Model Sensitivity Studies of the Decrease in Atmospheric Carbon Tetrachloride

Carbon tetrachloride is an ozone-depleting substance, which is controlled by the Montreal Protocol and for which the atmospheric abundance is decreasing. However, the current observed rate of this decrease is known to be slower than expected based on reported CCl₄ emissions and its estimated overall atmospheric lifetime. Here we use a three-dimensional (3- D) chemical transport model to investigate the impact on its predicted decay of uncertainties in the rates at which CCl₄ is removed from the atmosphere by photolysis, by ocean uptake and by degradation in soils. The largest sink is atmospheric photolysis (76% of total) but a reported 10% uncertainty in its combined photolysis cross-section and quantum yield has only a modest impact on the modelled rate of CCl₄ decay. This is partly due to the limiting effect of the rate of transport of CCl₄ from the main tropospheric reservoir to the stratosphere where photolytic loss occurs. The model suggests large interannual variability in the magnitude of this stratospheric photolysis sink caused by variations in transport. The impact of uncertainty in the minor soil sink (9% of total) is also relatively small. In contrast, the model shows that uncertainty in ocean loss (15% of total) has the largest impact on modelled CCl₄ decay due to its sizeable contribution to CCl₄ loss and large uncertainty range (157 to 313 years). With an assumed CCl₄ emission rate of 39 Gg/yr, the reference simulation with best estimate of loss processes still underestimates the observed CCl₄ (overestimates the decay) over the past two decades but to a smaller extent than previous studies. Changes to the rate of CCl₄ loss processes, in line with known uncertainties, could bring the model into agreement with in situ surface and remote-sensing measurements, as could an increase in emissions to around 45 Gg/yr. Further progress in constraining the CCl₄ budget is partly limited by systematic biases between observational datasets. For example, surface observations from the NOAA network are larger than from the AGAGE network but have shown a steeper decreasing trend over the past two decades. These differences imply a difference in emissions which is significant relative to uncertainties in the magnitudes of the CCl₄ sinks.