叶片碳同位素对城市大气污染的指示作用

Variation of carbon isotopic concentrations in tree leaves as an indication of urban air quality

沿南京市市中心-郊区-城市森林的梯度,观测了近地层大气CO2(pN-S(CO2))、SO2(pN-S(SO2))、NO2(pN-S(NO2))浓度,以及公路两侧悬铃木(Platanus acerifolia)叶片重金属Pb、Cr含量、稳定碳同位素13C和放射性碳同位素14C的变化规律,探讨叶片13C和14C对城市大气污染的指示作用。结果表明,沿市中心-郊区-城市森林梯度,pN-S(CO2)、pN-S(SO2)和pN-S(NO2)均呈现下降的趋势,与观测点距市中心距离间均存在极显著的负相关关系;与城市森林相比,市中心pN-S(CO2)、pN-S(SO2)和pN-S(NO2)分别高7.3%、88%和210%。沿市中心-郊区-城市森林梯度,叶片δ13C和14C含量有升高的趋势,其中,δ13范围为-27.9‰—-31.4‰,14C pMC范围为95.8%—101.2%,即叶片C有9.3%—14.4%来源于化石燃料;叶片14C含量与距市中心距离间存在极显著的线性相关。叶片1δ3C与污染参数pN-S(CO2)、pN-S(SO2)、pN-S(NO2)、Pb、Cr间均表现为负相关,其中与pN-S(CO2)、Pb间的相关性达到显著水平;而叶片14C含量与pN-S(CO2)、pN-S(SO2)、pN-S(NO2)、Pb和Cr间均表现出显著的负相关。表明,叶片14C含量可用于指示城市大气污染状况,且与叶片δ13C相比更为稳定。

The objectives of this study were to（1） explore the temporal variations of CO2,SO2 and NO2 concentrations in near-surface（2.5 m above the ground） atmosphere,termed as pN-S（CO2）,pN-S（SO2） and pN-S（NO2）,respectively,and lead（Pb）,chromium（Cr）,stable carbon 13C and radiocarbon ^14C concentrations in leaves of the tree Platanus acerifolia in environments with varying land covers and human activities,and（2） investigate whether ^13C or ^14C in plants can provide an indication of the atmospheric pollution in urban areas.A transect from the Nanjing urban center,across a suburban area to a forested area（urban forest） was selected as our study site（termed as urban-forest gradient in this paper）.pN-S（CO2） was measured with sensors mounted on a moving vehicle,and pN-S（SO2） and pN-SNO2 were measured at selected,representative,sites along the transect in summer of 2008.Meanwhile,leaves of Platanus acerifolia along the transect were sampled,and Pb,Cr,^13C,^14C concentrations were measured.Results showed that pN-S（CO2）,pN-S（SO2） and pN-S（NO2） were significantly negatively correlated with the distance to the urban center,with linear R^2 values between 0.84 and 0.97.pN-S（CO2）,pN-S（SO2） and pN-S（NO2） increased by as much as 7.3%,88% and 210%,respectively,from the urban forested area to the urban center.Heavy metal concentrations of Pb and Cr in leaves also were significantly negatively correlated with the distance to the urban center,with linear R2 values of 0.89 and 0.69,respectively.However,both ^13C and ^14C concentrations in leaves decreased along the forest-urban transect from forested area to the urban center,ranging from-27.9‰ to-31.4‰ and pMC from 95.8% to 101.2%,respectively.In other words,the contribution of fossil CO2 to the assimilated carbon in Platanus acerifolia leaves ranged from 9.3% to 14.4% along the forest-urban transect.Foliar 14C concentration was significantly correlated with each of the five commonly used parameters indicating the urban air pollution in this study,i.e.pN-S（CO2）,pN-S（SO2） and pN-S（NO2） in air,and Pb and Cr concentrations in leaves,with R2 values of 0.94,0.92,0.88,0.92 and 0.58,respectively.Although there also existed negative relationships between δ^13C and the five parameters,δ^13C was significantly correlated with only two of the five parameters,viz.,pN-S（CO2） and Pb with R^2 values of 0.74 and 0.67,respectively.It suggests that instead of stable carbon,radiocarbon concentration in urban plants is a good indication of the air pollution in urban environments.