**2. Methods**

Residential building is the most typical form of architecture in the city as some related research points out. From 2000 to 2010, area of residential land accounts for the largest proportion in urban land use type [12]. In this study, typical urban planning styles in China were selected and analyzed. Microscale residential districts in Xi'an were selected for representing the typical urban typology of residential districts that developed during different periods and used the urban simulation system scSTREAM (Software Cradle Co., 2011) to evaluate the impact of urban typology change on urban climate change.

#### **2.1. Urban typology in Xi'an city**

To estimate the impact of urban typology to urban ventilation and urban air quality, four typical residential areas were selected. The first area, Sanxuejie, is a low-rise area, a traditional residential zone rebuilt after the 1950s. It retains the urban form used in China Science the Ming and Qing Dynasties (about 600 years ago), with narrow streets between residential buildings, most of which have two floors. The second area, Xitiedaminggong, is a middle-rise neighborhood developed in the early 1990s. All of the residential buildings are five floors in height and perfectly represent the character of the building type constructed in the period between 1979 and the beginning of the 1990s. The third neighborhood, Jiaodayicun, is a mixrise area of low-rise and high-rise buildings, home to a variety of building styles developed around the 1980s and 2000s. The last selected neighborhood, Gongyuantianxia, is a high-rise residential neighborhood developed in 2009. The average building floor in this area was 17 floors in height, representing the common development style after 2000 (**Figure 2**).

open in 2012. In this area, there are one big shopping center, one supermarket, one hotel, one museum and an antique market. They are basically multistoried buildings. Therefore, it is a

) 123,200 189,280 153,000 95,850

Built area (%) 51 19 14 6 Green area (%) 22 28 41 49 Floor area (%) 106 153 242 254 Average floor 2 5 7 17

Site size (m) 280 × 440 520 × 364 450 × 340 410 × 300 Simulation size (m) 1080 × 1240 1520 × 1364 1450 × 1340 1410 × 1300

**Low-rise Middle-rise Mix-rise High-rise**

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**Low-rise Middle-rise Mix-rise High-rise**

new mixed-use shopping district.

Grid size (m) 0.6–6.0

Site area (m2

**Table 1.** Details of simulation domain size in the four selected areas.

**Table 2.** Land use and building height properties in the four selected areas.

**Figure 3.** Image selected areas and simulation domain data.

Besides, three typical shopping areas in different layout which present the typical urban typology of shopping district are selected. A domain of 500 × 500 m was chosen for the simulation models, and the detailed input domain data and weather data are presented in **Tables 3** and **4**.

The first is Xiao Zhai shopping district, which is located in Xi 'an Yan Ta district. Since 2001, it has become the second largest business circle in Xi 'an, and the area is mainly composed of small retail shops and large shopping centers, and these commercial buildings have no unified planning, and they are not built in the same period. In this district, they also have some residential architectures. Therefore, it can be said to be a traditional kind of mixed layout shopping district, and it is common in Xi'an. The second is Shu Yuan men shopping district, and it is located in the east side of the south gate in Xi'an. It has been developed since the Ming dynasty, and it has become a kind of antique commercial street with Ming dynasty and Qing dynasty architecture style. In this area, building density is relatively high, but building height is low, generally only 2–3 levels of height. Meanwhile, there are some old residential one-storied houses in the area. Therefore, it is a traditional historical and cultural district for shopping. The third is Tang West Market Group shopping district. It is the only project that was rebuilt on its original site in China. Through overall planning and design, this area was

**Figure 2.** Xi'an satellite photo with the four areas selected for this study and defined as low-rise area (Sanxuejie), middlerise area (Xitiedaminggong), mix-rise area (Jiaodayicun) and high-rise area (Gongyuantianxia). The area represented in this figure is indicated in **Figure 3** and **Tables 1** and **2**.

open in 2012. In this area, there are one big shopping center, one supermarket, one hotel, one museum and an antique market. They are basically multistoried buildings. Therefore, it is a new mixed-use shopping district.


**Table 1.** Details of simulation domain size in the four selected areas.

height and perfectly represent the character of the building type constructed in the period between 1979 and the beginning of the 1990s. The third neighborhood, Jiaodayicun, is a mixrise area of low-rise and high-rise buildings, home to a variety of building styles developed around the 1980s and 2000s. The last selected neighborhood, Gongyuantianxia, is a high-rise residential neighborhood developed in 2009. The average building floor in this area was 17

Besides, three typical shopping areas in different layout which present the typical urban typology of shopping district are selected. A domain of 500 × 500 m was chosen for the simulation models, and the detailed input domain data and weather data are presented in **Tables 3** and **4**. The first is Xiao Zhai shopping district, which is located in Xi 'an Yan Ta district. Since 2001, it has become the second largest business circle in Xi 'an, and the area is mainly composed of small retail shops and large shopping centers, and these commercial buildings have no unified planning, and they are not built in the same period. In this district, they also have some residential architectures. Therefore, it can be said to be a traditional kind of mixed layout shopping district, and it is common in Xi'an. The second is Shu Yuan men shopping district, and it is located in the east side of the south gate in Xi'an. It has been developed since the Ming dynasty, and it has become a kind of antique commercial street with Ming dynasty and Qing dynasty architecture style. In this area, building density is relatively high, but building height is low, generally only 2–3 levels of height. Meanwhile, there are some old residential one-storied houses in the area. Therefore, it is a traditional historical and cultural district for shopping. The third is Tang West Market Group shopping district. It is the only project that was rebuilt on its original site in China. Through overall planning and design, this area was

