Long-term trend of CH₄ at northern mid-latitudes: Comparison between ground-based infrared solar and surface sampling measurements

We report average tropospheric CH₄ volume mixing ratios retrieved from a 27 year time series of high spectral resolution infrared solar absorption measurements recorded between May 1977 and July 2004 at the US National Solar Observatory station on Kitt Peak (31.9°N, 111.6°W, 2.09 km altitude) and their comparison with surface in situ sampling measurements recorded between 1983 and 2004 at the Climate Monitoring and Diagnostics Laboratory (CMDL) station at Niwot Ridge, Colorado (40.0°N, 105.5°W, 3013 m altitude). The two measurement sets therefore overlap for the 1983-2004 time period. An average tropospheric volume mixing ratios of 1814 ± 48 ppbv (1 ppbv = 10,^-9 per unit volume) has been derived from the solar absorption time series with a best-fit increase rate trend equal to 8.26 ± 2.20 ppbv yr^-1 in 1983 decreasing to 1.94 ± 3.69 ppbv yr^-1 in 2003. The CMDL measurements also show a continuous long-term CH₄ volume mixing ratio rise, with subsequent slowing down. A mean ratio of the retrieved average tropospheric volume mixing ratio to the CMDL volume mixing ratio for the overlapping time period of 1.038 ± 0.034 indicates agreement between both data sets within the quantified experimental errors.