There are many types of air quality monitors available, but which one is the best choice for measuring gaseous air pollutant concentrations?
There are two classes of such air quality monitors: sampling monitors and in-situ monitors (“in-situ” = “in the original position”). Sampling monitors utilises a manifold through which the air to be analysed is pumped. A small air sample is then enclosed in a cell where its molecular content can be analysed. The analysis method varies depending on the optical properties of the molecule. By example, SO2 is often detected using UV fluorescence, while gas correlation is used to detect CO. The methods are quite different, and there is therefore usually a specific instrument for each specific type of molecule.
In contrast, an in-situ monitor detects molecular concentrations in their natural environment, without pumping and enclosing in any cell. The detection happens along an open light path, where all molecules present along the path contributes to characteristic absorption of certain wavelengths. Each type of molecule has its own absorption pattern, making it possible for a single monitor to detect the concentration of many different types of molecules.
So, should you choose a sampling or an in-situ monitor when you design your environmental monitoring system?
Looking at detection characteristics and data quality, both methods can give about the same results. However, that is on condition that all maintenance activities are conducted by the book. To that extent, sampling monitors require much more frequent and costly maintenance than in-situ systems. In practice, you often see that maintenance schemes are not upheld, resulting in doubtful monitoring results and even complete loss of data when trying to use sampling monitors.
In contrast, in-situ monitors require very little maintenance. In the long run, they therefore usually produce much more trustworthy data, at much higher capture rates than sampling monitors. Admittedly, an in-situ monitor is usually more expensive to acquire than a single or a few sampling analysers, but the total cost of ownership when also including maintenance aspects is in favour of the in-situ monitor.
A further advantage of an in-situ monitor over a sampling monitor, is the automatic averaging achieved along the light path. The monitoring results are therefore representative for the entire area where the monitoring takes place, and not just at the single point of the air intake to the sampling monitor.
In summary, the best choice of an air quality monitor for detecting gaseous pollutants is usually an in-situ model.