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.
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
The global monitoring of aerosol properties can only be done using remote sensing techniques from space. The space-based techniques currently utilised by the scientific community to retrieve aerosol vertical information are divided into two categories – active and passive techniques; active remote sensing techniques monitor aerosol properties by measuring the interaction of energy generated by the instrument with aerosols in the atmosphere, whereas passive techniques do the same by measuring the interaction of natural light with aerosol particles. There are several differences in the sensing principles between active and passive remote sensing of aerosols, specifically in terms of vertical resolution. Active sensors such as the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) instrument provide attenuated backscatter profiles resolved vertically at a vertical resolution as fine as 30 m for different channels over a spatial resolution as fine as 0.33 km (see Table 2 of
Several passive retrieval strategies that are either currently in their operational phase or are upcoming remote sensing missions utilise the interaction of incoming solar radiation with the aerosol particles to retrieve height information. Some notable mentions of missions that retrieve ALH are the Multi-angle Imaging SpectroRadiometer (MISR) on board the NASA Terra satellite
In Sect.
The TROPOMI ALH product is derived from measurements of the oxygen A-band in the near-infrared region between 758 and 770 nm. Within this spectral range, TROPOMI measures top-of-atmosphere radiances and solar irradiances with a spectral resolution between 0.34 and 0.35 nm and a spectral sampling of 0.126 nm. The retrieval algorithm exploits the absorption characteristics of molecular oxygen, which varies with the photon path length – the photon path length for an aerosol layer closer to the surface is longer, which appears as deeper oxygen absorption lines in the measured spectrum (see Fig. 1 of
The reported ALH is the height of a single aerosol layer for the entire atmospheric column within the scene measured by TROPOMI; in reality, however, there can be several cases where distinctly separated elevated and boundary layer aerosols are present in the same scene. In such cases, the retrieval algorithm is expected to retrieve an optical centroid pressure or height of the two (or more) aerosol layers, depending on the atmospheric level of the aerosol layer from which most of the photons are scattered back. For instance, if the elevated aerosol layer contributes significantly more than the boundary layer aerosols to the top-of-atmosphere measured spectra, the ALH retrieval algorithm is expected to retrieve values closer to the elevated layer.
The technique for retrieving ALH is based on optimal estimation
The
The surface reflectance model used in the algorithm is derived from
The forward model parameterises aerosols with a Henyey–Greenstein scattering phase function
Finally, the ALH retrieval algorithm implements a pixel selection scheme before committing to retrieving ALH estimates. This pixel selection scheme involves auxiliary data products from TROPOMI such as the UVAI (
The maximum solar zenith angle allowed is 75 If the pixel over water lies in the sun-glint region (a maximum sun-glint angle of 18 If the standard deviation of the surface elevation within the pixel is beyond 1000 m, the pixel is not processed and a flag is raised. If it is beyond 300 m, a warning flag is raised and the pixel is processed. If the surface covered by the pixel comprises both land and water, a warning indicating mixed surface type is raised and the pixel is processed regardless. If the pixel contains snow or ice, the pixel is not processed and a flag is raised. If the TROPOMI level-2 UV aerosol index product reports a value below 0.0, the pixel is not processed and a flag is raised. If the value is less than 1.0, a low UVAI flag is raised. If the reported cloud fraction values from the TROPOMI FRESCO product for the pixel is beyond 0.6, the pixel is not processed and a flag is raised. If the VIIRS average cirrus reflectance for the pixel is beyond 0.4, the pixel is not processed and a flag is raised. If it is beyond 0.01, a warning for possible cirrus clouds is indicated. If the difference between the scene albedo (calculated using a look-up table) from the level-2 UVAI product and the surface albedo from the The nominal TROPOMI pixels also contain radiances at a subpixel level, which are called small pixel radiances. If the standard deviation of the small pixel radiances is larger than
These relevant flags are reported in Table
Processing quality flags relevant for diagnosing Sentinel-5 Precursor (S5P) ALH product quality. The descriptions are derived from the S5P IODD (Input Output Data Definition).
