Thanks for that Sean, well researched. 
It doesn't say a huge amount (and appears to be based on BMW designs, probably very similar to Audi) but these are the key sections:
Quote:
The air quality sensor is a metal oxide sensor. This sensor is highly sensitive to various smells and pollutants that are typical of traffic.
The automatic air recirculation control sensor evaluates the concentration of carbon monoxide and nitrogen oxides in the intake fresh air.
The air quality sensor has reached its operating readiness after 30 seconds and responds to ambient air changes.
This sensor consists of two resistor elements and an electronic component.
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The 30 seconds "warm-up" time is consistent with the Audi system and I presume the resistors are the sensor and the electronic component converts that to a signal used by the HVAC system.
Funnily enough one of my neighbours specialises in developing sensors of this type, though mainly for use in the oil and gas industry, so I'll ask him what he knows when I see him next. 
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2003 D2 FL S8. Irish Green Pearl/Beige. Solar sunroof, auto-dim mirrors, electric rear seat functions, ski hatch retrofit; extended leather. Aftermarket DVB-T, reversing camera and full XCarlink (Bluetooth etc.).
2016 Volvo V40 T5 Cross Country (4WD) with ALL the toys including adaptive cruise etc. etc. Osmium Grey with Blonde/Charcoal leather interior. Polestar performance "optimisation".  (A much rarer model than a D2 S8 by the way!) Oh, and a brand new engine at just under 30,000 miles on the factory one! 
Finally: gone, but not forgotten.....
1998 D2 PF S8. AgateGrey/Platinum. Every option (I think) except electric rear seats, tiptronic steering wheel, ski hatch, towbar & dimming door mirrors.
e.g. Cruise control, NavPlus/TV, Bose, GSM, Xenons, Solar roof, Parking sensors, Alcantara/leather everywhere of course. (internal dimming mirror added later)
1998 (very early) Ford Focus 1.8 Zetec; ABS/TCS, Heated screen/mirrors, Aircon, Auto-dim mirror, Leather, Trip computer, Cruise control, OEM Ford SatNav with CD changer.
And before that a lot of Rover 800s, a few oddities, a lovely Triumph Dolomite 1850HL with Overdrive and way back in my schooldays an Austin Seven aka Mini 850!
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