With the accelerating pace of urbanization, urban pollutant dispersion has become a central concern in environmental fluid dynamics. Pollutant transport in street canyons is governed by complex, building-induced turbulent structures, yet the specific impacts of source dimensionality (line vs. point sources) and structural geometric irregularities on these dispersion processes remain insufficiently quantified. This study employs large eddy simulation (LES) to investigate the transient and time-averaged characteristics of tracer dispersion within idealized urban configurations. The research is conducted across two primary axes. First, the transport topologies of continuous line sources and isolated point sources are compared under both neutral (Ri = 0) and buoyancy-affected (Ri = 0.03) conditions. The results reveal that while line sources promote spanwise-uniform accumulation, point sources induce intense, localized scalar concentration peaks that are significantly mitigated by thermal buoyancy-driven vertical venting. Second, the impact of structural irregularities is systematically analyzed by comparing a regular urban array with a modified configuration featuring a 0.5h translational displacement of a single building obstacle. It is demonstrated that such localized geometric defects act as powerful aerodynamic triggers; they dismantle spanwise symmetry, generate non-uniform horizontal bypass jets, and provoke a strong upward pumping of low-momentum fluid. The structural misalignment further serves as an intensive turbulence catalyst, lofting Reynolds stress tensors (u′u′, v′v′, w′w′) into dome-shaped, high-altitude plumes that extend deep into the outer boundary layer. This geometry-induced turbulence amplification significantly enhances near-ground ventilation efficiency by rapidly dispersing trapped pollutants into upper atmospheric regions. These findings provide critical quantitative insights into how source characteristics and micro-scale structural non-uniformities dictate urban air quality, offering a scientific basis for more accurate street-level exposure modeling and urban ventilation planning.     «

With the accelerating pace of urbanization, urban pollutant dispersion has become a central concern in environmental fluid dynamics. Pollutant transport in street canyons is governed by complex, building-induced turbulent structures, yet the specific impacts of source dimensionality (line vs. point sources) and structural geometric irregularities on these dispersion processes remain insufficiently quantified. This study employs large eddy simulation (LES) to investigate the transient and time-av...     »