*2.4. Selection and Extraction of Urban Morphological Parameters*

## 2.4.1. Selection of Urban Morphological Parameters

Based on the research framework of urban morphology influencing air pollutant transport (Figure 4) and the generalization of existing studies, a system of six categories of urban morphological characteristics, including size, density, function, structure, shape and immaterial morphological characteristics, were constructed.

**Figure 4.** Framework of interaction mechanism between urban morphology and air quality.

The six morphological characteristics stated above have been proved to have a direct or indirect impact on air quality and the urban microclimate. In particular, urban size affects the urban microclimate, pollutant emissions, and dispersion transport, and pollutant concentrations increase significantly as urban size increases [26]. The association between land use and air pollutants is more obvious, and air pollution is severe in industrial sites and commercial districts with greater emission sources [27]. In contrast, green spaces and water bodies can improve the local microclimate [28] and reduce the concentration of PM2.5 in the region [29]. Different density characteristics demonstrate a broad range of heterogeneity in the routes of effect on air quality, such as the impact of building density on the alteration of local wind fields, which influences pollution dispersion [30,31], and the impact of road density on traffic pollutants [9]. Urban layout structure is a major factor affecting the wind environment, and a large number of scholars have conducted detailed studies on the elements of layout structure with wind environment and pollutant levels [32,33]. Most of the studies on shape features have focused on investigating the effects of different building shapes and combinations of building morphologies on the wind environment and pollutant dispersion [34]. Furthermore, since physical urban morphology is the spatial projection of non-physical urban morphology on land use [35], the adoption of the immaterial morphological indicator is highly relevant to the overall morphology of the neighborhoods.

Thus, ten morphological parameters were selected for investigation based on the principles of neighborhood scale, potential impact on pollutant levels, ease of implementation at the control and design stages, and the interaction mechanisms between the preceding morphological features and air quality, as well as the research progress of relevant literature (Table 2).


**Table 2.** Selection of characteristic morphological parameters of neighborhoods.
