The Spatial-Temporal Distribution And Transport Process Of Polycyclic Aromatic Hydrocarbons In Leaves Of Urban Typical Tree Species

Polycyclic aromatic hydrocarbons(PAHs),a group of persistent organic micro-pollutants,are widespread in the urban environment.They can pose a serious threat to the ecological environment and human health due to the potential carcinogenicity,teratogenicity and mutagenicity of some PAHs.Therefore,more and more studied from domestic and international scholars have been focused on PAHs in recent decades.As one of the typical super-large city of the Yangtze river delta,Shanghai is a celebrated economic,commercial and industrial center in China at the same time.There is an unprecedented pressure on the urban environment owing to the special geographical position and urban attribute of Shanghai.City greening plant is the core of urban eco-environmental construction.Due to the high dependency of plant on the surrounding environment,especially leaves which were rich in lipids can absorb gas-phase and particle-phase PAHs through gas diffusion,dry and wet deposition.Thus,plant leaves have been considered as the natural biological monitor to reflect the concentration levels and distribution characteristics of the organic pollutants in regional.In this study,leaf samples of Cinnamomum Camphora(camphora),Viburnum Odoratissimum(holly),Nerium Oleander(oleander)and Common Boxwood(boxwood)were collected at six sites in Minhang district of Shanghai in summer and winter.By the method of sequential extraction,leaves are divided into three fractions(foliar dust,cuticular wax and mesophyll).The contents of PAHs in these three tissues are determined simultaneously with leaf-wax content,tissue lipid,specific surface,stomata density and particulate size distribution.The results showed as follows:1)The ?16PAHs concentrations of leaves in Shanghai ranged from 225.47-6550.43 ng/g with a mean value of 1261.40 ng/g.Compared with other areas around the world,the level of total PAHs concentration collected in Shanghai is at a moderate level.Spatial variations of ?16PAHs in leaves were observed,which showed a changing trend of industrial and commercial area>traffic area>residential area>cultural and educational area or park green space.The seasonal ?16PAHs concentration order was winter>summer.2)The ?16PAHs concentrations of cuticular wax were 1103.64-5628.58 ng/g(mean value:5462.18 ng/g)for camphora leaves,863.68-3283.61 ng/g(mean value:809.53 ng/g)for holly leaves,132.61-2891.58 ng/g(mean value:880.00 ng/g)for oleander leaves and 437.73-3875.93 ng/g(mean value:1022.59 ng/g)for boxwood leaves.Overall,?16PAHs concentrations in leaf cuticular wax followed a distinct order of camphor>holly>oleander?boxwood during the summer and winter,which were consistent with the change trend of the wax content per unit area of leaves.It shows that the wax content is the principal factor to control the ?16PAHs concentrations in cuticular wax of leaves.3)The ?16PAHs concentrations of mesophyll were 253.85-921.85 ng/g(mean value:566.05 ng/g)for camphora leaves,96.36-950.16 ng/g(mean value:396.82 ng/g)for holly leaves,71.97-990.25 ng/g(mean value:457.61 ng/g)for oleander leaves and 63.74-699.36 ng/g(mean value:300.13 ng/g)for boxwood leaves.The PAHs adsorption capabilities of mesophyll were obvious difference among the species and seasons.In winter,the mesophyll?16PAHs levels showed a trend of oleander>camphor?holly>boxwood,whereas the ?16PAHs concentrations of camphor is significantly higher than holly,oleander and boxwood.4)The distribution of PAHs within leaves is very uneven.For mass concentrations alone,the ?16PAHs concentrations in cuticular wax were one or two orders of magnitude greater than those in mesophyll,indicated that the cuticular wax of leaves may accumulate PAHst more easily.However,the relative concentrations of gaseous PAHs(2-3 rings)were higher in the mesophull than in the cuticular wax in summer,while the relative concentrations of particulate PAHs(5-6 rings)is more gathered in cuticular wax.It shows that lighter PAHs were easier to migrate into the inner tissues than heavier PAHs.5)A good linear relationship between the wax contents and ?16PAHs concentrations in the cuticular wax was observed.It suggests that the wax content is the principal factor influencing the PAH levels in different species of leaves.Not only that,it also has a profound influence on the PAHs enrichment capability of intraspecies leaves in different seasons.We don't find the levels of ?16PAHs in mesophyll are closely related to the tissue lipid contents for all tree species.However,in winter,there was a significant linear positive correlation between stomata density and the ?16PAHs concentrations of mesophyll for the same plant leaves,indicated that stomata density affect the ?16PAHs concentrations in leaves of same tree species.6)Foliar dust,which has been regarded as a represent of the accumulation of particulate matter on the leaf surface,collected a certain amcount of PAHs.The ?16PAHs concentrations of foliage dust were 688.22-3578.85 ng/g(mean value:2553.37 ng/g)for camphora leaves,800.18-2484.33 ng/g(mean value:1233.60 ng/g)for holly leaves,739.01-2516.41 ng/g(mean value:1365.36 ng/g)for oleander leaves and 1553.89-3469.80 ng/g(mean value:2301.97 ng/g)for boxwood leaves.The results showed that the concentrations of ?16PAHs in foliage dust followed an obvious order of camphor>boxwood>holly>oleander,and this change trend were consistent with ?16PAHs concentrations of cuticular wax in different tree species.Due to disturbance of the external environmental,?16PAHs concentrations were no significant differences between summer and winter.7)By analyzing the correlation between the ?16PAHs concntrations in foliar dust and the composition of particle size,we found that the ?16PAHs concntrations in foliar dust is linear to the percentage of PM2.5,which were prone to accumulate gaseous PAHs,such as 3 or 4-ring PAHs.8)To further examine the migration abilities of PAHs in leaves,we evaluated the translocation factor(TF)of PAHs among different tissues.The TFf-w(foliar dust-leaf wax)range between 0.32 to 3.26,with a mean of 1.64.It indicated a strong translocation ability of PAHs from the foliar dust to the cuticular wax.However,the values of TFf-m(foliar dust-mesophyll)in most plant samples were less than 1,showed that PAHs are difficult migrated from the leaf surface to mesophyll.It further illustrated cuticular wax has a significant barrier function for the diffusion of the PAHs.9)Sources analysis showed that PAHs in leaves and foliage dust derived mainly from vehicle emissions and coal combustion.However,due to the human activities and meteorological condition varied in different periods,there are some seasonal sources.Furthermore,the results of ecological risk assessment indicated the toxicity of foliage dust varied greatly in sampling sites with the highest values of TTEF and TMEQ in industrial area both in summer and winter.But the lowest values of TTEF and TMEQ was existed in the residential area in summer,while the park green space in winter.Overall,the carcinogenic and mutagenic risk of PAHs in foliar dust was within the acceptable range.