PROCEEDINGS ARTICLE | September 10, 2005
Proc. SPIE. 5886, Ultraviolet Ground- and Space-based Measurements, Models, and Effects V
KEYWORDS: Visible radiation, Scattering, Aerosols, Ultraviolet radiation, NOx, Radiometry, Ozone, Atmospheric particles, Atmospheric modeling, Absorption
A significant database of simultaneous measurements of NO2 column amounts and aerosol optical properties has recently become available that permits partitioning between aerosol and gaseous absorption. The aerosol column absorption optical thickness, (AAOT) was inferred from the measurements of global and diffuse atmospheric transmittances by a UV-Multifilter Rotating Shadowband radiometer (UV-MFRSR), calibrated using AERONET CIMEL sun-sky radiometers. The NO2 column amounts were measured using a double-Brewer MK III spectrometer (#171) operated in direct-sun mode using a new 6-wavelength retrieval algorithm. Ancillary measurements of column particle size distribution and refractive index in the visible wavelengths (by AERONET sun-sky almucantar inversions), ozone (by Brewer) and surface pressure constrained the forward radiative transfer model input, so that a unique solution for AAOT was obtained in each UV-MFRSR spectral channel. In fall-winter months with typically dry conditions and low aerosol loadings, the NO2 absorption represented a significant source of error in aerosol absorption measurements. This was confirmed by UV-MFRSR AAOT retrievals at 325nm, where the NO2 absorption cross-section is only half the value at 368nm. Thus, the NO2 correction not only reduces AAOTs obtained from traditional aerosol remote sensing techniques (shadowband or Cimel sunphotometer), but also is capable of changing the spectral dependence of aerosol absorption, which could result in an incorrect interpretation of aerosol composition. To further confirm these findings, a new UV-MFRSR instrument was modified by adding a 440 nm channel to provide spectral overlap with AERONET AAOT inversions in the visible wavelengths.
