Carbon and PM 2.5 Reduction and Design Guidelines for Street Trees in Korea

: An increasing concentration of air pollutants, which negatively a ﬀ ect human health and living environment, present a serious environmental concern around the world. Street trees can help reduce carbon (C) and PM 2.5 in cities that lack su ﬃ cient greenspace. This study quantiﬁed C uptake and PM 2.5 deposition on street trees in the Republic of Korea and suggested sustainable design guidelines to enhance the e ﬀ ects of C and PM 2.5 reduction. The mean C uptake and the PM 2.5 deposition on street trees per unit area were 0.6 ± 0.1 t / ha / y and 2.0 ± 0.3 kg / ha / y, respectively. The major determining factors of the levels of C uptake and PM 2.5 deposition on street trees were the species, density, size, and layering structure of the planted trees. Street trees in the Republic of Korea annually o ﬀ set C and PM 2.5 emissions from vehicles by 1.4% and 180%, respectively. Based on these results, design guidelines are suggested that can contribute to sharing the value and the importance of planting street trees for the reduction of C and PM 2.5 levels in greenspaces.


Introduction
Streets are a major source of carbon dioxide (CO 2 ) and fine dusts produced during vehicle operation in urban living spaces [1]. Notably, citizens' outdoor activities are concentrated in these spaces; hence, the air quality can affect people detrimentally [2]. Airborne gaseous materials and dust that cause pollution have a negative impact on human health and the living environment [2]. CO 2 is a gas responsible for 50% of climate change [3]. The current generation is already experiencing climate change, such as abnormal temperatures and rainfall changes due to the increase in CO 2 concentration. According to the World Health Organization (WHO) [4], particulate matter is a Class 1 carcinogen; hence, it is very harmful to the human body, causing or contributing to respiratory diseases, pulmonary functional damage, and heart diseases. Notably, the number of days per year with high concentrations of particulate matter in the atmosphere has also been increasing in recent years, increasing concerns over the seriousness of the issue worldwide.
Recently, considering the seriousness of climate change, the international community has made diverse efforts such as specifying nation-specific greenhouse gas emission reduction goals, standardizing air quality monitoring equipment, controlling the emissions of air pollution matter, and reinforcing monitoring efforts in response to the increasing levels of air pollution [5]. The Republic of Korea has also established and implemented a master plan for responding to climate change, as well as comprehensive countermeasures for the control of particulate matter [6,7].

Selection of Study Cities and Characteristics
The study cities were selected by considering their regional distribution and size to quantify the C uptake and PM 2.5 of street trees in the Republic of Korea. The cities selected for the study included Seoul (the capital of the Republic of Korea, located in the northwest of country), Daejeon (in the center), Daegu (in the southwest), Chuncheon (in the northeast), and Suncheon (in the south) ( Figure 1). Seoul, Daejeon, and Daegu are either capital or metropolitan cities with a high population density, while Chuncheon and Suncheon are small and medium-sized cities with a relatively lower population density.
The total area by the study cities is approximately 243,302 ha (Seoul: 60,522 ha, Daejeon: 49,582 ha, Daegu: 79,792 ha, Chuncheon: 33,531 ha, and Suncheon: 19,875 ha). The total area is approximately 14% of the total area in the Republic of Korea [20][21][22][23][24]. The population density was 161 persons/ha in Seoul, 5-20 times higher than in Daejeon (30 persons/ha), Daegu (31 persons/ha), Chuncheon (8 persons/ha), and Suncheon (13 persons/ha). The mean annual temperature during the period of 10 years from 2012-2019 was the highest in Suncheon, located in the south of the peninsula (14.8 • C), whereas the temperature in Chuncheon was the lowest at 11.5 • C. The mean annual wind speed ranged from 1.4-2.5 m/s [20][21][22][23][24]. plotted at 400 m intervals in Seoul and Daejeon, 300 m in Daegu, and 100 m in Chuncheon and Suncheon, depending upon the size of the area occupied by the city. The points at which the circles and lines coincided were subject to sample extraction. One-lane roads or streets without sidewalks were excluded from this study. The samples were determined based on whether they contained a majority of the street tree species in the study city, the cost and time required for a field survey, and the statistical reliability of the research data.

