A rotating arm device to collect large samples of fogwater for chemical analysis was designed and fully characterized. This instrument was used to determine the chemical composition of fogwater at a large number of sites in California. Fogwater at both urban and non-urban sites was frequently found to contain important inorganic acidity. Chemical models were developed to interpret the origins of this acidity in terms of aqueous-phase S(IV) oxidation reactions and acid/base titration processes in the fog and the precursor atmosphere. These models were found to succesfully interpret the inorganic acidity in fogwater and aerosol during wintertime stagnation episodes in the San Joaquin Valley of California.