Near-global CFC-11 Trends as Observed by Atmospheric Infrared Sounder from 2002 to 2018

Recent studies have indicated the slowdown of the declination of CFC-11 concentration since 2012. Ground-based observations used in such studies cannot provide full spatial information of the trends over the globe. AIRS (Atmospheric Infrared Sounder) has been in operation since September 2002 and demonstrated remarkably stable calibration over the years and it covers CFC-11 absorption bands in the infrared. Using double differential method that we published before for estimating trace gas concentration from hyperspectral IR radiances such as AIRS, we explore here the feasibility of retrieving CFC-11 mixing ratio and its secular trend from a subset of nadir-view AIRS clear-sky observations over the near globe (55°S to 55°N). The clear-sky identification is based on MODIS imageries, the same method adopted by the NASA CERES algorithms. A double difference of brightness temperatures (dΔBTs) at two pairs of pre-selected AIRS channels in the CFC-11 absorption bands are used in the retrieval, which is based on a look-up-table approach. CFC-11 surface mole fraction is retrieved from dΔBTs with a spatial resolution of 10° in latitude by 30° in longitude over the near globe from September 2002 to December 2018. The trends of retrieved CFC-11 at 11 ground sites by NOAA and Japan Meteorological Agency are in good agreement with the trends based on in-situ measurements. Our results indicate that, from 55°S to 55°N, the CFC-11 trends from September 2002 to December 2012 are all negative with a magnitude ranging from 1 to 2.5 ppt/year. In contrast, the trends from January 2013 to December 2018 are slowed down by ~ 0.3-1 ppt/year. Our results also show that the northern-to-southern hemispheric-mean difference of CFC-11 had kept decreasing to 2011 and then became increasing afterwards. These findings confirm, in general, conclusions from the previous studies with a more comprehensive spatial perspective.