i think its only giving a concentration because to model the spatial dispersion pattern of a hypothetical volume of *some contaminant* in air, there has to be a starting concentration of *some contaminant*. it looks like they've used a dust based model, and the standard for quantifying concentrations of stuff in air when the exact composition is unknown (like dust / ash / etc.) is mass per cubic vol. for such hypotheticals the exact values are irrelevant and meaningless, but their relative differences are still useful as a vector.

and I wasn't sure of any of this either at first so i simply clicked the link OP provided to read about the model and its assumptions. There is an impressive amount of documentation available for HYSPLIT. I have learned some cool stuff already. Most of your questions / assumptions could have been addressed by reading the documentation, and doing so would probably even make clear how many of the shortcomings you've identified in the model could be improved. But it doesn't seem like you're interested in learning / contributing / helping bc all you've done so far is use hundreds of words to say nothing more than "youre dumb thats dumb you cant do that", and that seems like a waste of your knowledge

Well you asked "plumes of what" and then proceeded to tell us what would likely be there with more specificity than any other source ive seen here or elsewhere has offered, so it seems that was more just a complaint about the vagueness of the model.

HYSPLIT is primarily a model for predicting the movement of localized masses of air that start at a specific point, over a certain period of time, given local conditions/forecast. The only reason I've heard of it is because balloon pilots use it to predict flight paths. I didn't even know it could be used for tracing contaminants. You seem to be suggesting that it can't or shouldn't. But a model doesn't have to be perfect to be useful, and even a crude model is better than no model. I don't know this w/ certainty but I would assume that for gaseous contaminant dispersal, the biggest controlling factor on the dispersal pattern would simply be weather - regardless of gas composition. If that's true then HYSPLIT seems more than sufficient to at the very least say "its probably headed this way", which is all this model appears to be saying.

>Plumes of WHAT?

oh i thought you were asking sincere questions, but i can see from reading the other responses the questions are just a setup to flex your domain specifc knowledge in this area

>if something else, what?

the MSDS for vinyl chloride warns that burning it "produces toxic and corrosive fumes of hydrogen chloride and phosgene" the latter of which seems more concerning from a toxicity standpoint than anything else already mentioned.