AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Growth of Small Molecular Clusters: Comparison of Growth Rates Determined from Cluster Appearance Times and Collision–Evaporation Fluxes
Tinja Olenius, Ilona Riipinen, Katrianne Lehtipalo, HANNA VEHKAMÄKI, University of Helsinki
Abstract Number: 547 Working Group: Aerosol Physics
Abstract Particle growth rate is a well-established quantity used to characterize new particle formation events in the atmosphere, and to assess the probability that freshly formed particles survive to larger sizes before being scavenged. At present, instrumental development enables the detection of molecular clusters down to a mobility diameter of approximately 1 nm. Consequently, measured cluster concentrations have been recently utilized to extract cluster growth rates based on the appearance times of different cluster sizes. The appearance time is defined as the time at which the concentration of a certain size reaches 50% of its maximum value. On the other hand, it is not clear if the apparent growth rates determined from the appearance times coincide with the effective growth rates corresponding to the molecular fluxes between the clusters. In this work, we simulate the time evolution of a population of sub-3 nm molecular clusters and determine cluster growth rates (1) with the appearance time method, and (2) from the collision–evaporation fluxes between subsequent sizes. For simplicity, we consider a model substance with an evaporation profile corresponding to classical liquid droplet model and a situation where the growth occurs solely via monomer additions. We find that there may be significant differences between the growth rates determined with the two approaches, especially for the smallest clusters that have the highest evaporation rates. The appearance time –based growth rates are found to be generally higher than the flux-equivalent growth rates. Both quantities also depend qualitatively on ambient conditions, such as the magnitude of an external sink and the time evolution of the vapor monomer concentration. These aspects should be considered when analysing growth rates deduced from cluster measurements.