Stratospheric sulfate aerosols mainly comprise H2SO4 and H2O, with a H2SO4 weight percentage of 75 % on average (reviewed by ). The size of the particles depends on the strength of the release of volcanic precursors (mainly SO2) in the lower stratosphere and ranges from sub-nanometer molecular clusters, over a few hundred nanometers in the natural stratospheric background (volcanically quiescent periods e.g., ), up to a few micron in the aftermath of large volcanic injections e.g.,. Sulfuric acid aerosols are thermodynamically stable in the relatively cold lower stratosphere below about 35 km, forming a global layer – the so-called Junge layer . The main sources of sulfur for stratospheric sulfate aerosols are thought to be the transport of naturally produced COS (carbonyl sulfide) from the troposphere into the stratosphere, as well as the injection of SO2 and possibly other sulfur compounds into the stratosphere as a consequence of volcanic eruptions. COS, which has natural and anthropogenic sources, is the only sulfur compound with a sufficiently long lifetime to reach the stratosphere e.g.,. In the stratosphere COS is oxidized to form SO2, which is then converted to H2SO4 . The relative importance of photolysis of COS and its oxidation by O atoms or excited O atoms is still not well established. More detailed information on the current understanding of stratospheric sulfur chemistry was presented by .

Stratospheric aerosols are of fundamental importance for the atmosphere's radiative balance, because they scatter solar radiation, thereby enhancing the Earth's planetary albedo. Particles also contribute to the greenhouse effect, but the warming effect only dominates over the cooling effect for particles with sizes greater than about 2 micron . However, the typical particle sizes for large volcanic eruptions such as the Mount Pinatubo eruption in 1991 are significantly smaller e.g.,. After the eruption of Mount Pinatubo, the global mean surface temperature decreased by about 0.4 K during 1992/1993 in large parts of the world e.g.,.

During the last 15 years, the stratospheric aerosol load has exhibited significant variability, including an increase likely caused by a series of volcanic eruptions e.g.,. Other suggested causes for this increase include enhanced sulfur emissions, e.g., due to coal burning, mainly in China . This suggestion, however, was recently challenged by . The increased stratospheric aerosol load, associated with volcanic eruptions, is also one possible mechanism contributing to the recent global warming hiatus, as suggested by and further outlined in . The CMIP-5 model simulations used for the last IPCC (Intergovernmental Panel on Climate Change) assessment report assume constant stratospheric aerosol after 2000; i.e., they do not consider the changing radiative forcing associated with changes in stratospheric aerosols. An accurate knowledge of the temporal evolution of stratospheric aerosol optical depth and aerosol size is of fundamental importance to the assessment of the role of stratospheric aerosols during the current warming hiatus and also for future climate change.

Stratospheric sulfate aerosols are also of importance for stratospheric chemistry because they may facilitate chlorine activation through heterogeneous reactions , and they also act as nucleation nuclei for polar stratospheric clouds that play a crucial role in the formation of the Antarctic ozone hole, as well as catalytic ozone losses within the Arctic polar vortex during cold Arctic winters. Stratospheric aerosols are central to the discussion on geoengineering by solar radiation management e.g.,. Any potential testing, either using natural phenomena such as volcanic eruptions, or man made, for such complex management strategies requires precise measurements of stratospheric aerosol.

Measurement techniques employed to study stratospheric aerosol parameters include in situ particle sampling, e.g., by optical means e.g.,, single or multi-wavelength LIDAR observations e.g.,, satellite solar e.g., or stellar e.g., occultation measurements, as well as satellite observations of limb-scattered solar radiation .

In this article we present validation results for an improved version of the SCIAMACHY limb-scatter stratospheric aerosol extinction profile algorithm, that was developed at the Institute of Environmental Physics at the University of Bremen. In an earlier study presented an algorithm description, error budget and validation results for a previous version of the SCIAMACHY stratospheric aerosol retrieval. The new data set presented here is validated with co-located SAGE-II stratospheric aerosol profile measurements. Using different retrievals approaches, and also presented stratospheric aerosol profile retrievals from SCIAMACHY limb-scatter observations.

