Albert Barnes Commentary Revelation 9:2

"And he opened the pit of the abyss; and there went up a smoke out of the pit, as the smoke of a great furnace; and the sun and the air were darkened by reason of the smoke of the pit." — Revelation 9:2 (ASV)

And he opened the bottomless pit. It is represented previously as wholly confined, so that not even the smoke or vapor could escape.

And there arose a smoke out of the pit. Compare Revelation 14:11. The meaning here is that the pit, as a place of punishment or as the abode of the wicked, was filled with burning sulfur, and consequently emitted smoke and vapor as soon as it was opened. The common image of the place of punishment in the Scriptures is that of a lake that burns with fire and brimstone. Compare Revelation 14:10; Revelation 19:20; Revelation 20:10; Revelation 21:8.

See also Psalm 11:6; Isaiah 30:33; Ezekiel 38:22.

It is not improbable that this image was taken from the destruction of Sodom and Gomorrah (Genesis 19:24). Such burning sulfur would, of course, produce a dense smoke or vapor. The idea here is that the pit had been closed, and as soon as the door was opened, a dense column escaped that darkened the heavens.

The purpose of this is probably to indicate the origin of the plague that was about to come upon the world. It would be of such a character that it would appear as if it had been emitted from hell, as if the inmates of that dark world had broken loose upon the earth. Compare Barnes on Revelation 6:8.

As the smoke of a great furnace. So in Genesis 19:28, from which this image is probably taken: And he looked towards Sodom and Gomorrah, and all the land of the plain, and beheld, and lo, the smoke of the country went up as the smoke of a furnace.

And the sun and the air were darkened, etc. This will be the case when smoke ascends from a furnace. The meaning here is that an effect would be produced as if a dense and dark vapor should ascend from the underworld. We are not, of course, to understand this literally.