A long-acting ultrasonic dispenser that effuses a water-based deodorizer solution into ambient air as a dry fog in a closed environment, and a method for deodorizing noxious odors in that closed environment by the dry fog. The dry fog, containing the deodorizer, chemically reacts with materials producing the odors, thereby neutralizing them. The dispenser contains an electronic ultrasonic wave generator comprising a piezoelectric crystal and an ultrasonic membrane, which is always submerged under liquid. As the liquid evaporates, droplets larger than five microns are filtered out, and the remaining dry fog is emitted into the air. The dispenser is connected to a large reservoir. When the liquid level reaches a minimum permissible height, additional deodorizer solution is pumped into the dispenser. This system is ideal for use with central air conditioners in large office buildings to eliminate odors over a long period of time with minimum maintenance.