Fenceline monitoring is easy to describe and harder to picture. This is what the work looks like in action.
A campaign begins with the site, not the instrument. Where are the emission sources? What does the prevailing wind do, and what does it do when it is not prevailing? Where are the property boundaries, the receptors, and the locations where a measurement will actually mean something?
Getting this wrong produces data that is technically accurate and practically useless. A measurement upwind of everything tells you very little.
Real-time instrumentation on a mobile platform measures continuously rather than collecting a sample for later analysis. That difference matters more than it sounds.
A time-integrated sample shows the average concentration over the collection period. A real-time measurement shows you the shape of what happened. A brief high-concentration event and a steady low-level release can average to the same number, but they mean entirely different things operationally.
And because the platform moves, a reading is not a single point. It is a series of points that together describe where something is coming from.
This is where the real value sits. When a concentration rises, the question immediately becomes why, and a mobile platform can pursue that question while the event is still happening.
Move upwind. Measure again. Narrow the area. What began as an elevated reading at the property line resolves into a specific unit, a specific valve, or a source that is not on this site at all.
With conventional sampling, that entire sequence happens weeks later, in a report, about an event nobody can revisit.
A baseline of what normal looks like at that fence line. Identification of sources contributing to it. And where an anomaly occurred, there should be an account of its source rather than just a note that it happened.
For facilities preparing for HON corrective action requirements, that distinction is the entire point. Knowing a threshold was exceeded is the beginning of the obligation, not the end.
Related reading: PTR-MS: Moving mass spectrometry into the field.