A method for continuous determination of trace concentrations of ammonia in ambient air is described. The method is based on ammonia preconcentration in a wet cylindrical diffusion denuder and subsequent on-line effluent analysis with conductometric detection. The method provides fast, sensitive and selective determination of ambient ammonia. The detection limit of ammonia in air is 100 ppt for flow injection analysis and 20 ppt for continuous analysis of ammonia in denuder effluent.