Evalua&on  of  CCl4  Atmospheric  Loss  Processes,    Life&mes,  and  Uncertain&es    

NASA  Goddard  Space  Flight  Center,  Greenbelt,  Maryland,  USA      and    Science  Systems  and  Applica<ons,  Inc.,  Lanham,  Maryland,  USA.  fleming@kahuna.gsfc.nasa.gov  

Eric  L.  Fleming      Earth  System  Research  Laboratory,    Chemical  Sciences  Division,    Na<onal  Oceanic  and  Atmospheric  Administra<on,  Boulder,  Colorado,  USA.  James.B.Burkholder@noaa.gov  

Laboratory Studies of CCl4 Atmospheric Loss Processes

Conclusions * The 2-D model calculated global annually averaged CCl4 atmospheric lifetime is 48.7 (45.2–52.3) years

� The primary CCl4 loss is via short-wavelength UV photolysis in the stratosphere * The most critical atmospheric loss processes are well-characterized via precise laboratory studies * Uncertainty in the laboratory results leads to a ±~7% 2σ uncertainty spread in the CCl4 global lifetime * Differences among 3-D models is an additional source of uncertainty in model derived CCl4 lifetimes, which is larger than the photolysis and kinetic uncertainty, see SPARC (2013)

Acknowledgments This work was supported in part by NOAA’s Climate Goal (AC4) and NASA’s Atmospheric Composition Program

Reviews, evaluations, and recommendations for kinetic and photochemical parameters are available from the SPARC (2013) lifetime report and the NASA/JPL (2011) Data Evaluation. Note, the NASA/JPL (2015) Data Evaluation has adopted the SPARC (2013) recommendations.

James  B.  Burkholder  

2-D Atmospheric Modeling: Loss Processes and Lifetimes

Graphical Summary of Model Results (Global Annual Averages)

Altitude Dependence of Loss Processes Uncertainty (2σ) in Local Loss Rate CCl4 Atmospheric Profile

Estimated local (instantaneous) loss rate uncertainty due to the uncertainty in kinetic and photochemical loss process parameters

* No significant tropospheric loss processes

* Short wavelength UV photolysis (190-230 nm) is the most important stratospheric loss process

* O(1D) reaction is a minor process, while OH and Cl reactive loss are negligible

* Uncertainty in the troposphere is greatest, although there is negligible loss in the tropospheric region

* Uncertainty in the UV absorption spectrum dominates the uncertainty in stratospheric loss, the spectrum temperature dependence uncertainty accounts for the altitude dependence

* CCl4 is primarily removed in the stratosphere (by UV photolysis) * Maximum CCl4 loss rate at ~20 km * Negligible CCl4 above ~30 km

Tabulated Summary of Model Results (Global Annual Averages)

* UV photolysis is the primary loss process (>98%)

* CCl4 is primarily lost in the stratosphere * Lifetime range based solely on kinetic and photochemical estimated uncertainties

Atmospheric Lifetimes * Calculation of fractional contribution from known loss processes

* Results presented here were obtained using SPARC (2013) recommendations and NASA/JPL (2011) parameters otherwise

* Results are global annual averages

* Calculation of lifetime range (uncertainty) due solely to 2σ uncertainty in the kinetic and photochemical input parameters (i.e., does not include uncertainty in model representation)

* Model details are provided in Fleming et al. (2011)

Key References: Fleming, E. L.; Jackman, C. H.; Stolarski, R. S.; Douglas, A. R. A Model Study of the Impact of Source Gas Changes on the Stratosphere for 1850-2100. Atmos. Chem. Phys. 2011, 11, 8515-8541, doi:10.5194/acp-11-8515-2011. Ko, M. K. W.; Newman, P. A.; Reimann, S.; Strahan, S. E.; Plumb, R. A.; Stolarski, R. S.; Burkholder, J. B.; Mellouki, W.; Engel, A.; Atlas, E. L.; Chipperfield, M.; Liang, Q. Lifetimes of stratospheric ozone-depleting substances, their replacements, and related species, SPARC Report No. 6, WCRP-15/2013, 2013. Rontu Carlon, N.; Papanastasiou, D. K.; Fleming, E. L.; Jackman, C. H.; Newman, P. A.; Burkholder, J. B. UV absorption cross sections of nitrous oxide (N2O) and carbon tetrachloride (CCl4) between 210 and 350 K and the atmospheric implications. Atmos. Chem. Phys. 2010, 10, 6137-6149.JPL Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K.; Orkin, V. L.; Wine, P. H. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation Number 17, JPL Publication 10-6, Jet Propulsion Laboratory, California Institute of Technology Pasadena, California, 2011, http://jpldataeval.jpl.nasa.gov.

Introduction The understanding of the kinetic and photochemical processes that remove CCl4 from the atmosphere directly impacts our ability to determine its atmospheric lifetime.

In the determination of an atmospheric lifetime, it is also important to understand the regions of the atmosphere where loss occurs, i.e., troposphere or stratosphere.

Atmospheric loss processes are identified and quantified via laboratory studies and atmospheric lifetimes are evaluated using atmospheric models.

This poster provides an overview of the current understanding of the atmospheric loss processes for CCl4 and an analysis of its local and global lifetimes and their uncertainty due solely to the uncertainty in the kinetic and photochemical parameters using a 2-D model.

Loss Process

VUV photolysis (Lyman-α)

Possible Region of Importance General Relevance

UV photolysis (λ >295 nm)

UV photolysis (λ <295 nm)

O(1D) Reaction

OH and Cl Reaction

O3, NO3, … Reaction

Upper atmosphere (>60 km) Long-lived compounds (>500 years)

Stratosphere Important for halogenated compounds with lifetimes in the 40 to ~500 year range

Troposphere and Stratosphere Not significant for most stable chloro-compounds

Stratosphere

Troposphere and Stratosphere

Troposphere

Typically a minor (<10%) loss process

Significant for H-containing and unsaturated compounds Not significant for saturated halogenated compounds

Photolysis VUV

* Lyman-α cross section is well defined * Estimated 2σ uncertainty of 20%

* Rontu et al. (2010) parameterization is the basis for current recommendation

* Discrepancies on the order of 10% in the temperature dependence of the spectrum leads to greater uncertainty at stratospherically relevant temperatures

UV Critical λ region

UV-T dep. JPL10-6: Simon et al. (1988)

* 298 K spectrum well defined, estimated 2σ uncertainty in critical region is 6% at 298 K

O(1D) Reaction

* k(T) is well defined, estimated 2σ uncertainty is 15% * Recommended reactive branching ratio at room temperature is 0.79 ± 0.04 and estimated to be T-independent

OH and Cl Reactions

* Recommended k(T) upper-limit estimates are based on reaction thermochemistry and an estimated Arrhenius A-factor

* Experimentally determined k(T) upper limits do not provide realistic constraints on the reactivity

* Unit quantum yield

kOH(298 K) < 1 x10-20 cm3 molecule-1 s-1

kCl(298 K) < 1 x10-18 cm3 molecule-1 s-1