The release of hazardous materials originated from industrial/natural accidents or intentional attacks within a compact urban environment can be disastrous. Once released, the hazardous pollutants will be dispersed through the atmosphere instantaneously and threaten human health. This study aims at evaluating the overall effect of upstream obstacles and hazardous sources on the downstream targets through parametric investigations. A complex building morphology composed of obstacle group, target group, one main street canyon and several subsidiary street canyons is proposed. The dimensional parameters along with the sources are set as variables. An orthogonal numerical test with Large Eddy Simulation (LES) model is carried out, indicating that the source conditions play a more important role than the obstacles dimension in pollutants accumulation around targets. The flow and dispersion structures are categorized into five patterns, which are strongly relevant to the dimension parameters. Due to the upstream perturbation, the sidewalls of targets are found to be contaminated more seriously than the obstacles, while both their windward and leeward walls have a lower concentration of pollutants.