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Abstract Four ground cover plant species of Taiyuan （Iris， Hemerocallis， Sedum， Hosta） were selected to study their capacities in adsorbing PM2.5. Meanwhile， the concentration of PM2.5 in Taiyuan between April 2017 and August 2017 was recorded， and the characteristics of PM2.5 pollution in summer and autumn in Taiyuan were studied. The results showed that for the 4 plants， the ability to adsorb PM2.5 was in the order from great to small of Hosta， Iris， Hemerocallis， Sedum， in which H. plantaginea had the best effect to adsorb PM2.5. The fresh weight and dry weight per gram of H. plantaginea were 4.4 times and 2 times higher than those of S. spectabile， while the mass quality of dust adsorption was 2.8 times higher. The sorting result was the same as the ability to adsorb PM2.5 of unit leaf area. The result of the coefficient of purification showed that the purification coefficients of 3 vegetation structure were positive， and the combination of trees and shrubs in university campus had strong PM2.5 adsorption capacity. By comparing the seasonal variation of PM2.5 concentration in Taiyuan city， it found that the PM2.5 concentration was particularly high in late spring and early summer of Taiyuan， when most of the ground cover plants were not fully grown. Therefore， ground cover plants played an important role in the construction of landscape and the regulation of ecological environment in Taiyuan.
Key words Taiyuan； Ground cover plants； PM2.5； Adsorption capacity
As an important source of urban air pollution， PM2.5 not only affects the urban transport， deteriorates urban environment， but also carries toxic substances which can cause more serious harm to human health[1-2]. As for the long-term effects of PM2.5 pollution alone on human body， the research results at home and abroad show that for every 10 μg/m3 rise of PM2.5 in the air， the human mortality rate increases by 5.37%， and the death rate due to cardiovascular diseases and respiratory diseases respectively increases by 5.91% and 2.54 %[3]. However， in recent years， the concentration of PM2.5 in urban areas still shows a relatively upward trend.
As early as 2012， the daily average concentration of PM2.5 in Taiyuan was as high as 334 μg/m3. With the continuous development of industrial processes in the city， PM2.5 has become one of the major air pollutants in Taiyuan City. It is urgent to control PM2.5. Green plants and rivers and seawater are the nemesis of fine particulate matter （PM2.5， PM10， etc.）. In the related research process， scholars at home and abroad have explored the PM2.5 absorption capacity of common garden plants， such as Ligustrum lucidum， Platanus acerifolia[4]， Taxus chinensis， Ginkgo biloba[5]， finding that these greening tree species could all improve the air quality and block the adsorption of PM2.5 to a certain degree[6]. At present， ground cover plants are important plant materials for city landscaping. The research focuses mainly on the introduction[7]， breeding[8] and adaptability[9-11]， but there are few research reports on improving the ecological environment. Therefore， in this paper， 4 species of commonly used ground cover plants of Iris， Hemerocallis， Hosta， Sedum were used to determine the effect of PM2.5 adsorption by the four plants under different environmental conditions， and the plant species with strong PM2.5 absorption capacity were screened. Moreover， the concentration of PM2.5 in Taiyuan between April 2017 and August 2017 was recorded， and the characteristics of PM2.5 pollution in summer and autumn in Taiyuan were studied， indicating that ground cover plants play an important role in the landscape construction of urban air purification of Taiyuan City.
Materials and Methods
Materials
The 4 ground cover plant species （Iris， Hosta， Hemerocallis， Sedum） which were commonly used in the landscaping and planted widely in Taiyuan City were selected to carry out the test， and 2 varieties were selected from each ground cover plant species. The morphological characteristics of the 8 ground cover plants were shown in Table 1.
Method
Points setting and leaf collection The 4 ground cover plant species were collected from 3 collection points， namely the densely populated downtown area， traffic-intensive motor vehicle lane and the tree-lined university campus. Each collection point was provided with 3 distribution points， and each located 500 m away from each other. The leaves of each ground cover plant species were collected from the 9 distribution points.
