It was tear gas. Of course the article fails to mention it.
Diphenylchlorarsine (DA) is a sternutator. It was not developed in time for use in WW1 but it was supposed to penetrate the respirators then in use, causing a violent coughing/vomiting fit and the victim to remove his respirator, potentially exposing him to something even more unpleasant. The German respirator had no filter against such a particulate (neither did the British small box respirator for that matter).
Cheers
Steve