[cva66snipe]

I would be looking for buildings with boilers. If one is shut down but the gas control valve malfunctioned that could possibly do it but it should smell inside the boiler room though. But it could also be a multifunctional operating boiler with an unlit fire which would make even more sense as the forced draft blower would take the raw gas right on out the exhaust stack. Many boilers have pretty good sized gas lines feeding them.

[Cletus.D.Yokel]

Boiler feed lines?

Why not a malfunctioning NatGas odorant plant? Enough mercaptan there to stink up the entire metro area.

Does anyone know if these are underground in NYC?

[azhenfud]

I would first be looking for pulp paper plants or similar industrials using sulfuric acids within a fifty mile radius. Emissions output combined with stagnant atmospheric conditions, such as would usually result in fog, would put the odor(s) back along the ground.

Exactly what were the atmospheric conditions last night? Low clouds? fog? little to no wind?

Even light rain, at times, causes the paper mill 30 miles to the east of my hometown to make the outside air smell like an open sewer. Sulfur dioxide, I think, is the culprit.