HR: 1340h
AN: B13B-1193    [Abstracts]
TI: Effects of Water Vapor on the Data Quality of the Stable Oxygen Isotopic Ratio of Atmospheric Carbon Dioxide
AU: * Evans, C U
EM: Candice.Evans@colorado.edu
AF: Institute of Arctic and Alpine Research, University of Colorado at Boulder, Campus Box 450, Boulder, CO 80309, United States
AU: White, J W
EM: James.White@colorado.edu
AF: Institute of Arctic and Alpine Research, University of Colorado at Boulder, Campus Box 450, Boulder, CO 80309, United States
AU: Vaughn, B
EM: Bruce.Vaughn@colorado.edu
AF: Institute of Arctic and Alpine Research, University of Colorado at Boulder, Campus Box 450, Boulder, CO 80309, United States
AU: Tans, P P
EM: Pieter.Tans@noaa.gov
AF: National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Global Monitoring Division, R/GMD1 325 Broadway, Boulder, CO 80305, United States
AU: Pardo, L
EM: lpardo@ncar.edu
AF: Universidad Interamericana de Puerto Rico, Ave. Interamericana Carr. 102 Km. 30.6, San German, 00683, Puerto Rico
AB: The stable oxygen isotopic ratio of carbon dioxide can potentially track fundamental indicators of environmental change such as the balance between photosynthesis and respiration on regional to global scales. The Stable Isotope Laboratory (SIL) at the Institute of Arctic and Alpine Research (INSTAAR), University of Colorado at Boulder, has measured the stable isotopes of atmospheric carbon dioxide from more than 60 NOAA/Earth System Research Laboratory (ESRL) air flask-sampling sites since the early 1990s. If air is sampled without drying, oxygen can exchange between carbon dioxide and water in the flasks, entirely masking the desired signal. An attempt to investigate how water vapor is affecting the δ¹⁸O signal is accomplished by comparing the SIL measurements with specific humidity, calculated from the National Climatic Data Center (NCDC) global integrated surface hourly temperature and dew point database, at the time of sampling. Analysis of sites where samples have been collected initially without drying, and subsequently with a drying kit, in conjunction with the humidity data, has led to several conclusions. Samples that initially appear isotopically unaltered, in that their δ¹⁸O values are within the expected range, are being subtly influenced by the water vapor in the air. At Bermuda and other tropical to semi-tropical sites, the ‘wet' sampling values have a seasonal cycle that is strongly anti-correlated to the specific humidity, while the ‘dry' values have a seasonal cycle that is shifted earlier than the specific humidity cycle by 1-2 months. The latter phasing is expected given the seasonal phasing between climate over the ocean and land, while the former is consistent with a small, but measurable isotope exchange in the flasks. In addition, we note that there is a strong (r > 0.96) correlation between the average specific humidity and the percent of rejected samples for ‘wet' sampling. This presents an opportunity for determining a threshold of specific humidity, below which air flask samples can be trusted. This approach may allow segregation of suspect and trusted data, and thus provide an improved record of oxygen isotopic ratios of carbon dioxide over the past two decades.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0438 Diel, seasonal, and annual cycles (4227)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting