Monday, July 28, 2014

Greenhouse Gas Fluxes from the Tundra – How Are Those Measured?

Earlier in the week I mentioned that our NGEE Arctic team was interested in understanding how changes in landscape topography will impact the flux of CO2 and CH4 from the various polygons we are studying on the Barrow Environmental Observatory (BEO). Recall that we are investigating how ice-rich polygons might evolve in a warming environment and how surface-subsurface interactions, hydrology, biogeochemistry and vegetation dynamics might change for wet versus dry areas. The distribution of water varies dramatically across the landscape and it is likely that the flux of greenhouse gases do as well. Such information is needed for high-resolution models of land-atmosphere interactions and feedbacks to climate; however, how are those fluxes measured?

Scientists on our team are using a number of different approaches to get at this information. One of the best, especially if we want to characterize fluxes for specific locations among polygons, is the chamber-based approach. Using this technique, a clear or opaque chamber is placed over a corresponding collar that has been pressed into the tundra to form an airtight seal. The chamber can be attached to a gas analyzer and the flux of CO2 and CH4 quickly determined. A typical measurement sequence can provide information on net ecosystem exchange and respiration by switching between the two types of chambers and measurements can be completed in 5 to 10 minutes.
Oriana and Bryan, both from LBNL, have been conducting these measurements all week at our replicated field sites and along several transects. We are getting good data from dry and wet areas, and areas like the centers and troughs of polygons and small ponds where standing water is prevalent. Having CO2 and CH4 flux rates from these areas will be good since we have already determined the fraction of the landscape occupied by these geomorphological features. Therefore, upscaling from plot to landscape scales will be fairly straightforward.

In addition to measuring greenhouse gases from our polygon sites and transects, we are also collecting data from along the NGEE Arctic tram. This is a 60-meter long set of raised rails on which a cart carrying various sensors travels every 3 hours throughout the day. The sensor measures many components of the energy balance of the tundra plus pictures, thermal images, and albedo. The tram, installed by Bryan and colleagues at BNL and LBNL, has been in place since before snowmelt. Chamber measurements of CO2 and CH4 along the length of the tram, plus repeated measurements of soil moisture, temperature, and thaw depth, will help interpret seasonal changes in energy budgets across the summer and into early winter. In addition, we have an automated chamber nearby that records CO2 fluxes every 30 minutes so we can identify if, and how, CO2 fluxes vary over a 24 hour period.

The NGEE Arctic team will use this information to characterize greenhouse gas fluxes for the Arctic and compare results with an eddy covariance tower at our site. This way we can evaluate small-scale controls on large-scale fluxes, and use that knowledge, once validated, to improve the predictive skill of Earth system models.