The structure of the paper is as follows. In Sect.  we provide a description of the SCIAMACHY instrument focusing on the aspects most relevant to this study. In Sect.  the stratospheric aerosol retrieval algorithm is briefly described, and the differences with the previous version of the aerosol data product are highlighted. A brief description of the SAGE II stratospheric aerosol retrievals is given in Sect. . Section  presents comparisons of the SCIAMACHY stratospheric aerosol extinction profile retrievals with co-located SAGE II measurements. Sample results are presented in Sect. , and conclusions are given at the end.

The Stratospheric Aerosol and Gas Experiment (SAGE) II was a solar occultation instrument on ERBS (Earth Radiation Budget Satellite) that provided global measurements of stratospheric O3 and NO2 profiles and stratospheric aerosol extinction profiles at different wavelengths in the optical spectral range, including 525 nm. SAGE II operated from 1984 to 2005 and its stratospheric aerosol data set is generally considered to be one of the data sets with the highest accuracy. For this reason it is often used for validation studies and is well suited for comparisons with the SCIAMACHY stratospheric aerosol retrievals presented here.

A new version (7.0) of the SAGE II stratospheric aerosol data set was recently presented by , yielding stratospheric aerosol extinction profiles that sometimes differ significantly from the earlier version 6.2. In this study, we use both SAGE II versions for the validation of SCIAMACHY aerosol extinction profile retrievals.

We start by comparing globally averaged co-locations between SAGE II occultation and SCIAMACHY limb-scatter observations. The co-location criteria are 500 km spatial distance and 6 h temporal difference at most between the SAGE II and SCIAMACHY measurements. As a result of the complexity of the radiative transfer in limb geometry at large solar zenith angles (SZAs), SCIAMACHY limb measurements for SZAs exceeding 87∘ were not considered. All available co-locations between 1 January 2003 and 17 August 2005 were used. The SAGE II aerosol extinction values at a wavelength of 525 nm were used, and the SCIAMACHY extinction coefficients were evaluated at this wavelength using the assumed Ångstrøm exponent.

Figure  shows a comparison between SAGE II and SCIAMACHY aerosol extinction profiles at 525 nm for all co-locations. The left panel of Fig.  depicts the averaged SCIAMACHY (red solid line), SAGE II (version 6.2 shown as black and version 7.0 as blue solid line) profiles, together with the SCIAMACHY a priori profile (green solid line). The dashed colored lines show the corresponding standard deviations. The right panel of Fig.  shows the relative difference between the SCIAMACHY and SAGE II profiles relative to SAGE II, i.e., (SCIAMACHY – SAGE)/SAGE for SAGE II versions 6.2 (black solid line) and 7.0 (blue solid line). The dashed lines again correspond to the standard deviations of the relative differences. The total number of co-locations used for this comparison is 3589. We find average agreement to within about 10 % between SCIAMACHY and SAGE II version 7.0 aerosol extinction profiles for all altitudes between 15 and 35 km, which can be considered very good. Interestingly, the relative differences between SCIAMACHY and SAGE II version 6.2 are with values of up to about 30 % significantly larger than between SCIAMACHY and the new SAGE II version 7.0. This suggests that in a global average sense, differences between SAGE II version 6.2 and version 7.0 of up to about 20 % have to be expected. This estimate is in good agreement with the direct comparisons between the two SAGE II versions presented by .

The good agreement between SCIAMACHY and SAGE II version 7.0 aerosol extinction profiles in the global average is promising, but may not imply similar agreement in different hemispheres or for specific latitude bands. In order to identify possible interhemispheric differences in the agreement with SAGE II measurements, we show in Fig.  comparisons between SCIAMACHY and SAGE II stratospheric aerosol extinction at 525 nm for both hemispheres. The color/line convention is the same as in Fig. . SCIAMACHY aerosol extinction profiles are in very good agreement with SAGE II version 7.0 in the Southern Hemisphere (1635 individual co-locations) – with relative differences of less than about 10 % between 16 and 33 km. However, in the Northern Hemisphere (1954 co-locations) the differences reach about 20 % at altitudes above 25 km with SCIAMACHY aerosol extinction being lower than the SAGE values.

