No one knows for sure what the early atmosphere was. It most likely came from volcanos.
If the early came from volcanos it contains water vapor, oxygen, carbon dioxide sulfur compounds, and other compounds.
Carbon dioxide is very small percentage of the air less than 1%. In the past the Carbon dioxide levels were much higher. The rocks classified as Mesozoic Era show evidence of lush vegetation which indicates very high Carbon Dioxide levels in the atmosphere.
Sulfurs are very reactive compounds and have been mostly removed from the atmosphere. Except for acid rain there are almost no sulfur compounds in the present atmosphere.
The Early atmosphere was most likely an oxidizing environment.