Near‐Global CFC‐11 Trends as Observed by Atmospheric Infrared Sounder From 2003 to 2018

Recent studies have indicated a slowdown of the decline of CFC‐11 concentration since 2012. Ground‐based observations used in such studies have their limitations in terms of global coverage. Here we show that the CFC‐11 time‐varying behaviors can be seen by double differencing nadir‐view, clear‐sky brightness temperatures of four AIRS (Atmospheric Infrared Sounder) channels in an infrared CFC‐11 absorption band. Assuming that CFC‐11 is vertically well mixed through the troposphere, we retrieve CFC‐11 surface concentration and its secular trend using such AIRS observations over the near globe (55°S to 55°N) from January 2003 to December 2018. The retrieved trends of CFC‐11 at the 11 ground sites agree well with the trends derived from in situ measurements at those sites. Our results show that, from 55°S to 55°N, the CFC‐11 trends from January 2003 to December 2012 are all negative, ranging from −2.5 to −1 ppt/year. The trends from January 2003 to December 2018 are less negative by as much as ~0.5–1 ppt/year over the Shandong peninsula, the Arabian Peninsula, and north India and Nepal area, and such differences in the trends are statistically significant. Factors other than the CFC‐11 that can affect the retrievals and trends are also discussed. These findings can help us depict the near‐global spatial distribution of the CFC‐11 trends from 2003 to 2018. The analysis described here has the potential to be used with current and future hyperspectral sounders to help monitor the CFC‐11 from space.