  We study barrier discharges in CO$_2$ using optical emission spectroscopy and
electrical measurements. We record the spectra of weak CO$_2$ barrier discharge
emission in UV-VIS-NIR range. The CO spectral bands of 3rd positive, Angstrom
and Asundi systems are identified in the spectra, together with the CO+2 bands
of the UV-doublet and the Fox-Duffendack-Baker systems. The effect of CO$_2$
flow rate variations on the relative intensities of the CO and CO$_2$ optical
emissions is also investigated. We analyze the optical emission of selected
vibrational spectral bands and attempt to determine the E/N using intensity
ratio method with kinetic data available in the literature. The method, using
intensities from different spectral bands, is tested on sinusoidal driven
atmospheric pressure Townsend (APTD) and single-filament barrier discharges at
atmospheric pressure. For APTD at different flows and known CO conversion
factors, an attempt is made to use the ratio of selected CO to CO$_2$ band
intensities. The technique of time-correlated single photon counting is used to
obtain necessary sub-nanosecond temporal resolution and, importantly, high
signal sensitivity for the single-filament experiment and its utilization gives
a promising result, E/N of 330 Td in barrier discharge streamer. Nevertheless,
the uncertainties coming from different kinetic data and the low sensitivity of
some rate coefficient ratios complicate an easy utilisation of the method.
Future steps are therefore proposed, including uncertainty quantification and
sensitivity analysis of the simplified model and necessary CO+2 and CO
synthetic spectra fitting.
