Impacts of spectrally resolved irradiance on photolysis frequency calculations within a forest canopy

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.