### Abstract

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 p_{2}, 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 p_{2} with R^{2} = 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 p_{2} does not adequately describe variations in the optical properties of the atmosphere.

Original language | English |
---|---|

Pages (from-to) | 161-170 |

Number of pages | 10 |

Journal | Agricultural and Forest Meteorology |

Volume | 109 |

Issue number | 3 |

DOIs | |

Publication status | Published - 3 Sep 2001 |

MoE publication type | A1 Journal article-refereed |

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### Keywords

- Atmospheric integral transparency coefficient
- Direct radiation
- Photosynthetically active radiation
- Phytoactinometer

### Cite this

*Agricultural and Forest Meteorology*,

*109*(3), 161-170. https://doi.org/10.1016/S0168-1923(01)00269-6

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*Agricultural and Forest Meteorology*, vol. 109, no. 3, pp. 161-170. https://doi.org/10.1016/S0168-1923(01)00269-6

**Experimental study of ratio of PAR to direct integral solar radiation under cloudless conditions.** / Mõttus, M.; Ross, J.; Sulev, M.

Research output: Contribution to journal › Article › Scientific › peer-review

TY - JOUR

T1 - Experimental study of ratio of PAR to direct integral solar radiation under cloudless conditions

AU - Mõttus, M.

AU - Ross, J.

AU - Sulev, M.

PY - 2001/9/3

Y1 - 2001/9/3

N2 - 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 Ks 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 Ks, 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 Ks, the atmospheric integral transparency coefficient p2 does not adequately describe variations in the optical properties of the atmosphere.

AB - 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 Ks 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 Ks, 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 Ks, the atmospheric integral transparency coefficient p2 does not adequately describe variations in the optical properties of the atmosphere.

KW - Atmospheric integral transparency coefficient

KW - Direct radiation

KW - Photosynthetically active radiation

KW - Phytoactinometer

UR - http://www.scopus.com/inward/record.url?scp=0035801877&partnerID=8YFLogxK

U2 - 10.1016/S0168-1923(01)00269-6

DO - 10.1016/S0168-1923(01)00269-6

M3 - Article

VL - 109

SP - 161

EP - 170

JO - Agricultural and Forest Meteorology

JF - Agricultural and Forest Meteorology

SN - 0168-1923

IS - 3

ER -