Abstract
Abstract. We present a climatology of monthly and 10° zonal mean profiles of sulfur dioxide (SO2) volume mixing ratios (vmr) derived from MIPAS/Envisat measurements in the altitude range 15–45 km from July 2002 until April 2012. The vertical resolution varies from 3.5–4 km in the lower stratosphere up to 6–10 km at the upper end of the profiles, with estimated total errors of 5–20 pptv for single profiles of SO2. Comparisons with the few available observations of SO2 up to high altitudes from ATMOS for a volcanically perturbed situation from ACE-FTS and, at the lowest altitudes, with stratospheric in situ observations reveal general consistency of the datasets. The observations are the first empirical confirmation of features of the stratospheric SO2 distribution, which have only been shown by models up to now: (1) the local maximum of SO2 at around 25–30 km altitude, which is explained by the conversion of carbonyl sulfide (COS) as the precursor of the Junge layer; and (2) the downwelling of SO2-rich air to altitudes of 25–30 km at high latitudes during winter and its subsequent depletion on availability of sunlight. This has been proposed as the reason for the sudden appearance of enhanced concentrations of condensation nuclei during Arctic and Antarctic spring. Further, the strong increase of SO2 to values of 80–100 pptv in the upper stratosphere through photolysis of H2SO4 has been confirmed. Lower stratospheric variability of SO2 could mainly be explained by volcanic activity, and no hints of a strong anthropogenic influence have been found. Regression analysis revealed a QBO (quasi-biennial oscillation) signal of the SO2 time series in the tropics at about 30–35 km, an SAO (semi-annual oscillation) signal at tropical and subtropical latitudes above 32 km and annual periodics predominantly at high latitudes. Further, the analysis indicates a correlation with the solar cycle in the tropics and southern subtropics above 30 km. Significant negative linear trends are found in the tropical lower stratosphere, probably due to reduced tropical volcanic activity and at southern mid-latitudes above 35 km. A positive trend is visible in the lower and middle stratosphere at polar to subtropical southern latitudes.
Highlights
Sulfur dioxide (SO2) is one of the key species determining the aerosol content of the stratosphere (SPARC, 2006)
Main sources of stratospheric SO2 are the conversion of carbonyl sulfide (COS) (Crutzen, 1976; Brühl et al, 2012) and the direct transport of SO2 across the tropopause
The MIPAS observations show this maximum to be very pronounced in the tropics, as visible in Figs. 4 and 5, at altitudes of around 27–30 km
Summary
Sulfur dioxide (SO2) is one of the key species determining the aerosol content of the stratosphere (SPARC, 2006). This error contribution is estimated on the basis of dedicated retrieval simulations as described in detail in Höpfner et al (2009a) except that the values used for the line-of-sight uncertainty of single observations were 400 m for P1 and 300 m for the datasets during P2 These values have been determined from comparison of engineering tangent altitude values with those obtained by the pointing retrievals during the standard IMK/IAA data analysis (von Clarmann et al, 2003b, 2009; Kiefer et al, 2007). The lower altitude limit of the dataset is defined by the condition on the minimum number of limb scans used to calculate mean spectra This is determined by (a) the cloud coverage, (b) the scan pattern of MIPAS and (c) the lower limit of 15 km set to confine the retrievals mainly on the stratospheric situation. 0o - 10oS 10oS - 20oS 20oS - 30oS 30oS - 40oS 40oS - 50oS 50oS - 60oS 60oS - 70oS 70oS - 80oS 80oS - 90oS
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