Test methods and steps First， 15-20 ml of distilled water was used to rinse the plant leaves that were to be tested to clean the adsorbed fine particles[12]. Then， the leaves of the 4 ground cover plants were cut off and put into the ziplock bags， and then 5 leaves were randomly taken from each ground cover plant species to bring back to the laboratory. Weighing method was used to determine the weight of dust absorbed leaf， leaf fresh weight， leaf dry weight， and calculation was made to get the mass of absorbed dust， leaf dust adsorption， leaf fresh weight， leaf dry weight， and calculate the mass of absorbed dust， dust absorption amount of per gram of fresh weight， dust absorption amount of per gram of dry weight， dust-retention amount per unit leaf area， cumulative mass of absorbed dust and total leaf area of the plant. PM2.5 concentration observation Taking the university campus （vegetation structure： arbor-shrub-grass）， urban downtown （vegetation structure： arbor） and motor vehicle lane （vegetation structure： arbor-grass） as the sample plots， test points were distributed among the plant communities. The concentration of PM2.5 at a distance of 0.20 m from the ground was measured at the 3 monitoring points in each distribution points using Dust mate Dust Detector in non-rainy and snowy weather， and the concentration of PM2.5 at a distance of 0.20 m from the ground outside the distribution points were measured as the control. The measured value was recorded once every minute， 10 min each time. Thereby， the purification coefficients of the 3 kinds of vegetation structures were calculated （the difference between the average PM2.5 concentration outside the distribution point and the average PM2.5 concentration within the 3 monitoring points inside the distribution point）.
Determination of PM2.5 concentration in Taiyuan The data used in this study was determined by the Dust mate Dust Detector， including the daily average of PM2.5 concentration from April to August 2017at the 5 detecting points at Jiancaoping， Xiaodian， Taoyuan， Nanzhai and Jinyuan in Taiyuan City. The 5 detecting points were located in different parts of Taiyuan City， which could represent the pollution status of PM2.5 in Taiyuan City. The daily average concentration of PM2.5 in Taiyuan City was indicated by the average daily average value of PM2.5 concentration in these 5 detecting points， thus obtaining the monthly average PM2.5 concentration in Taiyuan.
Data analysis
The measured experimental data were calculated and plotted using Excel 2010.
Results and Analysis
Comparison on the PM2.5 adsorption capacities of common ground cover plants in Taiyuan City
In this test， the PM2.5 absorption capacities of 8 ground cover plant varieties were tested in the campus of Shanxi University， the Central-East motor express way and Liuxiang in city center. The weight of dust absorbed leaf of the plant leaves （Table 2） showed that H. plantaginea L. had the strongest capacity in absorbing fine particles， which was 99.3% higher than that of Sedum spectabile， which was the weakest. The dust absorption amount of per gram of fresh weight of plant leaves showed that H. plantaginea L. still had the strongest capacity in absorbing fine particles， which was 2.2 times higher than that of I. germanica， 4 times higher than that of H. ‘Batimore orioie’. Considering the observation sites， the 4 ground cover plant species might be affected by the motor vehicle exhaust， so the capacity in absorbing fine particles in the motor way was higher than that in university campus and urban downtown. Moreover， the fine particles absorbed by the 3 ground cover plant species of Iris， Sedum， Hemerocallis in university campus were less than those in the downtown area， which may be related to the larger green area. However， Hosta absorbed more fine particles in university campus than in downtown area， and it had stronger capacity in absorbing the static fine particles in the air， which might be related with the deep gully on the leaves. In addition， in terms of the leaf area of unit plant， the 2 Hosta varieties had greater advantages. Effects of leaves from different plants on the absorption capacity of PM2.5
Histogram was plotted using the test data from the sample plots to compare the differences in the retained and absorbed PM2.5 per unit area of the 4 ground cover plant species （Fig.1）. The measured data showed that for the 4 ground cover plant species， the capacity in absorbing PM2.5 per unit area was in the order of Hosta， Iris， Hemerocallis， Sedum， and Hosta had the strongest capacity in absorbing fine particles in the motor way. The cumulative amount of absorbed PM2.5 and the average absorbed amount of PM2.5 per unit area of each plant variety were shown in Fig. 2. The average value of absorbed fine particles amount of Hosta， Iris， Hemerocallis， Sedum was 0.73， 0.60， 0.50， 0.37 mg/cm2， respectively， so the capacity in absorbing PM2.5 was the poorest in the leaves of Sedum. The cumulative values and average values of leaf area of the 4 ground cover plant species were shown in Fig. 3， and Hosta had the largest cumulative leaf area， while Sedum was the smallest. The PM2.5 adsorption capacity of plants was positively correlated with the degree of leaf surface roughness[13]. Wang et al.[14] studied the amount of detained dust per unit leaf area， finding that the plants with infolded leaves had better effects in detaining dust， while the dust detaining effect was poor for the plants with smooth or waxy leaves. Sedum leaves were waxy and smooth， while Hosta leaves had deep folds， so the absorption amount PM2.5 per unit area was the highest in Hosta， while that of Sedum was relatively smaller.
