Washing Process Of Deposited Particulate Matter On Urban Greening Plants

Particulate matter(PM)pollution in urban cities is of great concern for public health due to its global and heavy burden of human health and ecosystems function in recent years.Vegetation control is an effective and eco-friendly way to alleviate PM pollution because greening plants can beautify the environment,improve regional climate and reduce atmospheric PM.This study focused on not only literature survey but also sampling and indoor analysis of throughfall,and its goals were to summarize the variations in PM deposition on vegetation and its influencing factors,and to reveal the relationship between precipitation factors and PM release process under wet precipitation.The results showed that:(1)The weekly leaf deposition and TSP yield in throughfall were both significantly different in seasons,averaged 1.71±0.05 g·m2·wk-1and 18.25±0.74 kg·hm'2,respectively.The weekly leaf deposition in winter was highest and averaged 3.35±0.41 g·m-2·wk-1,50.2%higher than that in summer.However,the TSP yield in throughfall was highest in summer compared to spring and winter,and PM yield in three particle sizes all showed the similar trend.(2)The washing rate of leaf PM was affected by particle size and wet precipitation characteristics.Larger particles were easy to be washed by rainfall,and the washing rates of PM0.4-3,PM3-10 and PM10-100 were respectively 15.4%,21.2%and 39.1%.With the increase of rainfall intensity and rainfall amount,the washing rate can rise to 5 1.4%under rainstorm.(3)The leaf deposition was obviously related to accumulation time.The dry deposition of PM on leaf surface will saturate on the 10th day during the sunny and windless days.Moreover,there was downwards parabola relationship between PM yield in throughdall and continuous dry days(CDD).The CDD was estimated 7-11 d when PM yield was modeled the maximum.This indicated that sufficient and continuous wet precipitation can wash off most of dry deposition on vegetation surface.(4)The PM yield was influenced by rainfall duration and accumulated rainfall,and PM concentration in throughfall was decreased with the increase of rainfall duration and accumulated rainfall,while PM yield increase on the contrary.The concentrations of PM0.4-3 and PM3-10 usually started its stagnation phase at 10th minute,when PM concentrations were respectively reduced by 73.6%and 62.0%compared to those at 1st minute.However,rainstorm can shorten the initial time of stagnation phase to 5th minute and improve washing rate.In summary,this study demonstrated the PM deposition and release on vegetation surface,which can be the theoretical basis of PM dynamic process on vegetation surface and species selection for efficient PM reduction.