# What is the current atmospheric concentration of $\small\sf{CO_2}$ in the atmosphere in atm (or bar)?

For some reason, I can only find the answer in ppm and I don’t know how to do this conversion.

What I think you are asking for is the partial pressure of carbon dioxide in the atmosphere.

The partial pressure of carbon dioxide can be determined, where,

$$P_{CO_2} \ = \ x_{CO_2}\cdot P_{atm}$$

One mole of any substance is equal to exactly 6.022⋅1023 molecules of that substance (Avogadro's number, $$N_A$$).

The number of moles of carbon dioxide is,

$$Moles\ of\ CO_2 \ = \ Number\ of\ molecules\ of\ CO_2 \cdot N_A$$

If the atmosphere has a carbon dioxide concentration of 400 ppm, then

$$n_{CO_2}\ = \ \frac{400}{1\ 000\ 000} \cdot N_A \ = \ 0.0004 \cdot N_A$$

Thus, the partial pressure of carbon dioxide in the atmosphere will be,

$$P_{CO_2} \ = 0.0004 \cdot 101\ 325 \ = \ 40.53 \ Pa \ = 0.0004 \ atm \ = \ 0.304 \ mmHg$$

Not nearly so elegant as @fred answer: 400 ppm = 0.04% , then CO 2 is 0,04 % or 0.0004 atm.

• More elegant :) Aug 18, 2022 at 6:13