Measuring Water When Computing Closed Path Flux for CO2 and Other Trace Gases

One frequently asked question about computing flux using a fast closed-path analyzer (e.g., LI-7000, and its predecessor LI-6262) is about the need to record a water signal when measuring CO2 concentrations.

There are two key reasons why it is important to have water vapor measured simultaneously with gas fluxes when using any fast closed-path trace-gas analyzer:

1. A reasonable estimate of water vapor concentration inside the cell is needed to determine the mean gas concentration. If the measured water concentration is incorrect, it can affect the mean gas concentration, and ultimately, the gas flux itself.
2. A good estimate of closed-path water vapor flux is also needed for the Webb-Pearman-Leuning (WPL) density term (Webb et al., 1980). This term is needed to compute correct flux, it is additive to the initial gas flux value and is quite significant in open-path systems. It may also become relatively significant in closed-path systems, especially when the gas flux is small and the water vapor flux is large. When gas flux is small, it is quite possible for the WPL term to exceed the uncorrected flux value.

For intake tubes with length to inner diameter ratios of 1000:1 to 500:1, the sensible heat flux (or the temperature) portion of the WPL term is usually considered negligible, (at least in the case of for CO2). Therefore, for normal magnitudes of for CO2 flux, it can generally be ignored. However, the water vapor (or latent heat flux) portion of the WPL term is still required.

Ideally, water vapor for the WPL term should be measured in the same sampling cell with the closed-path gas concentration. Actual closed-path water vapor fluctuations are attenuated in the intake tube, and as a result of this attenuation, water vapor flux measured inside the cell is significantly lower than the ambient one. Thus, using ambient water vapor flux can lead to overcorrecting the gas flux after the WPL term is applied.

Alternatives, such as Nafion tubes, chemical dryers, etc. are undesirable. They can lead to increased power requirements and maintenance demands, and to a decreased frequency response of the system. Effect of the drying arrangements on the gas flux frequency attenuation may also be difficult to predict and correct, due to the high variability and site-specific nature of such an arrangement, and due to lack of experimental research on the topic.

In summary, measuring water vapor concentration in fast closed-path gas analyzers is the most reliable way to insure accurate data.