南京夏季城市冠层大气CO2浓度时空分布规律的观测

Direct Observation on the Temporal and Spatial Patterns of the CO_2 Concentration in the Atmospheric of Nanjing Urban Canyon in Summer

城市是CO2重要排放源,直接观测城市中大气CO2浓度对于研究人类活动对大气温室气体的贡献至关重要,而在城市中多个站点、多个高度上观测大气CO2浓度则有助于认识城市CO2浓度的时空变化规律,确定其影响机制.本研究于2014年7月18日至7月25日在南京主城区东、西、南、北和中共5个方位(100 m左右高度),2014年8月3日至2014年8月9日在南京主城区中部站点的3个高度(30、65和110 m)观测CO2浓度.结果表明:1南京主城区垂直方向上CO2浓度存在明显梯度,近地面30 m处CO2浓度受人为活动影响明显,平均值达427.3×10-6(±18.2×10-6)(摩尔分数,下同),高层65m、110 m处CO2浓度混合均匀,平均值分别为411.8×10-6(±15.0×10-6)和410.9×10-6(±14.6×10-6).大气层结越稳定,CO2浓度越高,垂直梯度越大.2南京主城区CO2浓度的水平分布受风和大气稳定度的控制.观测期间盛行东北风向,导致CO2浓度分布整体呈现西南高,东北低的格局,城市主城区上下风向CO2浓度差为7.8×10-6.而且水平风速越大,越有助于将上风向的CO2传输至城市的下风向,CO2浓度差就越小.大气层结越稳定,整体CO2浓度越高.3南京主城区5个站点CO2浓度均有明显的日变化,日最高值出现在交通早高峰期间,谷值在17:00左右,在19:00左右有时会因交通晚高峰而出现次高值.

Direct observation of urban atmospheric CO2 concentration is vital for the research in the contribution of anthropogenic activity to the atmospheric abundance since cities are important CO2 sources.The observations of the atmospheric CO2 concentration at multiple sites / heights can help us learn more about the temporal and spatial patterns and influencing mechanisms.In this study,the CO2 concentration was observed at 5 sites( east,west,south,north and middle) in the main city area of Nanjing from July 18 to 25,2014,and the vertical profile of atmospheric CO2 concentration was measured in the middle site at 3 heights( 30 m,65 m and 110m).The results indicated that: 1 An obvious vertical CO2 gradient was found,with higher CO2 concentration [molar fraction of 427.3× 10- 6( ± 18.2 × 10- 6) ]in the lower layer due to the strong influences of anthropogenic emissions,and lower CO2 concentration in the upper layers [411.8 × 10- 6( ± 15.0 × 10- 6) and 410.9 × 10- 6( ± 14.6 × 10- 6) at 65 and 110 m respectively]for the well-mixed condition.The CO2 concentration was higher and the vertical gradient was larger when the atmosphere was stable.2 The spatial distribution pattern of CO2 concentration was dominated by wind and atmospheric stability.During the observation, the CO2 concentration in the southwest was higher than that in the northeast region with the CO2 concentration difference of 7.8 × 10- 6,because the northwest wind was prevalent.And the CO2 concentration difference reduced with increasing wind speed since stronger wind diluted CO2 more efficiently.The more stable the atmosphere was,the higher the CO2 concentration was.3 An obvious diurnal variation of CO2 concentration was shown in the 5 sites.A peak value occurred during the morning rush hours,the valley value occurred around17: 00( Local time) and another high value occurred around 19: 00 because of evening rush hour sometimes.