Zenith-sky observations of stratospheric gases: the sensitivity of air mass factors to geophysical parameters and the influence of tropospheric clouds

The retrieval of accurate vertical column amounts of stratospheric constituents from zenith-sky spectroscopy is dependent on accurately modeling the transfer of radiation through the atmosphere and calculating suitable air mass factors (AMFs). Using a vector radiative transfer (RT) model we evaluate potential error sources in the AMF calculation, arising from incorrect geophysical parameters and computational approximations. Ozone and NO2 AMFs were calculated using 19 different parameterizations, whereupon each was used to retrieve vertical column information from zenith-sky spectra recorded at a mid-latitude location. When the model was run with single-order scattering only, the derived AMFs were between 2 and 5% lower than those calculated using a multiple scattering scheme. Multiple scattering was also found to act in tandem with other parameters investigated, for example volcanic sulfate aerosols and surface albedo, and as such, we conclude that it is important to include multiple scattering in any RT model. Significant errors were also introduced by using a standard ozone profile rather than one derived from ozonesonde data. Substantially amplified AMFs were obtained from model runs that contained tropospheric clouds. Zenith-sky measurements recorded during cloudy conditions demonstrated that AMFs calculated from an RT model that neglects scattering by tropospheric clouds are unsuitable for analysis of spectra from overcast days. The feasibility of calculating AMFs tailored to account for the presence of clouds is also discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.