北京城区二氧化碳时空分布及湍流谱特征

利用北京325 m气象塔上安装的7层CO₂涡动相关系统在2014年12月到2015年11月的观测资料，分析了北京城区不同高度上CO₂浓度、通量时空分布及湍流谱的特征。结果表明:城市CO₂浓度日变化除了冬季都呈现双峰型，冬季由于人为碳源排放的大幅增加，双峰型不明显。每层的CO₂浓度、通量都有明显的季节变化:冬季最高，春末、夏季最低。CO₂浓度整体随高度的增加而降低。北京城区是CO₂源，CO₂通量的日变化不如CO₂浓度日变化规律明显。CO₂通量在47 m以下为负，47 m以上为正。通量在140 m以下随高度的增加而增加；140m以上随高度的增加而减少。根据对CO₂时空分布的分析可知:边界层CO₂浓度、通量强烈受到碳源、下垫面植被、大气稳定度、环境温度和天气过程等因素的影响。各变量谱与Kaimal等的研究结果接近:归一化速度谱和CO₂谱在惯性子区有－2/3的斜率，在低频区与稳定度参数（Z/L）有一定的关系。这说明复杂地形的城市下垫面的湍流谱结构与平坦地形相比没有太大的实质性差异。

Spatial and Temporal Distribution of CO2 and Its Spectrum Characteristic in Beijing Urban Area

Using the turbulence data measured by the open path eddy covariance system deployed at the Beijing 325-m meteorological tower, the concentration and flux and spectrum of CO₂ at seven different height levels are calculated for the period from December 2014 to November 2015. Analysis of the results indicate that the CO₂ concentration displays diurnal variation with double peaks except in the winter. Human activities must be considered in the winter since they reduce the diurnal variation of CO₂ concentration and lead to a relatively flat pattern. At all observational heights, the CO₂ concentration and flux show obvious seasonal variations with the maximum values appearing in the winter and the minimum values occurring at the end of the spring and summer. The CO₂ concentration decreases with height as a whole. Beijing is a carbon dioxide source. Daily changes in the CO₂ flux are not as obvious as daily changes in the CO₂ concentration. The CO₂ flux is negative below 47 m and positive above 47 m. The CO₂ flux decreases with height below 140 m and increases with height above 140 m. According to the analysis of spatial and temporal distribution of CO₂, it is found that the CO₂ concentration and flux in urban boundary layer are strongly affected by surface carbon emission source, underlying surface vegetation, atmospheric stability, environment temperature and weather process and so on. The results of turbulence spectra in the present study are very close to the results of Kaimal, i.e., the normalized velocity spectrum and the CO₂ spectrum have a slope of -2/3 in the inertial subregion and they both have relationships with the stability parameter (Z/L) in the low frequency zone. This indicates that the turbulence spectra of the city with complex topography are not quite different from those with flat topography.