城市微环境PM2.5浓度空间分异特征分析

以长沙市主城区为例，在203个地面点通过加密观测并获取PM_2.5浓度小时观测值，辅以同步常规稀疏国控点PM_2.5浓度观测数据，在点、面尺度对比分析加密、稀疏两种观测模式下城市微环境PM_2.5浓度空间分布的特征差异.结果表明：地面加密观测模式下PM_2.5浓度高值区主要集中在道路、地表扬尘、住宅小区、医院和工业园等人群、车辆活动的微环境场景；低值区主要出现在公园景区等高植被覆盖度区域.同一空间点位，地面加密观测PM_2.5浓度值均高于常规稀疏国控点PM_2.5浓度观测值，平均高出29.71μg/m³.反距离权重空间插值制图揭示地面加密观测模式下的PM_2.5浓度呈现明显的西北部高（>75μg/m³）、中部和南部居中（65~75μg/m³）、东部低（<55μg/m³）的三级阶梯式异质特征，剖面分析各向波动较大.相比，稀疏国控观测模式空间分布图仅能反映主城区PM_2.5浓度整体较低（<55μg/m³）、除北-南向之外各向剖面PM_2.5浓度相对无明显变化的格局.与此同时，稀疏国控观测模式在地面加密观测点估算的PM_2.5浓度同样显著低于实际观测值，所揭示的研究区高值PM_2.5浓度微环境为道路、地表扬尘、汽车站.研究结果证实，出于环境保护目标建立的空气质量国控监测点难以精确反映同点位近地面PM_2.5浓度，所识别的城市高低PM_2.5浓度值微环境与真实情景存在偏差，空气质量越优等级下偏差越大.

Spatial heterogeneity analysis of PM2.5 concentrations in intra-urban microenvironments

Combing the observed hourly PM_2.5 concentrations from 10 regular sites of national air quality monitoring network (sparse observation mode) and 203 ground portable air quality monitors (dense observation mode),we analyzed the micro-environmental distribution characteristic of PM_2.5 concentrations from point and area perspectives in the downtown area of Changsha city.Results showed that,under dense observation mode,relatively high PM_2.5 concentrations appeared in areas with intensive human and vehicles activities,such as road intersections,construction sites,residential districts,hospitals and industrial zones,while relatively low concentrations mainly happened in scenic regions with large vegetation coverage.More rarely differences of PM_2.5 concentrations at same location did exist under dense and sparse observation modes.Meanwhile,inverse distance weighting based spatial interpolated map of PM_2.5 concentrations from dense observation mode revealed that obviously heterogeneous characteristics of PM_2.5 variations were marked with the highest values (>75μg/m³) in northwest part,moderate values (65~75μg/m³) in the central south part,and the lowest ones (<55μg/m³) in the east part of the study area.These characteristics were furtherly demonstrated by the significant spatial anisotropy from directional profile analysis.Inversely,the interpolated PM_2.5 distribution map under sparse observation mode cannot reflect this inherent heterogeneity,with overall PM_2.5 concentration lower than 55μg/m³.The PM_2.5 concentrations at dense observation sites extracted from the sparsely interpolated distribution map were clearly lower than the real values,while the higher PM_2.5 concentrations only appeared in the road intersections,construction sites and bus stations.Results suggest that the PM_2.5 concentrations from the nationally regular sites established for protecting environmental was difficult to reflect the ground real PM_2.5 values at the same locations.The microenvironments in intra-urban area with higher PM_2.5 concentrations identified under sparse and dense observation modes are different.The greater concentration deviations of these microenvironments generally occurred with relatively good air quality.