It's really hard to "catch" these things in the moment. I work in chemical plants and there are tons of huge releases of extremely toxic gases all the time, which I can often smell from my apartment. I know what they are by smell because I manufacture them myself. Proper continuous monitoring for a broad range of chemicals would cost about $1 million per monitoring station and we'd need to build them along the perimeters of each plant so that leaks can be assigned to the offending companies, and they need to be built near housing so that we know how families are being affected. That sounds expensive, but really isn't that much added cost for a $100 million to $10 billion manufacturing site.

I live in the western hemisphere's largest integrated industrial complex (Freeport, TX integrated with the eastern edge of Houston as well). Note that Freeport, TX has ZERO state or federal EPA VOC analyzers which can actually detect which chemical is leaking. They can only detect "this amount of something with either {sulfur, N-O bonds, aromatic carbon rings} -- no clue what precisely though!". This is the same capability of the most advanced atmospheric pollution satellites. Completely fucking useless for an area which manufactures something like 15-20% of all USA domestic chemicals. The technology to measure individual chemicals exists, but the government isn't paying for it or installing it.

The ENTIRE east side of Houston metropolitan area is dedicated to or "next door" to massive chemical manufacturing. This is an industrial area nearly equal to the area encompassing all of Seattle+Bellevue+Redmond+Renton+Tukwila. That's not including nearby housing - that's just how big the manufacturing facilities themselves are! This massive area has only THREE air quality monitors which test for these kinds of chemicals[0]. During huge major events like the ITC fire[2], they typically show no increased pollution at all, which proves that it's woefully inadequate even during big emergencies that very obviously affect breathing air quality. I lived next to leaks every day and because I worked in the plants I knew the smells - one day acrylates, next day thiols, next day hydrocarbons, etc. But all 3 monitoring sites (over 10 miles from me) showed nothing at all.

Here is the one "correct" monitoring station near the chemical plants of Houston: [0]... but several of its analyzers are often offline/broken/pending maintenance. Here's a map of all the other ones: [1] Generally single/dual color dots mark "not-useful" monitoring sites which might measure only PM2.5 or Ozone, for example. The 4+ color dots are generally useful, they measure specific (large) families of chemicals so you can see very roughly what is leaking, even if it doesn't have "soot" in it.

The data used by ProPublica[3] is actually far worse than the woefully inadequate data collected by TCEQ/EPA air monitoring stations -- because what ProPublica used was "self-reported" data from the chemical plants. But I know from working in them and living next to them that many leaks are never reported and many leaks are never even known internally! Our government's data collection is a travesty. ProPublica couldn't use the real air quality measurements because having 2-3 points across 1000 mi^2 is completely useless for the wind models they wanted to apply to the problem.
We don't actually have any data. The government is failing us.

ITC fire which blanketed houston's sky in smoke: [2]

: https://tceq.maps.arcgis.com/apps/webappviewer/index.html?id...

1: https://tceq.maps.arcgis.com/apps/webappviewer/index.html?id...

2: https://abc13.com/deer-park-fire-2019-itc-houston-air-qualit...

3: https://www.propublica.org/nerds/visualizing-toxic-air