Abstract:As a representative coastal city in mid-latitude in China, the diffusion and transportation characteristics of air pollutants emitted by pollution sources near Shanghai coastline are analyzed by numerical simulation with the coupling effect of prevailing gradient wind and thermal circulation, and compared with the ideal situation that the sea-land temperature difference (SLTD) is ignored. The results show that the thermal circulation caused by the SLTD still has an important influence on the flow field near the coastline with the condition that the prevailing gradient wind dominates the urban wind field, and the spatiotemporal distribution of pollutant concentrations near-ground is quite different from that without SLTD. Pollutants are merely dispersed to the downwind direction areas of the prevailing gradient wind without SLTD. While the SLTD exists, the diffusion of pollutants is bi-directional. The time of the highest and lowest temperature on land in coastal cities corresponds to the most unfavorable releasing moment of pollutants (RTS-16:00 and RTS-04:00), and the total contaminated time and daily averaged concentration are both the largest. Pollutants near-ground are not only carried to the upwind areas of the prevailing gradient wind, but also increase the pollution concentration in the downwind areas, up to 4to 5times that without SLTD. Therefore, even when the speed of the prevailing gradient wind is high, the actual intensity and range of pollution on non-sea-land breeze day can significantly be underestimated when neglecting the influence of the thermal circulation.
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