Nucleation of α-pinene oxidation products with sulfuric acid

Particle nucleation from trace atmospheric vapours is important for climate since it gives rise to more than half of global cloud condensation nuclei. Sulfuric acid (H₂SO₄) has long been recognised to drive particle nucleation in the atmosphere and, more recently, highly oxygenated products of biogenic vapours-in particular monoterpenes such as α-pinene (C₁₀H₁₆)-have also been shown to nucleate under atmospheric conditions, without requiring additional vapours. This raises the question of whether a nucleation synergy exists between α-pinene oxygenated organic molecules (AP-OOM) and H₂SO₄, as has been suggested by early studies. Here we report new particle formation from AP-OOM and H₂SO₄ in the absence of base vapours such as ammonia (NH₃), measured in experiments performed with the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber at cool boundary layer temperatures of -10 °C and +5 °C. We find that AP-OOM nucleation rates increase strongly when H₂SO₄ concentrations exceed around 10⁶ cm^-3. The enhancement is synergistic and cannot be explained as a simple linear addition of independent chemical systems. Above this threshold, the nucleation rate depends approximately linearly on H₂SO₄ concentration, in contrast with the strong sensitivity to H₂SO₄ for H₂SO₄-NH₃ nucleation. Nucleation rates are 10-100-fold higher in the presence of ions from galactic cosmic rays or from the CERN pion beam. Based on these measurements, we have parameterised a temperature-dependent H₂SO₄-AP-OOM nucleation rate in the absence of base vapours and implemented it in the EMAC (ECHAM/MESSy Atmospheric Chemistry) Earth system model. In comparison with a parameterisation developed in an earlier study [Riccobono et al., Science, 2014, 344, 717-721.], the new parameterisation indicates sharply reduced nucleation rates in the boundary layer over warm regions, and increased rates over northern boreal forests.