西安市城市气溶胶污染岛长时间序列特征分析

Analysis of Long Time Series Characteristics of Urban Aerosol Pollution Island in Xi′an

城市气溶胶污染岛(UAPI)对人体健康、城市热环境、城市气候、大气辐射传输等有重要影响。该研究利用西安市2016—2019年城郊多个站点的PM_(2.5)数据定量化计算UAPI强度,分析PM_(2.5)和UAPI强度的日变化和年变化规律,并使用经验模态分解方法(EMD)对UAPI强度的长时间序列多尺度特征进行分析。结果表明,西安市具有明显的城市气溶胶污染岛效应,UAPI强度大于10μg·m^(-3)的比例达87.63%。在日内变化上,UAPI强度呈现夜晚高、白天低的特点;在年内变化上,呈现冬春高、夏秋低的“U”型分布特征;在年际变化上,2016—2019年UAPI效应呈逐年减弱的变化特征。利用EMD分解可以很好地揭示UAPI强度的多尺度特征,如IMF2、IMF4、IMF6分别体现了UAPI强度以周、月、年为周期的变化特征。对比PM_(2.5)和UAPI强度的时间序列,该方法可以较好地排除跨区域传输污染和大区域二次污染等对UAPI强度的影响。

Urban aerosol pollution island(UAPI)has significant impacts on human health,urban thermal environment,urban climate,atmospheric radiation transmission,etc.In this paper,the PM_(2.5) data of multiple stations in Xi′an from 2016 to 2019 were used to quantitatively calculate the intensity of UAPI,and then the daily and annual variations of PM_(2.5) and the intensity of UAPI were analyzed.Additionally,the empirical mode decomposition(EMD)method was employed to analyze the multi-scale characteristics of the long-term time series of UAPI intensity.The results show that there was obvious urban aerosol pollution island effect in Xi′an,and the proportion of UAPI intensity greater than 10μg/m^(3) reached 87.63%.In terms of intraday variations,UAPI intensity exhibited a characteristic of being higher at night and lower during the day time;in the annual variations,a"U"-shaped distribution was observed with higher values in winter and spring,and lower values in summer and autumn.On an interannual scale,the UAPI effect showed a gradual weakening trend from 2016 to 2019.The EMD decomposition can effectively reveal the multi-scale characteristics of UAPI intensity,such as IMF2,IMF4,and IMF6,which respectively reflect the characteristics of UAPI intensity with weekly,monthly,and annual cycles.By comparing PM_(2.5) time series with UAPI time series,this study can be helpful for the effective elimination of the influence of cross regional transmission pollution and large-scale secondary pollution on the intensity of UAPI.