What is going on? 5th Ave above the Prospect Expressway

THANK YOU! I thought I was the only one weirded out by all of this. Made me wonder what exactly they're using that it is worth polluting the air to detect another (presumably stronger) air pollutant.

If you walk by late nights you can actually see some of the work. They're apparently spraying some sort of "protective coating" on the underside of the bridge (the hazmat shack on the west side of the street has a few words of explanation on it). The work is pretty loud and apparently required a "noise mitigation plan" which is part of what I guess those heavy gray barrier curtains are about. But yeah, there are several scary looking and noisy tanker trucks involved, and big hoses come out from the hazmat shack and move around from the pressure of the mystery contents. The whole thing is a bit surreal/creepy, especially when you stumble upon it while alone with your dog at 2am as I did...