Experimental study of ratio of PAR to direct integral solar radiation under cloudless conditions

In comprehensive studies of radiation climate, remote sensing of the environment, radiation regime of plant canopy, photosynthesis and biomass production, data on direct and diffuse radiation are needed separately for integral and photosynthetically active radiation (PAR). For direct solar PAR, unlike for integral radiation, there exist no global networks of radiation measurement stations or standard instruments. Thus for estimating PAR from direct integral solar radiation parametric models are required. For measurement of direct solar PAR, we constructed a new instrument, the phytoactinometer, consisting of a LI-COR quantum sensor and a viewlimiting tube. With this instrument PAR was measured at Tartu Observatory, Estonia, concurrently with measurements with the Yanishevski actinometer during the foliated periods of 1996-2001. The ratio of direct solar PAR to direct integral solar integral radiation K^s was considered a function of the sine of solar height sin h and the atmospheric integral transparency coefficient p₂, both of which can be readily obtained from any actinometric measurements. Despite the large dispersion of data due to changes in atmospheric water and aerosol content, a semiempirical relationship between K^s, m = 1/sin h and p₂ with R² = 0.95 and error ±4% was obtained using irradiances averaged over 5 min periods. This model was qualitatively compared with the parametric models of [Agric. For. Meteorol. 78 (1996) 121; 93 (1999) 27]. We also concluded that for calculating K^s, the atmospheric integral transparency coefficient p₂ does not adequately describe variations in the optical properties of the atmosphere.