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

Abstract

The Ozone Monitoring Instrument (OMI), on board the NASA EOS Aura spacecraft since July 2004, provides a global view of surface spectral ultraviolet (UV) irradiance at 305, 310, 324, and 380 nm; erythemal dose rate both at overpass time and local noontime; and erythemal daily dose. Previous studies have shown comparisons of the OMI erythemal UV irradiances and ground‐based UV measurements in areas of midlatitude and high latitudes, predominantly in the Northern Hemisphere. In this study the noontime erythemal UV dose rates retrieved from OMI and measured from broadband instruments at four sites in Thailand were compared. The comparisons show a positive bias for the OMI data with respect to the ground‐based measurements. The differences between the two data sets were 30%–60% for all data and were 10%–40% for cloudless data. The differences for the cleanest site showed better agreement than those for the more urban sites. Using the Libradtran radiative transfer model, we show that aerosol is responsible for much of the positive bias in polluted areas. Since absorbing aerosol is not taken into account in the OMI surface UV algorithm, aerosol absorption correction factors have been introduced as a function of aerosol absorption optical thickness provided by OMI to improve the OMI UV data for urban and maritime sites. The differences between the corrected erythemal UV data and the ground‐based data were reduced to less than 20%.