Model sensitivity studies of the decrease in atmospheric carbon tetrachloride

Abstract

Abstract. Carbon tetrachloride (CCl 4 ) 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 CCl 4 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 CCl 4 is removed from the atmosphere by photolysis, by ocean uptake and by degradation in soils. The largest sink is atmospheric photolysis (74 % 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 CCl 4 decay. This is partly due to the limiting effect of the rate of transport of CCl 4 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,Published by Copernicus Publications on behalf of the European Geosciences Union. 15742 M. P. Chipperfield et al.: Model sensitivity studies of the decrease in atmospheric carbon tetrachloride the model shows that uncertainty in ocean loss (17 % of total) has the largest impact on modelled CCl 4 decay due to its sizeable contribution to CCl 4 loss and large lifetime uncertainty range (147 to 241 years). With an assumed CCl 4 emission rate of 39 Gg year −1 , the reference simulation with the best estimate of loss processes still underestimates the observed CCl 4 (overestimates the decay) over the past 2 decades but to a smaller extent than previous studies. Changes to the rate of CCl 4 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 47 Gg year −1 . Further progress in constraining the CCl 4 budget is partly limited by systematic biases between observational datasets. For example, surface observations from the National Oceanic and Atmospheric Administration (NOAA) network are larger than from the Advanced Global Atmospheric Gases Experiment (AGAGE) network but have shown a steeper decreasing trend over the past 2 decades. These differences imply a difference in emissions which is significant relative to uncertainties in the magnitudes of the CCl 4 sinks.

CCl<sub>4</sub> distribution derived from MIPAS ESA v7 data: intercomparisons, trend, and lifetime estimation

“…Similar features have also been observed by other authors in CFC-11 and CFC-12 trends . Recently some studies (Harrison et al, 2016;Mahieu et al, 2014;Ploeger et al, 2015) have shown that the trends in stratospheric trace gases are affected by variability in the stratospheric circulation. This has been shown for a number of halogen source gases and their complementary degradation products (i.e., HCl and HF).…”

“…Under the assumption of a well-mixed troposphere, we can consider the CCl 4 vertical distribution to be approximately constant (Chipperfield et al, 2016;Allen et al, 2009). We consider observations provided by two networks that regularly perform long-term, highly accurate near-surface measurements of various tracers, including CCl 4 : the NOAA/ESRL/HATS (http://www.esrl.noaa.…”

Emissions of halocarbons from India inferred through atmospheric measurements

“…Emissions of CFC-11 have been shown to be increasing since 2013 (Montzka et al, 2018), and evidence suggests new production from China which has not been reported to the United Nations Environment Programme (UNEP) (Rigby et al, 2019). While "bottom-up" emissions of CTC, estimated from reported consumption for feedstock use, are small (1-4 Gg yr −1 , Montzka et al, 2011), "top-down" studies, based on atmospheric observations, suggest actual global emissions still exceed 30 Gg yr −1 (Chipperfield et al, 2016;Liang et al, 2014;Lunt et al, 2018).…”

History of chemically and radiatively important atmospheric gases from the Advanced Global Atmospheric Gases Experiment (AGAGE)

“…Recent studies have attempted to reevaluate the global CCl 4 budget . Estimates of the soil and ocean partial lifetimes have been revised upward (Rhew and Happell, 2016;Butler et al, 2016) and several new industrial sources have been identified (Sherry et al, 2017), substantially reducing the gap between top-down and bottom-up estimates .…”