Spatial And Temporal Distribution Of CO₂ And Turbulence Characteristic In Beijing Urban Area

With the turbulence data measured by open path eddy covariance system of Beijing 325m meteorological tower,CO₂ concentration and flux and spectrum at 7different heights were calculated from December 2014 to November 2015.On the basis of data quality control and gap value filling,the temporal and spatial distribution characteristics of CO₂ concentration and flux in Beijing,including seasonal variation and annual variation,and the relationship between CO₂concentration and flux with meteorological elements were analyzed.By studying these uninterrupted and representative observation data,we can have a better understanding of the modal change of CO₂ exchange between cities and the influence of human factors on CO₂.In addition,the study of the turbulence spectrum and variance similarity of CO₂ as passive scalar is of great significance to study the turbulent transport mechanism in the urban boundary layer,and to understand,predict and mitigate the urban climate effect.The main contents and conclusions are as follows:（1）Data quality correction and quality control were carried out with EddyPro data processing and flux calculation software.Later,the 30 min turbulence results were filled with vacancies,and the observed data of 365d from December 2014 to2015 November were obtained.（2）At all observation heights,CO₂ concentration and flux have obvious diurnal variation,monthly variation and seasonal variation,accompanying with the weekend and domestic heating effects due to human activities.Beijing is a carbon dioxide source.The daily change of CO₂ flux is not as obvious as the daily change of CO₂concentration.With the change of height,the concentration and flux of CO₂ have their own changing laws,and the general formula can be synthesized.On the whole,CO₂ concentration value decreases with height as a whole.The CO₂ flux is negative under 47m and positive above 47m.CO₂ flux value decreases with height under140m and increases with height above 140m.According to the analysis of spatial and temporal distribution of CO₂,we know that CO₂ concentration and flux in urban boundary layer was strongly affected by surface carbon emission source,underlying surface vegetation,atmospheric stability,environment temperature and weather process and so on.（3）In addition to the influence of human activities,the concentration and flux of CO₂ are also related to meteorological factors such as temperature,humidity,wind and atmospheric pressure.In general,the relationship between the concentration and flux of CO₂ and the meteorological elements is basically the same:They are negatively correlated with temperature and water vapor concentration,and are positively correlated with atmospheric pressure.With the increase of wind speed,the concentration and flux of CO₂ gradually become smaller.The relationship between the CO₂ concentration and the wind direction is not obvious,and the CO₂ flux has a certain relationship with the wind direction in winter.（4）The normalized velocity spectrum and the CO₂ spectrum have a slope of-2/3in the inertial subregion and they have a relationship with the stability parameter（Z/L）in the low frequency zone.This indicates that the turbulence spectra of the city under complex topography are not quite different from those under flat topography.（5）For unstable boundary layer,the normalized standard deviation of wind speed and temperature and stability basically satisfy the local similarity relationship,but the concentration of water vapor and CO₂ do not correspond to the 1/3 law at some heights.Under stable conditions,the three dimensional wind speed is basically in accordance with the 1/3 law.The water vapor concentration and CO₂ are independent of the stability,which is a constant.The characteristics of the turbulence variance are influenced by the observed location,the observation time,the roughness of the underlying surface and so on.