Investigation of the dry deposition of air pollutants over urban surfaces using a building-resolving large-eddy simulation method

Dry deposition is a crucial process responsible for the removal of air pollutants from the atmosphere, significantly impacting air quality in urban environments. Currently, existing regional air quality models typically parameterize the turbulent transport and dry deposition of air pollutants in the atmospheric surface layers (ASL) based on the Monin-Obukhov similarity theory (MOST), which assumes that the underlying surface is flat and homogeneous. However, in modern cities, high-rise buildings with heights comparable to the ASL depth (approximately 100 meters) are prevalent. In this condition, applying MOST to parameterize turbulent transport in these urban settings may lead to inaccurate air quality predictions. To address this issue, this study investigates the effects of urban geometry on the turbulent transport and dry deposition of air pollutants in the ASL using a building-resolving large-eddy simulation (LES) method. Specifically, we develop an LES approach to simulate the dry deposition of ultrafine aerosol particles over four distinct real urban surfaces, each with unique morphological characteristics. By considering the turbulent transport within urban canopies and the deposition of aerosols on building roofs and facades, our results indicate that traditional parameterization methods may overestimate turbulent transport over urban surfaces and underestimate the corresponding aerodynamic resistance by 28% to 52%.