Effects of different vegetation structures on the adsorption capacity of PM2.5
In general， using difference value of PM2.5 concentrations in different plots to reflect the differences in PM2.5 purifying ability in different plots may be attributed to some errors in test data because of the differences in the concentrations of particulate matter in the sample plots and air flows. In this study， the average value of the differences between the PM2.5 concentration outside the sample plots and the concentration of PM2.5 in the 3 distribution points within the sample plots （purification coefficient） was used to represent the PM2.5 absorption capacity of the ground cover plants， avoiding the error caused by the above reasons， so the result had a certain accuracy. As shown in Fig. 4， the purification coefficients of the 3 vegetation structures were all positive. Under normal circumstances， if the average concentration of PM2.5 inside the plant community was significantly lower than that outside the plant community， then the plant community was considered to have a high dust-detaining effect[15]， indicating that the arbor-shrub-grass vegetation structure of the university campus had better effects in detaining and absorbing PM2.5， while the arbor vegetation structure in the downtown area and the arbor-grass vegetation structure in the motor way had relatively poorer effects in detaining and absorbing PM2.5. The capacities of the 3 vegetation structures in detaining and absorbing PM2.5 were in the order of arbor-shrub-grass > arbor-grass > arbor， indicating that rich plant community structure could not only improve the green coverage rate， achieving the corresponding landscape effect， but also could better purify the fine particles in the air， so as to create a better ecological environment. The results were consistent with the conclusion of Beckett et al.[16] that the plant canopy structure and the density of branches and leaves were related to the adsorption effect of atmospheric particulate matter. Significance of common ground plants in the landscape construction of Taiyuan City
In Taiyuan City， the air was dry in summer and autumn， when vehicle exhaust and dust weather contributed greater to PM2.5 in the air. The monthly average of PM2.5 concentrations in Taiyuan from April 2017 to August 2017 was shown in Fig. 5. The data in the figure showed that the monthly average PM2.5 concentration reached the highest in April 2017 in Taiyuan City， reaching 65.35 μg/m3. On the one hand， it may be related to the fact that the majority of garden plants had not well grown at this time. On the other hand， it may also due to the fact that central heating in urban areas had just ended and there were still many coal residues in the air. However， the monthly average PM2.5 concentrations in the other months were not significantly different from each other， and the air quality level maintained at a good level. Han et al.[17] measured the concentration of PM2.5 throughout the year in Taiyuan City， also believing that the concentration of PM2.5 was the lowest in summer and the pollution degree was the highest in winter， which was related to the increase of heating colas in winter.
