Eulerian Numerical Modeling Of Contaminant Transport In Lower Manhattan, New York City, From A Point-Source Release Under The Dominant Wind Condition: Insights Gained Via Les

Released pollutants, biological agents, and poisonous chemicals in populated urban areas can spread via wind flow and affect the safety and welfare of those close to the source and downstream. Being able to predict the transport of airborne contaminants in such urban areas could help stakeholders to plan emergency responses. Airflow due to the wind in urban areas with closely packed buildings results in a highly turbulent flow field around buildings that results in the transport of contaminant plumes. We conduct high-resolution large-eddy simulation (LES) of airflow of the dominant wind in the south to north direction with a mean-flow velocity of 3.58 m/s in a portion of Manhattan, New York, which is 2.5 km long in the north-south direction, 1.8 km wide in the east-west direction, and 0.6 km high, located at its southern tip. The study area is a highly-populated area with densely packed buildings in Manhattan. The LES captures turbulent flow dynamics around the buildings, which vary in height from several stories to skyscrapers, and, consequently, the transport of contaminant plume propagating downstream from a source point, which is located on the east side of the New York Stock Exchange building at 30 m above ground. The airflow tends to align with the streets that are reasonably parallel to the wind direction, creating high-velocity core regions. For as long as the source of contamination exists, the pollution plume rapidly travels through the streets, reaching downwind regions of the study area in about 20 min. Once the pollutant source is removed, the pollution plume propagates out of the study area. However, it takes about 40 min for the pollution to completely exit the study area due to the contaminants trapped in the low-momentum urban regions, such as open areas between tall buildings and narrow streets, which take longer to either exit the study area.