Intercomparison of aerosol layer height between geostationary and polar orbiting satellites

Aerosol Layer Height (ALH) is an important parameter for climate studies, air quality monitoring, aviation safety near volcanos and remote sensing of trace gases. The ALH is rapidly becoming available from many new sources, such as polar orbiting satellites like TROPOMI, EarthCARE, Sentinel-3 and 5, and geostationary instruments around the globe, such as GEMS, TEMPO and Sentinel-4. These instruments provide many opportunities, covering a large range of spatial and temporal scales, that are further strengthened by a number of regional lidar networks around the globe, like the Latin American Lidar Network (LALINET), the Asian dust and aerosol lidar observation network (AN-Net), Micro-Pulse Lidar Network (MPLNET) and the European Aerosol Research Lidar Network (EARLINET). The above-mentioned geostationary instruments have the unique capability of providing the temporal variation of ALH and other parameters during the day over a large area, and lidar stations and other ground-based layer height providing instruments can be used to compare and validate the hourly retrievals and their temporal variation. However, the instruments are not overlapping and so cannot be compared directly. Comparison between geostationary instruments must be performed by polar orbiting satellites covering the field of views of geostationary instruments, preferably by satellites with different local overpass times, to cover as much as possible the temporal variation over the day. 

The validation of the various ALH products suffers from a number of issues, next to instrument calibration most notably different measurement techniques, like e.g. oxygen absorption spectroscopy, polarization techniques, stereo photogrammetry or active sensing, and the spatial and temporal coverage of satellite instruments. Here, we provide an overview of the issues and opportunities that are available for ALH comparison and validation from the polar orbiting satellites Sentinel-5p and EarthCARE. Intercomparisons between ATLID average extinction profiles (like a weighted or effective extinction height) and ALH from TROPOMI, GEMS and Sentinel-3 will be shown, to illustrate the possibilities for validation of TEMPO, S4 and S5, and the possibility to intercompare these various geostationary satellites. It will demonstrate the issues that arise from different layer height definitions that exist between the instruments, and that cause problems especially when the extinction profiles become complicated (like e.g. in the case of multiple vertical layers). 
Theoretical treatment of the aerosol layer height problem will be combined with examples of ATLID extinction profiles, showing well-defined and less well-defined layer heights and their associated problems for ALH retrievals.