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Abstract. Flow tube reactors are often used to study aerosolkinetics. The goal of this study is to investigate how to best representcomplex growth kinetics of ultrafine particles within a flow tube reactorwhen the chemical processes causing particle growth are unknown. In atypical flow tube experiment, one measures the inlet and outlet particlesize distributions to give a time-averaged measure of growth, which maybe difficult to interpret if the growth kinetics change as particles transitthrough the flow tube. In this work, we simulate particle growth forsecondary organic aerosol (SOA) formation that incorporates both surface-and volume-limited chemical processes to illustrate how complex growthkinetics inside a flow tube can arise. We then develop and assess a methodto account for complex growth kinetics when the chemical processes drivingthe kinetics are not known. Diameter growth of particles is represented by agrowth factor (GF), defined as the fraction of products from oxidation ofthe volatile organic compound (VOC) precursors that grow particles during aspecific time period. Defined in this way, GF is the sum of all non-volatileproducts that condensationally grow particles plus a portion of semi-volatilemolecules that react on or in the particle to give non-volatile products thatremain in the particle over the investigated time frame. With respect toflow tube measurements, GF is independent of wall loss and condensationsink, which influence particle growth kinetics and can vary from experimentto experiment. GF is shown to change as a function of time within the flowtube and is sensitive to factors that affect growth such as gas-phase mixingratios of the precursors and the presence of aerosol liquid water (ALW) onthe surface or in the volume of the particle. A method to calculate GF from theoutlet-minus-inlet particle diameter change in a flow tube experiment ispresented and shown to accurately match GFs from simulations of SOAformation.