Iris， Sedum， Hemerocallis and Hosta are the ground cover plant species most commonly used in Taiyuan， and also the important plant materials for the landscape planting in summer and autumn in Taiyuan. Wang et al.[18] studied the physical and chemical properties of the particulate matter absorbed in coniferous leaves and found that the absorption density of fine particulate matter was larger in the lower leaves than in the higher leaves， indicating that ground cover plants had greater position advantages in absorbing PM2.5. High arbor trees can block atmospheric dust， and ground cover plants can absorb the dust on the ground. In the summer and autumn of Taiyuan， the concentration of PM2.5 increased due to the dust and tail gas. However， arbors and shrubs had limited capacity in detaining and absorbing PM2.5 at low altitude. Only the low ground cover plants could effectively absorb and block the fine particulates. As a result， common ground cover plants such as Iris， Sedum， Hemerocallis and Hosta can play an important role in dust retention in summer and autumn.
Conclusion
（1） According to the data from the test plots， for the 4 commonly used ground cover plants， the capacity in absorbing PM2.5 per unit area was in the order of Hosta > Iris > Hemerocallis > Sedum， which may be related to their leaf morphological characteristics. In the comparison of the absorption effects of fine particles，Hosta also had the strongest capacity in detaining and absorbing PM2.5 among the 4 commonly used ground cover plants. （2） According to the purification coefficients of 3 kinds of vegetation structures， the vegetation structure with strong ability of detaining and adsorbing PM2.5 was the combination of arbors， shrubs and grass， while the ability of detaining and absorbing PM2.5 was poor for the vegetation structure with arbors only.
（3） In summer and autumn， the concentration of PM2.5 was the highest in April in Taiyuan. Iris， Hosta， Sedum and Hemerocallis were the most commonly used ground cover plants in summer and autumn in Taiyuan City， which were not only the important plant materials for landscaping in Taiyuan in the two seasons， but also had an important ecological role.
References
[1] WU D， LIU QH， LIANG YG， et al. Hazy weather formation and visibility deterioration resulted from fine particulate （PM2.5） pollutions in Guangdong and Hong Kong[J] .Acta Scientiae Circumstantiae， 2012，32 （11）： 2660-2669 .
[2] DONG FM， MO YZ， LI GX， et al. Association between ambient PM10/PM2.5 levels and population mortality of circulatory diseases： a case-crossover study in Beijing[J]. Journal of Peking University （Health Sciences）， 2013，45 （3）： 398-404.
[3] XU YR， WANG DX， ZHANG JW， et al. General situation for the hazards， control and evaluation standard system of PM10 and PM2.5[J]. Occupation and Health， 2013，29 （1）： 117-119.
[4] FANG Y， ZHANG JC， WANG YH. Dustfall adsorbing capacity of major species of greening trees in Nanjing and its law[J]. Journal of Ecology and Rural Environment， 2007，23 （2）： 36-40.
[5] HWANG HJ， YOOK SJ， AHN KH. Experimental investigation of submicron and ultrafine soot particle removal by tree leaves[J]. Atmospheric Environment， 2011， 45 （38）： 6987-6994.
[6] GUO W， SHENTU YJ， ZHENG SQ， et al. Research advances on the mechanisms and rules of dust retention of urban green areas[J]. Ecology and Environment， 2010，26 （6）： 1465-1470.
[7] JIN LM， CAI ZY， YAO KD. Observation on twenty new species of evergreen ground cover plants in Suzhou area of Jiangsu[J]. Jiangsu Agricultural Sciences， 2006 （1）： 87-89.
[8] HUANG SZ， GU Y， SHAN A. The hybridization of Iris spp.[J]. Journal of Plant Resources and Environment， 1997，7 （1）： 35-36.
[9] WU SP， WANG JJ， YU ZX. A study on the shade-resistance of 11 ground covers[J]. Journal of Wuhan Botanical Research， 1994，12 （4）： 360-364.
[10] PEI BH， PENG WX， ZHANG DL. Studies on shade tolerance of Pachysandra terminalls[J]. Journal of Hebei Forestry College， 1994，9 （3）： 205-209. [11] SHI AP， WANG HL， GUO R， et al. The study on indexes of adverse circumstances-resistance about Viola yedoensis Makino[J]. Journal of Beijing Agricultural College， 1997，12 （1）： 48-52.
