AN ANALYSIS OF SULFUR DEPOSITION USING TWO DIFFERENT KINDS OF LONG-RANGE TRANSPORT MODEL FOR EAST ASIA

A long-range transport model for East Asia was developed to estimate the wet deposition of sulfate. The model is a trajectory type which is appropriate for long-term analysis. Trajectories of air masses are calculated by tracing the wind field which changes spatially and temporally. The processes of reactions, rainout removal, intake of sulfate in cloud water into rain water, and dry and wet depositions are considered. It is possible to calculate the concentration of sulfate in precipitation at a receptor by performing material balance in a grid box containing the receptor.

The results obtained by the trajectory model were evaluated through comparison with observation data of acidic deposition. The observation was conducted at 21 stations throughout Japan for one year. The calculated amount of wet deposition of sulfate in Japan was 0.22Tg/y in S equivalent, while the observed amount was 0.29Tg/y. The trajectory model can predict almost 80�� of observed wet deposition. The contributions of domestic anthropogenic sources and volcanic eruption to wet deposition of sulfate in Japan were estimated using the trajectory model. The ratio of the deposition of sulfate due to Japanese anthropogenic sources to that due to the Asian continental sources was about 1 to 2. The contribution of the sulfur oxides from volcanoes was about 20��.

We have also developed a hybrid long-range transport model to estimate the total of the dry and wet sulfur depositions. The hybrid model combines the trajectory model for the long-range transport with an Eulerian model for the short-range transport. The model shows the ability to predict the concentrations of sulfur oxides in air influenced by large nearby sources, which the trajectory model consistently underestimated. The total sulfur deposition in Japan calculated using the latest emission data was 0.43 Tg/y in S equivalent, while the observed amount was 0.53 Tg/y. The calculated deposition is in good agreement with the observed value. It is therefore possible to estimate the sources contributing to the total sulfur deposition in Japan using the hybrid model.

Acidic deposition began to attract attention as a regional-scale environmental problem in the beginning of the 1970's in Europe and North America. Recently, acid rain problems have also extended to Asia, because of a significant increase in atmospheric emissions resulting from high economic and population growth rate. CRIEPI(1992) established a nationwide network in Japan for the observation of acidic deposition in 1987 and have been conducting observations since then. In this paper, two different kinds of long-range transport models for East Asia were developed to analyze sulfur deposition and the validity of the models was examined on the basis of the data obtained from the above observation. A large number of long-range transport models of acidic substances for Europe and North America were developed in the 1970's and the 1980's. In the 1990's, some analyses of long-range transport for Asia were begun but there is hardly any case which predicts acidic deposition and examines the accuracy of the prediction all year round for various places in Japan. The results of our long-range transport models were evaluated through comparison with the data of acidic deposition observed at 21 stations throughout Japan for one year. This paper also describes the contributions of domestic anthropogenic sources and volcanic eruption to sulfur deposition in Japan.

There are two basic approaches to the analysis of long-range transport of acidic substances: use of a trajectory model and an Eulerian model. The trajectory model treats physical and chemical processes involved in acidic deposition simply and uses routinely obtained meteorological data. The model is suitable for long-term analysis, covering a year, for example. The Eulerian model enables detailed analysis of physical and chemical processes, but cannot provide good results without sufficient data on emission, meteorology, climate and geography, and an excellent computational environment. The model is suitable for episodal analysis within a limited period. Analyses for the period extending over at least one year are required to estimate the contributions of domestic anthropogenic sources and volcanic eruption to depositions. We therefore developed two kinds of approaches based on the trajectory model (Ichikawa & Fujita, 1995; Hayami & Ichikawa, 1995). Our trajectory model provides estimates of sulfur oxides (SO_x) concentration in air and sulfate concentration in rain. Trajectories of air mass are calculated by tracing the wind field, which changes spatially and temporally. Trajectories are traced every 3 hours for 10 days and calculated on an 850 hPa isobaric surface. The concentration distribution of pollutants is assumed to be normal in the horizontal direction and uniform in the vertical direction within the mixing layer. The height of the mixing layer is assumed to be 1000 m. Physical and chemical processes considered in this model are shown schematically in Fig. 1. The values of parameters vary widely depending on meteorological, geographical, and climatic conditions. In our model, the values are constant or simple functions of the precipitation intensity. The ratio of reactions, intake of cloud and rain water, and dry and wet deposition processes are assumed to be linearly proportional to concentration. Then, analytical solutions are obtained for sulfur dioxide (SO₂) and particulate sulfate concentrations in air, and sulfate concentration in cloud water. It is possible to calculate the concentration of sulfate in rain in a receptor by performing material balance in the grid box containing the receptor.

Fig.1 Schematic of the long-range transport model.

