Officials blocked off the area around Long Beach Cold Storage & Logistics at Magnolia Avenue and Anaheim Street.
As a precaution, authorities shut down Anaheim Street between Harbor Scenic Drive and Pacific Avenue. The Anaheim Street off-ramps from the northbound and southbound 710 Freeway were also closed.
Fire crews smelled the ammonia around 3 a.m. when they went to check on an alarm that went off at the facility.
Ammonia is a inhalation hazard, so residents were told to stay indoors. No evacuations have been ordered.
The same facility had a larger ammonia leak in February.