Characteristics of CO2 Concentration and Flux in Beijing Urban Area

As one of the most important green house gas discharged by human daily activities, CO2 concentration in urban area showed a significant augment tendency in recent years. While more than 80% of the world's CO2 emissions were emitted by urban areas, which account for less than 2.4% of the total land areas. The issues of the impact of anthropogenic carbon emissions on carbon balance, and the relationship between urbanization development and the change of atmospheric environment, have become the focus of carbon research. Under this background, the "Strategic Priority Research Program - Climate Change: Carbon Budget and Relevant Issues" of the Chinese Academy of Sciences has been implemented.

With the turbulence data measured by open path eddy covariance system of Beijing 325m meteorological tower, CO2, H2O, and wind field data of 10 Hz at 7 different heights were measured from February 2012 to now. As the urban surface structure is too complex to adapt EC's theoretical assumption, we have to make a series of corrections to obtain rather accurate and representative fluxes data to assure the validity and authenticity of study result. EDIRE and program developed by ourselves were used for flux data processing. The previous one was approval by Global FLUXNET and latter one was proved to have a good performance in data processing.

According to the analysis of data from 2012 to 2015, the results indicate that: CO2 yearly averaged concentration value decreases with height, and has an inverse relation with temperature. Its maximum value appears in winter by reason of vegetation withering, city heating and enhanced inversion; and the minimum value appears in summer due to the influence of strong convection system and vegetation carbon sequestration. Overall, CO2 concentration shows a generally uniform vertical distribution with about 10% difference between 8m and 280m level in summer and 6.8% in winter respectively.

At all observation heights, the diurnal variation of CO2 concentration displayed a very clear cycle with double peaks corresponding to city morning and evening transportation rush time. City heating must be considered in winter as it reduced the variation of concentration and lead to a relatively flat diurnal variation.

Take many factors into consideration, we can conclude that in the area close to ground, CO2 concentration was more strongly influenced by urban heating, vehicles transportations, surface vegetation distribution, etc. Its minimum value appears in summer and maximum in winter no matter at daytime or nighttime. While the other levels' concentrations are more affected by thermal convection or weather process, their extreme values appear in spring and winter respectively.

The vertical distribution type of CO2 concentration was different with season and certain time of day. Combination of nature and artificial factors, CO2 concentration in urban boundary layer was strongly affected by surface carbon emission source, underlying surface vegetation, atmospheric stability and weather process.