 isotopic
fractionation as a tracer of atmospheric chemical and dynamical processes. Coupled
with climate models, the ‘anomalous’ oxygen signature in CO2 can be used in turn to
study biogeochemical cycles, in particular to constrain the gross carbon fluxes
between the atmosphere and terrestrial biosphere.
Introduction
Of the many trace molecules that can be used to examine atmospheric transport
processes and chemistry (e.g., CH4, N2O, SF6, and the CFCs), carbon dioxide is unique
in the middle atmosphere, because of its high abundance (~370 ppmv in the stratosphere,
dropping to ~100 ppmv at the homopause). The mass independent isotopic fractionation
(MIF) of oxygen first discovered in ozone is thought to be partially transferred to
carbon dioxide via the reaction O(1D) + CO2 in the middle atmosphere. Indeed, while
the reactions of trace molecules with O(1D) usually lead to their destruction, the O(1D)
+ CO2 reaction regenerates carbon dioxide. This ‘recycled’ CO2 is unique in its potential
to trace the chemical (reactions involving O(1D) in either a direct or indirect way) and
dynamical processes in the middle atmosphere. When transported to the troposphere, it
will produce measurable effects in biogeochemical cycles involving CO 2 .
MIF of CO2
1. Yung et al. (1997) mechanism:
16O(1D) + C16O16O  C16O16O + 16O
3k
17O(1D) + C16O16O  C16O17O + 16O
2k  (1 + 1)
16O(1D) + C16O17O  C16O16O + 17O
k  (1 + 2)
18O(1D) + C16O16O  C16O18O + 16O
2k  (1 + 3)
16O(1D) + C16O18O  C16O16O + 18O
k  (1 + 4)
2. Isotopic fractionation of oxygen in CO2:
17O(CO2)  1 - 2 + 17O(1D) - 17O(CO2)t
18
18
1
18
 O(CO2)  3 - 4 +  O( D) -  O(CO2)t
where tropospheric values of 17O(CO2)t and
18O(CO2)t are 9 and 17 ‰, respectively, relative to
atmospheric O2.
3. Values of 1-4:
If scaled by reduced mass: 1 - 2 is of opposite sign
and similar magnitude to that from the quenching
reactions of O(1D) with O2 or N2. So 17O(CO2) and
18O(CO2) are equivalent to the case of 1-2=0=3-4
 Thiemens et al. 1995
 Lämmerzahl et al. 2002
O2 Lyman-
1. Model simulation:
Calculated by the Caltech/JPL one-dimensional KINETICS
model, which reproduces the the vertical profiles of the
1
Sources of O( D)
isotopic composition of O3 in the stratosphere (Liang et al. 2005)
2. Slopes:
1. Stratosphere:
a) ~1.6 in the stratosphere (dashed line)
O3 + h (230-310 nm)  O2 + O(1D)
b) <1.6 in the upper stratosphere
2. Mesosphere:
3
1
c) ~0.3 in the upper mesosphere (line AB)
O2 + Lyman-  O( P) + O( D)
d) ~0.5 above homopause (dash-dotted line)
3. Magnitude of 18O(CO2) in the mesosphere:
References
Measurements made by Thiemens et al. (1995) at 30N is likely
Yung et al. JGR, 1997; Thiemens et al. Science, 1995;
due
to
fresh
downwelling
air
at
this
latitude.
Lämmerzahl et al. GRL, 2002; Liang et al. JGR, 2005 (accepted)
xt = (0.80, 0.75, 0.70)
.
xm = (0, 0.05%, 0.1%)
Three-box model illustration:
a) 17O(CO2) = xt17O(CO2)t + xs17O(CO2)s +
17
17
xm O(CO2)m -  O(CO2)t
18O(CO2) = xt18O(CO2)t + xs18O(CO2)s +
xm18O(CO2)m - 18O(CO2)t
b) xm << xs < xt and xt + xs + xm = 1
c) different degree of air mixing from
troposphere, stratosphere, and mesosphere is
shown above.
Contact: see http://www.gps.caltech.edu/~mcl