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

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. Plain Language Summary: In regions dominated by tall and dense forest canopies, climate models need to resolve the vertical variation of sunlight to estimate assimilation of gases by foliage. Knowledge of sunlight disposition in forest canopies is also needed to determine light‐driven reactions of gases. Most climate models employ broadband sunlight to determine radiative transfer in plant canopies. Given that in‐plant canopy processes such as reactions of gases require knowledge of sunlight specified at high wavelength resolution, eight different radiative transfer models are considered to discern the most reliable approach to compute the disposition of solar radiation as a function of both canopy depth and wavelength. Results indicate it is necessary to estimate the in‐canopy disposition of sunlight as a function of wavelength to reliably compute dissociation of molecules susceptible to sunlight absorption. As part of the light spectra included in radiative transfer numerical models, a waveband resolution of at least 10 nm is necessary to reduce uncertainties of less than 30% of sunlight‐mediated dissociation of molecules such as ozone and nitrogen dioxide. The new approach developed in this study can be included in numerical modeling systems designed to investigate photochemical processes in vegetated landscapes. Key Points: Disposition of spectral sunlight in plant canopies is important to understand photolysis of molecules amenable to absorbing lightSpectral resolution is required to estimate reliable estimates of air chemistry‐specific quantities related to photolysis of moleculesA waveband resolution of at least 10 nm is sufficient to obtain accurate estimates for most photochemical reactions [ABSTRACT FROM AUTHOR]