More and more, we are talking about trace level ambient monitoring. This monitoring is in line with the measurement objectives of NCORE which is part of the Ambient Monitoring Technology Information Center of the US EPA. According to the EPA website, "NCore is a multi pollutant network that integrates several advanced measurement systems for particles, pollutant gases and meteorology. Most NCore stations have been operating since the formal start of the network on January 1, 2011." The measurement standards, as set it 40 CFR Part 58 Appendix A, are:
It can be a challenge to reach these levels due to a handful of error sources. These were well outlined in a presentation by Avraham Teitz and Mustafa Mustafa of US EPA Region 2. This presentation can be viewed here.
As they described, using a system that allows total flow to be measured at the mass flow control and not the just system output is a great way to minimize error in your flow measurement. In slide 18 and 19, you can see a drawing of the Series 6100 layout which is plumbed to allow direct measurement.
A second source of error is the zero air that is used. It is important to scrub the air of all moisture in addition to using charcoal, purafil and palladium scrubbers. Our ZAG provided all of these features, which are show on page 27 (and below). In addition, a hydrocarbon scrubber can be added for applications that need these to be removed. Proper ZAG maintenance (filters and media) is also important to keep the zero air as clean as possible.
In addition to these error sources, it is critical to have accurate and repeatable results from the calibrators and analyzers. For ozone and NO2, it is critical to have stable and linear readings for these very low ppb levels. For these measurements, the Series 6100 and 6103 both generate down to 2 ppb in a stable a repeatable manner.