How does the solubility of atmospheric carbon dioxide change with variations in pH and salinity?

Decreasing pH decreases the solubility; increasing salinity increases the salinity.

Explanation:

Atmospheric carbon dioxide in seawater is involved in the following equilibria:

(i)   CO₂(g) ⇌ CO₂(aq)

(ii)  CO₂(aq) + H₂O ⇌ H₂CO₃(aq)  

(iii) H₂CO₃(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HCO₃⁻(aq)  

(iv) HCO₃⁻(aq)+ H₂O(l) ⇌ H₃O⁺(aq) + CO₃²⁻(aq)

1. Effect of pH  

Decreasing pH increases [H₃O⁺].

According to Le Châtelier's Principle, the position of equilibrium of (iv) will be pushed to the left.  

This, in turn, pushes the positions of equilibrium of (iii), (ii), and (i) to the left.

The net effect is that CO₂ is pushed out of the solution and back into the atmosphere.

Thus, decreasing the pH decreases the solubility of atmospheric CO₂.

2. Effect of salinity

Salinity includes the measurement of all ions in seawater, not just Na⁺ and Cl⁻.

An important ion that contributes to salinity is Ca²⁺.

Ca²⁺ ions are involved in the equilibrium  

Ca²⁺(aq) + CO₃²⁻(aq) ⇌ CaCO₃(s).

Thus, increasing the salinity ([Ca²⁺]) removes CO₃²⁻ from the solution and pulls the position of equilibrium of (iv) to the right.

The effect goes back through the chain of equilibria.

The net result is that more atmospheric CO₂(g) dissolves in the seawater to replace the CO₃²⁻ that has been removed.

Increasing the salinity increases the solubility of atmospheric CO₂.