This isn't some dumb-downed whitewash. It's a must-read for a beginner taking his or her first steps toward understanding stats. There is already a background level of this in the air of say 4ppb. My normal emissions might raise this by 1ppb. Occasionally - a handful of times per year the process might give off a higher than usual emission, raising levels in the surrounding air to say 4000 ppb or more for a period of a few seconds or a few minutes.
Ambient measurement is very costly but for various reasons is the only option.
This appears to be the sort of problem where some sort of cumulative approach is called for. All rights reserved.