Survey and Analysis of Tree Planting
The sample points along the streets in the study cities were visited, and a survey plot for each point was established up to 40 m on each side in length from the point (a total of 80 m) and to the building boundaries along both sidewalks in width ( Figure 1). The field-surveyed data included the species, stem diameter (at breast height of 1.2 m (dbh) for trees, at 15 cm above ground for shrubs (≤2 cm in dbh)), the height, crown width, vertical structures, and pruning status. Vertical structures were divided into single-layered and multi-layered structures. If only trees or shrubs were planted, they were classified as single-layered structures. If trees and shrubs were planted together, they were classified as multi-layered structures.
Moreover, the potential planting space that can accommodate newly planted trees without interference from utility lines and street facilities were measured. In other words, the space for additional planting of trees with a height of 3 m or more and a crown width of 2 m or more were measured. The surveyed data were used to analyze structural characteristics (e.g., the density and cover per unit area, the dbh distribution, and the importance value of species). These data were hence applied to estimate the C uptake and the PM2.5 deposition in each study street.

Estimation of C Uptake and PM2.5 Deposition
The C uptake per tree was estimated using the quantification models developed for the urban trees of each species [11,[25][26][27][28][29][30] (Table 1). The quantification models, which apply the stem diameter as the independent variable, were derived from measurements of the seasonal CO2 rate of exchange of urban trees or were based on the direct harvesting method. In the absence of a quantification model for a particular species, regression models for the same genus or group were substituted to obtain the average C estimates.

Sampling of Study Streets
A total of 236 samples were selected by applying a systematic sampling method to a 1:1000-scale aerial photograph [25]. The number of samples collected in the study cities were 50 in Seoul, 47 in Daejeon, 50 in Daegu,45 in Chuncheon, and 44 in Suncheon. Eight straight lines radiating from the center of each of the study cities were drawn in equidistant directions and looping circles were plotted at 400 m intervals in Seoul and Daejeon, 300 m in Daegu, and 100 m in Chuncheon and Suncheon, depending upon the size of the area occupied by the city. The points at which the circles and lines coincided were subject to sample extraction. One-lane roads or streets without sidewalks were excluded from this study. The samples were determined based on whether they contained a majority of the street tree species in the study city, the cost and time required for a field survey, and the statistical reliability of the research data.

Survey and Analysis of Tree Planting
The sample points along the streets in the study cities were visited, and a survey plot for each point was established up to 40 m on each side in length from the point (a total of 80 m) and to the building boundaries along both sidewalks in width ( Figure 1). The field-surveyed data included the species, stem diameter (at breast height of 1.2 m (dbh) for trees, at 15 cm above ground for shrubs (≤2 cm in dbh)), the height, crown width, vertical structures, and pruning status. Vertical structures were divided into single-layered and multi-layered structures. If only trees or shrubs were planted, they were classified as single-layered structures. If trees and shrubs were planted together, they were classified as multi-layered structures.
Moreover, the potential planting space that can accommodate newly planted trees without interference from utility lines and street facilities were measured. In other words, the space for additional planting of trees with a height of 3 m or more and a crown width of 2 m or more were measured. The surveyed data were used to analyze structural characteristics (e.g., the density and cover per unit area, the dbh distribution, and the importance value of species). These data were hence applied to estimate the C uptake and the PM 2.5 deposition in each study street.

