---TITLE:  Winter CO2 efflux - IRGA

---AUTHORS:
  -Jeff Welker (PI), Jace Fahnestock (co-PI)
  -Natural Resource Ecology Laboratory, Colorado State University, Fort Collins,
CO 80523-1499
  -jwelker@nrel.colostate.edu, jace@nrel.colostate.edu
  -970)491-1796 (Jeff), 970)491-5262 (Jace), 970)491-1965 (Fax)

---DATA SET OVERVIEW:
  -This data represents estimates of wintertime carbon efflux from various
tundra and subarctic soils over several winters.  All of the measurements have
been taken in Alaskan arctic ecosystems and, in several cases, as indicated, in
specific experimental plots (e.g., fertilized, warmed, etc.).  Measurements
were collected with an infrared gas analyzer.
  -The major research sites are Barrow, Alaska (71.2N 156.4W), Toolik Lake,
Alaska (68.4N 149.4W), Dalton Highway sites from Fairbanks (65.1N 150.1W) to
Prudhoe Bay (70.2N 148.5W), and a transect of the Kuparuk Watershed which
started near Toolik Lake and ended at Prudhoe Bay.
  -Similar measurements have been collected at many of these sites using static
chemical absorption techniques (NaOH basetraps).  See associated data set
entitled "Winter CO2 efflux base traps"

---DATA COLLECTION and PROCESSING:
  -Measurements have been collected at various dates over several winters in
 different arctic ecosystems in Alaska.  The technique uses an infrared gas
 analyzer (we have generally used PP Systems, EGM-1 or EGM-2 IRGAs) connected to
 a stainless steel sampling probe with teflon tubing.  The probe is pushed into
 the snow at various depths down to the soil surface and concentrations of CO2
 (ppm) are measured.  We have found that the CO2 concentration gradient between
 the soil surface and the atmosphere is generally greater than any other
 sampling point within the snowpack; thus, we only report here measurements at
 the snow-soil interface and the atmosphere (2 m above the snow/ground surface).
  Measurements of snow density and snow and soil temperatures are obtained from
 a snow pit dug at each site where measurements are taken.  Carbon dioxide
 efflux is subsequently calculated using a simple derivative of Fick's flux:
Jg=Dg(d[CO2]/dz)ft
where Jg is Fick's flux estimate, Dg is a temperature dependent diffusion
 coefficient for CO2, d[CO2]/dz is the CO2 concentration gradient over depth, f
 is porosity (calculated as 1 - density of snow/density of ice), and t is an
 estimate of tortuosity (estimated as [1-((1-porosity)^2/3)]/porosity).  Jg is
 then corrected for prevailing temperature and pressure at the time of each
 measurement to generate an instantaneous estimate of CO2 efflux in mg CO2 m-2
 s-1.  This value can then be used to estimate daily efflux rates or combined
 with measurements taken in other days or months to estimate monthly or
 season-long CO2 efflux rates
  -Data span winters from 19960303 to 20001210

---DATA FORMAT:
  -Data set is space delimited ASCII
  -Column headers, in order, are: Site, Location description, Date (of
 measurement), CO2 concentration gradient (ppm), snow depth (cm), snow density
 (g/L), soil temperature (C), Fick's CO2 efflux estimate (ppm CO2 cm sec-1), CO2
 efflux estimate (mg CO2 m-2 d-1), SE (1 Standard Error of the mean CO2 efflux
 estimate)
  -Data version 1.0 (15 March 2002); FINAL

---REFERENCES:
  -More details on this technique and site descriptions can be found in 1)
 Fahnestock, JT, MH Jones and JM Welker. 1999. Wintertime CO2 efflux from arctic
 soils: implications for annual carbon budgets. Global Biogeochemical Cycles
 13:775-779; 2) Jones, MH, JT Fahnestock and JM Welker. 1999. Early and late
 winter CO2 efflux from arctic tundra in the Kuparuk River watershed, Alaska,
 U.S.A. Arctic, Antarctic, and Alpine Research 31:187-190; 3) Fahnestock, JT, MH
 Jones, PD Brooks, DA Walker and JM Welker. 1998. Winter and early spring CO2
 efflux from tundra communities of northern Alaska. Journal of Geophysical
 Research - Atmospheres 103:29023-29027