[12] CHAI YX， ZHU N， HAN HJ. Dust removal effect of urban tree species—a case study of Harbin[J]. Chinese Journal of Applied Ecology， 2002，13 （9）： 1121-1126.
[13] YANG HB， ZOU XD， WANG HY， et al. Study progress on PM2.5 in atmospheric environment[J]. Journal of Meteorology and Environment， 2012， 28 （3）： 77-82.
[14] WANG JH， LIU YQ， ZOU M. Dust detaining effect of main urban vegetation in Yongchuan， Chongqing[J]. Chinese Journal of Environmental Engineering， 2013， 7 （3）： 1079-1084.
[15] GU L， WANG C， WANG XL， et al. Variation characteristics of fine particulate matter PM2.5 concentration in three urban recreational forest in Hui Mountain of Wuxi City， Jiangsu Province of East China[J]. Chinese Journal of Applied Ecology， 2013，24 （9）： 2485-2493.
[16] BECKETT KP， FREER-SMITH PH， TAYLOR G. The capture of particulate pollution by trees at five contrasting urban sites[J]. Arboricultural Journal， 2000， 24 （2-3）： 209-230.
[17] HAN YL， WANG F. Study on temporal and spatial variation of fine particle pollutants PM2.5 in Taiyuan and its outdoor exercise decision[J]. Bulletin of Sport Science and Technology， 2017，25 （2）： 46-47.
[18] WANG L， HASI E， LIU LY， et al. Physico-chemical characteristics of ambient particles settling upon leaf surface of six conifers in Beijing[J]. Chinese Journal of Applied Ecology， 2007，18 （3）： 487-492.
Key words Taiyuan； Ground cover plants； PM2.5； Adsorption capacity
As an important source of urban air pollution， PM2.5 not only affects the urban transport， deteriorates urban environment， but also carries toxic substances which can cause more serious harm to human health[1-2]. As for the long-term effects of PM2.5 pollution alone on human body， the research results at home and abroad show that for every 10 μg/m3 rise of PM2.5 in the air， the human mortality rate increases by 5.37%， and the death rate due to cardiovascular diseases and respiratory diseases respectively increases by 5.91% and 2.54 %[3]. However， in recent years， the concentration of PM2.5 in urban areas still shows a relatively upward trend.
As early as 2012， the daily average concentration of PM2.5 in Taiyuan was as high as 334 μg/m3. With the continuous development of industrial processes in the city， PM2.5 has become one of the major air pollutants in Taiyuan City. It is urgent to control PM2.5. Green plants and rivers and seawater are the nemesis of fine particulate matter （PM2.5， PM10， etc.）. In the related research process， scholars at home and abroad have explored the PM2.5 absorption capacity of common garden plants， such as Ligustrum lucidum， Platanus acerifolia[4]， Taxus chinensis， Ginkgo biloba[5]， finding that these greening tree species could all improve the air quality and block the adsorption of PM2.5 to a certain degree[6]. At present， ground cover plants are important plant materials for city landscaping. The research focuses mainly on the introduction[7]， breeding[8] and adaptability[9-11]， but there are few research reports on improving the ecological environment. Therefore， in this paper， 4 species of commonly used ground cover plants of Iris， Hemerocallis， Hosta， Sedum were used to determine the effect of PM2.5 adsorption by the four plants under different environmental conditions， and the plant species with strong PM2.5 absorption capacity were screened. Moreover， the concentration of PM2.5 in Taiyuan between April 2017 and August 2017 was recorded， and the characteristics of PM2.5 pollution in summer and autumn in Taiyuan were studied， indicating that ground cover plants play an important role in the landscape construction of urban air purification of Taiyuan City.
