Why is sulfur dioxide used during commercial bromine production?

In the commercial production of bromine, sulfur dioxide is used to help concentrate the element from a dilute solution.

After bromine is extracted from ocean water and removed from the liquid by blowing air through it, the resulting free bromine gas is mixed with sulfur dioxide. These mixed gases are then passed up a tower while water trickles down. This process results in a mixture of acids that is significantly richer in bromide ions than the original seawater. This concentrated solution can then be treated again with chlorine to liberate and purify the bromine.1