Laboratories

Trace Gas Laboratory

The Biosphere 2 contains the instrumentation and tools to measure the main greenhouse gases (CO2, CH4, and N2O) and a man-made tracer (SF6) to track gas exchange and calibrate exchange volumes.

Greenhouse gases

Carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are three of the most dominant greenhouse gases.  All are both naturally produced and consumed by organisms in ecosystems and produced through human activity.  Our work focuses on measuring the dynamics of the greenhouse gases in natural ecosystems, in particularly in tropical ecosystems, which are among the most productive in the world, but we also measure the dynamics of these gases associated with the LEO slopes.

Tracer gas

Sulfur hexafluoride (SF6) is a purely manmade gas and thus an ideal gas to measure gas transfer rates, leak rates, and sample volumes because it does not interact with the biology of ecosystems.  One drawback of SF6 is it’s greenhouse potential (a whopping 40,000x CO2), therefore we use an ECD detector, which can measure SF6 in the ppt range (parts per trillion, or 1 part in 1,000,000,000,000 parts), which means that we only have to inject ~25ml in our 26,700 m3 rainforest to measure the leak rate.

Instruments

- Aerodyne Quantum Cascade Laser for CO2 concentration and isotope measurements.  This highly sensitive and precise instrument can be used to track carbon processes in ecosystem through measurements of the atmosphere across time and by isolating different parts of the ecosystem with specialized, automated chambers (see below).  Measurements are made at a rate of 1 Hz (or one measurement per second). The air entering the instrument is dried by a PermaPure counter flow air drier and flowing through the instrument at 400 ml per minute. The Aerodyne laser routinely has a precision of 0.01ppm for the CO2 concentration and 0.01‰ for both d13C and d18O, when we average isotope standards (filled by the CMDL lab at NOAA, Boulder CO) for a period of one minute.


- Gasmet DX4015 FITR gas analyzer for field portable automatic measurements of CO2, CH4, N2O and H2O. The Gasmet takes a measurement twice a minute with a precision of 0.2ppm, 0.02ppm, 2ppb, and 0.005% for CO2, CH4, N2O and H2O, respectively. Besides using the instrument for measurements inside Biosphere 2, we have taken the instrument to Brazil and Peru into forests and peatlands for greenhouse gas flux measurements.


- SRI greenhouse gas Gas Chromatograph (GC) for manual measurements for CO2, CH4, N2O and SF6.  We use 25cc syringes B&D plastic syringes with poly-carbonate, luer-lock stopcocks to collect air samples,  which we then –within 24hours- inject into the two 1 ml sampling loops of the GC.  The N2 carrier gas then takes the air from the sampling loops through a 2m Hayesep-D column, followed by a 25 cm molesieve 5a column to separate SF6 and N2O, to either a Flame Injection Detector (CO2 and CH4) or an Electron Capture Detector (N2O and SF6). Each analysis takes 2.5 minutes and every hour we run three high and low standards (supplied by Scott-Marrin, CA) to calculate the gas concentrations.  The precision of the instrument is ~+/-1% of the concentration for all gasses.

Sampling capabilities

- Soil flux chambers.  To measure gas fluxes from soil surfaces we have several types of soil chambers, which we use depending on the location and sampling technique.  The main trade-off with soil chambers is the need for a seal between the soil surface and the chamber and not cutting too many roots, which can greatly affect the gas fluxes because of the release of sugars and amino acids from the roots.  Aluminum chambers (1x1 ft) with a small moat to create a water seal between the base and lid are used in external, windy conditions and in very sandy soils.  In tropical soils, in the understory and at Biosphere 2 we use (8” diameter) circular chambers that are sealed to the soil surface with a strip of heavy-duty weather-stripping foam and tent stakes.  We also have used these chambers to measure the surface flux of CO2 on the LEO slopes, by applying a ~5 kg weight to the top of the chamber (both lid and base) to press the weather-stripping foam down onto the LEO basalt surface, that cannot be disturbed to preserve hydrological flow paths.
For automated sampling of the soil gas fluxes we have developed (14” diameter) flux chambers with automated lids that are driven by a small winch and optical sensors to ensure closure. The automated soil chambers can then be connected to the Aerodyne and Gasmet analyzers.


- Stem flux chambers. Stem respiration is a significant component of the overall carbon budget of tropical ecosystems and most chamber designs are rigid, thus have a limitation to the size of stems that can be measured.  We designed novel chambers based on hardware components to create a flexible chamber that can measure gas fluxes from tree stems greater that 5cm in diameter. Besides the flexibility the advantage of the chambers is that they are easy to install on the stems and can be adapted to rough bark or irregular stems using putty.


- Branch bags. We have deployed tedlar bags (60x30cm) to enclose a small part of a branch with several leaves to measure the CO2 exchange between the leaves and the air.  The bags can both be deployed for a short period of time for manual sampling, or up to two-months for automated sampling. Longer enclosures negatively impact the leaves enclosed, even though the bags for long installations are outfitted with small fans to rapidly circulate the air and cool the leaves.
 

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