Materials and Methods
Materials
The 4 ground cover plant species （Iris， Hosta， Hemerocallis， Sedum） which were commonly used in the landscaping and planted widely in Taiyuan City were selected to carry out the test， and 2 varieties were selected from each ground cover plant species. The morphological characteristics of the 8 ground cover plants were shown in Table 1.
Method
Points setting and leaf collection The 4 ground cover plant species were collected from 3 collection points， namely the densely populated downtown area， traffic-intensive motor vehicle lane and the tree-lined university campus. Each collection point was provided with 3 distribution points， and each located 500 m away from each other. The leaves of each ground cover plant species were collected from the 9 distribution points.
Test methods and steps First， 15-20 ml of distilled water was used to rinse the plant leaves that were to be tested to clean the adsorbed fine particles[12]. Then， the leaves of the 4 ground cover plants were cut off and put into the ziplock bags， and then 5 leaves were randomly taken from each ground cover plant species to bring back to the laboratory. Weighing method was used to determine the weight of dust absorbed leaf， leaf fresh weight， leaf dry weight， and calculation was made to get the mass of absorbed dust， leaf dust adsorption， leaf fresh weight， leaf dry weight， and calculate the mass of absorbed dust， dust absorption amount of per gram of fresh weight， dust absorption amount of per gram of dry weight， dust-retention amount per unit leaf area， cumulative mass of absorbed dust and total leaf area of the plant. PM2.5 concentration observation Taking the university campus （vegetation structure： arbor-shrub-grass）， urban downtown （vegetation structure： arbor） and motor vehicle lane （vegetation structure： arbor-grass） as the sample plots， test points were distributed among the plant communities. The concentration of PM2.5 at a distance of 0.20 m from the ground was measured at the 3 monitoring points in each distribution points using Dust mate Dust Detector in non-rainy and snowy weather， and the concentration of PM2.5 at a distance of 0.20 m from the ground outside the distribution points were measured as the control. The measured value was recorded once every minute， 10 min each time. Thereby， the purification coefficients of the 3 kinds of vegetation structures were calculated （the difference between the average PM2.5 concentration outside the distribution point and the average PM2.5 concentration within the 3 monitoring points inside the distribution point）.
Determination of PM2.5 concentration in Taiyuan The data used in this study was determined by the Dust mate Dust Detector， including the daily average of PM2.5 concentration from April to August 2017at the 5 detecting points at Jiancaoping， Xiaodian， Taoyuan， Nanzhai and Jinyuan in Taiyuan City. The 5 detecting points were located in different parts of Taiyuan City， which could represent the pollution status of PM2.5 in Taiyuan City. The daily average concentration of PM2.5 in Taiyuan City was indicated by the average daily average value of PM2.5 concentration in these 5 detecting points， thus obtaining the monthly average PM2.5 concentration in Taiyuan.
Data analysis
The measured experimental data were calculated and plotted using Excel 2010.
Results and Analysis
Comparison on the PM2.5 adsorption capacities of common ground cover plants in Taiyuan City
In this test， the PM2.5 absorption capacities of 8 ground cover plant varieties were tested in the campus of Shanxi University， the Central-East motor express way and Liuxiang in city center. The weight of dust absorbed leaf of the plant leaves （Table 2） showed that H. plantaginea L. had the strongest capacity in absorbing fine particles， which was 99.3% higher than that of Sedum spectabile， which was the weakest. The dust absorption amount of per gram of fresh weight of plant leaves showed that H. plantaginea L. still had the strongest capacity in absorbing fine particles， which was 2.2 times higher than that of I. germanica， 4 times higher than that of H. ‘Batimore orioie’. Considering the observation sites， the 4 ground cover plant species might be affected by the motor vehicle exhaust， so the capacity in absorbing fine particles in the motor way was higher than that in university campus and urban downtown. Moreover， the fine particles absorbed by the 3 ground cover plant species of Iris， Sedum， Hemerocallis in university campus were less than those in the downtown area， which may be related to the larger green area. However， Hosta absorbed more fine particles in university campus than in downtown area， and it had stronger capacity in absorbing the static fine particles in the air， which might be related with the deep gully on the leaves. In addition， in terms of the leaf area of unit plant， the 2 Hosta varieties had greater advantages. Effects of leaves from different plants on the absorption capacity of PM2.5
