| Boundary-layer measurements of concentration, carbon and oxygen isotopes of atmospheric CO2 over montane forests in Colorado, USA | |||||||||
| By:Chun-Ta Lai | |||||||||
| Composite of the NSF/NCAR C-130 research aircraft flying over the Rocky Mountains in Boulder. | |||||||||
| Introduction The mountain ecosystems, particularly areas along the Rocky Mountains, have been suggested as “islands” of high carbon uptake that are responsible for 25 to 50 % of US carbon uptake (Schimel et al., 2002). This high carbon uptake is surprising given a landscape of otherwise arid and semiarid ecosystems with relatively low carbon sequestration under current land-use practice. To understand mechanisms that control carbon exchange in forested mountain ecosystems, I participated in a collaboration project with scientists in the National Center for Atmospheric Research and the University of Colorado in 2004. The Airborne Carbon in the Mountains Experiment (ACME) was proposed under multi-institutional collaborations with a focused area along the Front Range of the Colorado Rocky Mountains. ACME emphasizes the importance of characterizing respiration and its isotopic signatures. A “Keeling Plot” approach can be applied to estimate the carbon isotope ratios of nighttime respired CO2 (d13CR). Flasks were filled in the early morning to measure concentrations, d13C and d18O of CO2 in the boundary layer using a C-130 aircraft. Flasks were also filled inside forest canopies by an automated sampler on the night before a research flight. Using flask samples collected in both ground-based and airborne measurements, we were able to compare d13CR values associated with surface respiration and those in the residual boundary layer. In addition, vertical profiles of CO2 and its isotope signatures in the convective boundary layer (CBL) were measured, which were compared to tower-based measurements conducted hourly during the day. | |||||||||
| Flask sample collection Flask samples were collected both on the ground and in the atmospheric boundary layer. Two sampling strategies were employed for airborne measurements: first is to fill flasks in the early morning to capture respiration signatures before the nocturnal boundary layer was broken from convective mixing. As the ground heated after sunrise and the depth of CBL increased, flasks were collected again to estimate isotope ratios of net CO2 exchange between the atmosphere and the biosphere. | |||||||||
| Collecting air samples by filling flasks on C-130. | |||||||||
| Equipment 1 | |||||||||
| Equipment 2 | |||||||||
| Study area 1 Colorado mountains | |||||||||
| Study area 2 Colorado mountains | |||||||||
Preliminary results | |||||||||
| Comparison between vertical profiles of CO2 concentration, d13C and d18O of atmospheric CO2 measured in the morning and in the afternoon of July 20, 2004. Lower altitude was achieved during morning flights in order to collect air samples in the nocturnal boundary layer (NBL). While respiration clearly showed its impact on NBL during morning hours, afternoon profiles showed a reversed trend with lower CO2 concentration and higher d13C values closer to the ground. This reversed vertical profile was due to CO2 uptake via photosynthesis where plants were known to discriminate against 13C, leaving the atmosphere enriched in d13CO2. Sampling height is referenced to the mean sea level (MSL). | |||||||||
| Comparison between diurnal variations of CO2 concentration, d13C and d18O of atmospheric CO2 measured above forest canopies and in the CBL. There is a rapid decrease in CO2 concentration above forests in the early morning, owing to the break up of NBL. Values of d13C largely mirror those of CO2 concentration. There were times that measurements in CBL were nearly identical to those above the canopy (e.g., early morning on May 20 and in the afternoon of July 29). However, there were also times when CBL measurements were clearly different from those measured above forests. For instance, CO2 concentration was higher while d13CO2 was lower in the CBL from 7 to 10 am on July 29. d18O values in CO2 measured in CBL were very comparable to those measured above forests, which clearly showed an enrichment effects of photosynthesis on the morning of July 29. | |||||||||
| Comparisons between carbon isotope ratios of respiration measured in CBL (d13CCBL) and within forest canopies (d13CR). For the 3 selected days, values of d13CCBL were slightly more positive, considerably more positive and indistinguishable from those of d13CR on May 20, May 28 and July 22 respectively. More positive values in CBL could be interpreted as that samples collected in CBL integrate fluxes over a larger spatial area, therefore they are likely to be affected by non-local sources of C4 plants afar from the mountain regions. Sampling height is referenced to the mean sea level (MSL). | |||||||||
References: Schimel, D., T.G.F. Kittel, S. Running, R. Monson, A. Turnipseed, D. Anderson, Carbon sequestration studied in western US mountains, EOS Transactions, 83: 445-449, 2002. | |||||||||