Column Abundances of Carbon Dioxide and Methane Retrieved from Ground-Based Near-Infrared Solar Spectra

Abstract

To predict future climate change, we must accurately predict future atmospheric concentrations of CO₂ and CH₄. The current budget has typically been inferred from top-down analyses of measurements from a global network of surface sites. These measurements are highly accurate, but have limited spatial coverage. In addition, accurate knowledge of local planetary boundary layer dynamics is necessary to determine fluxes. Column measurements, defined as the vertical integral of gas concentration, can complement the existing in situ network. Because column measurements sample a larger portion of the atmosphere, they exhibit less variability than surface data, while retaining information about surface fluxes. Column measurements are not influenced by planetary boundary layer dynamics, and do not suffer from the resulting correlation between exchange and transport. An automated observatory for measuring ground-based column abundances of CO₂, CH₄, and O₂ is described. Near-infrared spectra of the direct sun are obtained from 3,900–15,600 cm⁻¹ by a Bruker 125HR Fourier transform spectrometer. The observatory was assembled in Pasadena, California and then permanently deployed to Northern Wisconsin during May 2004. Under clear sky conditions, retrieved column CO₂ abundances demonstrate ~0.1% precision. Comparison of these column measurements with eight aircraft profiles of in situ CO₂ recorded during summer 2004 shows a small bias, but an excellent correlation. The observed secular increase and seasonal amplitude of column-average CO₂ observed during the period of May 2004 – March 2006 is 1.8 ppmv yr⁻¹ and 11 ppmv, consistent with theoretical predictions that the measurements will be representative of Northern Hemisphere CO₂ exchange over seasonal timescales. Comparisons with eddy covariance measurements show that the column measurements have potential for directly observing CO₂ exchange, but that this ability is constrained by the difficulty in accounting for atmospheric transport. Finally, the use of near-infrared spectral analysis is extended to observations of tropospheric column-average CH₄ concentrations. By employing a stratospheric slope equilibrium relationship between CH₄ and HF, the varying contribution of stratospheric CH₄ to the total column is inferred. This method is used to determine tropospheric column-average CH₄ VMRs from near-infrared solar absorption spectra recorded at the Kitt Peak National Solar Observatory during 1977–1995.