Li, Q; Li, CC; Mao, JT (2012). Evaluation of Atmospheric Aerosol Optical Depth Products at Ultraviolet Bands Derived from MODIS Products. AEROSOL SCIENCE AND TECHNOLOGY, 46(9), 1025-1034.
Solar ultraviolet (UV) radiation plays a significant role in climate, atmospheric chemical processes, and ecosystem balance. Aerosol optical depth (AOD) at UV wavelengths, the UV-AOD, is an important quantity for studying the extinction of UV radiation in the atmosphere. Ground-based UV-AOD observations, such as those from the AErosol RObotic NETwork (AERONET), are limited in spatial coverage. Current space derived UV-AOD from the Total Ozone Mapping Spectrometer (TOMS) or the Ozone Monitoring Instrument (OMI), on the other hand, are subject to large errors associated with low resolution, cloud contamination, assumed height of aerosol layer, and low sensitivity to aerosols in the lower troposphere. In this study, a new UV-AOD product is derived for the year 2009 by extrapolating the Moderate Resolution Imaging Spectra-radiometer (MODIS) visible bands AOD product (VIS-AOD) to 380 and 340 nm. Results are evaluated against UV-AOD measurements taken at AERONET sites. Over the oceans, four extrapolating methods are investigated by using two to five wavelengths. The best result, which has a correlation coefficient (R) of 0.90 at both wavelengths and root mean square errors (RMSE) of 0.062 and 0.068 at 380 and 340 nm, respectively, is achieved by combining a linear-extrapolation and a second-order polynomial fitting that takes into account the wavelength dependence of the Angstrom exponent. Moreover, more than 80% of the data fall within the uncertainty range of +/- 0.05 +/- 0.20 tau. Over land, UV-AOD is extrapolated using the Angstrom exponent derived from VIS-AODs at 470 and 660 nm, the only two wavelengths available from the MODIS AOD product. Compared with AERONET observations, the correlation coefficient is about 0.90 at both 380 and 340 nm, while the RMSE increases to 0.152 at 380 nm and 0.174 at 340 nm, due to the larger uncertainty of MODIS AOD over land. With the relatively low biases, this UV-AOD product will be valuable for climate and atmospheric chemistry research.