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Hiromasa Ueda, Shigeki Mitsumoto, and Hidemi Kurita

Abstract

Flow mechanism causing nighttime smog was investigated by analyzing 1) continuous records of meteorological data and concentration of Oxidants (Ox) for 15 days and 2) aircraft data along the transportation route of a polluted air mass.

It was found that an air mass which passed over the large emission sources along the coastline was transported inland as a sea breeze in the form of a gravity current. A high concentration layer was created in the upper part of the gravity current. It descended at the rear edge of a gravity-current head due to the internal circulation within the head, thus yielding the highest concentration of Ox near the ground there.

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Hidemi Kurita, Hiromasa Ueda, and Shigeki Mitsumoto

Abstract

The meteorological structure and transport mechanism of long-range transport of air pollutants from the coastal region to the mountainous inland region were investigated using joint field observation data. The observations were conducted during 26–30 July 1983 over a wide area from the coastal region around Tokyo Bay to the mountainous region located in central Japan.

Air, polluted around the Tokyo Bay area during the early morning remained there until noon, being transformed by photochemical reactions. It was then transported inland, at first by the extended sea breeze and thereafter by the large-scale wind system, toward the thermal low generated in the mountainous central region. At about 1600 Japan Standard Time (JST), polluted air penetrated into the mountainous region, and after sunset it drained down along the valley in the form of a gravity current. The path of the polluted air was in good agreement with the air trajectory started Tokyo at an altitude of 100 m, calculated from pilot-balloon data. Its transport speed reached a maximum value (7.5 m s−1) at about 1500 JST, when the thermal low was most fully developed.

Daytime temperatures over the mountainous central region (Saku) were higher than those over the Kanto Plain (Takasaki), and the thermal effects of the mountainous central region (average height; 1200 m) extended to more than 3000 m in height. The vertical temperature profiles at these locations showed a linear variation in the daytime. At the 700 m level, the temperature difference between these locations attained 3.6°C when the thermal low was most fully developed. The pressure difference between the mountainous central region (Matsumoto) and the Kanto Plain (Kumagaya) was calculated using the vertical temperature profiles observed at 1500 JST. The pressure difference between these locations decreased with altitude and became almost zero at an altitude of 1900 m, indicating the vertical extent of the thermal low. From these results it became obvious that the creation of the thermal low and the wind blowing toward the thermal low was due to the difference in temperature between the air above the plain and that on the plateau.

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Hidemi Kurita, Kazutoshi Sasaki, Hisao Muroga, Hiromasa Ueda, and Shinji Wakamatsu

Abstract

The long-range transport of air pollution on clew days under light gradient wind conditions is investigated from an analysis of all days with high oxidant concentrations in 1979 at locations in central Japan that are far from pollutant sources. Surface-level wind and pressure distributions over a 300×300 km area were analyzed, together with concentration isopleths of oxidants and suspended particles produced by photochemical reactions.

It was found that the transport mechanism consists of. 1) land/sea breezes; 2) a steady onshore wind driven by the diurnal-mean land–sea temperature difference; 3) the generation of a strong thermal low in the inland mountainous region in the daytime; and 4) a subsidence inversion accompanied by a synoptic-scale high pressure system. The last three mechanisms work to combine land/sea breezes and slope and valley winds into one large-scale high-speed wind field that transports pollutants a long distance inland into the mountainous region.

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