Abstract

Abstract. Vertically resolved distributions of sulfur dioxide (SO2) with global coverage in the height region from the upper troposphere to ~20 km altitude have been derived from observations by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat for the period July 2002 to April 2012. Retrieved volume mixing ratio profiles representing single measurements are characterized by typical errors in the range of 70–100 pptv and by a vertical resolution ranging from 3 to 5 km. Comparison with observations by the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) revealed a slightly varying bias with altitude of −20 to 50 pptv for the MIPAS data set in case of volcanically enhanced concentrations. For background concentrations the comparison showed a systematic difference between the two major MIPAS observation periods. After debiasing, the difference could be reduced to biases within −10 to 20 pptv in the altitude range of 10–20 km with respect to ACE-FTS. Further comparisons of the debiased MIPAS data set with in situ measurements from various aircraft campaigns showed no obvious inconsistencies within a range of around ±50 pptv. The SO2 emissions of more than 30 volcanic eruptions could be identified in the upper troposphere and lower stratosphere (UTLS). Emitted SO2 masses and lifetimes within different altitude ranges in the UTLS have been derived for a large part of these eruptions. Masses are in most cases within estimations derived from other instruments. From three of the major eruptions within the MIPAS measurement period – Kasatochi in August 2008, Sarychev in June 2009 and Nabro in June 2011 – derived lifetimes of SO2 for the altitude ranges 10–14, 14–18 and 18–22 km are 13.3 ± 2.1, 23.6 ± 1.2 and 32.3 ± 5.5 days respectively. By omitting periods with obvious volcanic influence we have derived background mixing ratio distributions of SO2. At 10 km altitude these indicate an annual cycle at northern mid- and high latitudes with maximum values in summer and an amplitude of about 30 pptv. At higher altitudes of about 16–18 km, enhanced mixing ratios of SO2 can be found in the regions of the Asian and the North American monsoons in summer – a possible connection to an aerosol layer discovered by Vernier et al. (2011b) in that region.

Highlights

  • The background aerosol loading of the stratosphere has been found to increase since about the year 2000 (Hofmann et al, 2009; Vernier et al, 2011b)

  • The plume of enhanced concentrations is clearly visible on June reaching from northern Africa over the mid-east to South East Asia at and 20 km, while no clear enhancements are visible at 22 km altitude

  • This global dispersion is similar to observations by IASI (Clarisse et al, 2014; Fromm et al, 2014)

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Summary

Introduction

The background aerosol loading of the stratosphere has been found to increase since about the year 2000 (Hofmann et al, 2009; Vernier et al, 2011b). While most analysis methods of nadir sounding observations provide vertical column amounts of SO2, various recent studies indicate that volcanic plume heights can be derived (Yang et al, 2010; Van Gent et al, 2012; Rix et al, 2012; Carboni et al, 2012; Clarisse et al, 2014; Fromm et al, 2014) Owing to their observation geometry, limb-sounding measurements are especially suited to obtain profile information of atmospheric constituents. In the following we present global altitude-resolved distributions of SO2 between about 10 and 20 km as retrieved from infrared limb-emission observations by MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) between June 2002 and April 2012 This data set is derived from single MIPAS limb spectra and complementary to the one presented in Höpfner et al (2013) which was reconstructed from monthly and 10◦ zonally averaged spectra, covering the height region between 15–20 and 40 km altitude.

Instrument
Retrieval
Error estimation
Comparison with ACE-FTS
Debiasing
Comparison of the debiased data set with in situ observations
SO2 distributions
Volcanic SO2 mass and lifetime
Global variability of background SO2
Conclusions
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