Chemical characteristics of aerosols transported from Asia to the East China Sea: an evaluation of anthropogenic combined nitrogen deposition in autumn
Introduction
Industrial growth is rapid in East Asia. The emission of anthropogenic substances in East Asia has been on the rise due to the use of fossil fuels, industrialization (e.g., Kato and Akimoto, 1992) and due to the lack of waste control practices (Huh and Chen, 1999). In particular, nitrogen oxide (NOx) emissions in East Asia increased by 65% from 1975 to 1987 (Akimoto and Narita, 1994). It is predicted that NOx emissions will increase approximately 5-fold from 1990 to 2020 (van Aardenne et al., 1999; Akimoto, 2003). The increasing industrial emissions have made several ecosystems significant recipients of atmospheric pollutants (Galloway et al., 1995). Sulfate and nitrate are the most common compounds contributing to atmospheric acidity, and their deposition affects the pH of surface water (Lestari et al., 2003). On the other hand, the atmospheric deposition of combined nitrogen, ammonium and nitrate, is an important nutrient supply to the oceans (Duce et al., 1991; Paerl, 1997; Spokes et al., 2000). It has been suggested that these deposits affect productivity and ecosystems in oceanic and coastal areas (e.g., Owens et al., 1992; Uematsu et al., 2004).
The western North Pacific receives a large influx of mineral particles and pollutants from East Asia through atmospheric long-range transport (Uematsu et al., 1983; Tsunogai et al., 1985; Gao et al., 1992). The East China Sea is one of the largest marginal seas in the world and is situated between the east coast of China and the western North Pacific (Lin et al., 2000). High biological activity in the East China Sea is maintained by nutrients supplied from the Changjiang River, one of the largest rivers in the world, and the Kuroshio upwelling (Chen and Chen, 2003).
Several of the previous atmospheric observations were made in spring due to the occurrence of frequent dust events under the influence of strong seasonal westerly winds (e.g., Gao et al., 1992), but only a few studies in autumn (e.g., Hatakeyama et al., 1995). Further, atmospheric nutrients supply to the East China Sea was not considered in the previous studies. The present attempts to investigate the nature and sources of aerosols through the study of their chemical properties, their transports to the study area, the deposition of the combined nitrogen species over the East China Sea and to evaluate their significance to biological fixation of carbon dioxide.
Section snippets
Methods
Fig. 1 shows the track of Leg 2 of the KH02-3 cruise conducted on board the vessel R/V Hakuho-maru between 26 September 2002 and 9 October 2002. A high-volume virtual impactor air sampler (Kimoto Electric Co., Ltd., AS-9) was used to collect atmospheric aerosols on Teflon filter papers (Toyo Roshi Kaisha, Ltd., PF040, 90 mm in diameter). The sampler used an inertial force to separate atmospheric aerosols according to their aerodynamic diameters, which were segregated into fine (<2.5 μm) and
Variations in particle number density
Fig. 2 shows the variations in particle number densities, where peaks in these reflect the times of ship passage through outflow of air masses blown from the continent. Four pronounced peaks were observed. Number densities of particles larger than 0.1 μm frequently exceeded 1000 cm−3 with the highest value of 4000 cm−3. Values below 100 cm−3 were very few. Matsumoto and Uematsu (2004) reported that number densities of particles larger than 0.1 μm were <100 cm−3 over the remote western North Pacific,
Conclusions
We made continuous measurements of atmospheric aerosols over the western North Pacific and the East China Sea. The conclusions of this study are:
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The highest concentration of total nss-SO42− observed over the central East China Sea was close to the highest value in an urban atmosphere in a highly industrialized area. However, the highest concentration of total NO3− was lower than that in polluted air. This is probably because NO3− was removed from the atmosphere faster than nss-SO42− and due to
Acknowledgments
We are thankful to the principal investigator Prof. M. Kawabe, Ocean Research Institute, University of Tokyo, and to the captain and crew of the R/V Hakuho Maru for the KH02-3 Cruise. We are grateful to the two anonymous reviewers for their constructive and encouraging comments on our manuscript. We are also thankful to Dr. M. Dileep Kumar of National Institute of Oceanography of India, for providing critical comments and help in revising the manuscript.
This study was carried out under the
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