do you have any suggestions??
Yes I do!
First find out what the legal emmissions limits are in your area. They may vary by time of day but you need a full breakdown of what is permitted and what isn't.
From that data work out what it is you need to detect (air quality is very subjective and would not stand much chance in court). - This will tell you what sensor(s) you need, what you need to detect and in what quantities (sensitivity of sensor).
Do you need auto data logging? Do you need a display with the current readings on it? Will you need a graph of the data?
These thoughts should get you thinking more along the right lines and yes I can build something like this, but there are a lot of things to consider first.