盛行梯度风下热力环流对近地空气质量的影响研究

以中纬度沿海城市上海为代表,采用数值模拟方法,分析了海岸线附近污染源排放的大气污染物在盛行梯度风和热力环流耦合作用下的扩散和输送特征,并与忽略海陆温差的理想情况作了对比.结果表明,即使在盛行梯度风主导城市风场时,海陆温差引起的热力环流对海岸线附近流场仍有重要影响,并使近地面污染物浓度时空分布与海陆无温差时截然不同.海陆无温差时,污染物仅向盛行梯度风的下风向区域扩散.而在海陆有温差时,污染物的扩散却可能是双向的.陆地最高和最低气温出现的时间分别对应着沿海城市污染物最不利释放时段(RTS-16:00和RTS-04:00),造成的污染总时长和日平均浓度均最大,不仅部分近地面污染物被海陆热力环流携带至盛行梯度风的上风向区域,并且下风向区域的日平均浓度最高达海陆无温差时的4~5倍.因此,即使在盛行梯度风较强时忽略海陆温差形成的热力环流影响,也会明显低估非海陆风日的实际污染强度和污染范围.

Influence of thermal circulation on air quality near-ground under the prevailing gradient wind

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.