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

摘要

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 degrees S to 55 degrees 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 degrees S to 55 degrees 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 similar to 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.