Growth of sulphuric acid nanoparticles under wet and dry conditions

Abstract. New particle formation, which greatly influences the number concentrations and size distributions of an atmospheric aerosol, is often followed by a rapid growth of freshly formed particles. The initial growth of newly formed aerosol is the crucial process determining the fraction of nucleated particles growing to cloud condensation nuclei sizes, which have a significant influence on climate. In this study, we report the laboratory observations of the growth of nanoparticles produced by nucleation of H₂SO₄ and water in a laminar flow tube at temperatures of 283, 293 and 303 K, under dry (a relative humidity of 1%) and wet conditions (relative humidity of 30%) and residence times of 30, 45, 60 and 90 s. The initial H₂SO₄ concentration spans the range from 2 × 10⁸ to 1.4 × 10¹⁰ molecule cm⁻³ and the calculated wall losses of H₂SO₄ were assumed to be diffusion limited. The detected particle number concentrations, measured by the Ultrafine Condensation Particle Counter (UCPC) and Differential Mobility Particle Sizer (DMPS), were found to depend strongly on the residence time. Hygroscopic particle growth, presented by growth factors, was found to be in good agreement with the previously reported studies. The experimental growth rates ranged from 20 nm h⁻¹ to 890 nm h⁻¹ at relative humidity (RH) 1% and from 7 nm h⁻¹ to 980 nm h⁻¹ at RH 30% and were found to increase significantly with the increasing concentration of H₂SO₄. Increases in the nucleation temperature had a slight enhancing effect on the growth rates under dry conditions. The influence of relative humidity on growth was not consistent – at lower H₂SO₄ concentrations, the growth rates were higher under dry conditions while at H₂SO₄ concentrations greater than 1 × 10¹⁰ molecule cm⁻³, the growth rates were higher under wet conditions. The growth rates show only a weak dependence on the residence time. The experimental observations were compared with predictions made using a numerical model, which investigates the growth of particles with three different extents of neutralization by ammonia, NH₃: (1) pure H₂SO₄ – H₂O particles; (2) particles formed by ammonium bisulphate, (NH₄)HSO₄; (3) particles formed by ammonium sulphate, (NH₄)₂SO₄. The highest growth rates were found for ammonium sulphate particles. Since the model accounting for the initial H₂SO₄ concentration predicted the experimental growth rates correctly, our results suggest that the commonly presumed diffusional wall losses of H₂SO₄ in case of long-lasting experiments are not so significant. We therefore assume that there are not only losses of H₂SO₄ on the wall, but also a flux of H₂SO₄ molecules from the wall into the flow tube, the effect being more profound under dry conditions and at higher temperatures of the tube wall. Based on a comparison with the atmospheric observations, our results indicate that sulphuric acid alone cannot explain the growth rates of particles formed in the atmosphere.