Reply on RC2

Abstract

Flow tube reactors are often used to study the growth of secondary organic aerosol (SOA). Because a significant amount of growth must occur over the short residence time of the flow tube, precursor mixing ratios in a flow tube experiment are generally much higher than ambient values. In this study, a model of SOA growth based on condensation of nonvolatile molecules, partitioning of semivolatile molecules, and reaction of semivolatile molecules in the particle volume to produce nonvolatile dimers, is used to compare particle growth under atmospherically relevant conditions to those under typical flow tube conditions. The focus is on the diameter growth of particles in the 10 to 100 nm diameter range, where growth rates can have a substantial impact on formation of cloud condensation nuclei. In this size range, both particle surface- and volume-limited kinetics may apply. Modelling shows that the higher precursor mixing ratios of a flow tube experiment cause surface-limited kinetics to be more prevalent in the flow tube than under atmospheric conditions. SOA formation is characterized by the growth yield (GY), defined as the yield of oxidation products that are to grow the particles. Defined in this way, GY is the sum of all nonvolatile products that condensationally grow particles plus a portion of semivolatile particles that react in the particle volume to give nonvolatile dimers. Modelling shows that GY actually changes as a function of time within the flow tube. The experimentally determined GY from the measured inlet-outlet diameter change of particles in a flow tube experiment closely tracks the average of the time-dependent GY obtained from modelling specific chemical processes. Modelling is also used to explore the effects of seed particle size (40, 60, 80 nm dia.), phase state (deliquesced vs. effloresced), and surface state (interfacial water), as well as precursor mixing ratio, all of which are shown to substantially influence SOA formation under the conditions studied.