Improving modeled light transfer within plant canopies: scheme comparisons and implications
Open Access
- Author:
- Moon, Zachary Leland
- Graduate Program:
- Meteorology and Atmospheric Science (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 15, 2021
- Committee Members:
- Jerry Harrington, Major Field Member
William Brune, Major Field Member
Jose Fuentes, Chair & Dissertation Advisor
Miriam Freedman, Outside Unit & Field Member
David Stensrud, Program Head/Chair - Keywords:
- plant canopies
land modeling
photolysis
canopy radiative transfer
actinic flux
isoprene
monoterpenes
air quality
spectral resolution
NOx
GOAmazon
Amazon rainforest
canopy-chemistry
optical properties
reflectance - Abstract:
- Light in plant canopies influences weather and climate indirectly through its impact on emissions of biogenic volatile organic compounds (BVOCs). In most land models, the solar light spectrum has not been well-resolved within plant canopies; most models split this region into only two or three broad bands for canopy radiative transfer (RT) calculations. Such a setup cannot accurately capture the wavelength dependencies in foliage optical properties, which are substantial in the near-infrared spectral region. This dissertation examines the impacts of improving the representation of in-canopy light in three parts: 1) the impact of spectrally resolved photolysis frequency calculations compared to a common simpler approximation; 2) sensitivity to the spectral resolution used for various relevant canopy variables such as canopy reflectance; and 3) the impact of improving photolysis frequencies by using spectrally resolved calculations and reaction-dependent data in the context of a one-dimensional (1-D) canopy-chemistry model. In the first part, we use field observations from the Borden forest to evaluate various 1-D canopy RT schemes, finding a commonly used Beer–Lambert scheme to perform poorly, underestimating light levels in and mid- and upper-canopy. Photolysis frequency profiles are then computed using the modeled spectral actinic flux, showing that percent errors incurred by using a common in-canopy approximation can be as high as 10–20 % in daily canopy integrated photolysis frequency. Next, we examine the sensitivity to canopy RT spectral resolution for various quantities. It is shown that with low resolution, error in canopy reflected irradiance can be as much as −70 W m⁻² (−38 %). Using a resolution of 70 nm reduces this error to ∼ 1 W m⁻². Similarly, we recommend 10-nm resolution for photolysis rate coefficients, which reduces errors from as high as 30 % to less than 1 %. In the third part, an improved photolysis module using spectral canopy RT and reaction-dependent data is incorporated into a 1-D canopy-chemistry model for the rainforest in the central Amazon. We find that the original photolysis scheme in this model results in a 3.5 % underprediction of above-canopy isoprene. Lastly, for the Amazon rainforest, we model the sensitivity to change levels of nitrogen oxides (NOx). NOx sensitivity experiments using a NOx range typical of biomass burning or urban transport events in the Amazon reveal increases in BVOC oxidation rates, leading to ∼ 50 % reductions in terpene levels. Overall, this dissertation recommends using a two-stream or more advanced canopy RT scheme and applying spectrally resolved canopy RT.