Naive. 

Attach a tank to a car, that has living plants, dead plant material or algae. This is in order to do capture CO2 from engine's exhaust. It is necessary to devise a storage tank system too because you woudn't be able to just hook up the exhaust to the device directly. It could be a trailed kart, it does not need to be something that every car is required to have if that is asking too much. From time to time poeple would replenish the biological component. 

I often wondered if crushed leaves could still do photosynthesis, or rather absorb CO2. It isn't very difficult to run an experiment to determine this. Oh, the itch.