Estimation of C Uptake and PM 2.5 Deposition
The C uptake per tree was estimated using the quantification models developed for the urban trees of each species [11,[25][26][27][28][29][30] (Table 1). The quantification models, which apply the stem diameter as the independent variable, were derived from measurements of the seasonal CO 2 rate of exchange of urban trees or were based on the direct harvesting method. In the absence of a quantification model for a particular species, regression models for the same genus or group were substituted to obtain the average C estimates. The PM 2.5 deposition per tree was computed by applying a dry deposition model [26,31], which was based on the deposition velocity, total leaf area, and resuspension ratio in consideration of the wind speed. The PM 2.5 concentration was obtained from data measured over the last three years of observations (2014-2017) in the study cities [32]. The PM 2.5 model applied in this study is as follows: Based on the estimations per tree, the mean C uptake and the PM 2.5 deposition per unit of street area were calculated per unit of crown cover. Subsequently, the average C uptake and PM 2.5 deposits per unit of street area in the study cities were applied to the total area of the street trees in the Republic of Korea to estimate the total C and PM 2.5 reduction of all the street trees.

Estimation of Emission Offset and Economic Value
Based on the annual estimates of the C and PM 2.5 emissions from vehicles in the study cities [33,34], the C and PM 2.5 reduction in the streets were calculated with respect to the C emission. Additionally, the economic value of the C uptake by street trees was estimated by taking into account the cost of C capture and storage applicable to each country's reduction [35]: an average of $116/t was considered. The economic value of PM 2.5 deposition was calculated by applying the treatment cost proposed by Murray et al. [36]. All of these values were calculated by considering an Organisation for Economic Co-operation and Development (OECD) annual inflation rate of $9074/t [37] for the period 1994-2017.

Characteristics of Study Street
The total street area in the Republic of Korea was 179,904 ha [38] and the area in which the trees were planted covered 71,962 ha. The total street area in the study cities ranged from 1328-8729 ha ( Table 2) and, of the total street area, only 25-45% was planted with street trees. According to this study, the road width excluding sidewalk in the study cities ranged from 6-36 m, with an average width being 14.2 ± 0.4 m. The sidewalk width ranged from 1-9 m, with an average width being 3.6 ± 0.1 m. The intervals at which street trees were planted ranged from 4-11 m, with an average interval being 7.3 ± 0.1 m. The standard interval for planting street trees in the Republic of Korea is 6-8 m [39] and is adjustable according to the surrounding street conditions, growth conditions of tree species, etc. Most of the street trees in the study cities were planted in 1 row and only 4% of them were planted in 2 rows.

Distribution of Street Trees
The density of the planted street trees ranged from 0.1 to 6.0 trees/100 m 2 in this study, with an average density being 1.0 ± 0.1 trees/100 m 2 (Table 3). Daejeon had the highest tree density (1.4 trees/100 m 2 ), followed by Seoul and Daegu (both with 1.0 trees/100 m 2 ), Chuncheon (0.9 trees/100 m 2 ), and Suncheon (0.8 trees/100 m 2 ). Compared with similar studies related to density of street trees, the planting density of street trees in this study was 50% lower than the density of 2.2 trees/100 m 2 (street planting interval of 3-5 m) in the street trees of Beijing [16]. The cover of the planted street trees and shrubs ranged from 0.5 to 39.0% in this study, with an average being 12.5 ± 1.4%. Daejeon had the highest tree cover (15.6 ± 1.2%), followed by Daegu (14.9 ± 1.0%), Seoul (12.1 ± 0.9%), Suncheon (11.7 ± 1.2%), and Chuncheon (8.4 ± 1.3%). The tree cover of streets in Davis (United States) and in Canada were reported to range from 14 to 22% [40,41]. Compared with similar studies related to cover of street trees, the tree cover in this study was 15.7-46.4% lower.
The dbh of the street trees averaged 21.0 ± 0.2 cm across all the streets in this study. Trees with a dbh of <20 cm accounted for 46.7% of all the planted trees, which was the highest percentage of Sustainability 2020, 12, 10414 6 of 12 trees, followed by 20-30 cm (32.1%) and >30 cm (21.2%) (Figure 2). The dbh of street trees in Italy and China were reported to range from 23.1 to 34.9 cm [15,16]. Compared to these studies, the dbh of trees obtained in this study were similar or up to 36.1% lower.
Concerning the vertical structure of planted trees in the streets of the study cities, single-layered structures (i.e., the planting of only trees or shrubs) accounted for 70.6%, whereas multi-layered structures (i.e., the planting of trees and shrubs) accounted for 29.4%. The single-layered planting might be unfavorable to a reduction of C and PM2.5 per unit area [45,46]. Concerning the ratio of planting deciduous and evergreen trees on the streets in the study cities, only deciduous trees were planted in 69.9% of the total streets, a mix of deciduous and evergreen trees were planted in 24.9%, and only evergreen trees were planted in 5.2%.

