Photochemical reflectance index (PRI) is an intriguing avenue for monitoring photosynthetic light use efficiency (LUE), which is a crucial physiological variable and the primary source of uncertainty in gross primary production (GPP) prediction. The correlations between PRI and LUE induced by the energy-dependent xanthophyll cycle are overall convincing at the leaf scale. However, their relationship becomes complicated and less reliable at the canopy scale due to several confounding factors, such as soil background, canopy structure and sun-observer geometry. A fundamental understanding of the interrelated effects of these factors is still missing. In this study, the roles of soil reflectance, canopy structure and sun-observer geometry were examined by investigating radiative transfer processes that determine canopy PRI. Theoretical analysis shows that the deviation between canopy- and leaf-level PRI is mainly attributed to soil background, namely the fraction of sunlit soil in the scene and the discrepancy between leaf and soil reflectance. Canopy structure and sun-observer geometry are not the direct drivers of variations in canopy PRI. They affect canopy PRI by altering the contribution of soil to canopy reflectance. Furthermore, several numerical experiments based on 1D and 3D radiative transfer models, which compare PRI of canopies with black soil (i.e., soil reflectance is zero) and non-black background, were conducted to confirm and validate the decisive role of soil background in explaining the deviation between canopy and leaf PRI. Multi-angular canopy PRI from PROBA-CHRIS data was employed to examine the angular dependency of canopy PRI changing with vegetation coverage. The numerical experiments demonstrate that canpy PRI differs from leaf PRI as large as 150% when canopy leaf area index (LAI) or the fraction of vegetation coverage is small. Canopy PRI is almost identical to soil PRI (i.e., PRI computed from soil reflectance) when the contribution of soil background to canopy reflectance is large. In contrast, when the soil reflectance is zero or LAI is greater than four, canopy PRI is almost identical to leaf PRI and independent of canopy structure and sun-observer geometry. Nonetheless, when leaf and soil reflectance are similar, canopy PRI is always close to leaf PRI regardless canopy structure and sun-observer geometry. The analysis of multi-angular PRI reveals that the angular dependency decreases with vegetation coverage due to weaker soil background effect. This study narrows down the causes of variation in canopy PRI to the fraction of observed soil background and soil reflectance. It suggests that estimating and accounting for the soil contribution to canopy PRI are necessary to mitigate angular and canopy structural effects on canopy PRI and eventually extract leaf physiological information from canopy PRI.

光化学反射指数 (PRI) 是监测光合光利用效率 (LUE) 的一个有趣途径，它是一个重要的生理变量，也是总初级生产 (GPP) 预测中不确定性的主要来源。由能量依赖性叶黄素循环引起的 PRI 和 LUE 之间的相关性在叶片尺度上总体上是令人信服的。然而，由于土壤背景、冠层结构和太阳观测器几何等多种混杂因素，它们的关系在冠层尺度上变得复杂且不那么可靠。仍然缺乏对这些因素的相互关联影响的基本理解。在这项研究中，通过调查确定冠层 PRI 的辐射传输过程，研究了土壤反射率、冠层结构和太阳观测器几何形状的作用。理论分析表明，冠层和叶层PRI的偏差主要归因于土壤背景，即场景中阳光照射土壤的比例以及叶片和土壤反射率之间的差异。冠层结构和太阳观测器几何形状不是冠层 PRI 变化的直接驱动因素。它们通过改变土壤对冠层反射率的贡献来影响冠层 PRI。此外，还进行了几个基于 1D 和 3D 辐射传输模型的数值实验，将冠层的 PRI 与黑土（即土壤反射率为零）和非黑色背景进行了比较，以确认和验证土壤背景在解释中的决定性作用。冠层和叶片 PRI 之间的偏差。采用来自 PROBA-CHRIS 数据的多角度冠层 PRI 来检查冠层 PRI 随植被覆盖度变化的角度依赖性。数值实验表明，当冠层叶面积指数（LAI）或植被覆盖率较小时，冠层PRI与叶片PRI相差150%。当土壤背景对冠层反射率的贡献很大时，冠层 PRI 与土壤 PRI（即从土壤反射率计算的 PRI）几乎相同。相反，当土壤反射率为零或 LAI 大于 4 时，冠层 PRI 几乎与叶片 PRI 相同，并且独立于冠层结构和太阳观测器几何形状。尽管如此，当叶片和土壤反射率相似时，无论冠层结构和太阳观测器几何形状，冠层 PRI 始终接近叶片 PRI。多角度PRI分析表明，由于土壤背景效应较弱，角度依赖性随着植被覆盖度的降低而降低。本研究将冠层 PRI 变化的原因缩小到观察到的土壤背景和土壤反射率的比例。这表明估计和解释土壤对冠层 PRI 的贡献对于减轻角度和冠层结构对冠层 PRI 的影响并最终从冠层 PRI 中提取叶片生理信息是必要的。