You would be better off taking the Impala or Riverside mini bus shuttle to Arusha. They come to the airport at 8:00am (more like 8:30) and will get you to Arusha by 2pm. Much better then sitting at the airport all day...cost is $25 USD for non residents. You can make a reservation at the airport kiosk. You would need to purchase a transit visa for Kenya.
I am taking the shuttle on Monday back to JKIA. Much cheaper and it brings you right into Arusha rather than JRO which is an hour away!