Why are greenhouse gases so difficult to measure?

Greenhouse gases are so difficult to measure, because carbone dioxide, methane, chlorofluorocarbons comes from multiple sources, some of which are relatively hard to measure.
For example, missions related to wetlands, deforestation, cattle, leakage from natural gas wells, storage facilities and distribution systems and because such leaks are highly variable and depend on factors such as well construction methods and maintenance systems for infrastructure.

It will fall off and tip over. 

What are the options? i can you if you give them.