NYC, DC, and Oklahoma City, among other places, have hosted a number of trials like this. Having just attended a conference on atmospheric transport and dispersion [0], I know multiple organizations use the resulting data to validate their transport models.
Subways are really interesting beasts from the point of view of people who try to model the transport and fate of airborne materials. The trains themselves pump material through the tubes, the cars entrain material in their wake, and the cars capture and carry material along with the passengers (meaning you can disperse material even after a car comes out from underground). All this is intermittent and bi-directional, following the train schedules. And then it's all tightly coupled to the city's above-ground airflow patterns, both because the train stations have massive ventilation exchange with the outdoors, and because there can be passive vents at street level.
Particles add another layer to the problem. As the article states, they deposit out of the air, and they can resuspend, e.g., due to human activity. They can also hitch a ride on people and things. They also get captured in ventilation system filters. Finally the rates at which they do all these things depends strongly on the particle size -- and the particle size can change due to things like coagulation.
Subways are really interesting beasts from the point of view of people who try to model the transport and fate of airborne materials. The trains themselves pump material through the tubes, the cars entrain material in their wake, and the cars capture and carry material along with the passengers (meaning you can disperse material even after a car comes out from underground). All this is intermittent and bi-directional, following the train schedules. And then it's all tightly coupled to the city's above-ground airflow patterns, both because the train stations have massive ventilation exchange with the outdoors, and because there can be passive vents at street level.
Particles add another layer to the problem. As the article states, they deposit out of the air, and they can resuspend, e.g., due to human activity. They can also hitch a ride on people and things. They also get captured in ventilation system filters. Finally the rates at which they do all these things depends strongly on the particle size -- and the particle size can change due to things like coagulation.
[0] http://camp.cos.gmu.edu/20th-announcement.html