Validation of TROPOMI SIF products based on spatial representativeness grading and atmospheric-geometric effect assessment

Satellite solar-induced chlorophyll fluorescence (SIF) is important for carbon cycle research, but its direct validation is often hindered by spatial scale mismatch between satellite footprints and in situ measurements. We developed a quantitative spatial representativeness grading framework for in situ SIF measurements. The framework uses two proxy metrics, the consistency index ( 𝐶 𝑠 𝑖 𝑚 ) and relative absolute error ( 𝑅 𝐴 ⁢ 𝐸 𝑠 𝑖 𝑚 ), to diagnose local SIF simulation consistency and point-to-footprint deviation. Using 6240 HyPlant pseudo-point/footprint samples, a domain-level threshold search yielded Level 1 criteria of 𝐶 𝑠 𝑖 𝑚 ∈ [ 0 . 4 , 1 . 0 ] and 𝑅 𝐴 ⁢ 𝐸 𝑠 𝑖 𝑚 ≤ 0 . 4 . Samples with the same 𝐶 𝑠 𝑖 𝑚 range but 𝑅 𝐴 ⁢ 𝐸 𝑠 𝑖 𝑚 > 0 . 4 were classified as Level 2 and required upscaling. We applied this framework to 28 global sites to evaluate three TROPOMI SIF products using Sentinel-2 and ERA5-Land inputs. The framework retained 41–42% of the original matched samples. After controlling spatial scale effects, validation R 2 increased from 0.19 to 0.31 for TROPOSIF 735 and from 0.44 to 0.69 for both TROPOSIF 743 and TROPOSIF Caltech . Sensitivity analyses showed that this improvement was generally preserved under alternative tolerance thresholds and reference confidence criteria, although the proxy thresholds remain empirical. We further assessed cloud and sun-sensor geometry effects. TROPOSIF 735 was sensitive to cloud contamination, whereas TROPOSIF 743 and TROPOSIF Caltech remained robust under moderate cloud cover. Under clear skies, retrieval accuracy declined when solar zenith angle (SZA) exceeded 60 ° , while viewing zenith angle effects were limited after excluding large-SZA observations. Phase angle mismatch introduced systematic bias through canopy SIF anisotropy. Overall, this framework improves direct validation reliability and provides practical guidance for evaluating satellite SIF products.