Three-dimensional behaviour of photochemical pollutants covering the Tokyo metropolitan area

The spatial distribution and transport process of photochemical pollutants covering the Tokyo Metropolitan Area in Japan were investigated from 31 July to 2 August 1979. In the experiment, the vertical profiles of pollutants were observed using four instrumented aircraft. This paper mainly considers the transport process of the polluted air mass using three-dimensional trajectory analysis in which the wind field was determined by objective analysis techniques from pilot-balloon observation data. In the Tokyo Metropolitan Area the sea-land breeze circulation is an important factor in photochemical oxidant formation inland when the geostrophic wind is weak. The night-time radiation inversion which persists into the early morning prevents the dispersion of primary pollutants emitted from the big coastal industrial zones around Tokyo Bay. These pollutants are then advected by the land breeze to the Sagami Bay area which acts as storage tank. Conversion of such pollutants results in a high O₃ air mass inland with the penetration of the sea breeze. The sea breeze layer is thermally stable and inhibits vertical mixing of NO_x. On the other hand, at the front of the sea breeze zone, a highly turbulent area transports the NO_x to 1000-1500 m above mean sea level. Polluted air masses, with O₃ concentration exceeding 100 ppb, were observed at 500-1000 m on both 31 July and 1 August 1979. The maximum O₃ concentration on the 2nd day exceeded that on the 1st day. These high O₃ air masses contained aged pollutant and were entrained into the mixing layer as the depth of the mixing layer increased. This accelerated the formation rate of secondary pollutants. These early morning, high O₃ concentrations, and the storage capacity of the Sagami Bay area, are important factors in the time scale of air pollution phenomena in this region.