Your endeavor is incredible. This endeavor has given me a lot to learn. I'd want to distribute it to the  xnx xnx honeywell analytics team so they can read it and apply something fresh to our ongoing efforts.

Do you have any modular sensor kits in mind? I'm interested in that for my BotSpine project ( https://hackaday.io/project/9006-botspine ). We currently connect our radiation sensors. The ability to easily add chemical sensors would be great!

Th approach is  a standard  ADC/ uC back end with plugin modules for the different chemical sensors.  For the moment I envisage perhaps allowing for  up to 8 simultaneous sensor modules per unit to provide a variety of  sensing and logging.

It may be reasonable to make each sensor module i2C compliant for simple scaling as the same bus can be used to store  local data logs into i2C memory should the  sensing device have to operate in a hazmat environment that disallows Wifi or RF signals. Once  withdrawn from the hazmat area the memory can upload into the cloud  or any secure devices  as appropriate for more limited access.

I hope to permit the system to 'tag' NFC stickers (to be affixed at the site) with a code that matches the chemical data set for later forensic investigations. Thus just  linking to the sticker with an app pulls up the archive record of the chemical event from the database for the investigator. Complete with secure time stamps and any images or video taken at the time. Can be useful for the legal documentation ramifications of emissions etc.

Do you have a brand of sensors in mind? How do you go about finding decent or state of the art chemical sensors? Thanks!

Maybe you could use ESP Serial Wifi Link, so you do not have to reinvent the wheel. Here is the github link 

https://github.com/jeelabs/esp-link

That looks useful, I'll have to investigate it for PIC.