I did a little research.
MQ-135 Sensor detect air particles from 10ppm to 300ppm (1000ppm benzene). 10ppm is high value - I think it is more applicable to alarms.
So we need more sensitive sensor.
TSG 2602 works from 1 to 10ppm - it is much better.
It seems that these sensors are more suitable for measuring indoor air quality. In datasheets don’t see information about sensitivity to S0x, NOx, C0x …etc, despite I think that because of their general sensitivity, will work in outdoor environment.
I created wiring diagram (see attachment, but circuit is untested) and formula for temperature compensation - these gas sensors is sensitive to temperature and slightly to humidity.
Schema is taken from hobby-boards Temperature/Sun sensor. I do not know whether I chose the optimal resistor values, but trimmer solves everything. 
Compensation formula (only for TSG2602):
Vcomp = Vin*(-0,028*T+1,6)
Where:
“Vin” is meausred voltage
“T” is Temperature in Celsius deg
Then for implemenation to SW is also needed know voltage for clear air (0,9~xV - depends on trimmer position) and voltage for contamined air (0.11V~xV depends on trimmer) - it will be simply readed from OneWireViewer.
Probably in the near future I’ll try to order parts and test it. 
EDIT: Added URLs, fixed bad ppm values (oh my memory… :))
EDIT2: Schema is moved to: http://discourse.weather-watch.com/