Kazuhide Matsuda

Model study on particle size segregation and deposition during Asian dust events in March 2002

[1] A size-segregated aerosol model that includes most of the major physical processes (generation, transport, and dry and wet deposition) is developed. This model is coupled with a Regional Air Quality Model (RAQM) and is applied to simulate Asian dust storms during the 10-day period of 15-24 March 2002. A nonhydrostatic mesoscale model (MM5) is used to provide meteorological fields. Model results are verified by available observational data including surface weather observations and size-segregated particle concentrations. The validation demonstrates a good capability of this model system in capturing most of the key features of dust evolution and reproducing the particle mass size distribution along the transport pathway of soil dust. An apparent feature has been both observed and reproduced by the model, showing a shift of size range with peak mass concentration from coarse mode to finer mode on the pathway from source regions to distant downwind areas. The maximum dust concentration averaged over 10 days is simulated to be 3000 μg m over the southern China-Mongolia border. Total dry deposition of soil dust for 10 days is up to 30 g m -2 in the Gobi desert along the southern China-Mongolia border. Distribution and magnitude of particle deposition are strongly dependent on both concentration and size-segregated dry deposition velocity and scavenging rate. While dry deposition dominates the removal of dust particles in or in the vicinity of source regions, the influence of wet deposition increases along the transport pathway of soil dust, with high removal efficiency for coarser particles (>2 μm) and very low efficiency for particles in the accumulation mode. Of the total dust emission (43.2 megatons), about 71% is redeposited onto the underlying surface by the dry deposition process, 6% is removed by the wet deposition process, and the remaining 23% is suspended in the atmosphere or subject to long-range transport.

Simulations of monthly mean nitrate concentrations in precipitation over East Asia

Abstract Monthly mean nitrate concentrations in precipitation over East Asia (10–55°N, 75–155°E) in April, July, September, and December of 1999 were simulated by using a regional air quality Eulerian model (RAQM) with meteorological fields four times per day taken from National Centers for Environmental Prediction. The distribution of the nitrate concentration in precipitation depends significantly on the emission patterns of nitrogen oxides (NOx=NO+NO2) and volatile organic compound (VOC) and seasonal precipitation variability. The downward trend is also revealed, particularly in July and December. Highest concentrations are found in the industrialized regions, i.e., the coastal area of the Mainland of China, the Bay of the Huanghai Sea and the Bohai Sea, Korea, and Southern Japan. Long-range transport may cause elevated concentrations in remote areas downwind of the industrialized regions under favorable meteorological conditions, e.g., low precipitation. Comparison of observations and simulations indicates that the RAQM model reasonably predicts synoptic-scale changes in different months (seasons) and simulated nitrate levels in 4 months fit observed data with the discrepancy within a factor of 2. Exclusion of liquid chemistry within clouds is feasible for regional (1°×1°) and long-term (monthly) nitrate simulations. The uncertainty originates mainly from that of the emission data and modeled precipitation amounts and initial and boundary conditions.

Wet and dry deposition of sulfur associated with the eruption of Miyakejima volcano, Japan

[1] Since July 2000 the activity of Miyakejima volcano has become brisk. Sulfur emitted from the volcano has flowed into the Japanese Archipelago and brought high-concentration episodes of SO 2 over a wide area. The daily average emission of SO 2 during the period from August 2000 to March 2001 was the same level as the recent emission of SO 2 originating from fossil fuel combustion in east Asia. Estimated total (wet plus dry) deposition of sulfur at Tanzawa before the eruption for the most remarkable high-concentration episodes was almost the same level as the recent total deposition of sulfur over Japan. Estimated total deposition of sulfur after the eruption, on the other hand, was more than three times higher than the level before the eruption. The contribution of wet deposition increased with increasing daily precipitation amount, which indicates that precipitation scavenging had an instant effect in the removal of the volcanic sulfur. In the warm season, in which the subtropical high-pressure system over the Pacific is dominant, high-concentration episodes at monitoring stations were sometimes linked together with a time lag, which was explained by the fact that volcanic plumes from Miyakejima were transported north by the prevailing wind system in this season. In the cold season, however, the predominant wind system over the Japanese Archipelago is northwesterly from the Asian continent; most of the volcanic sulfur was thought to flow over the Pacific Ocean. Although no direct correspondence was found between the SO 2 dry deposition and the SO 2 emission, there was a close parallel temporal variation between the nss-SO 2- 4 wet deposition and the SO 2 emission. It was thought that the interaction between the precipitation cloud and the volcanic cloud took place just after the eruption of Miyakejima and that wet deposition plays an important role in the removal of the volcanic sulfur inflow outside the region.

