乌鲁木齐近30a大气稳定度和混合层高度变化特征及与空气污染的关系

Variation characteristics of atmospheric stability and mixed layer thickness and their relation to air pollution in recent 30 years in Urumqi

基于1979-2008年乌鲁木齐1天4个时次的地面常规气象观测资料,运用Pasquill-Turner稳定度分类方法和修正的罗氏法计算得出了乌鲁木齐市近30 a大气稳定度和大气混合层高度随时间序列的变化特征,分析了2001-2008年混合层高度和稳定度与空气污染的关系。结果表明:近30 a乌鲁木齐大气混合层高度随时间基本呈现缓增的趋势。混合层高度在冬季夜间最低,说明此时大气垂向平均稀释能力最差。近30 a大气稳定度以D类频率出现最高,F类次之;其日变化为夜间多稳定层结,傍晚和清晨中性层结占居主导,中午弱不稳定层结出现最多。混合层高度与API相关系数-0.95,表明大气混合层高度是影响空气质量的重要因素,为开展空气污染潜势预报提供了一个很好的预报指标。F类稳定度的出现频率与API相关系数0.80,也是一个比较好的空气污染潜势预报因子。

With rapid development of economy in Urumqi,the local air pollution situation evolves worse.Atmospheric stability and mixed layer thickness are two of the most important factors which have great effects on air quality and also are important parameters for potential prediction model of air quality.It is worthy to be researched in Urumqi city which is over such complicated terrain with three sides surrounded by hills.In this paper,on the basis of surface observation data in Urumqi at 02∶00,08∶00,14∶00,20∶00(Beijing Standard Time) from 1979 to 2008,the atmospheric stability and atmospheric mixed layer thickness(AMLT) are calculated by using the Pasquill-Turner method and the Nozaki method,respectively.Annual and monthly variation characteristics of the atmospheric stability and AMLT are analyzed.In combination with daily air pollution index(API),the relation between API and the above factors also is discussed.The results show that annual mean AMLT increased step by step in recent 30 years with range from 527 m(1987) to 657 m(2002),which demonstrated that atmospheric dilution ability correspondingly went up.The distribution of monthly AMLT showed that summer values of AMLT were generally higher than winter's and the maximum of 803 m came out in June while the minimum of 348 m occurred in January.Diurnal variations of AMLT in four seasons showed the same phase with the maximums appearing in the afternoon or late afternoon and the lowest values occurring at 08:00.However,the lowest value of AMLT in winter generally occurred at 02:00 with value about 290 m,which further illuminated the reason why air quality was poor in winter.In terms of stability classification of Pasquill-Turner such as A,B,C,D,E,F which represents extreme instability,instability,slight instability,neutral stratification,slight stability and stability,respectively,the highest occurrence frequency of atmospheric stability is the D category(45.1%) and the F category(23.2%) ranked the second in the past 30 years.By comparison,the A category only accounted for very low proportion(less than 4.2%) in May,Jun and July while the D category distributeed relatively mean in 12 months.In general,the F accounted for over 20% in winter-half year,especially in January over than 40%.At the same time,the stable stratification dominated at night(11% for the F category) with neutral stratification in early morning and later afternoon(15.4% at 20:00 for the D category) while light instable stratification mainly occurred at noon with high value of 11.1% for the F category.The correlation coefficient between monthly mean AMLT and air pollution index(API) during 2001-2008 is-0.95 which passes the significance level of 0.001.Furthermore,the correlation coefficient between atmospheric stability of the F category and API is 0.80.The above better correlation revealed that both AMLT and atmospheric stability all had important effects on air pollution and are relative better factors for air pollution potential forecast.