The SPARC Lifetime Assessment report provides a unique opportunity to pair the 12-box inverse model with Chemistry Climate Models to derive global OH abundance and the lifetime of OH-removed Ozone Depleting Substances (ODSs) using observed and simulated CH3CCl3. Using the AGAGE and NOAA GMD surface measurements of CH3CCl3 from 1978 and 2011 in a 12-Box Bayesian inverse model, we estimated an atmosphere lifetime of 5.0 (4.8-5.4) yr for CH3CCl3 and a corresponding global mean OH ([OH]GM) of 1.09x106 molecules/cm3. While the 12-box model inverse method has been traditionally used with surface CH3CCl3 observations to derive global mean OH abundance, it is difficult to conclude whether the inferred OH concentration is representative of the true atmosphere due to a shortage of OH observations. Using model flux-based CH3CCl3 concentrations from the WACCM and GEOSCCM models as pseudo-observational constraints in the 12-box model, we derived [OH]GM and MCF lifetime accurate within ±5% of those calculated directly in the CCMs. This close agreement confirms the robustness of the 12-box inverse modeling method and the inferred [OH]GM is likely representative of the true global OH abundance. The four 3-D CCMs that participated in the Lifetime Assessment which calculates OH interactively in full tropospheric chemistry scheme all show [OH]GM 10-20% higher than this observation-derived OH. The OH difference among the models is more pronounced in the lower tropical troposphere, where OH loss plays a critical role in determining the atmospheric lifetime of OH-removed species. Despite this marked variance in modeled OH, our results suggest that the modeled and observation-derived OH partial lifetime (OH) of an OH-removed ODS are tightly correlated with that of CH3CCl3 (R2 = 0.98-0.99), independent of differences in their OH abundances. The multi-model regression slope of tau-OH of an ODS against tau-OH,CH3CCl3 agrees well with the reverse ratio of their thermal reaction rates with OH, kOH-CH3CCl3/kOH-ODS, at 272K, as noted in Spivakovsky et al. (2000). Based on our confidence in box-model inversed CH3CCl3 lifetime and the tight correlation of OH for the targeted ODSs, we are able to combine the scaled OH using OH,CH3CCl3 as a base lifetime with the modeled partial stratospheric lifetimes to derive a best estimate atmospheric lifetime for all OH-removed ODSs as well as the replacement HFCs.

Global OH abundance and lifetime of long-lived OH-removal species inferred from CH3CCl3: Implications from box model inversion analysis and flux-based CCMs

PITARI, Giovanni;
2013-01-01

Abstract

The SPARC Lifetime Assessment report provides a unique opportunity to pair the 12-box inverse model with Chemistry Climate Models to derive global OH abundance and the lifetime of OH-removed Ozone Depleting Substances (ODSs) using observed and simulated CH3CCl3. Using the AGAGE and NOAA GMD surface measurements of CH3CCl3 from 1978 and 2011 in a 12-Box Bayesian inverse model, we estimated an atmosphere lifetime of 5.0 (4.8-5.4) yr for CH3CCl3 and a corresponding global mean OH ([OH]GM) of 1.09x106 molecules/cm3. While the 12-box model inverse method has been traditionally used with surface CH3CCl3 observations to derive global mean OH abundance, it is difficult to conclude whether the inferred OH concentration is representative of the true atmosphere due to a shortage of OH observations. Using model flux-based CH3CCl3 concentrations from the WACCM and GEOSCCM models as pseudo-observational constraints in the 12-box model, we derived [OH]GM and MCF lifetime accurate within ±5% of those calculated directly in the CCMs. This close agreement confirms the robustness of the 12-box inverse modeling method and the inferred [OH]GM is likely representative of the true global OH abundance. The four 3-D CCMs that participated in the Lifetime Assessment which calculates OH interactively in full tropospheric chemistry scheme all show [OH]GM 10-20% higher than this observation-derived OH. The OH difference among the models is more pronounced in the lower tropical troposphere, where OH loss plays a critical role in determining the atmospheric lifetime of OH-removed species. Despite this marked variance in modeled OH, our results suggest that the modeled and observation-derived OH partial lifetime (OH) of an OH-removed ODS are tightly correlated with that of CH3CCl3 (R2 = 0.98-0.99), independent of differences in their OH abundances. The multi-model regression slope of tau-OH of an ODS against tau-OH,CH3CCl3 agrees well with the reverse ratio of their thermal reaction rates with OH, kOH-CH3CCl3/kOH-ODS, at 272K, as noted in Spivakovsky et al. (2000). Based on our confidence in box-model inversed CH3CCl3 lifetime and the tight correlation of OH for the targeted ODSs, we are able to combine the scaled OH using OH,CH3CCl3 as a base lifetime with the modeled partial stratospheric lifetimes to derive a best estimate atmospheric lifetime for all OH-removed ODSs as well as the replacement HFCs.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11697/32704
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