Validation of TROPOMI SIF Products With Improved Geolocation Match Between In Situ and Satellite Measurements

Abstract

Direct comparison with in situ measurements serves as the primary method for validating TROPOMI SIF products. However, due to geometric errors in satellite data, the exact spatial extent of the nominal validation pixel may not align with the in situ site perfectly. In addressing this challenge, this study proposed, for the first time, a method to precisely identify the validation pixels matching with in situ sites. Moreover, the accuracy of the TROPOMI SIF product was reevaluated with the improved geolocation match method between the satellite pixel and the corresponding in situ site. The results indicate that ignoring the geometric errors of TROPOMI pixels can result in a 49% probability of mismatch between the validation pixel and the in situ site. The errors caused by geolocation mismatch mainly come from two aspects. One is the incorrect extraction of the validation pixel, with a maximum error of 1.385 mWm⁻² sr⁻¹ nm⁻¹. The other is the pixel-scale reference “truth,” which resulted from the improper upscaling function of in situ measurements, and the maximum of this kind of error was 0.445 mWm⁻² sr⁻¹ nm⁻¹. With this improved geolocation match method, the TROPOMI SIF product showed a RMSE of 0.58 mWm⁻² sr⁻¹ nm⁻¹, a bias of 0.19 mWm⁻² sr⁻¹ nm⁻¹, and a R² of 0.51, which indicate a better performance than without considering geometric location matching errors.