In this Oct. 8, 2013 file photo, a convoy of chemical weapons disarmament experts depart the Four Seasons Hotel in Damascus, Syria. Two years after President Bashar Assad agreed to destroy his chemical arsenal and joined the Chemical Weapons Convention, activists said they have documented 18 cases of chlorine gas used in the country's rebel-held north since March 6, 2015, when the U.N. issued a resolution determining that chlorine was used in Syria and warning of repercussions.