I've been through a lot of FEMA courses and disaster training in my ARES group which used to be a part of the Arlington Fire Department.
The least effective threat is chemical. The next worst is nukes [still localized and detectable.] The worst by far is BIO [You don't know you've been hit until it's too late..massive casualties, no cures, and massive panic.]
Think like a terrorist. Chemical is the least effective, hardest to deliver, and least spectacular.
IOW, think nukes/bio/conventional or a combination of all 4 including chem, but chem alone is no threat IMO. Keep thinking nukes/bio....
But a chemical attack can be easily rigged (hook up chemical mines in series and plant upwind of a county fair--detonate on command). Chemical mines come with instructions done in pictogram's so that this soviet HORROR can be set up by any uneducated commie pawn anywhere in the world. It is as easy as can be. Or set up an attack via an industrial fire extinguisher from the top of a very tall building. This scenario would require training, and access, and a suicide attacker because as soon as the pressure is released into the atmosphere, the person doing so is the first to die, unless he's in MOPP 4 gear. But given a warm muggy day, and a gentle breeze over downtown and you could generate an effective downwind drift sufficient to cover most of downtown Miami, Florida (my home). We are just now coming into the prime season for such an attack.
Chemical munitions are easy to come by. The former USSR and it's satellites have thousands of the things languishing in almost unguarded warehouses all over the world. They look innocuous, like home plate for a baseball team, just a bit thicker, but still plastic. They are binary in nature so the contents are inert until mixed before dispersal. There has already been such an attack in Baghdad last year. Somebody tried to set off an IED by detonating an artillery shell that was a BINARY SARIN shell, but it failed to properly mix. A dedicated cell with the training and a specific target will have trained to overcome that problem.
Just how are you going to decontaminate a forty story building in the dry season? Even if you get your rain shower, how are you going to prevent the deadliest substances known to man (VX Nerve Agent) from leaking into the ground water and contaminating the aquifer? DECON is easy: Hot soapy water but you still have to contain the runoff of contaminated water so it doesn't soak into the earth. Everything touched must be decontaminated: Personnel, vehicles, aircraft, buildings, everything. There are still spots in FRANCE that hold trace amounts of a BLISTER agent (Mustard Gas) left over from the FIRST World War! A chemical attack, no matter how effective or not would undeniably show the world that another 911 had occurred. But a bio attack can be plausibly denied whether or not it succeeds. Think like a terrorist? You bet!
The only A.R.E. group I ever belonged to was Edgar Cayce ;)