Air pollution injury to the foliage of the terrestrial bamboo orchids (Arundina graminifolia) in Volcano, Hawaii.  The leaf spots resulted from the settling of "vog" (volcanic fog) droplets on the leaves. When the volcanic gas, sulfur dioxide, combined with a heavy fog, the chemical reaction with water created a dilute solution of sulfuric acid, which "burned" the leaves of this sensitive plant.

Hot steam, bearing sulfur dioxide gas, emerging from a vent at Volcanoes National Park on the island of Hawaii.