Nucleation experiments starting from the reaction of OH radicals with SO₂ have been performed in the IfT-LFT flow tube under atmospheric conditions at 293±0.5 K for a relative humidity of 13-61%. The presence of different additives (H₂, CO, 1,3,5-trimethylbenzene) for adjusting the OH radical concentration and resulting OH levels in the range (4-300) ×10⁵ molecule cm⁻³ did not influence the nucleation process itself. The number of detected particles as well as the threshold H₂SO₄ concentration needed for nucleation was found to be strongly dependent on the counting efficiency of the used counting devices. High-sensitivity particle counters allowed the measurement of freshly nucleated particles with diameters down to about 1.5 nm. A parameterization of the experimental data was developed using power law equations for H₂SO₄ and H₂O vapour. The exponent for H₂SO₄ from different measurement series was in the range of 1.7-2.1 being in good agreement with those arising from analysis of nucleation events in the atmosphere. For increasing relative humidity, an increase of the particle number was observed. The exponent for H₂O vapour was found to be 3.1 representing an upper limit. Addition of 1.2×10¹¹ molecule cm⁻³ or 1.2×10¹² molecule cm⁻³ of NH₃ (range of atmospheric NH₃ peak concentrations) revealed that NH₃ has a measureable, promoting effect on the nucleation rate under these conditions. The promoting effect was found to be more pronounced for relatively dry conditions, i.e. a rise of the particle number by 1-2 orders of magnitude at RH = 13% and only by a factor of 2-5 at RH = 47% (NH₃ addition: 1.2×10¹² molecule cm⁻³). Using the amine tert-butylamine instead of NH₃, the enhancing impact of the base for nucleation and particle growth appears to be stronger. Tert-butylamine addition of about 10¹⁰ molecule cm⁻³ at RH = 13% enhances particle formation by about two orders of magnitude, while for NH₃ only a small or negligible effect on nucleation in this range of concentration appeared. This suggests that amines can strongly influence atmospheric H₂SO₄-H₂O nucleation and are probably promising candidates for explaining existing discrepancies between theory and observations.