We further refine the comparisons by comparing co-located SCIAMACHY and SAGE II measurements for 20∘ latitude bins in Fig. . Table  lists relative differences between SCIAMACHY and both SAGE II versions for different altitudes and the different latitude bins. Figure  shows that there is no constant bias between SCIAMACHY and SAGE II aerosol extinction; the differences vary with latitude and altitude in a complex manner. At tropical latitudes the SCIAMACHY aerosol extinctions are generally lower than the SAGE II values, and the differences between the two SAGE II versions are smaller than for most other latitude bins. In the Northern Hemisphere the differences between SCIAMACHY and SAGE II version 7.0 decrease from low to mid latitudes and increase again at polar latitudes. In the Southern Hemisphere the differences between SCIAMACHY and SAGE II version 7.0 become smaller and change their sign. It is important to point out that some of the observed latitudinal variation of the difference between SAGE II and SCIAMACHY stratospheric aerosol extinction profiles is likely related to differences between the assumed scattering phase function – which is based on the assumed size parameters – and the actual scattering phase function in combination with the latitudinal variation of the scattering angle of the SCIAMACHY limb-scatter observations. The scattering angle varies from about 30∘ at high northern latitudes to about 150∘ at high southern latitudes. Errors in the assumed scattering phase function can easily lead to differences in retrieved aerosol extinction of several tens of % and may also lead to significant interhemispheric differences in the aerosol extinction retrieval errors from SCIAMACHY limb-scatter observations (see also Sect. ).

In terms of the agreement between the two SAGE II versions we find that the differences generally increase with increasing altitude and version 7.0 aerosol extinction is almost always smaller than the version 6.2 values. Furthermore, the differences are relatively small at low latitudes, increase towards mid-latitudes – particularly at altitudes above 25 km – and decrease again at the highest latitudes.

In this section we present results on the latitudinal and temporal variability of stratospheric aerosol extinction and stratospheric aerosol optical depth. In order to avoid contamination by tropospheric clouds, which can reach altitudes of up to 16–17 km at low latitudes, stratospheric aerosol optical depth is determined by integrating the extinction profiles from the Θ=380 K isentrope up to 40 km altitude, following .

Figure  shows the retrieved stratospheric aerosol extinction at a wavelength of 525 nm and at 18 km (top left panel), 22 km (top right panel), 26 km (bottom left panel) and 30 km (bottom right panel) altitude for the period from 1 January 2003 to 31 December 2011. Note that the aerosol extinction profiles were daily and zonally averaged. The white letters in the top left panel indicate volcanic eruptions (see Fig. caption) and the “Black Saturday” pyrocumulus event in February 2009 (E; e.g., ). During approximately the first 2.5 years of the SCIAMACHY mission, that were relatively unaffected by volcanic activity, the aerosol extinction at 18 km shows a pronounced annual cycle at mid-latitudes in both hemispheres with a wintertime maximum, and low values in the tropics and subtropics, where the aerosol layer resides at higher altitudes. This annual variation is consistent with LIDAR measurements of the stratospheric aerosol backscatter ratio observed from Boulder (40∘ N) and Hawaii (19∘ N) and is linked to seasonal variations in stratospheric temperature and moisture as well as the meridional transport associated with the Brewer–Dobson circulation . The volcanic eruptions lead to significantly enhanced stratospheric aerosol extinction values at 18 km altitude. The effects of mid and high latitude eruptions – e.g., Kasatochi in August 2008 e.g., and Sarychev Peak in June 2009 e.g., – are limited to the corresponding hemisphere, but can affect tropical latitudes.

The season during which eruptions occur has a significant impact on the meridional transport of the volcanic aerosol and of pyrocumulus injections into the upper troposphere. The aerosol detected after the Australian Black Saturday bushfires (E in Fig. ) in winter 2009 was predominantly transported equatorward, whereas the aerosol produced by the Mount Merapi eruption (G in Fig. ) in fall 2010 was mainly transported poleward.

The aerosol extinction fields at 18 and 22 km (top panels of Fig. ) show signatures of polar stratospheric clouds (PSCs) during hemispheric spring, particularly pronounced in the Southern Hemisphere at the highest latitudes during September and October. We have to point out that the SCIAMACHY stratospheric aerosol profile retrieval assumes a Mie phase function characteristic for stratospheric background aerosol, not for PSCs. The retrieved aerosol extinction for measurements affected by PSCs is therefore only an indicator of their enhanced extinction, and the extinction values may be affected by larger uncertainties than the stratospheric background aerosol retrievals. Whilst type Ib PSC particles are liquid and thus may be well described by Mie theory, type Ia PSC particles, which consist of solid nitric acid trihydrate, HNO3⋅3H2O or NAT and type II PSC particles, which consist of ice, are not spherical particles. More accurate phase functions for NAT and ice are required to obtain accurate extinction profiles.