C Uptake and PM2.5 Deposition
The C uptake by planted street trees per unit area in the study cities ranged from 0.1 to 3.9 t/ha/y, with an average uptake being 0.64 ± 0.08 t/ha/y (Table 4). Daejeon, with the relatively highest rates of tree density and cover, had the highest C uptake at 0.72 ± 0.07 t/ha/y, followed by Seoul at 0.70 ± 0.08 t/ha/y, Suncheon at 0.67 ± 0.09 t/ha/y, and Daegu at 0.66 ± 0.09 t/ha/y. On the other hand, Chuncheon, which had a lower tree cover ratio than the other cities, had a C uptake of 0.46 ± 0.06 t/ha/y (the lowest). C uptake by street trees in this study was about 51% lower than the C uptake of 1.3 t/ha/y in street trees of Beijing, China [16]. This result might be attributable to differences in the size and density of the planted trees. The C uptake by planted street trees per unit of tree cover ranged from 0.11 to 2.51 kg/m 2 /y depending on the study cities, with an average uptake being 0.53 ± 0.04 kg/m 2 /y. Meanwhile, the C uptake by Italy's street trees was reported to 0.24 kg/m 2 /y [15]. The average C uptake per unit tree cover of the trees in this study was larger than that in [16] studies. A total of 52 tree species were found in the study streets, indicating a diverse composition of species. The top 10 species (based on the importance values) were Ginkgo biloba (27.3%), Platanus occidentalis (15.4%), Nandina domestica (6.9%), Chionanthus retusa (5.9%), Rhododendron spp. (5.2%), Zelkova serrata (5.1%), Prunus yedoensis (4.6%), Buxus microphylla var. koreana (3.8%), Pinus densiflora (2.2%), and Metasequoia glyptostroboides (1.9%). These are major urban landscape tree species of relatively high importance in most cities of the Republic of Korea [42][43][44].
Concerning the vertical structure of planted trees in the streets of the study cities, single-layered structures (i.e., the planting of only trees or shrubs) accounted for 70.6%, whereas multi-layered structures (i.e., the planting of trees and shrubs) accounted for 29.4%. The single-layered planting might be unfavorable to a reduction of C and PM 2.5 per unit area [45,46]. Concerning the ratio of planting deciduous and evergreen trees on the streets in the study cities, only deciduous trees were planted in 69.9% of the total streets, a mix of deciduous and evergreen trees were planted in 24.9%, and only evergreen trees were planted in 5.2%.

