Column abundances of carbon dioxide and methane retrieved from ground-based near-infrared solar spectra

摘要

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.ududColumn 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.ududAn 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.ududThe 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.ududFinally, 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.