| Vegetation has always been an important component of the urban landscape.Urban vegetation has important impacts on both meteorological and environmental circumstances for its great influence on radiative transfer process and surface energy balance as well as ambient temperature and humidity.Besides,urban vegetation can affect air quality through both direct and indirect ways:by enhancing dry deposition process of air pollutants,which is its direct environmental effect,thus lead to improvement of urban air quality;while increasing biogenic volatile organic compounds(BVOCs)emissions,which is its indirect environmental effect,thus lead to an increase in urban 03 concentrations.An urban forest canopy model and an vegetation dry deposition parameterization scheme are established in this paper which could fully consider the impacts of vertical structure of tree morphology on radiative transfer,surface energy balance,air temperature and humidity as well as the influences of various meteorological and vegetation characteristic factors on dry deposition process.Based on this,we developed an online coupled urban boundary-vegetation-chemistry-radiation model system and this model system is verified adequately with the adoption of multiple observation data through both online and offline ways.Using this new model system,the meteorological and environmental effects of urban vegetation are systematically analyzed and quantified in Suzhou city,which is a typical city of Yangtze River Delta,China,as a case study.The results show that:The meteorological effects of urban vegetation include:(1)the current distribution pattern of urban vegetation within Suzhou has already contributed to a reduction of 0.40C in the ambient air temperature and tree planting is in general more beneficial in cooling the ambient air temperature than grass surfacing with the same coverage.The cooling effects of trees are more notable at noontime while for grass the cooling effects are stronger at night.(2)The urban heat island intensity(UHII)drops with the increasing of vegetation coverage.With the increasing of urban vegetation coverage from 0 to 20%and then to 40%,the daily mean UHII of Suzhou is 2.2,1.5 and 0.9 C,respectively.With a tree coverage of 40%,its mitigation effect on the urban thermal environment will exceed that of removing all anthropogenic heat in terms of the daily mean temperature.(3)Urban vegetation can alter the surface energy balance through decreases in net radiation and sensitive heat fluxes,while an increase in latent heat flux.Consequently,ambient humidity also enhances with increasing tree coverage.Noontime specific humidity in Suzhou reaches 13.4,15.0 and 16.9 g·kg-1 when the tree coverage is 0,20%and 40%,respectively.(4)The impacts of vegetation on urban meteorological conditions are more notable in summer than in winter.Urban vegetation could accelerate the dry deposition velocity(Vd)of air pollutant,i.e.enhancing its removal process.The results show that:(1)among the most common vegetation species in urban circumstance,trees are more efficient at promoting the dry deposition process of air pollutants than grass,moreover,the annual mean Vd of air pollutants over coniferous trees is generally larger than that over broadleaf species.Within major gaseous pollutants,SO2 exhibits the greatest Vd,the annual mean of which is 0.43 cm·s-1 over broadleaf trees,Vd of O3(0.39 cm·s-1)is smaller than that of SO2 and Vd of NO2(0.33 cm·s-1)is the smallest among the above three gaseous species;for particles,Vd of PM10(0.50 cm·s-1)is larger than that of PM2.5(0.13 cm·s-1).(2)The introduction of urban vegetation will cause substantial enhancement of dry deposition fluxes and among which the dry deposition fluxes over coniferous trees are the largest.Compared to urban underlying surfaces,annual dry deposition fluxes of SO2,NO2,O3,PM10 and PM2.5 over coniferous trees will increase by a factor of 3.5,3.1,3.2,1.3 and 0.64,respectively.(3)Current urban vegetation within Suzhou contributed to an annual removal of PM10 at about 1484.5 t·a-1,while for SO2,NO2,O3 and PM2.5,the annual removal amount is about 257.0.386.4,811.4 and 281.7t·a-1.Annual removal of SO2,N02,O3,PM10 and PM2.5 by urban vegetation account for 48.5%,50.0%,53.7%,29.1%and 25.3%of total dry removal amount within Suzhou.Urban vegetation could affect urban air quality through both direct and indirect environmental effects.Results show that:(1)current distribution pattern of urban vegetation within Suzhou has lead to a reduction in the concentration of urban major air pollutants and this improvement in air quality is more notable in summer than in winter.The reduction in pollutant concentrations is most significant in Kunshan which is consistent with the distribution of vegetation coverage.(2)With current urban vegetation coverage remains unchanged,the conversion of urban tree species causes little difference of pollutant concentrations in summer.Whereas in winter,replacement of broadleaf species by coniferous will lead to a further reduction of major pollutant concentrations.(3)The air quality improvement increases with increased percent vegetation coverage.With a tree coverage of 40%,the average pollutant concentrations in Suzhou will reduce by 9.7%(SO2),11.6%(N02),14.0%(03),5.5%(PM10)and 4.0%(PM2.5),respectively.(4)The introduction of forest ecosystems in peri-urban aeras will also lead to corresponding improvement of urban air quality.In summer,different adoption of tree species in peri-urban aeras will cause little discrepancy of major air pollutant concentrations within urban areas except for O3.While for O3,the scheme of broadleaf forests in peri-urban areas is more efficient in lowering the urban O3 concentrations.In winter,as LAI of broadleaf drops,adoption of coniferous forests in peri-urban aeras will contribute to a more notable reduction of urban pollutants,which is 7.7%(S02),12.2%(N02),17.2%(03),8.3%(PM10)and 7.9%(PM2.5),respectively.(5)BVOC emissions from broadleaf trees are substantially stronger than coniferous.Emission of BVOCs primarily occurs in the daytime and is highly dominating during summer.The urban vegetation within Suzhou emits 3.29 kt·a-1 of BVOCs which contributes to about 3.82%of the total VOC emissions within the same area.When the total tree coverage in Suzhou reaches 40%,the BVOC emissions emitted from urban trees will result in a decrease in the daily mean NOx concentration of about 3.2%whereas an increase of in the daily mean 03 concentration of about 2.3%.The positive environmental effects of urban vegetation(i.e.its direct environmental effects)as enhanced deposition surfaces for air pollutants outweigh its negative aspects(i.e.its indirect environmental effects)as BVOC emitters. |
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