C Uptake and PM 2.5 Deposition
The C uptake by planted street trees per unit area in the study cities ranged from 0.1 to 3.9 t/ha/y, with an average uptake being 0.64 ± 0.08 t/ha/y (Table 4). Daejeon, with the relatively highest rates of tree density and cover, had the highest C uptake at 0.72 ± 0.07 t/ha/y, followed by Seoul at 0.70 ± 0.08 t/ha/y, Suncheon at 0.67 ± 0.09 t/ha/y, and Daegu at 0.66 ± 0.09 t/ha/y. On the other hand, Chuncheon, which had a lower tree cover ratio than the other cities, had a C uptake of 0.46 ± 0.06 t/ha/y (the lowest). C uptake by street trees in this study was about 51% lower than the C uptake of 1.3 t/ha/y in street trees of Beijing, China [16]. This result might be attributable to differences in the size and density of the planted trees. The C uptake by planted street trees per unit of tree cover ranged from 0.11 to 2.51 kg/m 2 /y depending on the study cities, with an average uptake being 0.53 ± 0.04 kg/m 2 /y. Meanwhile, the C uptake by Italy's street trees was reported to 0.24 kg/m 2 /y [15]. The average C uptake per unit tree cover of the trees in this study was larger than that in [16] studies. The PM 2.5 deposition on planted street trees per unit area in this study cities ranged from 0.1 to 16.5 kg/ha/y, with an average deposition being 2.01 ± 0.32 kg/ha/y. Suncheon, which presented a relatively high ratio of multi-layered structures and evergreen trees, presented the highest PM 2.5 deposition on planted street trees per unit area (3.03 ± 0.56 kg/ha/y), followed by Daegu (2.45 ± 0.38 kg/ha/y), Seoul (2.42 ± 0.38 kg/ha/y), Daejeon (1.36 ± 0.16 kg/ha/y), and Chuncheon (0.80 ± 0.11 kg/ha/y).
The PM 2.5 deposition per unit of tree cover of this study cities ranged from 0.40 to 13.77 g/m 2 /y, with an average deposition being 1.71 ± 0.23 g/m 2 /y. Suncheon was characterized by the largest PM 2.5 deposition (3.03 ± 0.51 g/m 2 /y), followed by Seoul (3.32 ± 0.31 g/m 2 /y), Daegu (1.71 ± 0.21 g/m 2 /y), Daejeon (0.94 ± 0.09 g/m 2 /y), and Chuncheon (0.92 ± 0.06 g/m 2 /y). There are no indications about the global levels of PM 2.5 deposition on planted street trees per unit area and per unit of tree cover. The amount of PM 2.5 deposition per unit of urban tree cover in the United States was reported to be 0.25 g/m 2 /y [31]. Compared with that value, the average amount of PM 2.5 deposition calculated in our study is remarkably higher. These results might be attributable to differences in the atmospheric concentration levels, wind velocity, etc.
The amount of PM 2.5 deposition differed between deciduous and evergreen trees. In Seoul, for example, 56 g/y of PM 2.5 per week were deposited on average on deciduous trees (dbh = 22 cm), while 7 times more PM 2.5 (399 g/y) was deposited on evergreen trees. The PM 2.5 deposition was estimated by considering the seasonal average wind velocity in each city: the PM 2.5 deposition varied depending on the wind velocity and concentration levels of each city. The levels of C uptake by and PM 2.5 deposition on street trees registered in this study can be used to quantify the reduction effect of several variables (e.g., street area and tree cover).

Emission Offset and Economic Value
Based on the results above, the street trees in the Republic of Korea C uptake a total of 46,055 t/y and PM 2.5 deposit a total of 145 t/y ( Table 5). The total C and PM 2.5 emissions from vehicle use were 3,263,960 t/y of C and 80 t/y of PM 2.5 . The street trees in the Republic of Korea offset approximately 1.4% of the C emission from vehicle use. That is, the annual C emission per vehicle is 0.5 t/y [33], and the street trees in the Republic of Korea played a significant role in offsetting the C emissions from approximately 92,110 vehicles. PM 2.5 deposition of the street trees related to vehicle use was found to be about 181% larger than the amount that was emitted.
If economic value were applied to the total C uptake and PM 2.5 deposition of street trees in the Republic of Korea, their annual reduction effects would be calculated to be $5,996,423/y for C and $1,467,086/y for PM 2.5 . The sum of these annual values is $7,463,509/y: 14% of the annual planting budget for street trees in the Republic of Korea [43]. In the Republic of Korea, only 40% of the total street area is lined with trees. Although additional street trees are being planted every year (1932 ha/y) [43], they cover only 1% of total street area in the Republic of Korea. In order to improve the effects of C and PM 2.5 reduction, active tree planting is required in sections without street trees, as well as in potential planting spaces.

