My question involves criminal law for the state of: Pennsylvania

Recently i was arrested for public drunkeness and after i was arrested i was taken to the holding cell at my local police station while in the cell i spit out of the cell on the officer on duty and now they are charging me with aggravated assault. I was wondering how they can charge me with aggravated assault just for spitting and I'm a first time offender so i was wondering if anyone knows what kind of punishment I will recieve?