Acts 22:8 "'Who are you, Lord?' I [Paul] asked.
Whilst proceeding along a road, accompanied by others . . . . suddenly, a light shone round about Saul, Acts 9:3. He describes this light, later, as 'above the brightness of the sun', Acts 26:13.
Such was the light and the suddenness that Saul fell down on the ground, Acts 9:4, presumably in a state of shock.
He then heard a disembodied voice speaking to him, Acts 9:4.
The voice enquired of Saul why Saul was persecuting the person responsible for this incandescent light and this disembodied voice.
Saul did not know what to make of this. He did not know who was speaking to him. A lifetime of Jewish prayer addressing a Deity who was only known through reading scripture had not prepared Saul for this experience.
Was this an angel ?
Was this God - a God whom he now realised he had never met with before - and was unacquainted with His Person ?
Saul persecuted followers of Jesus. Saul did not believe in Jesus. Saul did not believe that Jesus of Nazareth, now crucified, dead and buried, and his body reportedly stolen by his followers - was Messiah.
So who was this, beyond the incandescent light and the disembodied voice ?
It must be - it must be - someone who deserved to be addressed as 'Lord'.
What other word could Saul have possibly used in such extreme circumstances ?
The Greek word κύριος can be used like the English word “sir” as an honorific address to an elder.
According to LSJ,1
B. Subst. κύριος, ὁ, lord, master, τοῖσι κ. δωμάτων A.Ch.658, cf. 689, S.Aj.734, etc.; ὁ κ. alone, head of a family, master of a house (cf. Sch.Ar.Eq.965), Antipho 2.4.7, Ar.Pl.6, Arist.Pol.1269b10, τοὺς κ. τῶν οἰκιῶν PTeb.5.147 (ii B.C.), owner or secure possessor, εἰ τριακονταετὴς παρέλθοι χρόνος καὶ ἡ κατοχὴ κυρίους τοὺς λαβόντας καταστήσειε Just.Nov.22.24; also, guardian of a woman, Is.6.32, PGrenf.2.15 i 13 (ii B.C.), etc.: generally, guardian, trustee, Is.2.10; D.43.15, 46.19, Men.Epit.89, etc., (in later Gr. freq. written κῦρις or κύρις w. late vowel shortening, e.g. Vit.Aesop.(G)30), SEG31.830 (iv/v A.D.).
b. later κύριε, as a form of respectful address, sir, Ev.Jo.12.21, 20.15, Act.Ap.16.30 (pl.), PFay.106.15 (ii A.D.), etc.
Paul did not recognize Jesus in the vision, so—as was custom to do to strangers—he responded to him formally.
1 p. 1013, κύριος, B., b.
Liddell, Henry George; Scott, Robert; et al. A Greek-English Lexicon. 9th ed. with revised supplement. Oxford: Clarendon, 1996.
The word "Lord" in Acts 22:8 is in the vocative form, Κύριε. There are two possible ways to understand this reply by Paul.
1. General Title of Honor
Note the extract from Thayer about the Greek κύριος = Lord:
a title of honor, expressive of respect and reverence, with which servants salute their master, Matthew 13:27; Matthew 25:20, 22; Luke 13:8; Luke 14:22, etc.; the disciples salute Jesus their teacher and master, Matthew 8:25; Matthew 16:22; Luke 9:54; Luke 10:17, 40; Luke 11:1; Luke 22:33, 38; John 11:12; John 13:6, 9, 13; John 21:15-17, 20f, etc., cf. 20:18; Luke 24:34; his followers salute Jesus as the Messiah, whose authority they acknowledge (by its repetition showing their earnestness (cf. Winer's Grammar, § 65, 5 a.)), κύριε, κύριε, Matthew 7:21; and R G in Luke 13:25; employed, too, by a son in addressing his father, Matthew 21:30; by citizens toward magistrates, Matthew 27:63; by anyone who wishes to honor a man of distinction, Matthew 8:2, 6, 8; Matthew 15:27; Mark 7:28; Luke 5:12; Luke 13:25; John 4:11, 15, 19; John 5:7; John 12:21; John 20:15; Acts 9:5; Acts 16:30; Acts 22:8.
In this form, it is equivalent to the English "Sir" and so translated in many places by some versions.
While this is indisputably correct, there is another possibility.
2. Paul recognized Jesus
Paul would have been a similar age to Jesus (approx) and as such, being in Jerusalem before the crucifixion, he may have seen Jesus and could have even been at, or at least observed one of the trials of Jesus that condemned Him.
IF this is true (and it is not impossible by any means), then when Jesus spoke to Saul on the Damascus road, Paul answered, "Who are you Lord?" This suggests that while Paul knew about Jesus (Saul was persecuting Jesus' followers) Saul had not yet met Jesus and did not know Him personally. Hence the question - he recognized the voice and countenance but did not know the person.
The first option above is certainly correct, but it is also possible that the second may also be true which might help to explain why Paul's conversion was so immediate.