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The morphology of mixed organic/inorganic particles can strongly influence the physicochemical properties of aerosols but remains relatively less examined in particle formation studies. The morphologies of inorganic seed particles grown with either -pinene or limonene secondary organic aerosol (SOA) generated in a flow tube reactor were found to depend on initial seed particle water content. Effloresced and deliquesced ammonium sulfate seed particles were generated at low relative humidity (<15% RH, dry) and high relative humidity (~60% RH, wet) and were also coated with secondary organic material under low growth and high growth conditions. Particles were dried and analyzed using SMPS and TEM for diameter and substrate-induced diameter changes and for the prevalence of phase separation for organic-coated particles. Effloresced inorganic seed particle diameters generally increased after impaction, whereas deliquesced inorganic seed particles had smaller differences in diameter, although they appeared morphologically similar to the effloresced seed particles. Differences in the changes to diameter for deliquesced seed particles suggest crystal restructuring with RH cycling. SOA-coated particles showed negative diameter changes for low organic growth, although wet-seeded organic particles changed by larger magnitudes compared to dry-seeded organic particles. High organic growth gave wide ranging diameter percent differences for both dry- and wet-seeded samples. Wet-seeded particles with organic coatings occasionally showed a textured morphology unseen in the coated particles with dry seeds. Using a flow tube reactor with a combination of spectrometry and microscopy techniques allows for insights into the dependence of aerosol particle morphology on formation parameters for two seed conditions and two secondary organic precursors. 

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.