Global modeling of CCl4: Using airborne and ground-based measurements to constrain the emissions and atmospheric losses for CCl4

The ozone-depleting substance carbon tetrachloride (CCl4) has a long lifetime ( 33 years), and displays significant spatial and seasonal variations, reflecting the balance between surface emissions, stratospheric photolysis (the predominant atmospheric sink), and ocean and land degradation. In this study, the observed spatial and temporal variations of CCl4 in the troposphere and stratosphere are used in global chemistry models to constrain the global emissions and total lifetime of CCl4. The seasonal cycle of CCl4 at a particular surface site reflects its proximity to surface emissions and sinks, including the injection of CCl4-depleted stratospheric air, and can differ greatly from place to place. A suite of tagged CCl4 tracers are used in the GEOSCCM 3-D model to track emissions from different geographic regions, and are compared with the observed seasonal cycle at GMD and AGAGE stations to improve regional emissions estimates. Additionally, stratospheric CCl4-CO2 tracer-tracer correlations from balloon and high-altitude aircraft measurements are used along with model sensitivity simulations to constrain the possible range of CCl4 photolysis lifetimes due to uncertainty in the photolysis cross sections. Due to the strong correlation between model age of air and CCl4 photolysis lifetime, stratospheric age of air observations are also used to constrain the most likely range of model CCl4 lifetimes among a suite of chemistry-climate models.