Picarro (http://www.picarro.com) produces highly sensitive and accurate gas sensing instruments which rely on a technology called cavity ring down spectroscopy. The company published a fascinating methodology to determine actual CO2 emissions from a city.This was performed during the Davos summit.You can see the results of such visualisation here:
CO2 emissions over Davos

Actual methodology is detailed in this publication from Picarro:
CityCarbon_Science_and_Data

This graph compares the emissions for cities of various scales and built around different economies: Davos, Seoul and Tokyo.

Even though sensors used for participatory sensing such as Senspods are not laboratory instruments, once properly calibrated and used by citizens to cover city wide areas, there is no reason why mobile sensors could not be used to quickly build a global environmental network.