Photosynthetically Active Radiation (PAR)
as Observed from Satellites

Department of Atmospheric and Oceanic Science
University of Maryland, College Park

Department of Atmospheric and Oceanic Science



Satellite Data

Available PAR Parameters


Data Distribution

Data Access





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A modified version of the Global Energy and Water Cycle Experiment (GEWEX) /Surface Radiation Budget ( SRB) model (Whitlock et al., 1995; Pinker et al., 1995) was used to generate PAR and related parameters. The approach is based on a physical inference scheme to derive surface spectral radiatiative components from satellite observations at the top of atmosphere. Model output parameters are:

  • total surface PAR (downwelling and upwelling)
  • diffuse surface PAR (downwelling and upwelling)
  • Top of the atmosphere (TOA) upwelling PAR
  • Conversion Factors (CF), defined as:

    CF = PAR / SW

    as computed from independently derived PAR and total shortwave radiative (SW) flux. Global climatologies of CFs are useful because these factors can later be applied to independently derived values of SW, produced with models which do not have capabilities to estimate PAR directly.

    The widely used quantum sensor for measuring PAR, provides photosynthetic photon flux density (PPFD) in units of Einstein per m**2 per second. It is an estimate of the number of photons in the 400-700 nm spectral region which are available to initiate photochemical reaction within a cell. It is the PPFD which should be related to dry matter production of plants or CO2 fixation in phytoplankton. Usually, when PAR is available in units of W/m**2, a nominal conversion factor at 485 nm is used to convert it into units of photon flux density. Preferably, such conversion should be done by spectral integration. Since the inference model we use is spectral, it is possible to get an improved estimate of PPFD.

    Maintained by Banglin Zhang and Chuan Li