Why does our porta potty smell like ammonia after a hot weekend?

Yes, the ammonia smell is from waste breaking down rapidly in the heat. A standard portable toilet uses a deodorizer in the holding tank to control odor and begin breaking down waste. In hot weather, bacteria multiply faster, speeding up the process that turns urea in urine into ammonia. Your unit was likely overloaded for its size and service schedule.

You had about 40 people using a single standard unit over a hot weekend. The guideline for a four-hour event starts at one unit per 50 to 75 people, but that's for short events. A weekend with food and alcohol means more use per person and longer duration. The heat accelerates the chemical reactions inside the tank. The deodorizer was probably overwhelmed because the unit's capacity and the service frequency were not matched to the intense, concentrated use.

For a future event of that size and length, especially in heat, you would need more units or a larger unit like a deluxe portable toilet with a bigger tank, and you should arrange for a service visit immediately after the event. Weekly service is the norm, but heavy use and high temperatures require more frequent pumping to prevent the system from being overloaded.