Arnaud's marker came on a chip shot over the head of Seattle goalkeeper Michael Gspurning off a feed from Felipe Martins.
Arnaud had another chance for the Impact (1-0-0) in the second half but Gspurning knocked it off the crossbar, preserving the victory in the first game for new head Coach Marco Schallibaum.
The best chance for the Sounders (0-1-0) came in the 80th minute when Eddie Johnson smacked a shot off the crossbar.
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