What reaction takes place when gaseous $\ce{NH3}$ and $\ce{SO2}$ are introduced into $\ce{BaCl2}$ solution and an insoluble white solid is produced?

I think that the white solid could be $\ce{BaSO4}$ or $\ce{BaSO3}$. If it is the former, then some redox takes place. But I'm not sure what $\ce{NH3}$ is doing here.