N2O Vertical Profiles Retrieved from Ground-based Solar Absorption Spectra Taken at McMurdo Station During Austral Spring of 1989

Abstract

N2O can be a tracer of atmospheric air motion due to its long life time. Ground-based FTIR solar spectra contain information on the vertical distributions of N2O due to pressure-broadening of absorption lines. We have combined the Chahine-Twomey' relaxation method with a line-by-line layer-by-layer radiative transfer code to retrieve N2O VMR profiles from ground-based solar absorption spectra. The spectra were taken at McMurdo station during the austral spring of 1989 with a 0.02 wavenumber resolution FTIR spectrometer. Since N2O is released from troposphere and is photolyzed in the stratosphere, the line shape of its absorption is mainly due Lorentz broadening. The 0.02 wavenumber resolution is high enough for us to retrieve N2O VMR profiles up to 25 kilometers. Figure 1 shows a typical observed N2O solar spectrum near 1993.15 wavenumber (solid line) and a calculated spectrum using our profile retrieval program. The best fit is obtained by iteratively adjusting N2O VMR profile according to the formulation of Chahine and Twomey. A contour plot of N2O VMR versus altitude and julian day number is shown in Figure 2. The lower tropospheric N2O VMRs have an average value around 310 ppb. Correlations of the N2O contour with that of temperature shows interesting features of tropospheric and lower stratospheric air motions. We have also compared the total N2O column amounts retrieved from this profile retrieval method and from the PC version of the non-linear least square spectral fitting algorithm (SFIT). The temporal variations of the N2O total column amounts retrieved from two methods show excellent correlation.