The AP100, designed for measuring atmospheric profiles of carbon dioxide (CO2) and water vapor (H2O), incorporates nearly 20 years of experience designing, building, and testing custom sampling systems for atmospheric trace gases or chamber concentrations. An insulated enclosure (16 in. by 18 in.), complete with mounting hardware, houses a CR1000 datalogger and 7-Ahr battery, LI-COR Model LI-840 CO2/H2O Gas Analyzer, and a miniature manifold where one of the eight profile intakes or up to three zero/span gases are selected for analysis. Swagelok connections (0.25 in. OD) are provided in the enclosure bottom. An optional heater provides operation to -30°C ambient temperatures.
The flow (~0.9 slpm) and pressure (~50 kPa) in the profile intake tubing is set by an orifice plumbed directly behind the intake filter. Operating the intake tubing at reduced pressure prevents condensation (within limits) and reduces the time required for H2O to equilibrate to a new concentration. The CO2 and H2O equilibrate to within 0.25% and 0.05%,respectively, of the concentration change from the previous intake for the following measurement sequence.
The entire profile (eight intakes) is sampled every 2 minutes, with each intake selected for 15 seconds. The first 8 seconds are allowed for equilibration, with measurements obtained over the last 7 seconds. These results are obtained using the AC powered pump (250 W) supplied with the system. Equilibration times have yet to be determined for lower flow rates obtained using a lower-powered 12-Vdc pump.