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Conversion from MegaJoule/sq.m/d to mol/sq.m/d

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  • Conversion from MegaJoule/sq.m/d to mol/sq.m/d

    I am working with term daily light integral (DLI). Does anyone know the conversion of megajoule/square meter/day (MJ/m2/d) to mol/square meter/day (mol/m2/d).

  • #2
    I don't think there is a direct link between MJ/m2/d and mol/m2/d. MJ/m2/d represents the total power over the surface, while DLI only uses the visual spectrum of light, which can change depending on environmental factors


    • #3
      Any conversion will be approximate as the actual spectrum of the sun varies with altitude of the sun, cloud cover, haze etc. The sun's total spectrum includes everything from UV to IR, while photosynthesis measurements usually only consider the part of the spectrum from 400 nm to 700 nm. I recommend the Wikipedia article on phosynthetically active radiation,

      They assume a color temperature of 5800 K as typical for sunlight They use the notation W* or J* for the fraction of total solar spectrum within the PAR spectrum. For the sun, they give J*/J = W*/W = 0.368. Notate that this varies with the color temperature of the source. Also for the solar (5800 K) spectrum, they give a photon flux to energy ratio of 4.56 µmol/J*

      Starting with your MJ/m2/d value

      1 MJ/m2/d x 106 J/MJ x 0.368 J*/J x 4.56 µmol/J* = 1.68 mol/m2/d

      Note that this is approximate and depends on several assumptions about the sun's spectrum as received at sea level. They do show the (horrible) integrals required to compute the factors for different spectra. However, I think instruments properly calibrated to read the PAR spectrum directly would be far superior to converting total solar energy results.