夏季南京市中心-郊区-城市森林梯度上的近地层大气特征

运用定点观测法和车载传感器的流动观测法,研究了夏季沿南京市市中心-郊区-城市森林梯度的近地层大气温度(T_(N-S))、CO_2浓度(p_(N-S)(CO_2))、湿度和污染气体浓度的变化规律.结果表明,沿市中心-郊区-城市森林梯度,T_(N-S)、p_(N-S)(CO_2)、相对湿度(RH_(N-S))和污染物浓度(p_(N-S)(SO_2)和p_(N-S)(NO_2))均表现为规律性的变化,与观测点距市中心的距离间的关系可用logistic方程进行较好的拟合(R~2在0.71～0.90之间).与城市森林(S9)相比,市中心(S1)的T_(N-S)和p_(N-S)(CO_2)在一天中的任何时段均有不同程度的升高,其中,T_(N-S)升高幅度在5:00～6:00时段最小,而在17:00～18:00时段最大,分别为1.3℃和4.7℃,白天和夜间平均分别升高了3.7℃和2.1℃;而p_(N-S)(CO_2)升高幅度在1:00～2:00时段最小,在13:00～14:00时段最大,分别为7.0μmol/mol和66μmol/mol,白天和夜间平均分别升高了55μmol/mol和20μmol/mol.S1和S9点间,绝对湿度(AH_(N-S))的全天平均值无显著差异;而RH_(N-S)除了5:00～6:00时段两点均接近饱和外,一天中S9点均高于S1点,两点最大差值出现在13:00～14:00时段,S1和S9分别为37.4%和52.9%,全天平均升高了7.0%.与S9点相比,S1点的p_(N-S)(SO_2)和p_(N-S)(NO_2)在上午10:00～11:40间分别升高了0.88倍和2.1倍.表明,当前城市的一些环境因子如T_(N-S)和p_(N-S)(CO_2)相当于全球数十年或更久以后的水平.

Near-surface atmospheric properties along an urban center-suburban-urban forest gradient in summer in Nanjing City

Because of differences in land covers and human activities, urban areas have significant different near-surface atmospheric conditions compared to suburban, rural or forested areas. Numerous studies documented the heat island phenomenon and air pollutions in urban areas but few researches focused on atmospheric compositions that are related to global climatic change such as atmospheric CO_2 along urban-forest gradients. The objectives of this study were to (1) explore the temporal and spatial variations of near-surface (2.5 m above the ground) atmospheric temperature (T_(N-S)), CO_2 concentration (p_(N-S)(CO_2)), humidity, SO_2 concentration (p_(N-S)(SO_2)) and NO_2 concentration (p_(N-S)(NO_2)) in an urban to urban forest environment with varying land covers and human activities, and (2) further investigate whether urban environment can mimic the future global climate condition. A transect from Nanjing urban center, across a suburban area to a forested area (urban forest) was selected as our study site (termed as urban-forest gradient or urban center-suburban-urban forest gradient in this paper). Atmospheric properties, including T_(N-S), p_(N-S)(CO_2) and atmospheric humidity, were measured with sensors mounted on a vehicle, and p_(N-S)(SO_2) and p_(N-S)(NO_2) were measured at selected, representative, sites along the transect in summer of 2008. Results showed that T_(N-S), p_(N-S)(CO_2), p_(N-S)(SO_2) and p_(N-S)(NO_2) increased significantly along the forest-urban transect from forested area to the urban center. This observed variation in atmospheric properties may mimic a temporal change of the global environment from current to future conditions. Furthermore, a significant correlation was found between the daily average relative humidity (RH_(N-S)) and the distance to the urban center, with a R~2 of 0.88. The urban center had consistently and significantly higher T_(N-S), p_(N-S)(CO_2), p_(N-S)(SO_2) and p_(N-S)(NO_2) throughout a day than those at urban forest site. The RH_(N-S) generally increased from urban center to urban forested area most time in a day except at 5:00-6:00 in the morning when the humidity nearly reached to saturation. The largest difference in T_(N-S) between the urban center and forested area were observed at 17:00-18:00 (4.7℃), while smallest at 5:00-6:00 (1.3℃). Similarly, T_(N-S) differed by 3.7℃(daytime) and 2.1℃ (nighttime). However, there is an asynchronce in p_(N-S)(CO_2): the maximum difference occurred at 13:00-14:00 (66μmol/mol) and minimum difference at 1:00-2:00 (7.0μmol/mol). On an average, urban center p_(N-S)(CO_2) was 55μmol/mol higher than urban forested site during the daytime and 20μmol/mol higher at nighttime. Although no significant difference in daily average absolute humidity (AH_(N-S)) was found between the urban and forested areas, daily average RH_(N-S) differed by as much as 7%. The largest difference in RH_(N-S) between the urban center and urban forested area were observed at 13:00-14:00 (15.5%). The near-surface atmospheric pollutants increased by as much as 88% (p_(N-S)(SO_2)) and 210% (p_(N-S)(NO_2)) from urban forested area to urban center. The gradients observed in atmospheric properties seem to mimic the future changes in global climate environment as predicted by some Global Climate Models (GCM).