**Figure 2.** Xi'an satellite photo with the four areas selected for this study and defined as low-rise area (Sanxuejie), middlerise area (Xitiedaminggong), mix-rise area (Jiaodayicun) and high-rise area (Gongyuantianxia). The area represented in

this figure is indicated in **Figure 3** and **Tables 1** and **2**.

floors in height, representing the common development style after 2000 (**Figure 2**).

64 Sustainable Cities - Authenticity, Ambition and Dream


**Table 2.** Land use and building height properties in the four selected areas.

**Figure 3.** Image selected areas and simulation domain data.

**Satellite image**

Shu Yuan men

**Table 3.**

Image selected areas and simulation domain data

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**CAD map**

**ENVI-met input data**

**Satellite image**

Xiao Zhai

Shopping district

**CAD map**

**ENVI-met input data**

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**Table 3.** Image selected areas and simulation domain data

#### **2.2. Simulation**

A commercial CFD code, scSTREAM (Software Cradle Co., 2011), was used to simulate the urban ventilation properties. Detailed distributions of air current and pressure per direction can be visualized. The simulation models were built according to the realities shown in **Figure 3**, and the detailed input data are presented in **Tables 1**, **2** and **5**.

on the summer day was 2 m/s, and the wind directions were N, ENN, EN, ENE, and E, the main being EN; the average wind speed on the winter day was 1.25 m/s, and the wind directions were N, ENN, and EN, the main direction being ENN. The average wind speed and all of the wind directions in different seasons served as the initial condition for the simulations in scSTREAM. The main wind directions were here used for detailed results analysis and

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The field measurement results of the PM10 concentration distribution in the summer of 2016 are shown in **Figure 5**. The air pollution concentration distribution was not averagely distributed in the city. Comparison of **Figure 4** and the satellite photo in **Figure 2** showed the distribution of air pollution to be partially but directly related to urban density. This is because the urban typology affects urban ventilation and accelerates aggregation, and the air pollution in

In order to clarify the mechanism of the air pollution concentration in high-density areas, the low-rise and middle-rise areas that located in highly polluted areas are selected for simula-

On a summer day, five wind directions (N, ENN, EN, ENE, E) were simulated at a wind speed of 2 m/s. On a winter day, three wind directions (N, ENN, EN) were simulated at a wind speed of 1.25 m/s. **Figures 6** and **7** present the simulation results in the selected four urban areas.

In summer (**Figure 6**), with the effects from trees, the median wind speed in the low-rise area was slower than in the other areas, and the wind speed in the mix-rise area was fastest. With a wind direction of EN, the median wind speed in mix-rise area was 0.35 m/s, which was 0.15 m/s higher than in the low-rise and middle-rise areas, respectively. The median wind speed in high-rise area was slightly slower (0.03–0.06 m/s) than in the low-rise and middlerise areas, but there were small areas of higher wind speed in the high-rise area, and these reached 2.13 m/s (2–3 times of the max wind speed in the low-rise and middle-rise areas). This could explain the results of high PM10 concentration measured in low-rise and middle-rise

In winter (**Figure 7**), without the effects from trees, the median wind speed in the mixed-rise area was higher than in the other areas. With a wind direction of ENN, the median wind speed in mixed-rise area was 0.24 m/s higher than in the low-rise area and 0.19 m/s higher than in the middle-rise area. The median wind speed in the high-rise area is 0.09 and 0.28 m/s lower than in the middle-rise and mixed-rise areas. This is to say, the low-rise area (high-built density) and the high-rise area (high urban roughness) are reducing the urban wind speed.

discussion.

**3. Results**

**3.1. Results of urban air quality and urban ventilation**

high-density urban districts is subsequently high.

tion. This phenomenon will be discussed with the simulation results.

*3.1.1. Air pollution distribution in Xi'an city*

*3.1.2. Wind environment simulation*

areas shown in **Figure 7**.

The wind environment (Xi'an Weather Station, 2016) on a typical summer day (21st July, 2016) and a typical winter day (21st December, 2015) is selected for analysis. The wind direction and wind speed between 6:00 a.m. and 8:00 p.m. are shown in **Figure 4**. The average wind speed


**Table 4.** Related parameters of three areas


**Table 5.** Details of ground surface material characteristics for simulation

**Figure 4.** The wind condition in a typical summer day (21st July, 2016) and a typical winter day (21st December, 2015). The main wind directions are shown in red.

on the summer day was 2 m/s, and the wind directions were N, ENN, EN, ENE, and E, the main being EN; the average wind speed on the winter day was 1.25 m/s, and the wind directions were N, ENN, and EN, the main direction being ENN. The average wind speed and all of the wind directions in different seasons served as the initial condition for the simulations in scSTREAM. The main wind directions were here used for detailed results analysis and discussion.
