Abstract
Abstract. The TROPOspheric Monitoring Instrument (TROPOMI) level-2 aerosol layer height (ALH) product has now been released to the general public. This product is retrieved using TROPOMI's measurements of the oxygen A-band, radiative transfer model (RTM) calculations augmented by neural networks and an iterative optimal estimation technique. The TROPOMI ALH product will deliver ALH estimates over cloud-free scenes over the ocean and land that contain aerosols above a certain threshold of the measured UV aerosol index (UVAI) in the ultraviolet region. This paper provides background for the ALH product and explores its quality by comparing ALH estimates to similar quantities derived from spaceborne lidars observing the same scene. The spaceborne lidar chosen for this study is the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission, which flies in formation with NASA's A-train constellation since 2006 and is a proven source of data for studying ALHs. The influence of the surface and clouds is discussed, and the aspects of the TROPOMI ALH algorithm that will require future development efforts are highlighted. A case-by-case analysis of the data from the four selected cases (mostly around the Saharan region with approximately 800 co-located TROPOMI pixels and CALIOP profiles in June and December 2018) shows that ALHs retrieved from TROPOMI using the operational Sentinel-5 Precursor Level-2 ALH algorithm is lower than CALIOP aerosol extinction heights by approximately 0.5 km. Looking at data beyond these cases, it is clear that there is a significant difference when it comes to retrievals over land, where these differences can easily go over 1 km on average.
Highlights
Aerosols are an important component of the Earth system, which provide the means for the formation of clouds by acting as cloud condensation nuclei, affecting the Earth’s radiation budget by absorbing or scattering incoming solar radiation (Twomey, 1974), and even nurturing forests from across oceans (Yu et al, 2015; Barkley et al, 2019)
Two sets of overall comparisons are done between Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) ALHext and TROPOspheric Monitoring Instrument (TROPOMI) aerosol layer height (ALH), one with all co-locations (Fig. 2a) regardless of cloud filtering and the other with a smaller subset of the dataset constrained by the cloud_warning flag from Table 1 (Fig. 2b)
With an average difference of −2.25 km, median difference of −1.62 km and a standard deviation of 3.83 km, the retrieved ALH from TROPOMI over land is reported to be systematically closer to the surface than CALIOP ALHext than in comparison with retrievals over the ocean, which has a mean difference of −0.41 km, a median difference of −0.29 km and a very high standard deviation of 6.86 km
Summary
Aerosols are an important component of the Earth system, which provide the means for the formation of clouds by acting as cloud condensation nuclei, affecting the Earth’s radiation budget by absorbing or scattering incoming solar radiation (Twomey, 1974), and even nurturing forests from across oceans (Yu et al, 2015; Barkley et al, 2019). While these measured backscatter profiles provide detailed quantitative information on the scattering components present in the atmosphere, spaceborne atmospheric profiling lidars have limited spatial coverage due to their limited beam width Owing to this particular feature of active remote sensing, spaceborne lidars currently do not revisit a specific point on Earth several times a day or even on a daily basis. Except for TROPOMI, there are currently no passive remote sensing missions that provide an operational stream of retrieved ALHs. In the fourth quarter of 2019, an operational data stream of retrieved ALHs derived from measured oxygen A-band spectra by TROPOMI was made available to the general public; the TROPOMI operational UVAI product augmented by the TROPOMI ALH product has the potential to further the operational monitoring of aerosol properties globally.
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