Satellite estimation and ground measurements of solar UV index and influence of aerosol in ground based UV data in Kathmandu, Nepal

This paper presents the ground based measurement and satellite estimation of the solar UV Index (UVI) as well as the influence of aerosol in ground based UV data of Kathmandu. Kathmandu (27.72°N, 85.32°E) is located at an elevation of 1350m from the sea level. The measurements were based on one year data from January to December 2009. The Ground Based (GB) measurement and satellite estimation were performed by NILU-UV irradiance meter and EOS Aura OMI spacecraft. The NILU-UV irradiance meter measures UV radiation in different spectral range whereas the Ozone Monitoring Instrument (OMI) is a spectrometer which monitors the solar radiation in spectral range of 270–500 nm. Besides, the Microtops II Sunphotometer was also used in this study for the measurement of aerosol optical depth (AOD). The relative percentage difference between OMI and GB UVI during pre-monsoon (March, April and May) was 165.5% and that in monsoon (June, July, August and September) was 105.5%. Likewise the monthly mean UVI in April and July was found to be 5.7±1.6 and 7.5±2.5 while the monthly mean AOD for the same months were 1.12 and 0.44 respectively. Meanwhile the correlation between GB UVI and AOD at 500nm before monsoon was -0.69 whereas during monsoon its value was -0.43. The above mentioned correlation in April was -0.86. During this study the higher values of AODs and OMI estimation were observed during the months of March to May. And this high aerosol content in Kathmandu valley was found capable of modifying the surface UV irradiance.