E), a coastal city in southwestern Japan, during three dust storm events in spring 2000. The elemental composition and size of individual dust particles and their mixture state with sea salt, sulfate, and nitrate were analyzed using electron microscopes and an energy dispersive X-ray spectrometer. About 60 $ 85% of dust particles were internally mixed with sea salt. Weather records indicated these particles were most probably formed by the collisions and coagulations of dust particles and sea-salt particles. The relative weight ratios of mineral components to sea salt in individual particles showed that the mixtures of particles were dominated by mineral, by sea salt, or by both. Size distributions of the particles segregated by the mixture levels of mineral and sea salt in the three dust storm events were similar and all distributions showed a diameter range of 1 $ 8 mm with maximum mode around 3 mm. Out of 1 $ 8 mm, dust particles were rarely detected. The combination of dust particles with sea salt caused an increase in size of the dust particles. Therefore the decrease of particle concentrations in the range of diameter >3 mm suggests the critical diameter for dust particle dispersion was possibly around 3 mm and a dust particle might be removed rapidly if it became larger than this scale in the marine atmosphere. Detection of sulfate and nitrate revealed that 91% or more dust particles contained sulfate and 27% or less contained nitrate. The comparisons of the relative weight ratios of sodium, sulfur, and chlorine in mixture particles and in sea-salt particles confirmed previous results that mineral materials could enhance particulate sulfate and nitrate formation and restrain chlorine depletion from the sea-salt components in mixture particles.

In order to investigate the characteristics of Asian dust particles before they leave the continent, particles were collected at a coastal site of Qingdao (120 27 0 E, 36 06 0 N) in northeastern China during three dust storm events in spring 2001. The elemental composition and size of individual dust particles, and their mixture state with sea salt, sulfate and nitrate were analyzed using electron microscopes and an energy dispersive spectrometer. It was found that far less than 10% of dust particles were internally mixed with sea salt. Although a number of dust particles contained Na, S, and Cl, the elements in most of the particles were more likely from crustal origins. The combination of reagent tests and elemental analysis revealed that 3.3-12.2% of dust particles contained sulfate and 6.5-10.0% contained nitrate on their surface. Number-size distributions of dust particles estimated from their electron microscope images had a distribution mode of around 3 mm diameter with the range of 1.0-8.0 mm. Out of this range, dust particles were rarely detected. r

[1] Asian dust particles usually refer to mineral particles which originate from arid and semiarid areas in the Asian continent and disperse eastward in a wide range in the atmosphere. Their appearances in the downstream marine areas are always accompanied by high concentrations of sulfate and nitrate, suggesting their significance as a medium for pollutant conversion and transportation. However, analysis of particles collected at a coastal site in east China before and after the cold front arrival of a heavy dust-loading low-pressure system in this study revealed that air masses polluted by anthropogenic emissions from local or regional areas and the natural Asian dust plume from northwest China were in different air parcels. Anthropogenic pollutants were in the prefrontal air while the so-called Asian dust particles were in the postfrontal air. There was a large amount of anthropogenic mineral particles in the polluted air masses, and the particles were abundant in sulfur, sodium, and calcium, while the so-called Asian dust particles were very ''clean'' (meaning lack of anthropogenic pollution). Continuous records of PM 10 , SO 2 , and O 3 concentrations indicated that the cold front was the boundary layer between the polluted air masses and the clean dust plume. Hence, at the coastal areas the Asian dust plume and polluted air masses did not mix with each other and were separated in two consecutive air parcels by the cold front although they were in the same lowpressure system. In addition, the analysis also revealed that calcium was not a good indicator for Asian dust from natural sources because the anthropogenic mineral particles contained significant calcium.

No abstract

Silicon (Si) is essential for growth of diatoms and other siliceous organisms, and plays a key role in marine ecosystems. We established a Si budget for the Bohai Sea and Yellow Sea (BSYS) on the basis of dissolved silicate (DSi) and biogenic silica (BSi) concentration measurements in two major rivers and in the water column and sediment of BSYS, and additional data from literature. The results show that BSi accounts for about 25 % of total reactive Si (DSi ? BSi) in the BSYS. The budget shows that the major Si source that supports primary production in the water column of BSYS is the benthic flux, accounting for 54 % of total Si input, followed by water exchange from the East China Sea, accounting for 26 %, direct riverine input (9 %), submarine groundwater discharge (7 %), surface runoff (3 %) and atmospheric deposition (\1 %). The dominant processes that remove dissolved Si (DSi) from the water column of BSYS are primary production and subsequent BSi sedimentation, and export to East China Sea, accounting for 80 and 20 % of the Si output, respectively. About 89 % of gross BSi production is recycled in the water column over the shelf of BSYS, and 11 % settles on the sea floor. The benthic flux to the water column of the BSYS is an important DSi source, but the budget indicates that overall there is a net burial of BSi amounting to 3.6 % of total BSi production.