Temperature Effects on Sulfuric Acid Aerosol Nucleation and Growth: Results from the TANGENT Study

New particle formation (NPF) consists of two steps: nucleation and subsequent growth. At present, chemical and physical mechanisms that govern these two processes are not well understood. Here, we report results obtained from the TANGENT (Tandem Aerosol Nucleation and Growth Environment Tube) experiments. The TANGENT apparatus enables us to study these two processes independently. The present study focuses on the effects of temperature on sulfuric acid nucleation and further growth. Our results show that lower temperatures enhance both the nucleation and growth rate. However, under temperatures below around 270 K the effects of temperature on the nucleation rate become less significant and the nucleation rate becomes less dependent on RH, indicating that particle formation takes place via barrierless nucleation at lower temperatures. We also examined the growth of newly formed particles under differing temperature conditions for nucleation and further growth. Our results show that newly nucleated clusters formed at low temperatures can indeed survive evaporation and grow in a warmer environment in the presence of SO₂ and ozone. These results indicate that it is feasible that newly nucleated particles formed in the free troposphere can be downward transported to the surface sites, e.g., in Amazon forests and marine boundary layer, to serve as a reservoir of nucleation mode particles. Our results also imply that some heterogeneous reactions involving nanoparticles may affect nucleation and growth of newly formed particles, which can in part explain the exceedingly high frequency of NPF observed in extremely polluted urban areas in China.