Mapping solar ultraviolet radiation from satellite data in a tropical environment Serm Janjai ⁎, Sumaman Buntung, Rungrat Wattan, Itsara Masiri Laboratory of Tropical Atmospheric Physics, Department of Physics, Faculty of Science, Silpakorn University, Nakhon Pathom 73000, Thailand abstract article info Article history: Received 5 May 2009 Received in revised form 12 November 2009 Accepted 16 November 2009 Keywords: Solar radiation Erythemally-weighted ultraviolet radiation Satellite data Radiation mapping Geostationary satellite This paper presents a technique for mapping erythemally-weighted solar ultraviolet (EUV) radiation from satellite data in a tropical environment. A satellite-based EUV radiation model was formulated for calculating the EUV daily dose from satellite-derived earth-atmospheric albedo, total column ozone and other ground- based ancillary data. The earth-atmospheric albedo was obtained from a geostationary satellite (GMS5) while the total column ozone was retrieved from a polar orbiting satellite (EP/TOMS). The model was validated against the monthly average EUV daily dose from the measurements at four solar radiation monitoring stations located in the tropical environment of Thailand. The monthly average EUV daily dose calculated from the model was in reasonable agreement with that obtained from the measurement, with root mean square difference (RMSD) and mean bias difference (MBD) of 12.3% and 0.7%, respectively. After the validation, the model was used to calculate the monthly average EUV daily dose over Thailand employing an 8-year period of data from GMS5, EP/TOMS and other ancillary surface data. Values of the monthly average of EUV daily dose were presented as monthly and yearly maps. These maps reveal that the tropical monsoons have a strong influence on the EUV in this region. © 2009 Elsevier Inc. All rights reserved. 1. Introduction Although solar ultraviolet (UV) radiation represents only a small part of the total solar spectrum, due to its high photon energy, it has important effects on human health, atmospheric photochemical reactions and biological ecosystems. Solar UV radiation is generally classified as UV-A, UV-B and UV-C. UV-C is completely absorbed by atmospheric constituents while traveling through the atmosphere and only UV-A and UV-B partly reach the earth's surface. The level of the surface UV-B, with high photon energy and inherent damaging effect, is essentially controlled by the stratospheric ozone. The depletion of stratospheric ozone by CFCs and related compounds has led to an increased awareness of UV effects on human health and biological environments, even in regions where actual ozone depletion to date has been small, but UV radiation remains naturally high. Consequently, a number of studies have been made to investigate the solar UV radiation levels in many parts of the world (Proffit et al., 1990; McKenzie et al., 1991; Webb, 1991; Blumthaler et al., 1992; Frederick and Weatherhead, 1992; Krzyscin and Jaroslawski, 1997; Bernhard et al., 1997; Nunez et al., 1997; Ilyas et al., 1999; Cappellani and Kochler, 1999; Thorseth and Kjeldstad, 1999; Udelhofen et al., 1999; Dubrovsky, 2000; Ziemke et al., 2000; Bais et al., 2003; Murillo et al., 2003; Ogunjobi and Kim, 2004; Palancar and Toselli, 2004; Cordero et al., 2005; Kift et al., 2006; Kalashnikova et al., 2007; Seckmeyer et al., 2008; Fichot et al., 2008). Accurate data on solar UV radiation are required for the study of the UV trend. These data are also needed for the study of atmospheric photochemical reactions and the protection of human health against UV effects. To obtain such data, UV monitoring networks have been set up in many countries (Roy et al., 1998; Sabburg et al., 2002) and guidelines for UV measurements have been made by the Scientific Advisory Group on UV measurements of the World Meteorological Organization (WMO) (Webb et al., 2006; Seckmeyer et al., 2006). However, the density of UV measuring stations in existing networks is still far too low to provide sufficient UV data, especially in tropical developing countries. An alternative solution to this problem is to derive solar UV radiation from satellite data. This approach has the advantage that it can provide UV data with a better spatial coverage, as compared to the ground-based measuring networks. In addition, availability of historical satellite data makes it possible to investigate long-term changes of UV levels. As a result, a number of methods have been developed for retrievals of solar UV radiation from satellite data. This can be briefly outlined as follows. An algorithm to derive UV radiation from measurements of the Total Ozone Mapping Spectrometer (TOMS) onboard different polar orbital satellites has been developed at NASA Goddard Space Flight Center (NASA/GSFC) (Eck et al., 1995; Krotkov et al., 1998, 2001; Herman et al., 1999). This algorithm consists of two steps. First, clear sky surface UV irradiance is determined by using total column ozone derived from the same instrument. Second, the clear sky UV irradiance is modified for the effect of clouds employing a cloud modification factor. The UV retrievals from TOMS have been intensively compared with ground-based measurements and a wide range of discrepancies Remote Sensing of Environment 114 (2010) 682–691 ⁎ Corresponding author. Tel.: + 66 34 270761; fax: + 66 34 271189. E-mail address: serm@su.ac.th (S. Janjai). 0034-4257/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2009.11.008 Contents lists available at ScienceDirect Remote Sensing of Environment journal homepage: www.elsevier.com/locate/rse