A Study On The Characteristics Of Horizontal Transport Of The Atmospheric Pollutant Over The Yangtze Delta

Transport mechanism and regional integrated control of air pollutant over urban agglomerations are the new tasks of current atmospheric environmental study. Dealing with Yangtze Delta, the Major Project of Chinese National Programs for Fundamental Research and Development(973 Program)-'Law of variation and principle of regulate operation in the quality of atmospheric and soil environment in Yangtze Delta and Pearl River Delta'investigates the pollution laws in urban agglomerations. As one component part of the subject, this study mainly simulates the transport field of the atmospheric pollutant over the Yangtze Delta with several methods, such as air mass trajectory analysis and mesoscale meteorological simulation. Combining with the analyses of API index of 2004 in some important cities, we also do some verification analysis to the simulation results. Main conclusions are as follows:(1)Analysis of API index of January, April, July and October in 2004 in representative cities showed that there was perfect regional congruence and synchronization in the pollution processes. Because of the spatial distance, pollution processes in Wuhan city were later than that in Yangtze Delta.(2)The quality of atmospheric environment changed clearly with seasons: frequency of occurrence of each air quality level was different in four seasons; variation of ranks showed the influences of seasonal wind to the air quality of downwind cities. In the four representative months of 2004, air quality of Wuhan city situated to the southeast of Yangtze Delta was the worst except in July. In winter, air quality of cities in the northern part of Yangtze Delta, such as Nanjing, Nantong, was much better than that of cities in the southern part. In spring, cities in the east had better air quality than that in the west. In summer, it's opposite to the winter time. However, in autumn, it's same to the spring time. That was to say, from east to west, the air quality went worse gradually.(3)With the analysis of atmospheric circulation and simulation of the meteorological field, we achieved the main motion systems and the changing characteristics of the dominant wind in January, April, July and October in 2004. In January, situated in the rear of low-pressure groove, Yangtze Delta was controlled by the northern one of East Asian current and northwest wind was rife. Transport currents arriving Yangtze Delta mainly came from the inland region in the west. At this time, Shanghai and Hangzhou were the downwind cities, therefore had a poor air quality. In April, with the reinforcement of anticyclone over the western Pacific, east wind was prevailing in Yangtze Delta. There is a significant transport current from southeast to northwest. Pollutants from Hangzhou and Shanghai began to influence cities situated in the northwest of Yangtze Delta, such as Nanjing.In July, with the reinforcement of subtropical high in western Pacific, south wind dominated this area. In October, anticyclone of high pressure transformation over polar begin to control the eastern part of the continent south to the 40oN line. Yangtze Delta was under the influence of north wind.(4)Analysis of backward trajectories exhibited that characteristics of transport from other places to Yangtze Delta changed clearly with seasons. From January to April, influenced by continental high, air masses arriving Yangtze Delta originated from the central continent of North Asia, via Jingjintang district and eastern coastal provinces. In April, there is another important transport pathway—southwest transport airflow. Influenced by the anticyclone over the western Pacific, the airflow went by the East Sea, Taiwan Strait, Fujian or Guangdong provinces. In July, the Yangtze Delta was mainly controlled by southwest and southeast monsoon and in October, it had the same character with January.(5)With calculation of forward trajectories of representative cities, mesoscale transport influence of discharge in Yangtze Delta to other places was obtained. The results from the investigation suggested that movements of East Asia monsoon played an important part in the mesoscale transport of pollutant in Yangtze Delta. Winter monsoon was a main mechanism which moved the air pollutant in Yangtze Delta to South China and West Pacific ocean. Another important transport system was the subtropical anticyclone over the western Pacific ocean which controlled the east coast of our country in Spring and summer. This circulation system mainly affected the inland areas of our country and Korea, Japan, and so on. (6)The 72h backward trajectories of Wuhan city confirmed that problem of pollutant input from Yangtze Delta did exist in 500m level.