Laboratory study on new particle formation from the reaction OH + SO₂: influence of experimental conditions, H₂O vapour, NH₃ and the amine tert-butylamine on the overall process

Abstract. 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.