In America people will quietly blow their noses in public.
In Hungary people will loudly honk their noses in public.
In China people blow their noses and loogies onto the street.
Koreans will not blow their noses in public - my students ask to leave the room to blow.
In America we keep our snot on a handkerchief or kleenex and save it in our pockets.
In China people do not keep a snotty rag in their pockets. Gross!
Whether it's loud or quiet, in our pockets or onto the street, private or public, what is considered polite nose-blowing etiquette is cultural. So it's no use getting grossed out when the person next to you on the tram blows a super loud snot ball onto a tissue about 10 inches from your face. It's cultural!