Homogenous nucleation of sulfuric acid and water at close to atmospherically relevant conditions

Abstract. In this study the homogeneous nucleation rates in the system of sulfuric acid and water were measured by using a flow tube technique. The goal was to directly compare particle formation rates obtained from atmospheric measurements with nucleation rates of freshly nucleated particles measured with particle size magnifier (PSM) which has detection efficiency of unity for particles having mobility diameter of 1.5 nm. The gas phase sulfuric acid concentration in this study was measured with the chemical ionization mass spectrometer (CIMS), commonly used in field measurements. The wall losses of sulfuric acid were estimated from measured concentration profiles along the flow tube. The initial concentrations of sulfuric acid estimated from loss measurements ranged from 10⁸ to 3 × 10⁹ molecules cm⁻³. The nucleation rates obtained in this study cover about three orders of magnitude from 10⁻¹ to 10² cm⁻³ s⁻¹ for commercial ultrafine condensation particle counter (UCPC) TSI model 3025A and from 10¹ to 10⁴ cm⁻³ s⁻¹ for PSM. The nucleation rates and the slopes (dlnJ/dln [H₂SO₄]) show satisfactory agreement when compared to empirical kinetic and activation models and the latest atmospheric nucleation data. To the best of our knowledge, this is the first experimental work providing temperature dependent nucleation rate measurements using a high efficiency particle counter with a cut-off-size of 1.5 nm together with direct measurements of gas phase sulfuric acid concentration.