Abstract Although photolysis frequencies are wavelength-dependent and the dependence varies among chemical species, previous canopy radiative transfer models did not consider more than three broad bands (ultraviolet, photosynthetically active radiation (PAR), and near-infrared). In this study, high spectral resolution and wavelength-dependent idealized leaf optical properties allow us to determine the disposition of the light spectrum within a mixed deciduous forest canopy. Four radiative transfer approaches of varying complexity are applied to obtain vertical profiles of spectral actinic flux. Broad-band radiation measurements made above and below a mixed deciduous forest provide the necessary information to verify the fidelity of each radiative transfer approach. Model comparison results indicate that the Beer–Lambert scheme gives less total actinic flux, while the other three schemes give similar actinic flux profiles. Spectral actinic flux profiles are used to calculate in-canopy photolysis for different chemical species and to assess the importance of in-canopy photochemistry in modifying biogenic volatile organic compounds transported to the overlying atmospheric boundary layer. We find that, depending on the time of day and chemical species, percent errors in photolysis frequencies incurred by using a common in-canopy approximation based on weighting by relative PAR profiles can be as high as ± 50% in lower regions of the canopy, or 10–20% in daily canopy integrated photolysis frequency. Results obtained using a one-dimensional photochemical model suggest that choice of canopy radiative transfer scheme can have substantial impacts on in-canopy chemical reactions and concentrations in the overlying atmospheric boundary layer air. Such effects caused in-canopy gas concentration differences ranging from 8% for ozone and 35% for hydroxyl radical to 77% for nitrate radical.

中文翻译：

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光谱分辨辐照度对森林冠层内光解频率计算的影响

摘要 尽管光解频率与波长有关且依化学物种而异，但先前的冠层辐射传递模型并未考虑超过三个宽带（紫外线、光合有效辐射 (PAR) 和近红外）。在这项研究中，高光谱分辨率和波长相关的理想化叶片光学特性使我们能够确定混合落叶林冠层内光谱的分布。应用了四种不同复杂度的辐射传输方法来获得光谱光化通量的垂直剖面。在混合落叶林上方和下方进行的宽带辐射测量提供了必要的信息，以验证每种辐射传输方法的保真度。模型比较结果表明，Beer-Lambert 方案给出的总光化通量较小，而其他三种方案给出了相似的光化通量分布。光谱光化通量分布用于计算不同化学物种的冠层内光解作用，并评估冠层内光化学在改变运输到上覆大气边界层的生物挥发性有机化合物方面的重要性。我们发现，根据一天中的时间和化学物种，使用基于相对 PAR 剖面加权的常见冠层内近似值所导致的光解频率百分比误差在冠层下部区域可高达 ± 50%，或每日冠层综合光解频率的 10-20%。使用一维光化学模型获得的结果表明，冠层辐射传输方案的选择会对冠层内的化学反应和上覆大气边界层空气中的浓度产生重大影响。这种影响导致冠层内气体浓度差异范围从臭氧的 8% 和羟基自由基的 35% 到硝酸根自由基的 77%。