Northern and southern hemisphere ground‐based infrared spectroscopic measurements of tropospheric carbon monoxide and ethane

Abstract

Time series of CO and C2H6measurements have been derived from high‐resolution infrared solar spectra recorded in Lauder, New Zealand (45.0°S, 169.7°E, altitude 0.37 km), and at the U.S. National Solar Observatory (31.9°N, 111.6°W, altitude 2.09 km) on Kitt Peak. Lauder observations were obtained between July 1993 and November 1997, while the Kitt Peak measurements were recorded between May 1977 and December 1997. Both databases were analyzed with spectroscopic parameters that included significant improvements for C2H6relative to previous studies. Target CO and C2H6lines were selected to achieve similar vertical samplings based on averaging kernels. These calculations show that partial columns from layers extending from the surface to the mean tropopause and from the mean tropopause to 100 km are nearly independent. Retrievals based on a semiempirical application of the Rodgers optimal estimation technique are reported for the lower layer, which has a broad maximum in sensitivity in the upper troposphere. The Lauder CO and C2H6partial columns exhibit highly asymmetrical seasonal cycles with minima in austral autumn and sharp peaks in austral spring. The spring maxima are the result of tropical biomass burning emissions followed by deep convective vertical transport to the upper troposphere and long‐range horizontal transport. Significant year‐to‐year variations are observed for both CO and C2H6, but the measured trends, (+0.37±0.57)% yr−1and (−0.64±0.79)% yr−1, 1 sigma, respectively, indicate no significant long‐term changes. The Kitt Peak data also exhibit CO and C2H6seasonal variations in the lower layer with trends equal to (− 0.27±0.17)% yr−1and (−1.20±0.35)% yr−1, 1 sigma, respectively. Hence a decrease in the Kitt Peak tropospheric C2H6column has been detected, though the CO trend is not significant. Both measurement sets are compared with previous observations, reported trends, and three‐dimensional model calculations.