Calculating atmospheric SO2 with pH and K values given

Oct 2019
Hello, I am currently doing a take home exam and I'm completely stuck on a problem. I've been scouring the internet and my textbook for hours upon and and I'll be honest - I have no idea how to even attempt this problem. I was hoping someone could at least nudge me in the right direction or tell me the set up so I can solve it myself.

"Assuming that the acidity of a water body is caused by atmospheric SO2, calculate the concentration of atmospheric SO2 in ppb (and STP) when the pH of the water body was equal to 4.39."

KH = 1 mol/ (lit/atm)
K1 = 1.7 x 10^-4 mol/lit
K2 = 6.4 x 10^-8

Is this an equilibrium constant problem? Help is greatly appreciated.
May 2018
Elephant Island, Antarctica
Use the pKas of H2SO3 to obtain the concentration of SO2 dissolved in water (SO2 + H2O --> H2SO3) at pH 4.39 using the Henderson Hasselbach Eq.
From there I think you can use the solubility of SO2 to find the mol fraction and then pV=nRT to determine the concentration in air.

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