Atmos. Chem. Phys. Discuss., 11, 12269-12322, 2011
www.atmos-chem-phys-discuss.net/11/12269/2011/
doi:10.5194/acpd-11-12269-2011
© Author(s) 2011. This work is distributed
under the Creative Commons Attribution 3.0 License.
Review Status
This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
A study of uncertainties in the sulfate distribution and its radiative forcing associated with sulfur chemistry in a global aerosol model
D. Goto1, T. Nakajima1, T. Takemura2, and K. Sudo3
1Atmosphere and Ocean Research Institute, the University of Tokyo, Kashiwa, Chiba, Japan
2Research Institute for Applied Mechanics, Kyusyu University, Kasuga, Fukuoka, Japan
3Graduate School of Environmental Studies, Nagoya University, Nagoya, Aichi, Japan

Abstract. The direct radiative forcing by sulfate aerosols is still uncertain, mainly because the uncertainties are largely derived from differences in sulfate column burdens and its vertical distributions among global aerosol models. One of possible reasons of the large difference in the computed values is that the radiative forcing delicately depends on various simplifications of the sulfur processes made in the models. In this study, therefore, we investigated impacts of different parts of the sulfur chemistry module in a global aerosol model, SPRINTARS, on the sulfate distribution and its radiative forcing. Important studies were effects of simplified and more physical-based sulfur processes in terms of treatment of sulfur chemistry, oxidant chemistry, and dry deposition process of sulfur components. The results showed that the difference in the aqueous-phase sulfur chemistry among these treatments has the largest impact on the sulfate distribution. Introduction of all the improvements mentioned above brought the model values noticeably closer to in-situ measurements than those in the simplified methods used in the original SPRINTARS model. At the same time, these improvements also led the computed sulfate column burdens and its vertical distributions in good agreement with other AEROCOM model values. The global annual mean radiative forcings due to aerosol direct effect of anthropogenic sulfate was thus estimated to be −0.3 W m−2, whereas the original SPRINTARS model showed −0.2 W m−2. The magnitude of the difference between original and improved methods was approximately 50% of the uncertainty among estimates by the world's global aerosol models reported by the IPCC-AR4 assessment report. Findings in the present study, therefore, may suggest that the model differences in the simplifications of the sulfur processes are still a part of the large uncertainty in their simulated radiative forcings.

Citation: Goto, D., Nakajima, T., Takemura, T., and Sudo, K.: A study of uncertainties in the sulfate distribution and its radiative forcing associated with sulfur chemistry in a global aerosol model, Atmos. Chem. Phys. Discuss., 11, 12269-12322, doi:10.5194/acpd-11-12269-2011, 2011.
 
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