Histogram was plotted using the test data from the sample plots to compare the differences in the retained and absorbed PM2.5 per unit area of the 4 ground cover plant species （Fig.1）. The measured data showed that for the 4 ground cover plant species， the capacity in absorbing PM2.5 per unit area was in the order of Hosta， Iris， Hemerocallis， Sedum， and Hosta had the strongest capacity in absorbing fine particles in the motor way. The cumulative amount of absorbed PM2.5 and the average absorbed amount of PM2.5 per unit area of each plant variety were shown in Fig. 2. The average value of absorbed fine particles amount of Hosta， Iris， Hemerocallis， Sedum was 0.73， 0.60， 0.50， 0.37 mg/cm2， respectively， so the capacity in absorbing PM2.5 was the poorest in the leaves of Sedum. The cumulative values and average values of leaf area of the 4 ground cover plant species were shown in Fig. 3， and Hosta had the largest cumulative leaf area， while Sedum was the smallest. The PM2.5 adsorption capacity of plants was positively correlated with the degree of leaf surface roughness[13]. Wang et al.[14] studied the amount of detained dust per unit leaf area， finding that the plants with infolded leaves had better effects in detaining dust， while the dust detaining effect was poor for the plants with smooth or waxy leaves. Sedum leaves were waxy and smooth， while Hosta leaves had deep folds， so the absorption amount PM2.5 per unit area was the highest in Hosta， while that of Sedum was relatively smaller.
Effects of different vegetation structures on the adsorption capacity of PM2.5
In general， using difference value of PM2.5 concentrations in different plots to reflect the differences in PM2.5 purifying ability in different plots may be attributed to some errors in test data because of the differences in the concentrations of particulate matter in the sample plots and air flows. In this study， the average value of the differences between the PM2.5 concentration outside the sample plots and the concentration of PM2.5 in the 3 distribution points within the sample plots （purification coefficient） was used to represent the PM2.5 absorption capacity of the ground cover plants， avoiding the error caused by the above reasons， so the result had a certain accuracy. As shown in Fig. 4， the purification coefficients of the 3 vegetation structures were all positive. Under normal circumstances， if the average concentration of PM2.5 inside the plant community was significantly lower than that outside the plant community， then the plant community was considered to have a high dust-detaining effect[15]， indicating that the arbor-shrub-grass vegetation structure of the university campus had better effects in detaining and absorbing PM2.5， while the arbor vegetation structure in the downtown area and the arbor-grass vegetation structure in the motor way had relatively poorer effects in detaining and absorbing PM2.5. The capacities of the 3 vegetation structures in detaining and absorbing PM2.5 were in the order of arbor-shrub-grass > arbor-grass > arbor， indicating that rich plant community structure could not only improve the green coverage rate， achieving the corresponding landscape effect， but also could better purify the fine particles in the air， so as to create a better ecological environment. The results were consistent with the conclusion of Beckett et al.[16] that the plant canopy structure and the density of branches and leaves were related to the adsorption effect of atmospheric particulate matter. Significance of common ground plants in the landscape construction of Taiyuan City
In Taiyuan City， the air was dry in summer and autumn， when vehicle exhaust and dust weather contributed greater to PM2.5 in the air. The monthly average of PM2.5 concentrations in Taiyuan from April 2017 to August 2017 was shown in Fig. 5. The data in the figure showed that the monthly average PM2.5 concentration reached the highest in April 2017 in Taiyuan City， reaching 65.35 μg/m3. On the one hand， it may be related to the fact that the majority of garden plants had not well grown at this time. On the other hand， it may also due to the fact that central heating in urban areas had just ended and there were still many coal residues in the air. However， the monthly average PM2.5 concentrations in the other months were not significantly different from each other， and the air quality level maintained at a good level. Han et al.[17] measured the concentration of PM2.5 throughout the year in Taiyuan City， also believing that the concentration of PM2.5 was the lowest in summer and the pollution degree was the highest in winter， which was related to the increase of heating colas in winter.