The impact of the volcanic eruptions decreases with increasing altitude, as expected. At 30 km (bottom right panel of Fig. ) a volcanic signature is hardly apparent in the latitudinally resolved time series and the main variability component at low latitudes has a period characteristic of the Quasi-Biennial Oscillation (QBO, reviewed by ). The enhanced tropical aerosol extinction values at 30 km altitude occur during easterly shear conditions of the QBO . The bottom right panel of Fig.  also shows the -30 m s-1 (i.e., easterlies) contour line of the monthly and zonally averaged zonal wind at 10 hPa taken from the ERA Interim reanalysis. The results demonstrate that enhancements in aerosol extinction at 30 km altitude are linked to strong easterlies at the 10 hPa pressure level. These enhancements in aerosol extinction are caused by an upward motion, which is a manifestation of the QBO secondary oscillation induced by meridional temperature gradients caused by the wave-driven vertical wind shear . recently discussed the underlying physical mechanisms, also based on SCIAMACHY stratospheric aerosol measurements, in more detail.

The top panel of Fig.  shows the latitude and time variation of stratospheric aerosol optical depth (between Θ=380 K and 40 km altitude) at a wavelength of 525 nm. The effects of the volcanic eruptions are again clearly visible and are associated with local enhancements of the aerosol optical depth of up to a factor of about 5. The bottom panel of Fig.  shows the temporal evolution of stratospheric aerosol optical depth averaged over latitude and weighted with the surface area of the latitude bins, from 60∘ S to 60∘ N, again for the period 1 January 2003–31 December 2011. The optical depth is shown at wavelengths of 525 and 750 nm, in order to facilitate comparisons with previous studies that present optical depth at different wavelengths. Note that the ratio of the optical depths at these two wavelengths is constant and determined by the fixed Ångstrøm exponent mentioned in Sect. 2.2. In 2003 and 2004 the globally averaged aerosol optical depths are 3–4 × 10-3 and around 2 × 10-3 at 525 and 750 nm, respectively. The temporal evolution exhibits intermittent enhancements associated with the volcanic eruptions as evident from the comparisons of the upper and lower panel of Fig. . The maximum stratospheric aerosol optical depth values of about 8 × 10-3 and 4–5 × 10-3 at 525 and 750 nm, respectively, during the time period covered by SCIAMACHY measurements occur in summer 2011 after the eruption of Nabro, and correspond to roughly 2 times the 2003/2004 values. Following , we can convert stratospheric aerosol optical depth (OD) to radiative forcing (RF) using the formula RF =-25×OD(λ=525 nm), as in . The right axis of the bottom panel of Fig.  shows the radiative forcing caused by the stratospheric aerosol. The radiative forcing varies from about -0.1 W m-2 in 2003/2004 to a peak value of about -0.2 W m-2 immediately after the Nabro eruption in summer 2011. Please note that the radiative forcing shown in the bottom panel of Fig.  applies to the optical depth at 525 nm (blue line).

Similar to the top panel of Fig.  we show in Fig.  time series of stratospheric aerosol optical depth and radiative forcing for different latitude bins with 20∘ meridional extent. For the 20–40∘ and 40–60∘ latitude bins and during volcanically undisturbed conditions, the winter/spring maximum is clearly visible in both hemispheres. Particularly the low latitudes exhibit significant variability indicating that the stratospheric aerosol layer is highly variable even during periods without major volcanic eruptions . This implies that the Brewer–Dobson circulation and the QBO both play an important role for the exchange of COS and any other sources of stratospheric aerosol. The overall increase – with high variability – of the stratospheric aerosol loading during the SCIAMACHY mission period is in good general agreement with the results presented by .

