We have developed an automated observatory for measuring atmospheric column abundances of CO2 and O2 using near‐infrared spectra of the Sun obtained with a high spectral resolution Fourier Transform Spectrometer (FTS). This is the first dedicated laboratory in a new network of ground‐based observatories named the Total Carbon Column Observing Network. This network will be used for carbon cycle studies and validation of spaceborne column measurements of greenhouse gases. The observatory was assembled in Pasadena, California, and then permanently deployed to northern Wisconsin during May 2004. It is located in the heavily forested Chequamegon National Forest at the WLEF Tall Tower site, 12 km east of Park Falls, Wisconsin. Under clear sky conditions, ∼0.1% measurement precision is demonstrated for the retrieved column CO2 abundances. During the Intercontinental Chemical Transport Experiment–North America and CO2 Boundary Layer Regional Airborne Experiment campaigns in summer 2004, the DC‐8 and King Air aircraft recorded eight in situ CO2 profiles over the WLEF site. Comparison of the integrated aircraft profiles and CO2 column abundances shows a small bias (∼2%) but an excellent correlation.
[1] Observations of the column-averaged dry molar mixing ratio of CO 2 above both Park Falls, Wisconsin and Kitt Peak, Arizona, together with partial columns derived from aircraft profiles over Eurasia and North America are used to estimate the seasonal integral of net ecosystem exchange (NEE) between the atmosphere and the terrestrial biosphere in the Northern Hemisphere. We find that NEE is $25% larger than predicted by the Carnegie Ames Stanford Approach (CASA) model. We show that the estimates of NEE may have been biased low by too weak vertical mixing in the transport models used to infer seasonal changes in Northern Hemisphere CO 2 mass from the surface measurements of CO 2 mixing ratio.
[1] The column-averaged volume mixing ratio (VMR) of CO 2 over Kitt Peak, Arizona, has been retrieved from high-resolution solar absorption spectra obtained with the Fourier transform spectrometer on the McMath telescope. Simultaneous column measurements of CO 2 at $6300 cm À1 and O 2 at $7900 cm À1 were ratioed to minimize systematic errors. These column ratios were then scaled by the mean O 2 VMR (0.2095) to yield columnaveraged vmrs of CO 2 . These display similar behavior to the Mauna Loa in situ surface measurements. During the period 1977 -1995, the column-averaged mixing ratio of CO 2 increased at an average rate of 1.49 ± 0.04 ppmv/yr with seasonal variations of $7 ppmv peak-to-peak. Our retrievals demonstrate that this remote technique is capable of precisions better than 0.5%.
High-resolution solar absorption spectra obtained from Table Mountain Facility (TMF, 34.3S0N, 1 17.68OW, 2286 m elevation) have been analyzed in the region of the 0 2 A-band. The photon paths of direct sunlight in clear sky cases are retrieved from the 0 2 absorption lines and compared with ray-tracing calculations based on the solar zenith angle and surface pressure. At a given zenith angle, the ratios of retrieved to geometrically derived photon paths are highly precise (-0.2%), but they vary as the zenith angle changes. This is because current models of the spectral lineshape in this band do not properly account for the significant absorption that exists far from the centers of saturated lines. For example, use of a Voigt function with Lorentzian far wings results in an error in the retrieved photon path of as much as 5%, highly correlated with solar zenith angle. Adopting a super-Lorentz function reduces, but does not completely eliminate this problem. New lab measurements of the lineshape are required to make further progress.
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