乌鲁木齐冬季大气边界层结构特征及其对大气污染的影响

Analysis on Atmospheric Boundary Layer Structure and Its Effect on Air Pollution over Urumqi City in Winter

基于2005年12月11～20日乌鲁木齐每日6次的低空1.5 km的加密气象探空观测资料,分析乌鲁木齐大气边界层的风、温、混合层高度、逆温等特征。观测期内5 d是重污染,其他时间为轻微污染,重点分析由轻微污染向重污染演变的气象条件。结果表明:观测期平均混合层高度274 m;低空逆温的出现频率明显多于接地逆温,且以逆温底高度小于100 m的频率最高;轻微污染时,傍晚没有出现接地逆温;重污染时段的风速小于轻微污染时段,且气温垂直分布恰好满足了阴雾天气形成的温度条件。冬季大气边界层内整层风速同步减小、近地面主导风向不稳定、地面气温不断升高、烟囱排烟高度以下形成不稳定层结是重污染形成的重要原因,接地逆温的再形成并稳定维持,使重污染得以长时间持续。

Effect of meteorological conditions of atmospheric boundary layer on air pollutant dispersion is significant,but there have been few studies on atmospheric boundary layer characteristics over Urumqi yet.During December 11-20,2005,the sounding experiment in urban atmospheric boundary layer over Urumqi City was performed six times a day.Atmospheric boundary layer conditions,such as wind,temperature,mixing layer depth and temperature inversion,were analyzed.The results show that wind speed near ground surface was often lower than 3 m/s,and it was increased gradually with the increase of height below 600 m.The temperature inversion of ground-based and low level-elevated was higher than 1 ℃/100 m,and the daily mean mixing layer depth was 274 m.The meteorological conditions in two durations of air pollution at severe and light levels were compared.Wind speed was lower and temperature was higher during severe air pollution than that during light air pollution.In addition,the temperature inversion intensity during light air pollution was very high（4.5 ℃/100 m）,but its inversion depth was low（166 m）.The results reveal that the severe air pollution in Urumqi was mainly caused by the simultaneous decrease of wind speed in atmospheric boundary layer,labilization of the prevailing wind direction near ground surface,continuous surface temperature increase,and formation of unstable stratification below exhaustsmoke height of chimneys.The formation and stable continuance of the thick temperature inversion resulted in a long duration of severe air pollution.