The Cloud-Aerosol lidar with Orthogonal Polarisation (CALIOP) instrument is a part of the payload for the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission
Histogram of differences between CALIOP
In this paper, the level-1 total backscatter profiles from the 532 nm channel are used as curtain plots to visualise the vertical structure of the atmosphere. Level-2 aerosol extinction profiles from the 532 nm channel are then used to compute an aerosol weighted extinction height,
TROPOMI–CALIOP co-locations between 1 May 2018 to the 28 February 2019 are selected. Two sets of overall comparisons are done between CALIOP
A map of cloud-filtered and sun-glint-filtered differences between co-located TROPOMI ALH and CALIOP ALH
From Fig.
Scatter density plots of the difference between TROPOMI ALH and CALIOP ALH
The distribution of the differences between TROPOMI ALH and CALIOP ALH
Retrieved ALH over land (if successful) can be closer to the surface than where the aerosol layer actually is situated vertically. The TROPOMI ALH product, unlike the CALIOP
The analysis presented in the previous section alone is insufficient to fully quantify the quality of the retrieved TROPOMI ALHs, due to the manner in which clouds are handled by both aerosol heights; TROPOMI pixels are affected by the presence of undetected clouds, whereas CALIOP aerosol extinction profiles do not consider clouds. Another significant source of departure between TROPOMI and CALIOP is their different sensing principles. Making conclusions on the quality of the current TROPOMI ALH product requires case-by-case studies of selected scenes. In line with this, four cases are selected to represent a very good mix of scenes containing elevated aerosol layers as well as aerosol layers close to the surface, high and low UV aerosol index, clear and cloudy scenes, clouds over and below aerosol layers, multiple aerosol layers, and retrievals over land and the ocean.
First column: corrected reflectance for the four selected cases as measured by the Suomi NPP/VIIRS imager. The yellow line represents the CALIOP ground track. Second column: the TROPOMI level-2 UV aerosol index product. The black line passing through the TROPOMI level-2 retrievals on this plot represents the ground track of the CALIPSO mission. Third column: retrieved AOT from TROPOMI. Fourth column: operational TROPOMI ALH.
The cases selected are Saharan desert dust and biomass burning events: three off the west coast of Sahara (desert dust) in June 2018 and one off the southern Saharan coast (biomass burning) in December 2018. All four cases have very good co-locations between TROPOMI and CALIOP, with the CALIOP ground track over the aerosol plumes (plotted with a yellow line over the VIIRS images in Fig.
CALIOP level-1 backscatter curtain plots for measurements in the 532 nm channel for the four selected cases in Fig.
It is important to note that spaceborne lidars, while having the advantage of being able to map more than one vertical layer in the atmosphere, suffer from attenuation of the signal in the presence of strongly backscattering components such as clouds or aerosols with a large optical depth. In the presence of a primary strongly backscattering aerosol layer, the attenuation of the signal may lead to undetected secondary aerosol layers beneath the primary layer. These layers, not apparent in the CALIOP curtain plots of the measured attenuated backscatter profiles, may be detected by the level-2 aerosol extinction profile product from the CALIOP mission, using the formula described in Eq. (
The retrieved TROPOMI ALH in Fig.
A visual inspection of Fig.
While scenes not contaminated with clouds show a smooth spatial distribution of the retrieved ALH, the presence of clouds may or may not add spatial variability in the ALH product. For instance, the presence of low clouds is clear in case b (Fig.
Comparison between the CALIOP weighted extinction heights (
From Fig.
For cases a and b, retrieved TROPOMI ALH does not seem to coincide with large values of the received backscatter signal in the level-1 data, whereas it does for case c and to a certain extent for case d (over land it tends to be closer to the surface). Parts of the CALIOP curtain plots for cases a, b and c suggest the existence of a possible second layer beneath the layer that is visually obvious or that the desert dust layer extends deeper to the surface and the CALIOP signal is simply too attenuated to detect it.
