Investigation of Aerosol Optical Properties in the Ultraviolet Spectrum

Abstract

Atmospheric aerosols can reduce the amount of ultraviolet (UV) radiation reaching the Earth's surface by scattering this radiation towards space and mitigating the increase of UV irradiance due to stratospheric ozone depletion. The objective of this study is to determine the amount of scattering of ultraviolet radiation by aerosol particles in the earth's atmosphere. Aerosol single scattering albedo (SSA) at UV wavelengths is an important aerosol radiative parameter in determining how aerosols affect the surface UV irradiance. An ultraviolet multi-filter rotating shadowband radiometer (UVMFR-SR) situated in Raleigh, NC, is used to gather the UV irradiances and the aerosol optical depth (AOD). These data are collected at 300 nm, 305 nm, 311 nm, 318 nm, 325 nm, 332 nm, and 368 nm. A total of 15 cloudless days were studied in the Raleigh, NC area from March to September of 2004 at a solar zenith angle of 45 degrees. The values of aerosol SSA in this study ranged from 0.68 to 0.99, with an overall average value of 0.867, indicating that aerosols are scattering most of the incident UV radiation. These results are in good agreement with previous studies. Two of these clear days were also studied for trends in SSA and black carbon levels during different times of the day. It was found that less scattering occurs during the middle of the day. A tropospheric radiative transfer model (TUV) was used to determine the SSA when values of AOD, diffuse-to-direct ratios, solar zenith angle, and time are known. The asymmetry parameter and ground albedo were given assumed values in the model, 0.70 and 0.04, respectively. The SSA values were determined by comparing the output diffuse-to-direct ratios in the model to those actually observed. More values of SSA in the ultraviolet spectrum will allow for better estimation of this parameter for future UV radiative transfer modeling and also reduce the error in estimation of surface UV irradiances.