O3 Ozone & SO2 Sulphur Dioxide Sensors would be great! — to complite an Air Quality Index Kit.

like:
http://www.sainsmart.com/sainsmart-mq131-gas-sensor-ozone-module-for-arduino-uno-mega2560-r3-raspberry-pi.html

http://www.sainsmart.com/sainsmart-mq136-gas-sensor-hydrogen-for-arduino-mega2560-uno-r3-raspberry-pi.html