The hail, fire (lightning?) and blood burn up 1/3 of trees and grass on earth, so the mixture must be something that kills (burns up) vegetation.
Review Exodus chapter 9 again. Thunder and lightning lead to hail. The Lord says to Job, "Have you...seen the storehouses of the hail, which I reserve for times of trouble...?" (Job 38:23).
"Coals of fire" in Psalm 18:12&13 in the King James version, is translated as "bolts of lightning" in the New International Version. When a person looks up mixed or mingled in a Bible concordance, a person finds passages dealing with recipes of one type or another.
Is it possible that various types of pollution released into the air will fall as hail? And this concoction will act as a herbicide? Or, is the hail debris from a volcanic eruption? An earthquake (8:5) could precede volcanic eruptions (8:7).
In any case, the Bible says 1/3 of trees and grass on earth will burn up as a result of the "...hail and fire mixed with blood..." (8:7).