Evaluating Numerical Methods to Investigate Spectral Solar Radiative Transfer in Plant Canopies

Abstract

The disposition of spectral solar irradiance in plant canopies is crucially important to understand processes such as photolysis of molecules amenable to absorbing actinic light. Thus, one objective of this study is to evaluate the most commonly applied radiative transfer approaches to estimate spectral irradiance as a function of plant canopy depth. Eight radiative transfer approaches are ascertained. Another objective is to determine the impacts of the spectral resolution assumed in radiative transfer calculations and model choice on key processes such as canopy absorption and reflection of irradiance. By comparing results from broadband-only and spectrally-resolved canopy radiative transfer, we aim to quantitatively determine the uncertainties associated with failing to resolve the sunlight spectra. We determine the optimal spectral resolution required to estimate canopy radiative transfer results such as air-chemistry-specific quantities related to photolysis of a select group of molecules. In addition, we evaluate techniques for binning leaf and soil optical properties. Results showed that high spectral resolution is ideally desired to compute photolysis of molecules such as ozone, nitrogen dioxide, nitrate radical, nitrous acid, and formaldehyde. For in-canopy photolysis of molecules, a waveband resolution of at least 10 nm is sufficient to obtain accurate estimates for most photochemical reactions. Positive reaction-dependent uncertainties in canopy-mean relative photolysis values for individual molecules can be as high as 30% compared to estimates derived with broad-band solar irradiance.