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 CO2 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 (TN-S),CO2 concentration (pN-S(CO2)),humidity,SO2 concentration (pN-S(SO2)) and NO2 concentration (pN-S(NO2)) 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 TN-S,pN-S(CO2) and atmospheric humidity,were measured with sensors mounted on a vehicle,and pN-S(SO2) and pN-S(NO2) were measured at selected,representative,sites along the transect in summer of 2008. Results showed that TN-S,pN-S(CO2),pN-S(SO2) and pN-S(NO2) 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 (RHN-S) and the distance to the urban center,with a R2 of 0.88. The urban center had consistently and significantly higher TN-S,pN-S(CO2),pN-S(SO2) and pN-S(NO2) throughout a day than those at urban forest site. The RHN-S generally increased from urban center to urban forested area most time in a day except at 5:006:00 in the morning when the humidity nearly reached to saturation. The largest difference in TN-S between the urban center and forested area were observed at 17:0018:00 (4.7℃),while smallest at 5:006:00 (1.3℃). Similarly,TN-S differed by 3.7℃(daytime) and 2.1℃ (nighttime). However,there is an asynchronce in pN-S(CO2):the maximum difference occurred at 13:0014:00 (66μmol/mol) and minimum difference at 1:002:00 (7.0μmol/mol). On an average,urban center pN-S(CO2) 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 (AHN-S) was found between the urban and forested areas,daily average RHN-S differed by as much as 7%. The largest difference in RHN-S between the urban center and urban forested area were observed at 13:0014:00 (15.5%). The near-surface atmospheric pollutants increased by as much as 88% (pN-S(SO2)) and 210% (pN-S(NO2)) 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).