An investigation of solar erythemal ultraviolet radiation in the tropics: a case study at four stations in Thailand

Abstract

Our study examines a 5-year data set of erythemal ultraviolet radiation (EUV) collected at four locations in Thailand: Chiang Mai (18.78°N, 98.98°E), Ubon Ratchathani (15.25°N, 104.87°E), Nakhon Pathom (13.82°N, 100.04°E) and Songkhla (7.20°N, 100.60°E). Seasonal changes are strongly influenced by the wet and dry season in this tropical environment, with maximum daily and noontime irradiances being reached in April or May, prior to the onset of the wet season. Transmission by aerosols, estimated by comparison of cloudless measured irradiance with a cloudless sky model, ranges from 51% at Chiang Mai to 83% at Songkhla during the dry season. By contrast, higher transmissions characterise the wet season when values around 90% are reached. Cloud cover further depletes the EUV irradiance and wet season transmissions range from 68% at Chiang Mai to 79% at Songkhla. Three of the four stations record increases in EUV irradiance over the study period.

Comparison of erythemal UV irradiances from Ozone Monitoring Instrument (OMI) and ground‐based data at four Thai stations

“…Before 2003, one site instrument per year was sent to Solar Light for characterization and recalibration and then used to check the calibrations at the other sites through intercomparison. The cosine responses showed little change while the spectral responses altered gradually with time causing changes in calibration factors that were within 1.5% over an 8 year period [ Janjai et al , 2009a]. The spectral response of the fifth, traveling standard instrument was checked in Thailand in 2008, and both cosine and spectral responses were independently checked in Manchester in 2009: The cosine response showed little change from the original (within the measurement uncertainties ∼2%), while there had been a small shift in the spectral response at longer UVA wavelengths.…”

“…However, the spectral response check in 2008 in Thailand showed no significant change in response. Since we use data up until 2007 only, we take the statement of Janjai et al [2009a] that this traveling instrument was stable for the period of concern. Therefore, the original cosine and spectral responses from the manufacturer have been used throughout since any observed changes during recalibration have been small.…”

“…The instruments at the four sites are well maintained with daily cleaning of the dome, regular changing of the desiccants, and annual field calibrations under clear‐sky conditions. The estimated overall uncertainty of the radiometer is within ±8% [ Janjai et al , 2009a].…”

“…Its effects on human health require an understanding of the global distribution of solar UV [ United Nations Environmental Programme ( UNEP ), 2007; World Meteorological Organization ( WMO ), 2007]. For example, for public health purposes, the UV Index (UVI, 1 unit equals 25 mW.m −2 ) is used to express the level of erythemally effective UV, which can be up to the value of about UVI 12 in the Tropics [ Ilyas et al , 1999; Janjai et al , 2009a], whereas at high latitudes, e.g., Southern Ontario, Canada, the value does not exceed about 6 [ Fioletov et al , 2004].…”

Comparison of UV index from Ozone Monitoring Instrument (OMI) with multi‐channel filter radiometers at four sites in the tropics: effects of aerosols and clouds

“…Owing to its high photon energy, UV radiation has detrimental effect on human health and ecosystems (Tevini, ). In this connection, a great deal of efforts has been made to monitor UV radiation both from ground‐ and satellite‐based approaches in the past few decades (Bhattarai et al ., ; WMO, ; Janjai et al ., and references therein). For the satellite approach, a number of UV measuring instruments onboard various satellites have been deployed to monitor UV (Arola et al ., ; Tanskanen et al ., ), including the Ozone Monitoring Instrument (OMI) onboard Aura satellite.…”