Ammonia enhances atmospheric aerosol formation and, once in the aerosol form, enhances the light extinction characteristics of those aerosols. In this paper, an ammonia emissions inventory is developed for the Lower Fraser Valley (LFV) of British Columbia and used in conjunction with ambient ammonia and ammonium measurements to explain: (i) the observed temporal and spatial variability of fine inorganic aerosol composition and concentrations in the valley, and (ii) the severity and spatial variability of visibility degradation in the LFV. It is proposed here that advection of urban emissions of nitrogen and sulphur oxides over agricultural areas in the eastern and central valley with higher ammonia emissions favours subsequent ammonium nitrate and sulphate formation. This leads to higher fine mass concentrations and lowest visibility in the predominantly agricultural regions of the valley. (C) 1998 Elsevier Science Ltd. All rights reserved.