Retrieval of atmospheric CO2 vertical profiles from ground-based near-infrared spectra
Abstract. We evaluate vertical profile retrievals of CO 2 from 0.02 cm −1 resolution ground-based near-infrared solar absorption spectra with the GFIT2 algorithm, using improved spectroscopic line lists and
line shapes. With these improvements, CO 2 profiles were obtained from
sequential retrievals in five spectral windows with different vertical
sensitivities using synthetic and real spectra. A sensitivity study using
synthetic spectra shows that the leading source of uncertainty in the
retrieved CO 2 profiles is the error in the a priori temperature
profile, even with 3-hourly reanalysis a priori profiles. A 2  ∘ C
error in the temperature profile in the lower troposphere between 0.6 and
0.85 atm causes deviations in the retrieved CO 2 profiles that are
larger than the typical vertical variations of CO 2 . To distinguish the
effect of errors in the a priori meteorology and trace gas concentration
profiles from those in the instrument alignment and spectroscopic
parameters, we retrieve CO 2 profiles from atmospheric spectra while
using an a priori profile built from coincident AirCore, radiosonde, and surface in
situ measurements at the Lamont, Oklahoma (USA), Total Carbon Column
Observing Network station. In those cases, the deviations in retrieved
CO 2 profiles are also larger than typical vertical variations of
CO 2 , suggesting that remaining errors in the forward model limit the
accuracy of the retrieved profiles. Implementing a temperature retrieval or
correction and quantifying and modeling an imperfect instrument alignment
are critical to improve CO 2 profile retrievals. Without significant
advances in modeling imperfect instrument alignment, and improvements in the accuracy of the temperature profile, the CO 2 profile retrieval with GFIT2 presents no clear advantage over scaling retrievals for the purpose of ascertaining the total column.