Publication Type:Journal Article
Source:Atmospheric Research, Volume 231, p.104658 (2020)
Keywords:Atmospheric mixing ratios, Background levels, CHCCl, emissions, long-term trend
Although CH3CCl3 is an important ozone-depleting substance regulated by the Montreal Protocol on Substances that Deplete the Ozone Layer and its Amendments, the levels of atmospheric CH3CCl3 mixing ratios in China are poorly understood. Long-term in-situ measurements by GC-ECD have been conducted at the Shangdianzi Station in northern China since October 2006. In addition, air samples have been collected daily/weekly at seven stations and analyzed by GC/MS system since 2010. The two methods show comparable precisions at around 1% and all the measurements were calibrated by standards linked to the Advanced Global Atmospheric Gases Experiment (AGAGE) reference scale Scripps Institution of Oceanography (SIO)-05. Mixing ratios for both “background” and “polluted” conditions are reported. The atmospheric CH3CCl3 mixing ratios during background conditions showed a significant and consistent decreasing trend with a decline rate from 1.9 ppt/yr (October 2006 to December 2007) to 0.3 ppt/yr (November 2015 to December 2017) at the Shangdianzi Station. Measured background mixing ratios increased from 41% in 2007 to 95% in 2017. The mean mixing ratios and their enhancement of polluted conditions have decreased from 15.1 and 2.0 ppt in 2007 to 2.7 and 0.3 ppt in 2017 at Shangdianzi. The mixing ratios in air at background conditions at the seven stations in China agree well with each other and all decrease gradually. The percentage of pollution events to the total number of measurements (POL/SUM) at Lin'an has decreased significantly from 2010 to 2017, especially after 2013. In 2017, POL/SUMs of all stations were <8% and the enhancements above background were <0.7 ppt. Both mixing ratios and enhancements have decreased by one or two orders of magnitude when comparing the results from previous studies, in the early 2000s, with the results in 2017 from this study. Chinese emissions of CH3CCl3, which were estimated by a tracer-ratio method, have decreased from 1.6 kt/yr in 2007 to 0.3 kt/yr in 2013, indicating that CH3CCl3 has been phased-out in China in accordance with the Montreal Protocol.