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Volume 17, issue 18 | Copyright

Special issue: The Geoengineering Model Intercomparison Project (GeoMIP):...

Atmos. Chem. Phys., 17, 11209-11226, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 21 Sep 2017

Research article | 21 Sep 2017

Sulfate geoengineering impact on methane transport and lifetime: results from the Geoengineering Model Intercomparison Project (GeoMIP)

Daniele Visioni1,2, Giovanni Pitari1, Valentina Aquila3, Simone Tilmes4, Irene Cionni5, Glauco Di Genova2, and Eva Mancini1,2 Daniele Visioni et al.
  • 1Department of Physical and Chemical Sciences, Università dell'Aquila, 67100 L'Aquila, Italy
  • 2CETEMPS, Università dell'Aquila, 67100 L'Aquila, Italy
  • 3GESTAR/Johns Hopkins University, Department of Earth and Planetary Science, 3400 N Charles Street, Baltimore, MD 21218, USA
  • 4National Center for Atmospheric Research, Boulder, CO 80305, USA
  • 5ENEA, Ente per le Nuove Tecnologie, l'Energia e l'Ambiente, 00123 Rome, Italy

Abstract. Sulfate geoengineering (SG), made by sustained injection of SO2 in the tropical lower stratosphere, may impact the CH4 abundance through several photochemical mechanisms affecting tropospheric OH and hence the methane lifetime. (a) The reflection of incoming solar radiation increases the planetary albedo and cools the surface, with a tropospheric H2O decrease. (b) The tropospheric UV budget is upset by the additional aerosol scattering and stratospheric ozone changes: the net effect is meridionally not uniform, with a net decrease in the tropics, thus producing less tropospheric O(1D). (c) The extratropical downwelling motion from the lower stratosphere tends to increase the sulfate aerosol surface area density available for heterogeneous chemical reactions in the mid-to-upper troposphere, thus reducing the amount of NOx and O3 production. (d) The tropical lower stratosphere is warmed by solar and planetary radiation absorption by the aerosols. The heating rate perturbation is highly latitude dependent, producing a stronger meridional component of the Brewer–Dobson circulation. The net effect on tropospheric OH due to the enhanced stratosphere–troposphere exchange may be positive or negative depending on the net result of different superimposed species perturbations (CH4, NOy, O3, SO4) in the extratropical upper troposphere and lower stratosphere (UTLS). In addition, the atmospheric stabilization resulting from the tropospheric cooling and lower stratospheric warming favors an additional decrease of the UTLS extratropical CH4 by lowering the horizontal eddy mixing. Two climate–chemistry coupled models are used to explore the above radiative, chemical and dynamical mechanisms affecting CH4 transport and lifetime (ULAQ-CCM and GEOSCCM). The CH4 lifetime may become significantly longer (by approximately 16%) with a sustained injection of 8Tg-SO2yr−1 starting in the year 2020, which implies an increase of tropospheric CH4 (200ppbv) and a positive indirect radiative forcing of sulfate geoengineering due to CH4 changes (+0.10Wm−2 in the 2040–2049 decade and +0.15Wm−2 in the 2060–2069 decade).

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Sulfate geoengineering (SG), the sustained injection of SO2 in the lower stratosphere, is being discussed as a way to counterbalance surface warming, mimicking volcanic eruptions. In this paper, we analyse results from two models part of the GeoMIP project in order to understand the effect SG might have on the concentration and lifetime of methane, which acts in the atmosphere as a greenhouse gas. Understanding possible side effects of SG is a crucial step if its viability is to be assessed.
Sulfate geoengineering (SG), the sustained injection of SO2 in the lower stratosphere, is being...