Urban Ventilation And Its Impact On Urban Heat Island And Air Quality

Urbanization is leading to higher density of population and buildings in cities, which deteriorates urban ventilation, heat island effect and air quality and increases the risk of infecting epidemic in the cities. Improving urban ventilation is an effective method to solve above urban environment and health problems. This study analyzed the influencing factors and improving measures of urban heat island intensity (UHI) and inhalable particulate matters (PM10) from the urban ventilation and meteorological parameters perspective, which was helpful for making effective control strategies of above urban environmental problems.The urban ventilation model, including urban space, the concept of urban ventilation and urban ventilation coefficient, was proposed. Mixing layer height (MLH) was the most important parameter of urban ventilation, but it is difficult to obtain. So the MLH of Changsha was computed by HYSPLIT model, and surface meteorological parameters were downloaded from famous meteorological web http://www.underground.com. The characteristics of MLH in Changsha during 2005 to 2009 and its relations to surface meteorological parameters were analyzed in this study, and prediction models of MLH were established and tested. Secondly, the characteristics of UHI and PM10 concentration in Changsha during 2008 were studied. Finally, the relations between UHI and meteorological parameters, including MLH, were investigated. And so were the relations between PM10 concentration and meteorological parameters. The effects of urban ventilation coefficients in improving urban heat island intensity and PM10 concentration were discussed.The result of this study was that (1) The MLH in Changsha displayed seasonal difference and daily variation rule. The MLH was significantly higher during daytime than during nighttime. Its variation was acute during daytime, however was steady during nighttime. The MLH was mainly affected by solar radiation and relative humidity during daytime, however only by wind speed during nighttime. The prediction model 2 and model 3 could accurately predict the MLH in different times and places. (2) The difference between urban area temperature and suburban area temperature was slight during daytime, but it was very apparent during nighttime. The average UHI in Changsha during nighttime was 2.5℃, and the UHI displayed daily variation characteristics. MLH and wind speed were meteorological factors that significantly affected the UHI. When the MLH and wind speed were higher, the UHI was lower. (3) The daily variation of PM10 concentration was a double-peak curve. The first peak was present in about 9:00, and the other was in about 19:00. However, the morning peak held for a short time. The PM10 concentration was 112.6±49.4,81.6±32.0,91.5±41.6 and 137.7±71.8μg/m3 during spring, summer, autumn and winter, respectively. The days whose PM10 concentration was higher than national daily concentration standard were 20.9%,3.3%,6.6%and 33.3%of the total days during spring, summer, autumn and winter, respectively. MLH and wind speed were the main meteorological factors that affected the PM1o concentration, and they were both negatively relative to the PM1o concentration in every season.Because the urban heat island effect and air pollution are serious in most Chinese cities, strategies should be taken to enhance mixing layer height and surface wind speed. Urban ventilation coefficient could be improved by elevating MLH and wind speed, which could effectively cut down urban heat island effect and air pollution.