When exposed to open air and in normal condition ammonia evaporates at a rate of 0.12 grams per second after the first 2 minutes. Half a minute later (2 minutes in total), the evaporation rate decreases to half of the initial rate, which is 0.06 grams per second. However, it is important to note that the rate of evaporation can be affected by various factors such as temperature, humidity, and airflow.