Effects Of Urban Forests And Meteorological Factors On Particulate Matter Deposition

With the global climate change and rapid urbanization,urban forests play an important role in regulating climate and environment,especially in alleviating pollution caused by atmospheric particulate matters which drew attention in recent years.However,previous studies on the related aspects mainly focused on the unilateral and single scale effects,and lack oflacking comprehensive research on the effects of urban forest on dry and wet deposition of atmospheric particulate matters under different scales combined with climate meteorological factors.Based on stand scale and regional scale,this study analyzed the effects of urban forest and meteorological factors on dry and wet deposition of on atmospheric particulate matters and its mechanism.At the stand scale,studies have carried on microclimate and deposition(dry deposition and wet deposition)on urban plantation in Harbin from Jan.2018 to Dec.2019.At the regional scale,on the basis of dry-wet evolution calculated by Standardized Precipitation Evapotranspiration Index in Heilongjiang province,the capacities of particulate matters(PM2.5,PM 10)deposition on urban forests were estimated by i-Tree model in 10 cities in Heilongjiang Province from 2015 to 2019.Results showed:(1)The microclimate characteristics of urban plantations showed different degrees of differencesin different forest types,inside and outside of the forests and different sites within the forests.Relative humidity and photosynthetically active radiation were similar in different urban plantations,but other factors such as wind speed,temperature and precipitation showed differently.The average velocity in Larix gmelinii plantation was higher than in Fraxinus mandshurica Rupr.plantation.Temperature in Larix gmelinii plantation was relatively lower in warm period but higher in cold period.Throughfall volumes in urban plantation were related to rainfall intensity:the throughfall in broad-leaved plantations was more than in coniferous plantations with great rainfall intensity,but less when rainfall intensity was weak.The differences in microclimate characteristics of urban plantations,such as temperature,wind speed and precipitation,directly affect the deposition of atmospheric particles(dry deposition and wet deposition)by urban plantations.The differences of main microclimate characteristics between inside and outside urban plantations indicated:Temperature in forests was lower than outside from July to September,but it turned to be higher than outside from October to November.Relative moisture was higher than outside from July to November,but the difference decreased after October.Photosynthetically active radiation in forests was lower than outside,but the difference decreased after October.Wind velocity was higher outside than in forests mostly.The deviation of meteorological factors in urban plantations was mainly reflected in the temporal and spatial distribution of throughfall.The deviation of the throughfall volume(?t,j)depended on the rainfall intensity and leaf area index.The ?t,j of different samples in the same plantation decreased when LAI increased with the maximum rainfall,but it's opposite with the minimum rainfall.In addition,with the increase of daily mean temperature,the variability of Modified throughfall volumes(MTV)increased first and then decreased,but MTV decreased when diurnal temperature range or wind velocity increased.The throughfall volumes directly determined total amount of particulate matter deposition by urban plantations.(2)Coniferous plantations was more effective in PM dry deposition,for both PM2.5 and PM10 dry deposition velocities and flux in coniferous plantations were larger than in broad-leaved plantations,which was related to the leaves smaller and more complex of coniferous tree species.PM2.5 and PM10 dry deposition velocities reached to the maximum value at noon or later,but turned to smaller in the morning or evening,which was related to wind velocity.With wind velocity increasing,PM2.5 and PM 10 dry deposition velocities increased first and then decreased.(3)Coniferous plantations was more effective in PM wet deposition if the amount of precipitation was not in consider.The wet deposition on PM(particle diameters classified by 10 ?m?2?m?0.45 ?m)metal elements by snowfall or rainfall in coniferous plantation was more effective.The metal concentrations in snowfall samples(K?Ca?Mg?Fe?Zn?Cu?Mn?Pb?Ni?Cr,mg/L)and in rainfall samples(Mg?Fe?Mn?Zn?Cu?Pb,?g/L)in urban coniferous plantations were both higher than in broad-leaved plantations.When taking account of precipitation amount,the total deposition of every metal was similar.Based on the results,it can be seen that stand factors and meteorological factors work together in the particulates deposition process;however,only in terms of plantation types,coniferous plantations have a certain comparative advantage in atmospheric particulate matter deposition in the study area.(4)At regional scale,the climate tended to be wet in winter but be dry yearly or in summer in Heilongjiang province.The weekly change of air quality was concerned with working and holiday cycles.The removal of PM2.5 and PM 10 on urban forests in Harbin was the largest among in Heilongjiang province,resulting from the higher PM concentration and larger greening area.(5)The main coupling relationships among "urban forests-meteorological factors-atmospheric particulate matters" showed:urban plantation has the function of regulating temperature,humidity and wind,and the distribution of throughfall in the plantation has spatial difference;different types of urban plantation affected the temporal and spatial variation of atmospheric particulate matters concentration and PM deposition velocities,and the coverage areas of urban forests were in direct proportion to PM deposition;the daily mean temperature increased with the increase of PM2.5 and PM 10,while the diurnal temperature range decreased with it,and the dry deposition rate of atmospheric particulate matters by urban plantation was related to wind speed.Among the relationship "meteorological factors-urban forests-atmospheric particulate matters",the three elements are related and interacted with each other,with urban forests acting as"regulators".Based on this research,it could provide references for sustainable urban forests management and dominant tree species selection,which is critical to ecological benefits of urban forests and urban forestry constructions.