The Isotopic Composition of Carbon Dioxide in the Middle Atmosphere

1. The Isotopic Composition of Carbon Dioxide in the Middle Atmosphere Mao-Chang Liang1, Geoffrey A. Blake1, Brenton R. Lewis2, and Yuk L. Yung1 1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, USA 2Research School of Physical Sciences and Engineering, The Australian National University, Canberra, Australia • Abstract • 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 CO2. • MIF of CO2 • 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 • 18O(CO2)  3 - 4 + 18O(1D) - 18O(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 xt = (0.80, 0.75, 0.70) O2 Lyman- xm = (0, 0.05%, 0.1%) • Model simulation: • Calculated by the Caltech/JPL one-dimensional KINETICS • model, which reproduces the the vertical profiles of the • isotopic composition of O3 in the stratosphere (Liang et al. 2005) • 2. Slopes: • a) ~1.6 in the stratosphere (dashed line) • b) <1.6 in the upper stratosphere • c) ~0.3 in the upper mesosphere (line AB) • d) ~0.5 above homopause (dash-dotted line) • 3. Magnitude of 18O(CO2) in the mesosphere: • Measurements made by Thiemens et al. (1995) at 30N is likely • due to fresh downwelling air at this latitude. Three-box model illustration: a) 17O(CO2) = xt17O(CO2)t + xs17O(CO2)s + xm17O(CO2)m - 17O(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. • Sources of O(1D) • Stratosphere: • O3 + h (230-310 nm)  O2 + O(1D) • Mesosphere: • O2 + Lyman-  O(3P) + O(1D) References Yung et al. JGR, 1997; Thiemens et al. Science, 1995; Lämmerzahl et al. GRL, 2002; Liang et al. JGR, 2005 (accepted) Contact: see http://www.gps.caltech.edu/~mcl