On the stability of radiometric ratios of photosynthetically active radiation to global solar radiation in Tsukuba, Japan

The precise measurement of incident photosynthetically active radiation (PAR) is crucial for the estimation of ecosystem vegetation productivity. However, reliable values of PAR are seldom measured routinely. Instead, it is sometimes estimated based on solar radiation (R_S). One popular estimation method is by use of the conversion ratio PAR/R_S. Depending on whether PAR is expressed in energy units (P^E, W m^-2) or photon units (P^P, μmol m^-2 s^-1), there are two types of conversion ratios: P^E/R_S (unitless), or P^P/R_S (μmol J^-1). Moreover, to translate PAR expressed in one unit to another, the ratio P^P/P^E (μmol J^-1) is also important. However, past studies have not led to a general understanding of these ratios, mainly due to instrumental errors such as cosine errors. To reduce such errors, we developed a new PAR measurement system using grating spectroradiometers mounted on sun trackers to measure direct and diffuse PAR separately. The present study aims to clarify the characteristics of these three ratios using this new, more precise measurement system and radiative transfer simulation. We carried out measurements for one year in Tsukuba, Japan (36.05°N, 140.13°E). P^E/R_S increased with water vapor pressure (e) from 0.40 to 0.47 and increased with decreasing clearness index (k_T), but did not strongly depend on solar zenith angle (θ). P^P/R_S also increased with e from 1.9μmol J^-1 to 2.2μmol J^-1. Its dependence on climatic factors was similar to that of P^E/R_S. P^P/P^E varied by about 3% around the value of 4.57μmol J^-1.