Estimation of IPAR and Direct Forcing of Aerosol over Dehradun and Surrounding Area (India) Using MODIS Derived Atmospheric Products: A Case Study

Scattering and absorption of solar radiation by anthropogenic aerosols reduce the photosynthetic active radiation (PAR) incident on the Earth surface and the changes incident PAR can cause contradictory responses in terrestrial plants. The reduction of incident PAR is mainly varied according to the regional characteristics and forcing efficiency of aerosols. The MODIS derived atmospheric products are widely used for many such environmental investigations. In this paper a radiative transfer model is developed in cloudless clear sky condition to estimate instantaneous PAR and radiative forcing of aerosol. The estimated instantaneous PAR is validated with the ground measured (AccuPAR model LP-80) incident PAR during 2009. The Mean Absolute Error (MAE) of MODIS based incident PAR simulations was 20.09 w/m2 and Absolute Percentage Difference (APD) was 6.9 % while the MAE of Microtops II based estimations of incident PAR was 7.5 w/m2 and APD was 2.4 % . Inter comparison of estimations incident PAR with sun-photometer and MODIS measurements showed that the MODIS based PAR is over estimated mainly due to the under estimation of MODIS AOD over the study area. The model can generate spatial maps of instantaneous PAR and aerosol forcing efficiency. An Improved PAR map was generated (MAE 7.6 w/m2 and APD 2.5 %) by correcting MODIS AOD based on Microtops II measurements.