A Case Study on Estimation of Dry Deposition of Sulfur and Nitrogen Compounds by Inferential Method

In order to estimate dry deposition, deposition velocity calculation and concentration measurement were carried out in Niigata, Japan. Deposition velocities of SO2 and HNO3 for some surfaces such as coniferous forest, deciduous forest, agricultural land, and water were calculated taking into account diurnal variations of meteorological elements using routine meteorological data. Deposition velocities of fine and coarse aerosols were also estimated respectively. Concentrations of SO2, HNO3, sulfate and nitrate in fine and coarse aerosols respectively were measured from July to December in 1998 using filter pack and denuder methods. The results indicate that HNO3 dry deposition for the high aerodynamic roughness surfaces such as forests is quite large. It is suggested that sulfate and nitrate aerosols depositions as well as the gases depositions should be taken into account to estimate dry deposition of sulfur and nitrogen compounds. It is also indicated that dry depositions of sulfur and nitrogen compounds are unable to ignore compared with their wet depositions.

Simulated impacts of SO2 emissions from the Miyake volcano on concentration and deposition of sulfur oxides in September and October of 2000

Abstract A regional air quality Eulerian model was run for 2 months (September and October of 2000) with and without SO2 emissions from the Miyake volcano to investigate effects of the changes in the volcanic emissions on SO2 and sulfate concentrations and total sulfur deposition around the surrounding areas. Volcanic emissions were injected into different model layers in different proportions within the planetary boundary layer whereas the other emissions were released in the first model layer above the ground. Meteorological fields four times per day were taken from National Centers for Environmental Prediction (NCEP). Eight Japanese monitoring sites of EANET (Acid Deposition Monitoring Network in East Asia) were used for the model evaluation. Simulations indicate that emissions from the Miyake volcano lead to increases in SO2 and sulfate concentrations in the surrounding areas downwind in the PBL by up to 300% and 150%, respectively, and those in SO2 levels in the area found ∼390 km north away from the Miyake site in the free troposphere (FTR) by up to 120%. Total sulfur deposition amounts per month are also increased by up to 300%. Daily SO2 concentrations in different model layers display strong variability (10–450%) at sites significantly influenced by the volcano. Comparison shows that the RAQM model predicts daily SO2 variations at relatively clean sites better than those at inland sites closer to volcanoes and the model well captures the timing of SO2 peaks caused by great changes in SO2 emissions from the Miyake volcano at most chosen sites and that monthly simulated sulfate concentrations in rainwater agree quite well with observations with the difference within a factor of 2. Improvement in spatial and temporal resolutions of meteorological data and removal of the uncertainty of other volcanic emissions may better simulations.

A Literature Review of Uncertainties in Studies of Critical Loads for Acidic Deposition

Uncertainties in the assessment of critical loads for acidic deposition are caused by the choice of biological indicators (BI), critical chemical values (CCV), the current methods used to determine critical loads for an ecosystem, and deficient field data. This paper focuses on the present steady-state mass balance (SSMB) approach, dynamic models and the importance of changes in atmospheric base-cation deposition (BCD), particularly in China. It is argued that 1) for the SSMB approach much uncertainty may come from the choice of BI and the related CCV, and long-term and large-scale monitoring data on weathering rates and growth uptake are urgently needed, especially in China, 2) significant uncertainty may be caused by changes in BCD during SO2 emission controls, particularly in China, 3) constructing a mechanistic Al submodel may be a promising direction for dynamic models, and 4) the nutrient cycle in the vegetation through biogeochemical processes should be incorporated into dynamic models but the input requirements should be moderate for broad application considerations. Generally higher BCD, different soil components and characteristics and different vegetation types in China compared to Europe and North America suggest that more field investigations on BI and their corresponding CCV be carried out before application of current approaches to specific areas, particularly in China.