Atmospheric aerosol formation trough nucleation is commonly modelled using binary nucleation theory applied to the H2SO4H2O mixture (e.g. Raes and Van Dingenen, 1992). Other nucleation pathways have been proposed, of which ternary nucleation of H2SO4, H2O and NH3 seems to be a likely candidate (Coffman and Hegg, 1995). In order to test the effect of NH3 on the nucleation of H2SO4 and H2O, a series of experiments have been performed in the Ispra laminar flow reactor AERREA2.The reactor is described by Roth et al. (1993). It consists of a 9 meter thermostated quartz tube, with an inner diameter of 4.5 cm. It is vertically positioned, and surrounded by 5 sections of UV lights. In the experiments described here, the flow was 5 l/min. The averaged irradiation time on the axis of the reactor was increased from 17 sec to 85 sec, by switching on one, two, .. up to five light sections: starting with the section near the outlet. The aerosol size distribution was measured with a twin DMA system, covering the particle diameter range between 3 nm and 600 nm. Sampling was done either from the central core of the reactor, without disturbing the laminar flow (core-sampling), or from the complete reactor cross-section after rapid mixing (cup-sampling). The results shown below where obtained with cup-sampling.In the first series of experiments, several gas-mitures were mixed before entering the reactor at the top. The mixture consisted of 0.3 ppm of SO2, 0.35 ppm of HONO, 50% RH of H2O, and 20 ppm of O2 in N2. UV irradiation produces OH radicals, which oxidise SO2 into H2SO4. Modelling of the photochemistry in the reactor, has shown that apart from H2SO4 also HNO3 is formed (Roth, 1996). Figure 1 shows the size distribution of the aerosol generated in the reactor after 17 and 85 sec. These distributions are not yet corrected for diffision broadening in the DMA's, neither for the counting efficiency of the CPC's.In the second series of experiments, 0.5 ppm of NH3 was added to the mixture. All other conditions remained the same. The resulting size distributions after 17 and 85 sec are shown in Fig. 2. They are again uncorrected.There is a clear effect of NH3 on particle nucleation. After 17 sec. the particle concentration increases with a factor of 130 when adding NH3. After 85 sec, the concentration is higher by only a factor of 2, most likely as a result of coagulation. Without NH3 nucleation is completely quenched after 85 sec, whereas with NH3 it seems to be still going on.The observed effect of NH3 on nucleation is in line with the theoretical prediction, however no quantitative comparison with the theory can yet be made. Such a comparison will require the modelling of the coupling between hydrodynamics, photochemistry and aerosol dynamics. The latter should consider at least 3 nucleation pathways: H2SO4H2O, NH3H2SO4H2O and NH3HNO3. This modelling is in progress (Wilck et al, this conference).▪