Design Guidelines
The planting structure of street trees included in the study cities was analyzed, and problems related to C and PM 2.5 reduction were found to be mainly due to the following reasons: a single-layered structure, low density, limited growing space, insufficient composition of tree species, and poor management such as excessive pruning (Figure 3).

PM2.5 offset (%)
Value ($/y) If economic value were applied to the total C uptake and PM2.5 deposition of street trees in the Republic of Korea, their annual reduction effects would be calculated to be $5,996,423/y for C and $1,467,086/y for PM2.5. The sum of these annual values is $7,463,509/y: 14% of the annual planting budget for street trees in the Republic of Korea [43]. In the Republic of Korea, only 40% of the total street area is lined with trees. Although additional street trees are being planted every year (1932 ha/y) [43], they cover only 1% of total street area in the Republic of Korea. In order to improve the effects of C and PM2.5 reduction, active tree planting is required in sections without street trees, as well as in potential planting spaces.

Design Guidelines
The planting structure of street trees included in the study cities was analyzed, and problems related to C and PM2.5 reduction were found to be mainly due to the following reasons: a singlelayered structure, low density, limited growing space, insufficient composition of tree species, and poor management such as excessive pruning (Figure 3). Within the limited space available for street tree planting, it is desirable to form multi-layered structures of high-density where possible to improve the reduction of C and PM2.5 ( Figure 4). According to previous studies [45,46], it is reported that multi-layered structures are more advantageous in terms of C uptake capacity than single-layered structures. In this study also, the C uptake capacity of multi-layered structures was 1.1-1.2 times higher than single-layered structures. Thus, planting small trees at low density should be avoided and planting in multi-layered structures preferred instead. According to this study, the C uptake of deciduous trees is higher than that of evergreen trees, whereas the PM2.5 deposition is higher for evergreen than for deciduous trees. To increase effects of C and PM2.5 reduction, mixed planting of deciduous and evergreen trees is advisable. However, in the streets examined in this study, only deciduous trees were planted in approximately 70% of the section. In addition, it is recommended that larger trees be planted, rather than small trees, as well as tree species with good growth rates such as Zelkova serrata, Prunus yedoensis, Pinus koraiensis, and Pinus densiflora [45]. Within the limited space available for street tree planting, it is desirable to form multi-layered structures of high-density where possible to improve the reduction of C and PM 2.5 ( Figure 4). According to previous studies [45,46], it is reported that multi-layered structures are more advantageous in terms of C uptake capacity than single-layered structures. In this study also, the C uptake capacity of multi-layered structures was 1.1-1.2 times higher than single-layered structures. Thus, planting small trees at low density should be avoided and planting in multi-layered structures preferred instead. According to this study, the C uptake of deciduous trees is higher than that of evergreen trees, whereas the PM 2.5 deposition is higher for evergreen than for deciduous trees. To increase effects of C and PM 2.5 reduction, mixed planting of deciduous and evergreen trees is advisable. However, in the streets examined in this study, only deciduous trees were planted in approximately 70% of the section. In addition, it is recommended that larger trees be planted, rather than small trees, as well as tree species with good growth rates such as Zelkova serrata, Prunus yedoensis, Pinus koraiensis, and Pinus densiflora [45].
Moreover, tree planting space for tree growth should be expanded. As a result of this study, many streets were found with insufficient tree planting or with wide planting intervals that require supplementary tree planting. The potential planting space in the study cities was at a total of 12,418 ha. If these were actively planted with trees and shrubs, approximately 8457 t/y of C could be absorbed while additionally depositing 3 t/y of PM 2.5 . If the growth of a tree is poor, its C uptake and PM 2.5 deposition also becomes unfavorable; thus, it is necessary to provide good soil for growth and appropriate post-planting care [44]. Regardless of the tree size, the streets included in this study mostly had one tree in a 1-1.2 m 2 tree planting space. Because the roots of street trees spread widely (as opposed to deep into the ground), securing sufficient width (rather than depth) is more important for tree growth [44]. Therefore, normal growth can be promoted by forming elongated planting areas in the form of a belt. tree pruning may further reduce the already insufficient urban plant leaf area and undermine normal growth and functioning [3]. To ensure the normal growth and proper functioning of street trees, it is desirable to avoid excessive pruning and involve specialists in the process. In addition, existing overhanging utility lines could be placed underground to provide further space for tree growth and to minimize disruption of the streetscape. These design guidelines could help improve C and PM2.5 reduction, microclimate amelioration, rainfall interception, and more.
.  Furthermore, by road diet, unnecessary road lanes could be reduced while expanding tree planting to create additional tree areas. Where it is not possible to plant street trees, it will be necessary to increase the tree biomass per unit area by employing artificial ground greening (such as on walls and rooftops). Tree pruning was conducted for reasons such as overhead utility line protection and to prevent the visual disturbance of commercial signs in the study cities. Severe tree pruning to protect utility lines was found at approximately 76% of all the study city streets. Excessive tree pruning may further reduce the already insufficient urban plant leaf area and undermine normal growth and functioning [3]. To ensure the normal growth and proper functioning of street trees, it is desirable to avoid excessive pruning and involve specialists in the process. In addition, existing overhanging utility lines could be placed underground to provide further space for tree growth and to minimize disruption of the streetscape. These design guidelines could help improve C and PM 2.5 reduction, microclimate amelioration, rainfall interception, and more.

