Suzuki, Y; Watanabe, S; Hasebe, M
After the 1990s, sulphur oxide (SOx) emissions from human activities, one of the main causes of acid rain, have been consistently increasing in the Asian region. Especially, in mainland China showing rapid industrialization, a large amount of SOx (mainly as SO2) is discharged every year, which can be transported to Japan by atmospheric advection, which can become an issue of serious public concern. If the current tendency of global warming continues and advances, variations of the atmospheric field in the near future would affect cross-border pollution by acid materials, and therefore, countermeasures should be devised based on environmental impact assessment considering climate change in the future.
In the current study, with the aim of achieving local assessment in East Asia under the influence of future climate change, advection and dispersion simulations about SO2 discharged from mainland China were conducted using the numerical model HYSPLIT4 (Hybrid Single-Particle Lagrangian Integrated Trajectory Model Version 4). As atmospheric datasets for the model input, the current study employed GCM (Global Climate Model) output, provided by the Innovative Program of Climate Change Projection for the 21st Century (KAKUSHIN Program) in Japan. The datasets were computed with a boundary condition of sea surface temperature representing the effect of future global warming (A1B scenario of IPCC). Wintertime datasets for 25 years of 1980 to 2004 were used for the present condition and those of 2075 to 2099 were for the future condition.
Firstly, to see how the atmospheric field (wind velocity and water vapour) in East Asia tends to vary with climate change in the future, a pattern classification technique of Self-Organizing Maps (SOMs) was applied to the atmospheric datasets with 6-hour intervals. SOMs, one of Artificial Neural Networks (ANNs) with unsupervised learning, can provide a two-dimensional map classifying a complicated pattern of nonlinear and multi-dimensional data. Consequently, the atmospheric field on the present and future conditions were classified into mainly 7 patterns, and the occurrence frequency of each atmospheric pattern was computed respectively for the present and future conditions. The minimum frequency among the 7 patterns is around 2 % and the maximum is around 20 %, and two of them show a significant increase of the frequency on the future condition, while two patterns show a significant decrease and the remaining three show a slight change only.
Secondly, advection and dispersion simulations by HYSPLIT4 were conducted using datasets of the atmospheric field representing each of the 7 patterns, and the air concentration and deposition amount of SOx was computed to see how and where the influence of SO2 discharged from mainland China could become intensified. In the model, puffs or particles were traced using the Lagrangian approach, and three processes of advection, dispersion and deposition are computed in turn. The horizontal resolution was set to 0.15 by 0.15 degrees, and vertical layer from the ground to the altitude of 3000 m was divided into 8 layers. The amount of SO2 discharged from China was determined based on data for 2005 reported in China, and 9 points located in the eastern part of China with a relatively large SO2 emission were chosen as emission sources.
Simulation results show, in the atmospheric pattern with a frequency increase on the future condition, SOx tends to be advected in the northern or northeastern direction, and the total amount of its deposition tends to be larger in the northern part of China and Japan or around the Korean Peninsula. In this pattern, the amount of SOx deposition as a whole including that in other Asian regions shows a tendency to be larger than other atmospheric patterns, which can be attributed to its higher specific humidity in the atmosphere, producing greater wet deposition. This result is consistent with the expectation that the amount of atmospheric water vapour will increase in the future due to the progress of global warming.