Due to the success of the Montreal Protocol, in the last two decades, scientists had been used to seeing atmospheric levels of chlorofluorocarbons (CFCs) decline year after year. However, starting in 2013, an unexpected signal began to emerge; there was a slowdown in the rate at which CFC-11, one of the most abundant CFCs, was declining.
This finding was based on the examination of trends in CFC-11 in the remote atmosphere, far from potential emissions sources. It showed that someone, somewhere, was emitting CFC-11 again, despite the global production ban that came into force in 2010.
To find who was responsible, we needed to look at measurements closer to potential source regions. The key clue came from monitoring stations in Korea and Japan; when air arrived at these stations from China, elevated concentrations of CFC-11 were seen, and the size of these elevations increased after 2013.
The next step was to determine how much CFC-11 was being emitted. To do this, we needed to use models that simulate the dispersion of gases in the atmosphere. These simulations, such as the UK Met Office NAME model, use three-dimensional estimates of wind fields, produced by meteorological centres, to simulate the advection and turbulent diffusion that gases experience as they are transported through the atmosphere.
Armed with the observations, and model estimates of atmospheric dispersion, we could work backwards to infer the emissions responsible for the elevated concentrations we observed. We used a range of Bayesian methods to do this, and tested a few different atmospheric models, for good measure. Each approach came back with a similar answer: emissions from eastern China had increased by around 7,000 ± 3,000 tonnes per year after 2013. This was enough to explain about half of the apparent global increase. Because of the sparse global monitoring network, it remains a mystery where the remaining emissions could originate from, or whether errors in the models are responsible for the gap.
As a result of our work, there have been investigations by non-governmental organisations and the media, which have confirmed that CFC-11 has been used in China in recent years. The Chinese government has announced a crack-down on the production of ozone depleting substances, and there are tentative signs that things may be returning to normal…
For more information:
Rigby, Montzka and Western: How we traced ‘mystery emissions’ of CFCs back to eastern China, The Conversation, 22nd May, 2019
Montzka, S. A., Dutton, G. S., Yu, P., Ray, E., Portmann, R. W., Daniel, J. S., Kuijpers, L., Hall, B. D., Mondeel, D., Siso, C., Nance, J. D., Rigby, M., Manning, A. J., Hu, L., Moore, F., Miller, B. R. and Elkins, J. W.: An unexpected and persistent increase in global emissions of ozone-depleting CFC-11, Nature, 557(7705), 413–417, doi:10.1038/s41586-018-0106-2, 2018.
Rigby, M., Park, S., Saito, T., Western, L. M., Redington, A. L., Fang, X., Henne, S., Manning, A. J., Prinn, R. G., Dutton, G. S., Fraser, P. J., Ganesan, A. L., Hall, B. D., Harth, C. M., Kim, J., Kim, K.-R., Krummel, P. B., Lee, T., Li, S., Liang, Q., Lunt, M. F., Montzka, S. A., Mühle, J., O’Doherty, S., Park, M.-K., Reimann, S., Salameh, P. K., Simmonds, P., Tunnicliffe, R. L., Weiss, R. F., Yokouchi, Y. and Young, D.: Increase in CFC-11 emissions from eastern China based on atmospheric observations, Nature, 569(7757), 546–550, doi:10.1038/s41586-019-1193-4, 2019.