Aerosol ultraviolet absorption experiment (2002 to 2004), part 2: absorption optical thickness, refractive index, and single scattering albedo

Compared to the visible spectral region, very little is known about aerosol absorption in the UV. Without such information it is impossible to quantify the causes of the observed discrepancy between modeled and measured UV irradiances and photolysis rates. We report results of a 17-month aerosol column absorption monitoring experiment conducted in Greenbelt, Maryland, where the imaginary part of effective refractive index k was inferred from the measurements of direct and diffuse atmospheric transmittances by a UV-multifilter rotating shadow-band radiometer [UV-MFRSR, U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Network]. Colocated ancillary measurements of aerosol effective particle size distribution and refractive index in the visible wavelengths [by CIMEL sun-sky radiometers, National Aeronautics and Space Administration (NASA) Aerosol Robotic Network (AERONET)], column ozone, surface pressure, and albedo constrain the forward radiative transfer model input, so that a unique solution for k is obtained independently in each UV-MFRSR spectral channel. Inferred values of k are systematically larger in the UV than in the visible wavelengths. The inferred k values enable calculation of the single scattering albedo omega, which is compared with AERONET inversions in the visible wavelengths. On cloud-free days with high aerosol loadings [tau(ext)(440) >0.4], omega is systematically lower at 368 nm (<omega(368)>=0.94) than at 440 nm (<omega(440)>=0.96), however, the mean omega differences (0.02) are within expected uncertainties of omega retrievals (similar to 0.03). The inferred omega is even ;lower at shorter UV wavelengths (<omega(325)>)similar to <omega(332)>=0.92), which might suggest the presence of selectively UV absorbing aerosols. We also find that a) decreases with decrease in aerosol loading. This could be due to real changes in the average aerosol composition between summer and winter months at the Goddard Space Flight Center (GSFC) site. Combing measurements of tau(ext) and w, the seasonal dependence of the aerosol absorption optical thickness, tau(rbs)=tau(ext)(1 - omega) is derived in the UV with an uncertainty of 0.01 to 0.02, limited by the accuracy of UV-MFRSR measurement and calibration. The tau(abs) has a pronounced seasonal dependence with maximum values similar to 0.1 occurring in summer hazy conditions and <0.02 in the winter and fall seasons, when aerosol loadings are small. The measured tau(abs) is sufficient to explain both the magnitude and seasonal dependence of the bias in satellite estimates of surface UV irradiance previously seen with ground-based UV measurements. (c) 2005 Society of Photo-Optical Instrumentation Engineers.