Validation Summary
Sentinel-5p TROPOMI provides one operational ozone profile data product: L2_O3_PR Ozone Profile
Accurate retrieval of nadir ozone profile data requires the recalibrated TROPOMI Level-1b data (v2). Verification based on a diagnostic data set has demonstrated the value of Level-1b recalibration for improved ozone profile retrieval. Operational TROPOMI ozone profile data (v2.3.1) based on L1b v2.0 have become publicly available since November 29, 2021. A switch to L2 processor version 2.4.0 has been made in July 2022, based on L1b v2.1 with improved soft-calibration. Routine validation results for the NRTI and OFFL ozone profile streams are provided here.
Comparison of the Sentinel-5p TROPOMI L2_O3_PR data (both NRTI and OFFL channels of the operational NL-L2 processors v02.03.01 and v02.04.00) with ozonesonde and lidar measurements concludes to a median agreement better than 10 % in the troposphere and up to the upper troposphere/lower stratosphere (UTLS), and to higher values (20 %) of dispersion. In the stratosphere comparisons reveal vertically oscillating biases of 5-10 % (positive and negative), with smaller dispersion than in the troposphere (order of 10 %). Chi-square tests demonstrate that on average the observed differences confirm the ex-ante satellite and ground uncertainty estimates in the stratosphere, except at low SZA for comparisons with the stratospheric lidar measurements at Table Mountain between 30 and 35 km. Around the tropopause and below (around 15-20 km and lower), the mean chi-square value increases up to about two. Here, the predicted (random) satellite uncertainty is smaller than what is actually observed. The information content of the ozone profile retrieval is characterised by about six vertical sub-columns of independent information (estimated from the Degree of Freedom of the System) and a vertical sensitivity nearly equal to unity at altitudes from about 20 km (UTLS) to 50 km, and decreasing rapidly at altitudes above and below. The altitude registration of the retrieved profile information (estimated as the altitude of the barycentre of the associated averaging kernels) usually is close to the nominal retrieval altitude in the 20-50 km altitude range, and shows positive and negative offsets of up to 10 km below and above the 20-50 km altitude range, respectively. The effective vertical resolution of the profile retrieval (estimated as the Full Width at Half Maximum (FWHM) of the averaging kernels) usually ranges within 10-15 km, with a minimum close to 7 km in the middle stratosphere. Increased sensitivities and higher effective vertical resolutions (FWHM) can be observed for high solar zenith angles, as can be expected. On the other hand, a reduced DFS, sensitivity, and retrieval quality are sometimes observed for scenes that have both high SZA and high surface albedo, especially over the sea around the Antarctic.
Global maps of the six ozone profile sub-columns show the geographical coverage of the data, but also a slight along-orbit striping, especially in the middle stratosphere (24-32 km sub-column). Global maps of the integrated L2_O3_PR ozone profile data and of the L2_O3 total column data look mutually consistent. On the other hand, orbit curtain plots reveal that for some ground pixels the retrieved ozone profile deviates strongly and non-physically from the a-priori profile used in the retrieval. This issue needs further examination. Finally, in the absence of clouds, data files sometimes contain negative surface albedo values. The ground pixels affected by this anomaly are usually located at the east and west edges of the TROPOMI measurement swath.
| Product ID | Stream | Version | Orbits | Bias (req. 30 %) | Disp. (req. 10 %) | Special features |
|---|---|---|---|---|---|---|
| L2_O3_PR | NRTI | 02.03.01 02.04.00 |
21325-24697 24697-now |
10 % | 5-20 % | Bias below 10 % in the troposphere up to UTLS, and higher dispersion. Vertically oscillating bias of 5-10 % (positive to negative) in the stratosphere, with a smaller dispersion. |
| OFFL | 02.03.01 02.04.00 |
21207-24654 24655-now |
10 % | 5-20 % |
The graphs below show the comparison between S5P L2_O3_PR OFFL ozone number density profile data and all co-located ground-based reference measurements (November 2021 to September 2022, top: ozonesondes, bottom: lidar measurements). From left to right: the difference and the percent relative difference between S5P and ozonesonde data, the chi-square profile, the vertical sensitivity, the altitude registration offset, and the averaging kernel FWHM associated with the S5P retrieval. The colour scale indicates retrieval degrees of freedom. Black dashed lines show mean values (thick lines) and standard deviations (thin lines, around the mean), while grey dashed lines indicate the mean difference between the a-priori profile and the reference measurement. Dotted black lines indicate the total ex-ante (inductive) uncertainty of TROPOMI and the reference measurements combined (around the mean difference).
A detailed description of the method and a comprehensive discussion of validation results can be found in the Quarterly Validation Report #18: April 2018 - February 2023.