Iris， Sedum， Hemerocallis and Hosta are the ground cover plant species most commonly used in Taiyuan， and also the important plant materials for the landscape planting in summer and autumn in Taiyuan. Wang et al.[18] studied the physical and chemical properties of the particulate matter absorbed in coniferous leaves and found that the absorption density of fine particulate matter was larger in the lower leaves than in the higher leaves， indicating that ground cover plants had greater position advantages in absorbing PM2.5. High arbor trees can block atmospheric dust， and ground cover plants can absorb the dust on the ground. In the summer and autumn of Taiyuan， the concentration of PM2.5 increased due to the dust and tail gas. However， arbors and shrubs had limited capacity in detaining and absorbing PM2.5 at low altitude. Only the low ground cover plants could effectively absorb and block the fine particulates. As a result， common ground cover plants such as Iris， Sedum， Hemerocallis and Hosta can play an important role in dust retention in summer and autumn.
Conclusion
（1） According to the data from the test plots， for the 4 commonly used ground cover plants， the capacity in absorbing PM2.5 per unit area was in the order of Hosta > Iris > Hemerocallis > Sedum， which may be related to their leaf morphological characteristics. In the comparison of the absorption effects of fine particles，Hosta also had the strongest capacity in detaining and absorbing PM2.5 among the 4 commonly used ground cover plants. （2） According to the purification coefficients of 3 kinds of vegetation structures， the vegetation structure with strong ability of detaining and adsorbing PM2.5 was the combination of arbors， shrubs and grass， while the ability of detaining and absorbing PM2.5 was poor for the vegetation structure with arbors only.
（3） In summer and autumn， the concentration of PM2.5 was the highest in April in Taiyuan. Iris， Hosta， Sedum and Hemerocallis were the most commonly used ground cover plants in summer and autumn in Taiyuan City， which were not only the important plant materials for landscaping in Taiyuan in the two seasons， but also had an important ecological role.
References
[1] WU D， LIU QH， LIANG YG， et al. Hazy weather formation and visibility deterioration resulted from fine particulate （PM2.5） pollutions in Guangdong and Hong Kong[J] .Acta Scientiae Circumstantiae， 2012，32 （11）： 2660-2669 .
[2] DONG FM， MO YZ， LI GX， et al. Association between ambient PM10/PM2.5 levels and population mortality of circulatory diseases： a case-crossover study in Beijing[J]. Journal of Peking University （Health Sciences）， 2013，45 （3）： 398-404.
[3] XU YR， WANG DX， ZHANG JW， et al. General situation for the hazards， control and evaluation standard system of PM10 and PM2.5[J]. Occupation and Health， 2013，29 （1）： 117-119.
[4] FANG Y， ZHANG JC， WANG YH. Dustfall adsorbing capacity of major species of greening trees in Nanjing and its law[J]. Journal of Ecology and Rural Environment， 2007，23 （2）： 36-40.
[5] HWANG HJ， YOOK SJ， AHN KH. Experimental investigation of submicron and ultrafine soot particle removal by tree leaves[J]. Atmospheric Environment， 2011， 45 （38）： 6987-6994.
[6] GUO W， SHENTU YJ， ZHENG SQ， et al. Research advances on the mechanisms and rules of dust retention of urban green areas[J]. Ecology and Environment， 2010，26 （6）： 1465-1470.
[7] JIN LM， CAI ZY， YAO KD. Observation on twenty new species of evergreen ground cover plants in Suzhou area of Jiangsu[J]. Jiangsu Agricultural Sciences， 2006 （1）： 87-89.
[8] HUANG SZ， GU Y， SHAN A. The hybridization of Iris spp.[J]. Journal of Plant Resources and Environment， 1997，7 （1）： 35-36.
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