**1. Introduction**

Rapid urbanization has resulted in air pollution issues that had a negative impact on many sectors of human lives. According to the Beijing State of the Environment Bulletin 2020 [1], the number of days that met air quality standards in 2020 was 276 days, accounting for 75.4%. Although the overall air quality has improved compared to the previous period, the distribution of pollution still showed north–south differences, and the concentration of pollutants such as PM2.5 and PM10 in ecological zones in the north and northwest is significantly lower than that in the southern high-density built-up areas and high-density population areas, which showed a "Low-Northwest while High-Southeast" situation. As urban planning became a more essential component of the development of livable cities, how to enhance air quality by optimizing urban morphology evolved into a focus of investigation in relevant disciplines [2,3].

Based on the foregoing, studies on urban morphology and air quality were steadily established [4]. Studies mainly involve two scales: regional-urban [5] and neighborhoods [6]. At the regional-urban scale, most scholars had focused on exploring the intrinsic effects

**Citation:** Zhang, P.; Cheng, H.; Jiang, Z.; Xiang, F. How Sensitive Morphological Parameters Influence on the PM2.5 Diffusion: An Empirical Study of Two Neighborhoods in Central Beijing. *Atmosphere* **2022**, *13*, 921. https://doi.org/10.3390/ atmos13060921

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 12 May 2022 Accepted: 5 June 2022 Published: 6 June 2022

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of different urban morphological features on air quality. Research elements included physical spatial characteristics such as city size and urban shape, [7] vegetation cover [8], and non-physical characteristics such as population density and employment density [9]. In addition, studies have also been conducted to analyze the correlation between urban morphology and air quality from a spatial and temporal perspective [10].

At the neighborhood scale, most studies had conducted comparative studies of neighborhoods with different morphological characteristics for the correlation between urban morphology and pollutants [11,12] with air pollutant monitoring data from urban observatories. However, the specific relationship between urban morphological parameters and PM2.5 concentration as well as diffusion has not been investigated clearly. In addition, the generalization of urban morphological characteristics needed a systematic and comprehensive framework. Since the selection of vegetation and non-physical morphological indicators is relatively weak in existing studies, they are unable to provide universal laws.

In recent years, the development of simulation techniques such as Computational Fluid Dynamics (CFD) has provided the technical support to establish the correlation between urban morphology and pollutant dispersion at the neighborhood scale with the Fluent simulation software being the most widely utilized. Studies could be divided into two categories. First, ideal-neighborhood simulation based on traditional settlement patterns [13] was constructed (lineal type, point group type, etc.). Different building combinations [14] or vegetation layouts [15] were explored individually, and the effect of different morphological features on the dispersion of pollutants based on simulation results was qualitative or quantitative analyzed. Secondly, a simulation based on actual cases was constructed. Different urban design schemes for the same neighborhoods [16] or comparisons of different neighborhoods [17] have been studied to promote the air quality. In general, the correlation between urban morphology and air quality had been gradually established; however, the quantitative guidance was limited. The simulation studies of ideal neighborhoods were separated from the complicated morphology of the building arrangement in the actual environment, and the simulation data were based on empirical data. In addition, in terms of modeling, the impact of the integrated neighborhood environment of buildings and vegetation on pollution dispersion has not been considered in previous studies, while the simulation studies of actual cases were aimed at promoting the air quality of specific public spaces and neighborhoods, which lacked the general application.

The study, which focused on two typical residential neighborhoods with different features in central Beijing, explored the quantitative rules of affection between urban morphology and air quality. We devised an urban morphology and air quality mechanism for selecting morphological parameters. Through the neighborhood-scale CFD simulation, which includes the calibration of vegetation factors and multi-source data, the Sensitive Morphological Parameters (SMPs) impacting air quality (PM2.5, for example) at the neighborhood scale were filtrated before statistical models. Therefore, the quantitative rules of the influence of SMPs on the "pollutant-wind environment" could be estimated.

#### **2. Data and Methods**

#### *2.1. Study Area*

Taking a traditional residential area in central Beijing as the study area, the study selected a low-rise residential neighborhood (Neighborhood A) and a high-rise residential neighborhood (Neighborhood B) as the core study area based on the street network, buildings' layout, and its group form (Figure 1a). Neighborhood A, which was built in the 1990s, is dominated by enclosed low-medium-rise residential buildings with schools and underlying retail; Neighborhood B, which was built in the early twenty-first century, is dominated by row-slab high-rise residential buildings with a few underlying retails. The aforementioned two neighborhoods differ in terms of building periods, functional placement, and spatial arrangement, which might illustrate the main features of Beijing's residential neighborhood morphology.

**Figure 1.** The study area: (**a**) the study area; (**b**) core study area, A refer to Neighborhood A, B refer to Neighborhood B.
