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Figure 1: The top panel shows the spatial distribution of averaged CO2 at 1 m above the ground over about a 1 km x 1km area and the bottom panel shows the CO2 along the distance across Como Creek (140 m). The plot demonstrates that nighttime CO2 accumulation was transported toward low lying grounds, even toward the 2-m-wide creek along the main slope; however, the spatial distribution of CO2 was also affected by low respiration from the water and local mixing generated by the thermal contrast between the water and its surrounding land, leading to low CO2 right above the water (not included in the top panel). This work advances our understanding of CO2 transport processes, which can lead to better estimates of the terrestrial carbon budget which is important to understanding global warming.
The spatial distribution of CO2 reflects how respired CO2 is transported at night. The current long-term CO2 monitoring networks completely miss horizontal transport of CO2, which significantly affects annual terrestrial CO2 budgets. The data analysis from CME and ACME will have significant impacts on the long-term CO2 observation community. Future data analysis from both CME and ACME will focus on problems such as how terrain topography affects CO2 transport differently during day and night, how this deviates from CO2 transport over flat terrain, and how regional and global terrestrial carbon budgets are affected. |
The difficulty in understanding the carbon dioxide (CO2) budget over complex terrain, and particularly its contribution to the global carbon balance, has led to two complementary field studies: the Carbon in the Mountain Experiment (CME) and the Airborne Carbon in the Mountain Experiment (ACME-04). Both supported the ESSL science priorities of understanding the representation of precipitation and chemical processes, including multiscale atmospheric chemical constituent transport, dispersion and transformations. CME was an intensive ground-based field campaign over the foothills of the Colorado Front Range from 10 June to 5 October 2004. ACME was an airborne campaign conducted on a large-scale area of the Front Range from mid-May to the beginning of August, 2004. This unique combination allowed us to “scale” up the CO2 budget from several hundreds of meters to regional scales.
The field campaigns confirmed our speculations that nighttime respired CO2 accumulated in low lying areas through drainage flows. Drainage flows are sensitive to land surface types and topography. On the large-scale, our aircraft data analysis indicates that nighttime respired CO2 accumulated in North Park, Colorado, a bowl-shaped area surrounded by mountains. The disappearance of the accumulated high CO2 concentration in the morning due to the reverse of the slope flow and convective mixing was slower than expected. The CO2 concentration decreased exponentially with height even around 10 a.m. On the local scale, we found that at the Niwot Ridge CME site, the CO2 concentration increased with decreasing altitude. High CO2 was even transported toward Como Creek, which runs along the main slope and is only about 2 m wide (Figure 1 upper panel). However the spatial variation of CO2 was also affected by the low respiration of CO2 from the cold water in Como Creek and local mixing generated by the thermal contrast between the creek and the surrounding land. As a result, the CO2 concentration was lower just over the water surface than it was over its banks (Figure 1 lower panel). |
