Discussion on:

Mobile lab tracks greenhouse gas emissions

Raytheon developed a high gain laser spectrometer for use by the EPA back in the late 1970s. They patented it in 1979. Being a truck mounted rig it was used to verify emissions at the source. Right at the top of the smoke stack.

Maybe it does not work for CO2.

...started shooting her mouth off about CO2 emissions, I and some fellow geeks started hypothesizing an infra-red or laser-based measuring system that could use triangulation to estimate emissions from a not-too-distant location. Basically, the lasers would criss-cross the airspace around the suspect emitter. It would have been interesting and fun to study the emission patters of those who are so vocal about other people's environmental impact. I'm actually quite surprised that this kind of thing doesn't already exist.