PAR Irradiance Research Articles

Urban aerosol and clear skies spectra for global and diffuse photosynthetically active radiation

From a data archive of spectral energy distribution of global and diffuse solar irradiances measured in Athens during a field experiment, the influence of atmospheric turbidity caused by urban aerosol on spectral composition of visible radiation and transparency of urban atmosphere has been determined for cloudless conditions. The effect of urban aerosol is significant in the PAR part of solar spectrum, where attenuation of global PAR irradiance surpasses 18% on very polluted atmospheric conditions. The increase in diffuse PAR irradiance caused by urban aerosol ranges in the interval 7%–51%. The ratio of global PAR to total global irradiance was found to depend slightly on air pollution whereas ratio's dependencies on site elevation were not identified clearly. Nevertheless, this ratio was found to be in good agreement with those reported in the literature. The ratio of diffuse-to-global PAR irradiance was found to depend on both, air mass and turbidity. For very turbid atmosphere and large air masses the 40% of PAR irradiance results from diffuse radiation. Finally, some biological implications of the results are briefly discussed.

Lake optics and depth limits for photogenesis and photosynthesis in charophyte meadows

Two series of lakes with increasing attenuation were examined for trends in spectral composition. They became the basis for an evaluation of the light environment at the lower boundary (LB) of Nitella meadows in three other series of lakes. Increased attenuation (KdPAR) was marked by progressive erosion of the ‘blue window’ and caused primarily by humic substances. An increase in KdPAR from 0.06 to 0.81 produced, at the floor of the euphotic zone, a shift in Kdmin from 440 to 580 nm. Regressions of boundary depths of Nitella meadows on water clarity produced similar slope coefficients for the three series of lakes. Several trends became evident: 1, PAR irradiance at the LB increases with depth of the LB; 2, red light (Ed660) declines from richness at shallow LB to near extinction in deep water LB in clear lakes; while 3, blue light (Kd450) increases to an asymptote. Blue light appears to substitute, although less effectively, for red light irradiance in the growth regulation of charophytes. These data support an hypothesis that spectral quality is involved in the determination of lower boundary depths for benthic macro-algae.

Characterization of the light field in laboratory scale enclosures of eutrophic lake water (Lake Loosdrecht, The Netherlands)

Light conditions in laboratory scale enclosures (LSE) of shallow, eutrophic Lake Loosdrecht (The Netherlands), including a method for simulating a ‘natural’ incident light course, are described. Total PAR (400–700 nm) and spectral irradiance distribution were measured at sestonic chlorophyll a and dry weight concentrations ≥ 100 mg m−3 and 16 g m−3, respectively. Phytoplankton was dominated by Oscillatoria spp. The euphotic depth (Z eu) was 0.7–1.0 m. Shortly after filling the LSE with lake water, diffuse attenuation coefficients ranged from 14 m−1 for blue to 5 m−1 for red light. Around Zeu, attenuation in the blue region was markedly lower and irradiance reflectance (R) continued to increase; these anomalies were caused by lateral incident light from the LSE’s waterbath. Spectrophotometry indicated that absorption was mainly by particles, but dissolved humic substances (gilvin) were also important. The particles were likely to be dominated by detritus absorbing more blue relative to red light. Subsurface R in lake water in the LSE had a maximum around 705 nm and low values in the blue band, but was lower than that previously reported for measurements in situ. Wash-out of detritus, presumably both dissolved and particulate fractions, by flow-through with synthetic medium, greatly affected the spectral reflectance measured outside the LSE. The maximum value of R decreased from 0.022 to 0.009, and the peak shifted to 550 nm.

Electron-electron and electron-phonon interactions in irradiated N-methyl pyridinium derivatives with TCNQ studied by optical spectroscopy

Electron irradiation of the organic semiconductors of NMePy(TCNQ) 2 type produces defects which change their vibrational and electronic spectra. The spectral changes are described and discussed. The electron-phonon coupling constant changes are deduced; the defect concentration as well as their volume are calculated from spectral measurements. The role of electron-electron interactions are emphasized and we suggest, that the N-methyl pyridinium derivatives with TCNQ are Mott insulators Etude experimentale et discussion des modifications des spectres d'absorption IR et electroniques en presence des defauts induits par irradiation electronique. Deduction des variations des constantes de couplage electron-phonon, de la concentration et du volume des defauts. Role des interactions electron-electron

On the coloration properties of CaF2 : Na

We have investigated the dichroic properties and the existence of metastable excited states of several colour centres created by X-irradiation in CaF 2 :Na. According to our results absorption bands peaking at 600, 385 and 322 nm are associated to the same centre which behaves optically as orthorhombic. The lack of observation of a metastable state is in favour of the F 2A + model for this centre. On the other hand the band at 500 nm and another absorption at 322 nm are proved to belong to the same centre which behaves optically as tetragonal. A metastable excited state of this centre with lifetime of 0,83 s at 77 K has been detected. Two possible models are discussed for this centre Etude des proprietes dichroiques et de l'existence d'etats excites metastables de plusieurs centres colores crees par irradiation X