Dry deposition strongly depends on the ground surface concentrations of SO_x. When a large source exists in the vicinity of the receptor and the model does not take vertical concentration distribution into account, SO_x concentration near the ground surface may be underestimated by a one-layer model such as our trajectory model. In that case, it is important that the model be expanded to include the vertical distribution of emissions and concentration. The Eulerian model could express the vertical distribution and bring better results, but it is difficult to prepare the necessary detailed emission data over the East Asian domain. Substances emitted from far sources are expected to be well mixed during the long-range transport and uniformly distributed in the vertical direction which means the long-range transport could be represented with one-layer model. Based on the above consideration, we proposed a hybrid long-range transport model, which combines a trajectory model with an Eulerian model. The combination could bring more accurate predictions by retaining the strong point of the both models. Figure 2 shows the concept of the hybrid model. The trajectory and Eulerian models are respectively used to calculate the sulfur depositions due to the far sources and nearby sources. The Eulerian model covers a box containing the receptor with approximately 80 km and 1.6 km in the horizontal and vertical directions, respectively. The vertical domain of the box has six non-uniformly spaced layers. The horizontal eddy diffusivities are assumed to be constant. The vertical eddy diffusivities vary diurnally and seasonally. At present, the hybrid model treats the same chemical species and the same processes of reaction, cloud and deposition as the trajectory model. The hybrid model, however, can incorporate more complex processes.

Fig.2 Concept of the hybrid model.

Anthropogenic emissions of SO₂ per grid square in East Asia were obtained by CRIEPI. Figure 3 shows the emission distribution of anthropogenic SO₂ for 1986. Recently, we updated the emission data. The pattern for 1990 is similar to Fig.3, although there has been an increase of 20�� in the total SO₂ emissions of East Asia between 1986 and 1990. The region of the study includes Japan, China, Taiwan, South Korea, North Korea, Mongolia and Far East part of Russia and was divided into 54 by 54 grids, with grid size equivalent to 80km by 80km in the center of the region. The grid system with three times higher resolution in the horizontal and vertical directions provides the Eulerian part of the hybrid model with the emission data. Anthropogenic SO₂ emissions from East Asia for 1990 were about 25 �~ 10⁶ t/y. The breakdown of this figure is as follows: 1.0 �~ 10⁶ t/y for Japan; 21 �~ 10⁶ t/y for China; 0.6 �~ 10⁶ t/y for Taiwan; 1.6 �~ 10⁶ t/y for South Korea; and 0.7 �~ 10⁶ t/y for North Korea, 0.3 �~ 10⁶ t/y for others. The ratio of SO₂ emissions from the lower (0-50m), middle (50-300m) and upper (300m-) layers in Japan are respectively 35��, 36�� and 29��. These values were estimated considering plume rise.

Fig.3 Emissions of anthropogenic sulfur dioxide per grid square in East Asia.

There were 12 active volcanoes in Japan at the end of the 1980's. The SO₂ emissions from volcanoes were estimated to be approximately 1.5 �~ 10⁶ t/y. Two thirds of the volcanic emissions was from the Kyushu island. The volcanic SO₂ are emitted at the top layer in the Eulerian model.

High-altitude wind data were obtained from the Aerological Data of Japan and weather charts, edited by the Japan Meteorological Agency (JMA) and published by the Japan Weather Association (JWA). There are 54 observation points in East Asia. Winds are observed at 0 and 12 Greenwich mean time. Data of precipitation for each day were obtained from SDP (JMA weather station) data and World data. Both data are edited by JMA. There are 345 observation points in East Asia. In addition, hourly surface wind data from the AMeDAS (Automated Meteorological Data Acquisition System) operated by the JMA were used for the Eulerian part of the hybrid model. The observation points of the AMeDAS are placed with the space of about 20 km throughout Japan.

The meteorological fields needed for the both models were obtained from the interpolation and extrapolation of the above observation data using the objective analysis technique.

The long-range transport models were evaluated using observation data of acidic deposition. The location of observation stations is illustrated in Figure 6. The observation stations indicated by white circles (��) represent the areas divided by solid lines on the main Japan Islands according to climatic and geographical conditions. Black circles (��) and white circle with plus sign (��) show the observation stations at small islands and our research institute, respectively.

A wet-only sampler with an aperture of 190 cm² was used to collect precipitation samples. The precipitation sensor responds to rain droplets with a diameter larger than 0.5mm. Precipitation samples were collected in a five-liter polyethylene bottle installed in the sampler. Particulate and gaseous samples were collected using a two stage low volume air sampler with 80 mm diameter filter holders. A teflon filter (AF-07P) was mounted on the upper stage of the sampler to collect particles, and an alkaline filter (QAST-2500, impregnated with 6�� K2CO3 solution) was mounted on the lower stage to collect sulfur dioxide. The filters were cut into small pieces and treated ultrasonically using distilled water for one hour. Then, the resulting aqueous sample was filtered through a millipore filter and filled up to 50 ml.

