Global atmospheric sulfur budget under volcanically quiescent conditions: Aerosol‐chemistry‐climate model predictions and validation

Abstract

The global atmospheric sulfur budget and its emission dependence have been investigated using the coupled aerosol-chemistry-climate model SOCOL-AER. The aerosol module comprises gaseous and aqueous sulfur chemistry and comprehensive microphysics. The particle distribution is resolved by 40 size bins spanning radii from 0.39 nm to 3.2 μm, including size-dependent particle composition. Aerosol radiative properties required by the climate model are calculated online from the aerosol module. The model successfully reproduces main features of stratospheric aerosols under nonvolcanic conditions, including aerosol extinctions compared to Stratospheric Aerosol and Gas Experiment II (SAGE II) and Halogen Occultation Experiment, and size distributions compared to in situ measurements. The calculated stratospheric aerosol burden is 109 Gg of sulfur, matching the SAGE II-based estimate (112 Gg). In terms of fluxes through the tropopause, the stratospheric aerosol layer is due to about 43% primary tropospheric aerosol, 28% SO 2 , 23% carbonyl sulfide (OCS), 4% H 2 S, and 2% dimethyl sulfide (DMS). Turning off emissions of the short-lived species SO 2 , H 2 S, and DMS shows that OCS alone still establishes about 56% of the original stratospheric aerosol burden. Further sensitivity simulations reveal that anticipated increases in anthropogenic SO 2 emissions in China and India have a larger influence on stratospheric aerosols than the same increase in Western Europe or the U.S., due to deep convection in the western Pacific region. However, even a doubling of Chinese and Indian emissions is predicted to increase the stratospheric background aerosol burden only by 9%. In contrast, small to moderate volcanic eruptions, such as that of Nabro in 2011, may easily double the stratospheric aerosol loading.

Positive trends in Southern Hemisphere carbonyl sulfide

“…Previous studies suggested that OCS is a dominant contributor to the stratospheric aerosol layer during volcanically quiescent periods [Crutzen, 1976;Pitari et al, 2002]. This finding is corroborated by a recent model-based study that indicates that although 90% of the OCS transported into the stratosphere returns unprocessed to the troposphere, the remaining OCS contributes~56% to the stratospheric aerosol burden [Sheng et al, 2015]. There has been some debate, however, on the magnitude of the OCS flux to the stratosphere and on the relative contribution of OCS to the stratospheric aerosol loading [Brühl et al, 2012;Chin and Davis, 1995;Myhre et al, 2004].…”

“…A dominant indirect source results from CS 2 emissions from the rayon industry [Campbell et al, 2015]. With a tropospheric lifetime of~2-7 years [Blake et al, 2004], OCS is sufficiently long-lived in the troposphere that it is transported into the stratosphere where it is photooxidized to form sulfate particles [Sheng et al, 2015]. Previous studies suggested that OCS is a dominant contributor to the stratospheric aerosol layer during volcanically quiescent periods [Crutzen, 1976;Pitari et al, 2002].…”

Anthropogenic contributions to global carbonyl sulfide, carbon disulfide and organosulfides fluxes

“…However, their analysis of the temporal trend of stratospheric OCS between 1978 and 2005 did not suggest that an increase in background levels of sulfate aerosol could arise from an increased OCS source (Coffey and Hannigan, 2010). Sheng et al (2015) have modelled the global atmospheric sulfur budget under volcanically quiescent conditions, including stratospheric processes and suggest that 30-40 Gg S a -1 in net is transferred across the tropopause, which is in line with the estimate of 35…”

“…Net fluxes to the stratosphere have been estimated in the range of 34-66 Gg S a -1 with a global stratospheric lifetime of 64 ± 21 a (Barkley et al, 2008;Sheng et al, 2015). Coffey and Hannigan (2010) examined conflicting claims about increases in the sulfate loading of the stratosphere from increasing OCS.…”

The Interactive Stratospheric Aerosol Model Intercomparison Project (ISA-MIP): Motivation and experimental design

“…The total net sulphur mass flux from the troposphere into the stratosphere is estimated to be about 181 Gg S/yr 286 based on simulations by Sheng et al (2015a) using the SOCOL-AER model, 1.5 times larger than reported in 287 ASAP2006 (KTH2016). This estimate, however, could be highly dependent on the specific characteristics of the 288…”