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In comets the atomic oxygen green to red-doublet emission intensity ratio(G/R ratio) of 0.1 has been used to confirm H$_2$O as the parent speciesproducing oxygen emission lines. The larger ($>$0.1) value of G/R ratioobserved in a few comets is ascribed to the presence of higher CO$_2$ and COrelative abundances in the cometary coma. We aim to study the effect of CO$_2$and CO relative abundances on the observed G/R ratio in comets observed atlarge ($>$2 au) heliocentric distances by accounting for important productionand loss processes of O($^1$S) and O($^1$D) in the cometary coma. Recently wehave developed a coupled chemistry-emission model to study photochemistry ofO($^1$S) and O($^1$D) atoms and the production of green and red-doubletemissions in comets Hyakutake and Hale-Bopp. In the present work we applied themodel to six comets where green and red-doublet emissions are observed whenthey are beyond 2 au from the Sun. In a water-dominated cometary coma and withsignificant ($>$10%) CO$_2$ relative abundance, photodissociation of H$_2$Omainly governs the red-doublet emission, whereas CO$_2$ controls the green lineemission. If a comet has equal composition of CO$_2$ and H$_2$O, then ~50% ofred-doublet emission intensity is controlled by the photodissociation ofCO$_2$. The G/R ratio values and green and red-doublet line widths calculatedby the model are consistent with the observation. Our model calculationssuggest that in low gas production rate comets the G/R ratio greater than 0.1can be used to constrain the upper limit of CO$_2$ relative abundance providedthe slit-projected area on the coma is larger than the collisional zone. If acomet has equal abundances of CO$_2$ and H$_2$O, then the red-doublet emissionis significantly ($\sim$50%) controlled by CO$_2$ photodissociation and thusthe G/R ratio is not suitable for estimating CO$_2$ relative abundance.

Photochemistry of atomic oxygen green and red-doublet emissions in comets at larger heliocentric distances