When onions are cut, they release an enzyme called alliinase. After the enzyme reacts with the amino acid in onion, the amino acid is converted into sulfinic acid. Disulfuric acid molecules rearrange to form thiopropionaldehyde -S- oxide, which will stimulate tears when released into the air. So it is gas, but gas also contains liquid. The molecular components in the liquid will be distributed in the air.