From @ottinger on the air quality list "Unfortunately, to my knowledge there are few sensors comparable to the Purple Air for oil refinery pollution--specifically, sensors that can produce readings for toxic gases (e.g. hydrogen sulfide, which is of concern around refineries) in real time.
In Benicia, CA, we experimented some with the AWAIR (about a $200 instrument) but that only reads total VOCs, not individual gases. We also found it more difficult to access data than with the Purple Air (no API) and hard to keep online. (folks in Pittsburgh have more extensive experience with these, and have also found that the absolute values creep up over time)
I believe some of my colleagues in Northern CA are investigating the possibility of using OPSIS systems for this purpose, but I don't know how expensive they are or how amenable to a DIY approach.
The best instrument I know of for this purpose is the UV Hound by Argos Scientific. They monitor the toxic gases of concern around oil refineries, in real time, at reasonably low detection limits, and they aren't terribly hard to use--you have to be able to calibrate them and manage the data upload to a laptop, but there are no moving parts to the monitor itself. They aren't cheap (a couple thousand dollars, I think), so you couldn't spread them throughout a neighborhood, but they are portable, and I'm almost certain that you could rent one for a while for less, if it would be helpful to do a study of a limited duration.
If you were willing to take samples instead, there are additional strategies you could use, especially the "bucket" air sampler.
Hope that helps!"