Emission Characteristics Of BVOCs From Typical Urban Greening Tree Species And Their Regional Total Assessment

Biogenic volatile organic compounds（BVOCs）,an important precursor of urban ozone（O₃）and secondary organic aerosol,have a significant impact on regional air quality and climate changes.In recent years,the emissions of BVOCs have received much attention due to the more and more serious O₃ and particulate pollution in China.This thesis focused on the BVOCs emission characteristics of different types（deciduous vs evergreen species）of greening species in Nanjing and Beijing,explored the emission characteristics of BVOCs and their relationships with leaf physiological variables by measuring the leaf-level emission rate of BVOCs and gas exchange parameters of 31 urban greening tree species in situ.Moreover,based on the measured emission rate of BVOCs and the emission factors in related literatures,a detailed emission inventory of dominant tree species was established,and the emission of BVOCs from forest sources in the two cities in 2015 was estimated by using the data of forest stock volume and G93 model.Our study will provide theoretical basis and suggestions for the selection of urban greening tree species,regional emission reduction and urban air pollution control.The results are as follows:From the perspective of the emission rate,deciduous trees mainly emitted isoprene,with the average emission rate as 20.5±6.0μg·g^-1·h^-1,but monoterpenes and sesquiterpenes had a lower emission rate.Populus euramericana cv.‘74/76’and Populus deltoides cv.‘55/56’×P.deltoides cv.‘Imperial’were high isoprene-releasing trees,with the emission rates of93.1±8.3μg·g^-1·h^-1 and 91.8±10.0μg·g^-1·h^-1,respectively,followed by Broussonetia papyrifera,Sophora japonica Linn.var.japonica and Sophora japonical.The emission level of BVOCs from evergreen tree species was low,with the average emission rate of 2.3±1.3μg·g^-1·h^-1.With the exception of Elaeocarpus decipiens,all greening species were found to mainly emit monoterpenes and sesquiterpenes,of which,species from Cinnamomum camphora and Magnolia grandiflora were the higher monoterpenes emitters,with the emission rates that ranged from 8.7±1.6μg·g^-1·h^-1to 7.2±0.7μg·g^-1·h^-1.Osmanthus fragrans,Eriobotrya japonica,Ligustrum lucidum and Viburnum odoratissimum mainly emitted sesquiterpenes.From the composition of BVOCs,all greening tree species emitted 1 to 6monoterpenoids except for Platanus orientalis Linn and Viburnum odoratissimum.Monoterpenes emitted from deciduous plants were mainly limonene,ocimene andβ-pinene.The higher average contribution rates of monoterpenes emitted from evergreen plants wereα-pinene and ocimene.On the basis of gas exchange parameters（net photosynthetic rate（P_n）,stomatal conductance（g_s）,intercellular CO₂ concentration（C_i）and transpiration rate（T_r））,there were significant differences among tree species,and this difference was mainly caused by plant-specific characteristics.Among all tree species,P_n from Broussonetia Papyrifera was the highest(22.1±1.7μmol·m^-2·s^-1),and the lowest P_n was observed for Osmanthus fragrans,with the rate of 5.3±0.4μmol·m^-2·s^-1.Moreover,g_s and C_i from all tree species ranged from0.1-0.4 mol·m^-2·s^-1and 168.6-340.5μmol·mol^-1,respectively.By analyzing the relationship between each BVOCs component and gas exchange parameters,we found that no significant correlations were found between BVOC emission and Pn or C_ifor any species.However,deciduous plants showed a significant positive correlation between isoprene emissions and stomatal conductance（g_s）or transpiration rate（T_r）.Moreover,a significant positive correlation between monoterpenes and T_r was also observed in evergreen tree species.The total BVOCs emission in Nanjing were mainly derived from Slash pine,Poplar and Quercus.Among the dominant tree species in Beijing,Quercus,Poplars and Pinus tabuliformis Carr.had the highest BVOCs emissions.Both cities mainly emitted isoprene,while the emissions of monoterpenes and other VOCs were relatively low.From the total emissions,the VOCs emissions from forest sources in Beijing were 2 to 3 times those of Nanjing,which were 72114.1 t·a^-1 and 28025.3 t·a^-1,respectively.The seasonal changes of all types of BVOCs emissions were unimodal,with the highest emissions in summer and the lowest in winter,and there was little difference in emissions between spring and autumn.In Nanjing,middle-aged forests and near-mature forests were the main emission sources of BVOCs.However,middle-aged forests and young forests were the main emission sources in Beijing.The contribution to total BVOCs emissions in each tree age class was mainly determined by its leaf biomass,but the type of dominant tree species in each tree age class also affected the emission of BVOCs to a certain extent.