We now discuss how the stratospheric aerosol optical depths retrieved in this study compare to previously published measurement and model results. The comparison of the stratospheric aerosol optical depth derived from the SCIAMACHY retrievals with earlier results is not straightforward, because different definitions of the stratospheric aerosol optical depth are used in different studies. use the 185 hPa pressure level as the lower boundary, integrate from 15 to 40 km altitude, and integrate from the Θ=380 K isentrope up to 40 km altitude. Moreover, aerosol optical depth is presented at different wavelengths (530 nm in , presumably 525 nm in – this is not explicitly mentioned – and 750 nm in and ) which further complicates direct comparisons. In order to simplify the comparison of our results with these published results we show in Fig.  (bottom panel) and Fig.  stratospheric aerosol optical depth at two wavelengths (525 and 750 nm).

show the temporal evolution of stratospheric aerosol optical depth from 1994 to 2010 for the tropics and the 50∘ S–50∘ N latitude range (their Fig. 2). For 2003/2004 they find values of about 5 × 10-3 for both latitude ranges. These values are slightly larger than the 3–4 × 10-3 found in our study. The differences may be related to the differences in the lower altitude limit. present measurement and model results of stratospheric aerosol optical depth from 2002 to 2011 for the 20∘ S–20∘ N and the 45–70∘ N latitude ranges (their Fig. 11). For the tropical latitude range optical depths of around 4 × 10-3 at 530 nm are found in 2003/2004 in both the EMAC model results and the SAGE/CALIPSO measurements. These values are in good agreement with the results presented here. The quantitative enhancements in aerosol optical depth due to the volcanic eruptions are slightly different in compared to our results, e.g., for the case of the Soufriere Hills and Tavurvur eruptions in 2006, but the overall agreement can be considered good. We can also compare the aerosol optical depth time series for the 45–70∘ N latitude range in with the 40–60∘ N results shown in the bottom right panel in Fig. . In 2003/2004 find values of around 4 × 10-3 (at 530 nm), which compares well with the 3–5 × 10-3 at 525 nm in our results. Good agreement is also found in terms of the optical depth enhancements associated with the eruptions of Kasatochi, Sarychev Peak, and Nabro.

The Sarychev results presented by allow a comparison of our results with the OSIRIS aerosol optical depth during 2009. During the 2 months prior to the Sarychev eruption report aerosol optical depths of about 3 × 10-3 at 750 nm and latitudes between 50–60∘ N (their Fig. 5, middle panel). The optical depth for this latitude range increases to about 1 × 10-2 in August and September. Our results also show optical depths of about 3 × 10-3 at 750 nm and latitudes between 40–60∘ N (bottom right panel of Fig. ) before the eruption and values of about 1 × 10-2 in August and September; i.e., the results presented by are in very good agreement with our results. Similar agreement is found between the Kasatochi results presented by and our results.

In summary, the stratospheric aerosol optical depths presented here are in good quantitative agreement with previously published results in terms of both the relatively quiet conditions in 2003/2004 and the volcanic enhancements that occurred since 2006.

Stratospheric aerosol extinction profiles in the visible spectral range were retrieved from SCIAMACHY limb-scatter observations using an improved retrieval method. A near-global stratospheric aerosol extinction profile data set covering the altitude range from the tropopause up to about 40 km has been retrieved for the period from August 2002 to April 2012. The retrieved aerosol extinction profiles were validated by comparison with co-located solar occultation measurements with SAGE II between January 2003 and the end of the SAGE II mission. A global mean comparison with the latest SAGE II data set (version V7.0) shows agreement within 10 % for altitudes above 15 km. An earlier version of the data set was using a Henyey–Greenstein scattering phase function and showed a strong interhemispheric difference of the comparisons with SAGE II. Comparisons made for different latitude ranges in opposite hemispheres showed that the retrieval improved significantly with the implementation of a Mie scattering phase function based on realistic aerosol particle sizes. We also find relative differences between aerosol extinction coefficients in SAGE II version 6.2 and 7.0 data of up to 40 % for certain altitudes and latitudes. The SCIAMACHY stratospheric aerosol extinction profile data set covers a period characterized by small and medium-sized volcanic eruptions and provides important measurements for the determination of the climate effects of stratospheric aerosol evolution from 2002 to 2012. Over the period of SCIAMACHY measurements, the changes in aerosol extinction have been shown to be strongly dependent on (a) the volcanic activity up to about 28 km and (b) dynamical processes, such as the Brewer–Dobson circulation and the quasi biennial oscillation. We find good quantitative agreement of the stratospheric aerosol optical depth derived in this study with previously published results. The radiative forcing of the stratospheric aerosol over the period 2003–2011 has also been assessed. In summary, the SCIAMACHY observations are providing a unique and milestone global record of the stratospheric aerosol between the tropopause and about 40 km altitude for the period between 2002 and 2012.