Abstract. A new instrument for the on-line determination of ammonia was developed. Since ammonia is a rather sticky compound, sampling losses were minimised with a new sampling device where the ammonia was transferred to the liquid phase only 5 mm after the inlet tip. The liquid phase was then analyzed by long pathlength absorption spectrophotometry using the Berthelot reaction with phenol and hypochlorite as reagents. The measurements were made during the CLOUD3 campaign at CERN where the influence of ammonia on the nucleation rate was studied. At stable conditions the detection limit reached with this instrument was 35 pptv (air flow rate of 2 l min−1, liquid flow rate of 0.3 ml min−1), although occasionally the instrument was affected by background problems. The range of mixing ratios during this campaign was varied from the background contamination (<35 pptv) up to around 2 ppbv. The measured ammonia concentration was correlated with the rate of ammonia injected into the chamber, but with a response time of several hours due to the high tendency of ammonia to adsorb to or to desorb from surfaces. Since it was found that ammonia strongly increases the nucleation rate already at the lowest measured concentration, future work will focus on further decreasing the detection limit of the instrument.