Accounting for surface reflectance variations along the satellite line of sight for UV-Vis ozone limb retrievals

Abstract. Retrievals of stratospheric ozone profiles from space-borne limb-scatter measurements in the UV-Visible spectral range were found to produce a bias if the effective reflectance of the underlying scene has a strong gradient along the instrument line of sight. This effect is particularly pronounced when the underlying scene between the sub-satellite and the tangent point passes from desert to ocean or from cloudy to cloud-free conditions, especially over the ocean. For stratospheric ozone columns retrieved from Ozone Mapper and Profiler Suite Limb Profiler (OMPS-LP) observations, regional biases of up to 7–10 DU were detected. When applying the limb-nadir matching technique, i.e. combining stratospheric and total ozone column information to obtain the residual tropospheric column, these underlying scene related biases amount to about 10–30 % of typical tropospheric ozone column values.

In this manuscript we present an improved retrieval algorithm, which substantially mitigates the observed biases in the stratospheric and tropospheric ozone. The algorithm employs an extended version of the SCIATRAN radiative transfer model (v4.8) capable of accounting for variations of the effective reflectance of the underlying scene along the instrument line of sight. VIIRS observations are used to prescribe the variation of the effective reflectance of the underlying scene along the instrument line of sight in the radiative transfer model. The manuscript describes the implementation strategy, discusses the results in terms of stratospheric ozone changes with respect to the standard version and presents updated OMPS-LP/OMPS-NM and new OMPS-LP/TROPOMI tropospheric ozone column data sets along with the validation of the results with respect to correlative observations. Any retrieval of observations from solar limb sounding spectrometers is potentially affected by this issue and should take it into account.