Not surprising, I have just a few verses that I believe justify preaching in the open air:.
For though I preach the gospel, I have nothing to glory of: for necessity is laid upon me; yea, woe is unto me, if I preach not the gospel!
1 Corinthians 9:16 KJV
Saying with a loud voice, Worthy is the Lamb that was slain to receive power, and riches, and wisdom, and strength, and honour, and glory, and blessing.
Revelation 5:12 KJV
Go up to Lebanon, and cry; and lift up thy voice in Bashan, and cry from the passages: for all thy lovers are destroyed. I spake unto thee in thy prosperity; but thou saidst, I will not hear. This hath been thy manner from thy youth, that thou obeyedst not my voice. The wind shall eat up all thy pastors, and thy lovers shall go into captivity: surely then shalt thou be ashamed and confounded for all thy wickedness.
Jeremiah 22:20-22 KJV
Cry aloud, spare not, lift up thy voice like a trumpet, and shew my people their transgression, and the house of Jacob their sins.
Isaiah 58:1 KJV
Definition: Preach
to publish, proclaim openly: something which has been done
And he said unto them, Go ye into all the world, and preach the gospel to every creature.
Mark 16:15 KJV
Therefore disputed he in the synagogue with the Jews, and with the devout persons, and in the market daily with them that met with him.
Acts 17:17 KJV
John did baptize in the wilderness, and preach the baptism of repentance for the remission of sins.
Mark 1:4 KJV
From that time Jesus began to preach, and to say, Repent: for the kingdom of heaven is at hand.
Matthew 4:17 KJV
By mercy and truth iniquity is purged: and by the fear of the Lord men depart from evil.
Proverbs 16:6 KJV
Be not wise in thine own eyes: fear the Lord , and depart from evil.
Proverbs 3:7 KJV
A wise man feareth, and departeth from evil: but the fool rageth, and is confident.
Proverbs 14:16 KJV
The fear of the Lord is a fountain of life, to depart from the snares of death.
Proverbs 14:27 KJV
As many as I love, I rebuke and chasten: be zealous therefore, and repent.
Revelation 3:19 KJV
When I say unto the wicked, Thou shalt surely die; and thou givest him not warning, nor speakest to warn the wicked from his wicked way, to save his life; the same wicked man shall die in his iniquity; but his blood will I require at thine hand.
Ezekiel 3:18 KJV
And Jonah began to enter into the city a day's journey, and he cried, and said, Yet forty days, and Nineveh shall be overthrown. So the people of Nineveh believed God, and proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them.
Jonah 3:4-5 KJV
But when he saw many of the Pharisees and Sadducees come to his baptism, he said unto them, O generation of vipers, who hath warned you to flee from the wrath to come? Bring forth therefore fruits meet for repentance:
Matthew 3:7-8 KJV
Thou shalt not hate thy brother in thine heart: thou shalt in any wise rebuke thy neighbour, and not suffer sin upon him.
Leviticus 19:17 KJV
Romans 10:14 KJV