Abstract
Abstract. The sulfur cycle and radiative effects of sulfate aerosol on climate are studied with a Global tropospheric Climate-Chemistry Model in which chemistry, radiation and dynamics are fully coupled. Production and removal mechanisms of sulfate are analyzed for the conditions of natural and anthropogenic sulfur emissions. Results show that the 1985 anthropogenic emission tripled the global SO2 and sulfate loadings from its natural value of 0.16 and 0.10 Tg S, respectively. Under natural conditions, the fraction of sulfate produced in-cloud is 74%; whereas with anthropogenic emissions, the fraction of in-cloud sulfate production slightly increased to 76%. Lifetimes of SO2 and sulfate under polluted conditions are estimated to be 1.7 and 2.0 days, respectively. The tripling of sulfate results in a direct radiative forcing of −0.43 W m−2 (clear-sky) or −0.24 W m−2 (all-sky), and a significant first indirect forcing of −1.85 W m−2, leading to a mean global cooling of about 0.1 K. Regional forcing and responses are significantly stronger than the global values. The first indirect forcing is sensitive to the relationship between aerosol concentration and cloud droplet number concentration which requires further investigation. Two aspects of chemistry-climate interaction are addressed. Firstly, the coupling effects lead to a slight decrease of 1% in global sulfate loading for both the cases of natural and anthropogenic added sulfur emissions. Secondly, only the indirect effect of sulfate aerosols yields significantly stronger signals in changes of near surface temperature and sulfate loading than changes due to intrinsic climate variability, while other responses to the indirect effect and all responses to the direct effect are below noise level.
Highlights
Aerosol particles affect the Earth’s energy budget directly by absorbing or scattering short-wave and long-wave radiation, and indirectly by influencing the structure and radiative properties of clouds through acting as cloud condensation nuclei and ice nuclei (Twomey, 1974; Albrecht, 1989)
By comparing simulations with and without the coupling of aerosol radiative forcing, we examined the meteorological responses to the forcing as well as feedbacks to the meteorology and subsequently to the aerosol fields
The annual means from global climate-chemistry model (GCCM) are on the whole lower than the observed concentrations at these stations, with differences ranging from 12% to 48%
Summary
Aerosol particles affect the Earth’s energy budget directly by absorbing or scattering short-wave and long-wave radiation, and indirectly by influencing the structure and radiative properties of clouds through acting as cloud condensation nuclei and ice nuclei (Twomey, 1974; Albrecht, 1989). Many chemical transport models (CTMs) have been developed to simulate the sulfur cycles using prescribed (offline) meteorology to drive the chemistry. Another approach has been to use prescribed aerosol for calculating radiative forcing in global climate models Many studies indicated that feedbacks might be more influential than expected (Kaufman and Freaser, 1997; Cerveny and Bailing, 1998; Audiffren et al, 2004; Resenfeld, 2000), models that do not include coupled chemistry, radiation and dynamics may have large errors in the estimates of the feedback mechanisms occurring in the climate system and impact the model results (Zhang, 2008). In this study we incorporated an interactive tropospheric sulfur chemistry scheme into a global climate-chemistry model (GCCM) (Wong et al, 2004) to estimate radiative forcing of sulfate aerosols, including direct aerosol effect and aerosol-cloud albedo effect. By comparing simulations with and without the coupling of aerosol radiative forcing, we examined the meteorological responses to the forcing as well as feedbacks to the meteorology and subsequently to the aerosol fields
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