Abstract
Abstract. The Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) instrument on Envisat provided, between August 2002 and April 2012, measurements of solar and Earthshine spectra from the UV to the shortwave infrared spectral region in multiple viewing geometries. We present a new approach to derive stratospheric aerosol extinction profiles from SCIAMACHY solar occultation measurements based on an onion-peeling method similar to the onion-peeling differential optical absorption spectroscopy (DOAS) retrieval, which has already been successfully used for the derivation of greenhouse gas profiles. Since the retrieval of aerosol extinction requires as input measured transmissions in absolute units, an improved radiometric calibration of the SCIAMACHY solar occultation measurements has been developed, which considers various instrumental and atmospheric effects specific to solar occultation. The aerosol extinction retrieval can in principle be applied to all wavelengths measured by SCIAMACHY. As a first application, we show results for 452, 525 and 750 nm. The SCIAMACHY solar occultation time series has been processed, covering a latitudinal range of about 50–70∘ N. Reasonable aerosol extinctions are derived between about 15 and 30 km with typically larger uncertainties at higher altitudes due to decreasing aerosol extinction. Comparisons with collocated Stratospheric Aerosol and Gas Experiment II (SAGE-II) and SCIAMACHY limb aerosol data products revealed good agreement with essentially no mean bias. However, dependent on altitude, differences of up to ±20 %–30% to SAGE-II at 452 and 525 nm are observed. Similar results are obtained from comparisons with SAGE-III. SCIAMACHY solar occultation data at 750 nm have been compared with corresponding SAGE-III, Optical Spectrograph and InfraRed Imager System (OSIRIS) and SCIAMACHY limb results. The agreement with SCIAMACHY limb data at 750 nm is within 5 %–20 % between 17 and 27 km. SAGE-III and OSIRIS show at this wavelength and altitude range on average about 40 % and 25 % smaller values, with some additional 10 %–20 % modulation with altitude. The altitude variations in the differences are mainly caused by systematic vertical oscillations in the SCIAMACHY occultation data of up to 30 % below about 25 km. These oscillations decrease to amplitudes below 10 % with increasing number of collocations and are no longer visible in monthly anomalies. Major volcanic eruptions as well as occurrences of polar stratospheric clouds (PSCs) can be identified in the time series of aerosol extinction data and related anomalies. The influence of the quasi-biennial oscillation (QBO) is visible above 25 km.
Highlights
OSIRIS (Optical Spectrograph and InfraRed Imager System; see, e.g. Llewellyn et al, 2004; Bourassa et al, 2012; Rieger et al, 2014, 2019), ACE-MAESTRO, CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization; Winker et al, 2007; Vernier et al, 2011), OMPS (Ozone Mapping Profiler Suite; Jaross et al, 2014; Loughman et al, 2018; Chen et al, 2018) and SAGE-III are still operational and deliver data on stratospheric aerosols
For consistency reasons and because the fitting windows are optimised for the aerosol extinction retrieval, we use a specific sequence of retrievals such that information obtained in one retrieval can be used in other retrievals
To illustrate the outcome of the different calibration and retrieval parts described in the previous section, we present in this subsection as an example the results for orbit 8014
Summary
Stratospheric aerosols play an important role in climate as they affect radiative forcing either by scattering and absorption of light (direct effect) or by their impact on clouds and Published by Copernicus Publications on behalf of the European Geosciences Union. OSIRIS (Optical Spectrograph and InfraRed Imager System; see, e.g. Llewellyn et al, 2004; Bourassa et al, 2012; Rieger et al, 2014, 2019), ACE-MAESTRO (the Atmospheric Chemistry Experiment Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation instrument; McElroy et al, 2007; Sioris et al, 2010), CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization; Winker et al, 2007; Vernier et al, 2011), OMPS (Ozone Mapping Profiler Suite; Jaross et al, 2014; Loughman et al, 2018; Chen et al., 2018) and SAGE-III (from ISS; Cisewski et al, 2014) are still operational and deliver data on stratospheric aerosols. We describe a new method to derive stratospheric aerosol extinction profiles from SCIAMACHY solar occultation data. Some details on the methods used in this study are given in the Appendix
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