Spectral Composition and Quanta-to-Energy Ratio of Diffuse Photsynthetically Active Radiation under Diverse Sky Conditions
Dye, D.G.(a) and Yasuoka, Y.(a,b), JAMSTEC-FRSGC (a), University of Tokyo (b)
Data on the beam and diffuse components of photosynthetically active radiation (PAR, 400-700 nm) are often required for detailed process modeling of terrestrial photosynthesis and ecosystem-atmosphere carbon exchange. Such models typically require PAR data in quantum units (photosynthetic photon flux density) however PAR is often reported in energy units (irradiance). Reliable conversion of diffuse PAR data from energy to quantum units requires knowledge of how the quanta-to-energy ratio for diffuse PAR varies across a broad range of sky conditions that includes variations in cloudiness. This study employs a time-series of spectral measurements of diffuse irradiance of PAR collected during spring and summer at the Southern Great Plains (SGP) site of the Atmospheric Radiation Measurement (ARM) program. The influence of a diverse range of observed sky conditions on the spectral composition of diffuse PAR and on the corresponding quanta-to-energy ratio at 1-minute and daily time scales are examined. The results indicate that enhanced contribution of green-to-red (550-700 nm) light to the spectral composition of diffuse PAR, induced by atmospheric scattering and reflectance by clouds, causes the quanta-to-energy ratio at the daily time scale to increase nonlinearly by up to 7% relative to clean, cloudless sky conditions. The relation is explained well (R=0.98) by variation in the diffuse fraction of the daily total irradiation of global (beam + diffuse) PAR. An emprical equation is presented that may enable improved accuracy when converting diffuse PAR data between energy and quantum units for environments where clouds are a major agent of atmospheric scattering.
Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).
