Use the simple past tense to express action completed at a specific time in the past. To form the past tense, add the ending -ed to the base form of the verb. (But see Irregular Verbs for verbs that form the past tense differently.)
In the Los Angeles basin during December 1982, acid fog
reached a level of acidity comparable to that of
vinegar.
--C. Donald Ahrens, Meteorology Today