A direct comparison of the CALIOP
This paper discusses the quality of the soon to be released TROPOMI ALH product by comparing it with CALIOP data of co-located measurements of scenes containing aerosols between the two instruments. In order to do so, CALIOP weighted extinction heights from the 532 nm channel were calculated following Eq. (
From the analysis presented in this paper, TROPOMI's neural network ALH retrieval algorithm retrieves ALH values that compare well with CALIOP weighted extinction heights in cloud-screened cases following the cloud screening strategy using the TROPOMI ALH level-2 processing quality flags discussed in Table
There is a clear distinction between TROPOMI ALH retrievals over land and the ocean as photons scattering back from bright surfaces tend to influence ALH estimates closer towards the surface than an elevated aerosol layer. Retrieved ALH over land, if successful, can be closer to the surface if measured signal in the top of atmosphere contains more photons scattered back from the deepest atmospheric layer, which is the surface, in comparison to elevated aerosol layers which are higher up in the atmosphere. This, however, can change depending on the amount of aerosol information available in the spectrum compared to the same from the surface. Any attempt at retrieving ALH over the desert generally fails, with very few exceptions. There are several challenges that will need further development.
The TROPOMI level-2 UVAI product is currently an ingredient in selecting pixels containing aerosols for retrieving ALH. While this choice works quite well for cloud-free scenarios, it does not do a great job when a scene that contains both aerosols and clouds. These cloudy scenes do not seem to be detected by the current cloud-filtering schematic in the level-2 algorithm, and they will require a significant update in deciding whether a pixel is cloudy or not. For cases of scenes with a low aerosol load, square-shaped artefacts resulting from a surface albedo database with a resolution significantly lower than TROPOMI exist. Currently, the GOME-2 surface LER product derived from
Finally, space-based lidar (such as the CALIOP instrument on the CALIPSO mission) is a very good tool to retrieve aerosol vertical information to validate the TROPOMI ALH product. While the CALIOP level-1 backscatter profiles may be attenuated in cases of very strong signals from the top of the aerosol layer, the weighted extinction heights in conjunction with the backscatter profiles are sufficient for validation activities. These CALIOP profiles will be very important in assessing the impact of future development activities of the TROPOMI ALH product.
The co-location between TROPOMI and CALIOP ground pixels is done in the following manner. First, the geographic coordinates of CALIOP level-1 backscatter profiles and level-2 aerosol extinction profiles are converted into the Cartesian coordinate system. These CALIOP coordinates are fed into a
A map of all TROPOMI–CALIOP co-locations considered for Fig.
Sensitivity analysis of UV aerosol index to show the influence of different aerosol properties on the UVAI. The aerosols in these analyses have a Henyey–Greenstein scattering phase function with an asymmetry factor of 0.7; an Ångström exponent of 1.0; viewing zenith angle of 0
It is well documented that the UVAI depends on ALH
All TROPOMI data are publicly available. For the latest information on accessing these data, please check
The research analysis was conducted by SN under the supervision of MdG, JPV and PFL. The operational level-2 algorithm on which the research is based was developed together by JdH, MS, MtL and SN under the supervision of JPV and PFL. JS provided valuable insights into the role of UV absorbing index and the aerosol layer height.
The authors declare that they have no conflict of interest.
TROPOMI on Sentinel-5 Precursor: first year in operation (AMT/ACP inter-journal SI) SI statement: This article is part of the special issue “TROPOMI on Sentinel-5 Precursor: first year in operation (AMT/ACP inter-journal SI)”. It is not associated with a conference.
This publication contains modified Copernicus Sentinel data. This research is partly funded by the European Space Agency (ESA) within the EU Copernicus programme. We acknowledge the use of VIIRS imagery from the NASA Worldview application (
This research has been supported by the ESA (TROPOMI Science Contract).
This paper was edited by Jhoon Kim and reviewed by two anonymous referees.