10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Growth of Atmospheric Clusters by Organic Vapors: Resolving the Growth Mechanism
JENNI KONTKANEN, Tinja Olenius, Markku Kulmala, Ilona Riipinen, University of Helsinki
Abstract Number: 1141 Working Group: Aerosol Physics
Abstract New particle formation (NPF) is a significant source of atmospheric aerosol particles. According to current understanding, the key compounds in NPF are sulfuric acid, bases (e.g. ammonia and amines) and oxidized organic compounds. Therefore, there is a need for a robust physical description of NPF involving these compounds. One of the mechanisms suggested to depict this process is the nano-Köhler theory, which describes the activation of inorganic clusters to growth by condensation of a soluble organic vapor. In this work, we use molecular-resolution cluster kinetics simulations to investigate if the nano-Köhler theory is able to describe the growth of atmospheric molecular clusters. We simulated the time-development of atmospheric cluster concentrations starting from vapor monomers up to clusters with mass diameter of ~3 nm by solving the discrete general dynamic equation for each cluster. The simulated systems involved two model compounds: a quasi-unary sulfuric acid–base mixture and an oxidized organic compound. In most simulations, the properties of a sulfuric acid–dimethylamine mixture and a representative low-volatile organic compound were used for the inorganic and organic vapors. We performed several simulation sets to investigate the effects of the volatilities and concentrations of the organic and inorganic vapors on the dynamics of the cluster population. From the simulated cluster concentrations, we determined the contributions of different vapor monomers and clusters to the growth over selected threshold sizes. We also determined the apparent cluster growth rates (GR) using the method that is applied for measured particle size distributions. We found that the dominant cluster growth mechanism is determined by the organic vapor saturation ratio (SORG) and the ratio between organic vapor and sulfuric acid concentrations (CORG/CSA). When SORG = ~4–40 and CORG/CSA = ~10–10000 nano-Köhler type behavior is observed: the organic vapor starts to contribute to the growth after a certain size is reached. With lower SORG and CORG/CSA, sulfuric acid dominates the growth at the studied sizes, while with the larger values the organic compound dominates. We compared the cluster activation size determined from the simulations to the predictions by the nano-Köhler theory and found that the two values can differ significantly. Thus, the nano-Köhler theory cannot be readily used to determine the cluster activation size. On the other hand, cluster GRs were often observed to start to increase around the activation size determined from the simulations. However, other dynamic processes, such as varying vapor concentrations, can also cause a similar increase in GR.