The accumulation of dust, pollen, chemical residues, and other contaminants on the optics of an unfiltered enclosed gas analyzer can lead to measurement drift and cause gaps in datasets. This is not a significant problem if an instrument is filtered and regularly maintained. If an instrument is unfiltered or not maintained, however, there is risk that contaminants will affect measurements.
Innovations to the optics and electronics ensure that the LI-7200RS collects more accurate and dependable data—even as contaminants begin to accumulate on the optics.
Data from 13 instruments at 5 sites—with exposure to different levels of contamination—consistently show that the LI-7200RS provides more stable water vapor measurements than the original LI-7200 (Figure 1). CO2 measurements (not shown) are similar to or slightly better than those of the LI-7200.
High-speed temperature and pressure measurements of sampled air—the actual air in the gas analyzer cell—provide the data required to compute fast dry mole fraction, and thus compute accurate fluxes. The LI-7200RS measures air temperature and pressure with a sophisticated system of thermocouples and pressure sensors. This patented configuration ensures true alignment of temperature, pressure, and gas densities, to ensure the most accurate flux results.
Although the absence of fast air temperature and pressure measurements may appear negligible on half-hourly fluxes, the problems become apparent when computing carbon budgets (Figure 3), which can accrue large differences.