Abstract
Abstract. The budgets of seven halogenated gases (CFC-11, CFC-12, CFC-113, CFC-114, CFC-115, CCl4 and SF6) are studied by comparing measurements in polar firn air from two Arctic and three Antarctic sites, and simulation results of two numerical models: a 2-D atmospheric chemistry model and a 1-D firn diffusion model. The first one is used to calculate atmospheric concentrations from emission trends based on industrial inventories; the calculated concentration trends are used by the second one to produce depth concentration profiles in the firn. The 2-D atmospheric model is validated in the boundary layer by comparison with atmospheric station measurements, and vertically for CFC-12 by comparison with balloon and FTIR measurements. Firn air measurements provide constraints on historical atmospheric concentrations over the last century. Age distributions in the firn are discussed using a Green function approach. Finally, our results are used as input to a radiative model in order to evaluate the radiative forcing of our target gases. Multi-species and multi-site firn air studies allow to better constrain atmospheric trends. The low concentrations of all studied gases at the bottom of the firn, and their consistency with our model results confirm that their natural sources are small. Our results indicate that the emissions, sinks and trends of CFC-11, CFC-12, CFC-113, CFC-115 and SF6 are well constrained, whereas it is not the case for CFC-114 and CCl4. Significant emission-dependent changes in the lifetimes of halocarbons destroyed in the stratosphere were obtained. Those result from the time needed for their transport from the surface where they are emitted to the stratosphere where they are destroyed. Efforts should be made to update and reduce the large uncertainties on CFC lifetimes.
Highlights
This study presents a trend and budget analysis of seven halogenated gases: CFC-11 (CCl3F), CFC-12 (CCl2F2), CFC-113 (CCl2FCClF2), CFC-114 (CClF2CClF2), CFC-115 (CClF2CF3), CCl4 and SF6
The major sources of uncertainties in the atmospheric cycle of long lived halocarbons are related to their emissions, their lifetimes and their measurement calibration. These uncertainties are relatively small for CFC-11, CFC-12 and CFC-113 (WMO, 2007), these gases are used to discuss the validation of our chemistry model
According to WMO (2007), inter-calibration differences between ESRL and AGAGE are lower than 2% for CFC-11, CFC-12 and CFC-113
Summary
This study presents a trend and budget analysis of seven halogenated gases: CFC-11 (CCl3F), CFC-12 (CCl2F2), CFC-113 (CCl2FCClF2), CFC-114 (CClF2CClF2), CFC-115 (CClF2CF3), CCl4 and SF6. Whereas all the studied gases are emitted mainly in the Northern Hemisphere, their concentrations are similar in both hemispheres because their very long lifetimes allow for their mixing in the whole atmosphere Such long lifetimes explain the important time lag between the emission decrease of CFCs and CCl4, and the decrease of their concentrations. The firn diffusion model has to be constrained with a sitespecific effective depth-diffusivity profile It is calculated using both the known long term atmospheric trend and concentrations in firn air of a reference gas (such as CO2 or CH4). The combined use of an atmospheric chemistry model and a firn diffusion model allows the discussion of the model/data comparison in terms of budget understanding for our target species. It contains additional chemistry model results which are presented below only for selected species
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