**5. Conclusions**

This study aims to investigate the mechanisms underlying the correlation between urban form and atmospheric pollutants (PM2.5, for example), and two types of typical residential neighborhoods in Beijing were selected as the study area. Morphological parameters were selected according to the research pathway of urban morphology affecting air quality, and a sensitivity analysis of morphological parameters with PM2.5 and wind speed was carried out through field monitoring and CFD numerical simulation.

In the sensitivity filtrating, six morphological parameters, such as BD, BED, etc., showed high sensitivity to PM2.5 concentrations and wind speed within the neighborhood, which are called the Sensitive Morphological Parameters (SMPs). The different correlations of SMPs between PM2.5 and wind speed were observed. This demonstrates the existence of a tripartite relationship between morphological characteristics, wind environment, and pollutant dispersion.

The SMPs showed different influent rules on the PM2.5 diffusion. It revealed a positive and cosine curve trend of BD and BED with PM2.5. PM2.5 is lowest when BD is around 10% and BED around 3 and continues to rise when BD is above 10%, reaching a maximum of PM2.5 when BD is at around 20% and BED is at 5. GCR was significant to dust retention along with vertical canopy height, with a most noticeable effect in the vertical zone beneath the canopy and a progressive decrease in the zone above the canopy. When ABV = 40,000 m3 and ABF = 20F, the lowest PM2.5 concentration was observed. Increased SDH could promote airflow and enhance the capacity of PM2.5 diffusion.

To optimize the circumstances of pollutant dispersion, three residential planning strategies were proposed. First, the BED of residential buildings along the street should be minimized while commercial buildings along the street should be expanded, and BD should be reasonably managed. Second, vegetation species of appropriate height should be considered, and GCR should be increased. Third, building height should be increased appropriately, as should a proper division of building height disparities in neighborhoods. Furthermore, acceptable control of individual building mass and the total number of buildings in the neighborhood should be considered.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/atmos13060921/s1, Table S1: The Details of XL68 Equipment.

**Author Contributions:** Conceptualization, H.C. and P.Z.; methodology, H.C., P.Z. and Z.J.; software, P.Z. and Z.J.; validation, P.Z.; formal analysis, P.Z. and H.C.; investigation, P.Z.; data curation, P.Z. and F.X.; writing—original draft preparation, P.Z.; writing—review and editing, H.C. and F.X.; supervision and funding acquisition, H.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China, grant numbers 52170174 and 51808009.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of this study are available from the Beijing Municipal Ecological and Environmental Monitoring Center, http://www.bjmemc.com.cn (accessed on 1 January 2020).

**Conflicts of Interest:** The authors declare no conflict of interest.
