I would suggest using the static chamber. Electrochemical sensors may not be a good choice because they often have limited sensitivity. In you case, I would imagine the NOx concentration is very low (sub-ppm level). Chemiluminescent NOx analyzer should work but make sure you choose the right model (Some models are for stack emission tests).

We have MRU varrio plus multigas analyser it has NDIR based and electrochemical sensors with detectable limit for NOx for low concentrations, we will be first testing it in the field