"I though perhaps, if the rebel forces had a cache of chemical weapons, taken at an earlier date and concealed in the area now under conflict, the cache may have been hit in the "sustained barrage" of conventional shelling."

That I believe was your original contention wasn't it Akenaton?

Now it would appear that you have decided that it wasn't chemical weapons that were taken and concealed it was storage tanks containing chemical or biological warfare agents, this change probably prompted by what your "BBC" experts stated. Rather odd though that in Iraq in 1991 Saddam's chemical and biological weapons sites were hit yet none of the inevitable ensuing vapours killed anybody - possible explanation?