I suggest that you consider adding sensors for Hydrogen Sulfide and Sulpher Dioxide.
Both of these gases have objectionable odour and can be dangerous even fatal ( H2S especially).
H2S is insidious in its effects. If present in the environment it has a bad smell but as concentration increases the nose loses sensitivity to its bad odour but the gas can be fatal at levels ~ 100 ppm. There are other health effects too.
The gases are found in industrial situations, near coal power stations and in areas with volcanic activity, such as Taiwan, New Zealand, Iceland and others.
The sensors are inexpensive ($Usd 20) but highly accurate (because each unit is calibrated) and can be obtained from
https://spec-sensors.com
Their sensors would need to be interfaced to the arduino environment (but its fairly easy) and a library developed. Its not too hard because there is extensive documentation available and the vendor will likely cooperate.
Their wide range of products could also be a second source for some of your existing high end sensors.
