Partitioning between aerosol and NO₂ absorption in the UV spectral region

Abstract

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.