Samples were collected at 10-day intervals. Sulfate and sodium ion in precipitation were analyzed by ion chromatography and atomic absorption spectrometry, respectively. Sulfate concentration originated from non-seasalt sources was estimated using sodium ion concentration.

Calculations using the long-range transport model were carried out for the period from October 1988 to September 1989. This period corresponds to the second year of our observation. The selection of this period is based on the facts that insufficient observation data were obtained in the first year and that the data on volcanic emissions were not reliable after the third year.

Figures 4 and 5 show the back trajectories arriving at Tokyo for summer and winter seasons, respectively. The trajectories were calculated on an 850 hPa isobaric surface. In summer, air flows from various directions to Japan, as shown in Fig.4. Air stream from the direction of the Asian continent dominates from October through May, as shown in Fig.5.

Fig.4 Back trajectories arriving at Tokyo (Summer) .

Fig.5 Back trajectories arriving at Tokyo (Winter).

The SO₂ emission data for 1986 was used for the calculation in this section. This data didn't include the emissions from Russian Far East part and Mongolia. Figure 6 compares the annual wet depositions of sulfate predicted by the trajectory model with those observed. Numbers accompanying the squares correspond to the observation points indicated on the map of Japan. Calculation was carried out for the grid with size equivalent to about 80 km². The predicted total wet deposition in Japan was 0.22 Tg/y in S equivalent, while the observed one was 0.29 Tg/y in S equivalent. It is concluded that the trajectory model can predict wet deposition of sulfate with high accuracy.

Fig.6 Comparison of wet depositions predicted by the trajectory model with those observed at 21 points of the CRIEPI sampling network for acidic deposition.

The contributions of domestic anthropogenic and volcanic sources to wet deposition of sulfate in Japan were estimated using the trajectory model. Values in percent under the circle graph in Figure 7 indicate the contribution of volcanoes. The contribution amounted to 20�� for the whole area of Japan. The circle graph in Figure 7 shows the contributions of East Asian countries to the total wet deposition in Japan excluding the wet deposition arising from volcanic sources. For the whole of Japan, the contributions of China, Japan, and Korea are approximately one-half, one-third, and one-sixth of the total contribution, respectively. Less than 1 �� is due to anthropogenic emissions from Taiwan. Emissions from both volcanic and anthropogenic sources in Japan account for about 50 �� of the total wet deposition of sulfate in Japan. The contribution of domestic sources to the region which faces the Sea of Japan is low as compared with that to other regions. The contribution of Asian continental sources to the coastal region of the Japan Sea amounted to more than 85 �� in winter. In warm season (April through September), the contribution of Asian continental sources to wet deposition in the whole area of Japan is about 40 %.

Fig.7 Sources contributing to wet deposition of sulfate in Japan.
Circle graphs show the contributions of anthropogenic emissions excluding those of volcanic emission.

The SO₂ emission data for 1990 were used for the calculation by the hybrid model. Figure 8 compares the annual dry deposition of sulfur predicted by the hybrid model with that estimated from observation. The hybrid model underpredicted at No.12, Takamatsu. This might be due to the influence of the local wind such as sea and land breeze and the emission from navigation unconsidered in the model. The calculated amount of the total of the dry and wet depositions in Japan was 0.43 Tg/y in S equivalent, which the observed amount was 0.53 Tg/y. Our tentative prediction showed that the emission from both volcanic and anthropogenic sources in Japan account for about 60�� of the total deposition.

Fig.8 Comparison of dry depositions of sulfur predicted by the hybrid model with those observed at 21 points of the CRIEPI sampling network for acidic deposition.

Two different kinds of long-range transport model were developed to estimate sulfur deposition. The results calculated by the models were evaluated through comparison with observation data of acidic deposition. The observation was conducted at 21 stations throughout Japan for one year. The trajectory model showed relatively high accuracy of prediction for wet deposition. The contributions of domestic anthropogenic and volcanic sources to the total wet deposition of sulfate in Japan were estimated using the trajectory model. The contribution of the emissions from both domestic anthropogenic and volcanic sources was approximately 50 ��. It was also concluded that the hybrid model can predict the total of the dry and wet sulfur depositions with high accuracy. It is therefore possible to estimate the sources contributing to the total sulfur deposition in Japan using the hybrid model.

The long-range transport model is a useful tool for the assessment of the source-receptor quantitative relationship. We should use the model to adopt measures for the reduction of emissions and acid depositions in Asia.

Ichikawa, Y., S. Fujita. "An analysis of wet deposition of sulfate using a trajectory model for East Asia.", Water, Air and Soil Pollution 85(1995):1927-1932.

Hayami, H., Y. Ichikawa. "Development of hybrid LRT model to estimate sulfur deposition in Japan", Water, Air and Soil Pollution 85(1995):2015-2020.

CRIEPI research group of acidic deposition. "Acidic deposition in Japan", CRIEPI Report ET91005(1992).