You can use two temperature sensors with a small heating element.
That is how cheap car emission analysers work.

The one sensor is housed with the element in the path of the gas.
The other sensor is housed so that it is as close as possible to
the ambient temperature of the heated sensor, but without the
cooling effect of the moving gas. For a constant gas flow, there
would be a correlation between the level of CO2 and the temperature
difference. I have no idea what this relationship is, though.

--
Friendly Regards

Tjaart van der Walt
mailto:tjaart@wasp.co.za
 _____________________________________________________________
| WASP International http://www.wasp.co.za/~tjaart/index.html |
|       R&D Engineer : GSM peripheral services development    |
|   Vehicle tracking | Telemetry systems | GSM data transfer  |
|    Voice : +27-(0)11-622-8686 | Fax : +27-(0)11-622-8973    |
|              WGS-84 : 26010.52'S 28006.19'E                 |
|_____________________________________________________________|