Dry bulb onions should be kept in a cool, dry well ventilated place. Do not store whole onions in plastic bags. Lack of air movement will reduce their storage life.
Sweet onions have a higher water content than storage onions, making them more susceptible to bruising, and a shorter shelf life than storage varieties.
One way to extend the shelf life of a sweet or high water content onion is to wrap each one in paper towels or newspaper and place them in the refrigerator to keep them cool and dry.