Mitigating particulate matter dispersion from urban earthen sites: A case study of city walls in Zhengzhou, China

Abstract

Historic sites are valuable urban development and renewal assets, contributing to cultural identity, historical research, built environment, and tourism economy. A study conducted near the Zhengzhou Shang Dynasty Site in China collected complaints from tourists and residents regarding surrounding air environments, negatively affecting attitudes toward site conservation. This study measured wind environments and air qualities in an outdoor parkway and indoor bedrooms adjacent to the exposed city wall. Computational fluid dynamics simulations were conducted using the particle transport and plant canopy models in Cradle scSTREAM to analyse the dispersion process and spatial distribution of particulate matter (PM). The results show: 1) Desiccating earthen sites increased PM₁₀ concentrations after rainfall. 2) PM₁₀ concentrations were primarily influenced by vehicle emissions during peak periods and by earthen city walls at other times. 3) Steady north-south canyon winds generated shear ventilation along residential facades but did not facilitate effective indoor-outdoor air exchange. 4) Site winds and spatial forms were key factors influencing the PM₁₀ dispersion pathway and spatial concentrations. The study summarises multi-scale mitigation measures and simulated three implementable measures: land use and block form, tree placement in the canyon, and street canyon permeability. These measures complement source control in the existing studies from the dispersion control. This study contributes to the sustainable renewal of historic districts and improves the social image of urban sites.