Conclusions
Street trees can help reduce C and PM 2.5 in cities that lack sufficient greenspace as well as play a key role in enhancing diverse values. This study concerned the quantification of C and PM 2.5 reduction conducted thus far; planting design for enhancing the diversity of street trees has not yet been reported. This study quantified C uptake and PM 2.5 deposition on street trees in the Republic of Korea and suggested sustainable design guidelines to enhance effects of C and PM 2.5 reduction.
The density and cover of street trees in the study cities averaged approximately 1.0 trees/100 m 2 and 12.5%, respectively. C uptake and PM 2.5 deposition per unit area were 0.6 t/ha/y and 2.0 kg/ha/y, respectively. This result might be attributable to differences in the size and density, species, and layer structure of the planted trees. The planted trees across all streets in the Republic of Korea played a significant role in offsetting approximately 1.4% of C emissions from vehicle use and deposited 181% of the PM 2.5 emissions. The economic value of C and PM 2.5 reduction amounted to $7,463,509/y, which is approximately 14% of the annual planting budget for street trees in the Republic of Korea.
The street trees in the study cities were limited in optimizing C and PM 2.5 reduction because of a single-layered structure, low density, limited growing space, insufficient composition of tree species, and poor management such as excessive pruning. To enhance effects of C and PM 2.5 reduction in the street trees, this study proposed design guidelines including multi-layered structures of high-density, the mixed planting of deciduous and evergreen trees with a good growth rate, active tree planting in the potential planting spaces, the securing of ground and space for plant growth (belt-type planting space), road diet, artificial ground greening, and avoiding excessive tree pruning.
Most cities in the Republic of Korea only record the species and number of planted street trees, and data on important variables for assessing the diversified values of street trees such as planted tree density or size are not available. In the future, it is necessary to advance the open data system so that big data on relevant items that are lacking can be easily used by experts and the general public. This study contributes to expanding this fundamental information, which supplements the information gathered during our on-site investigation. The indicator of the amount of C uptake and PM 2.5 deposition is useful in quantifying the role of street trees in offsetting C and PM 2.5 . Any subsequent study would need to establish a C uptake and PM 2.5 deposition indicator according to the type of greenspace (such as parks and gardens), then compare it with the case of street trees, and establish design guidelines for each type of greenspace for improved effects of C and PM 2.5 reduction.