  Released pollutants and poisonous chemicals in the highly-populated urban
areas can spread via wind flow and affect the health, safety, and wellbeing of
those close to the source and others 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 complicated aerodynamics around buildings
that, in turn, can lead to intricate contaminant transport phenomena. We
conduct high-resolution large-eddy simulation (LES) of airflow in Southern
Manhattan, a highly-populated area in New York City, due to a dominant wind
blowing from south to north. The LES results show complex flow dynamics around
the buildings of various heights and, consequently, complicated contaminant
transport processes from a source-point which is located in front of the New
York Stock Exchange building at 30 m above ground. The flow tends to align with
the streets that are fairly parallel to the wind direction creating
high-velocity core regions, while the airflow loses its momentum markedly
farther downwind in the urban area. For as long as the source of contamination
exists, the pollution plume rapidly travels through the streets reaching
downwind areas. When the source is removed, contaminants that are trapped in
the low-momentum urban area linger to exit the flow domain.
