Simulation of the tropospheric sulfur cycle in a global model with a physically based cloud scheme

Abstract

[1] The treatment of the sulfur cycle in the CSIRO global climate model (GCM) is described. It is substantially based on the scheme developed previously for the European Center/Hamburg (ECHAM) model, but the treatment of wet scavenging has been completely rewritten to better reflect the different properties of liquid and frozen precipitation, and the treatment of these in the model's cloud microphysical scheme. The model is able to reproduce the observed finding that wet deposition of sulfur over Europe and North America is larger in summer than in winter, but the seasonal cycle of sulfate over Europe is not well simulated. The latter is improved when the amplitude of the seasonal cycle of European emissions is increased. Below-cloud scavenging makes an important contribution in our scheme: On omitting it, the global sulfate burden increases from 0.67 to 0.93 Tg S. On reverting to the less efficient scavenging treatment used in ECHAM, the global sulfate burden again increases from 0.67 to 0.93 Tg S, and excessive sulfate concentrations are obtained in Europe and North America. Some deficiencies in the simulation are investigated via further sensitivity tests. In particular, during the Arctic winter, the modeled sulfur dioxide (SO 2 ) concentrations are too large, and the modeled sulfate concentrations are too small (as in most global sulfur-cycle models). Recent laboratory experiments suggest that SO 2 oxidation in ice clouds is nonnegligible. We obtain a much improved Arctic simulation when a simple treatment of SO 2 oxidation in ice clouds is included.

DMS cycle in the marine ocean-atmosphere system – a global model study

“…where w 10 is the 10 m wind speed and SC the Schmidt number for DMS which is calculated analogous to Saltzman et al (1993). The undetermined parameters are derived from a fit of the simulated DMS sea surface concentrations to observed DMS sea surface concentrations.…”

“…The Kettle and Andreae (2000) climatology of the DMS seawater concentration is widely used in global atmospheric models (e.g. Boucher and Pham, 2002;Jones et al, 2001;Berglen et al, 2004;Gondwe et al, 2003;Rotstayn and Lohmann, 2002). The response of the DMS emission to climate change can then only be assessed in models with prescribed DMS sea surface concentrations through changes in the sea-air exchange rate which varies with wind speed and temperature.…”

Evaluation of climate model aerosol seasonal and spatial variability over Africa using AERONET

“…Aerosol emissions across the RCP scenarios through the latest year simulated here (2012) are similar (van Vuuren et al, 2011). Within CCAM, of the SO 2 emissions from fossil fuel and smelting, 3 % are emitted as sulfate directly (Rotstayn and Lohmann, 2002); a similar fraction is assumed in other global models to represent rapid in-plume transformation of SO 2 to sulfate (Liu et al, 2005;Chin et al, 2000;Koch et al, 1999). The model has three prognostic variables to represent the sulfur cycle: dimethyl sulfide (DMS), SO 2 , and sulfate.…”

“…The model has three prognostic variables to represent the sulfur cycle: dimethyl sulfide (DMS), SO 2 , and sulfate. Additional minor sources of model sulfate aerosol are volcanic SO 2 emissions and biogenic DMS emissions, which can be oxidized to sulfate (Rotstayn and Lohmann, 2002). Concentrations of sulfur oxidants (OH, NO 3 , H 2 O 2 , and O 3 ) are prescribed, with the amount of SO 2 dissolved into cloud water described by Henry's Law.…”

“…Secondary organic aerosol (SOA) formation is not treated in the model. All prognostic aerosol species are removed via wet and dry deposition, while dust is additionally removed through gravitational settling (Rotstayn and Lohmann, 2002;Ginoux et al, 2001). …”