Additional details about the experimental configurations and transmission electron microscope (TEM) imaging protocols, further methods and details on particle spreading and additional modes in scanning mobility particle size (SMPS) spectra, additional visual and numerical data pertaining to scanning electron microscopy (SEM) and atomic force microscopy (AFM) experiments, and a description of the assessment of secondary organic-and organic proxycoated samples over time. MethodsSamples of several inorganic and organic/inorganic salt systems were prepared for TEM and SEM analysis. Particles with two components comprised ammonium sulfate as well as succinic acid (>99.0%, TCI America), 2-methylglutaric acid (98%, Alfa Aesar) or PEG400 (BioUltra, Sigma-Aldrich) in a 1:1 ratio for a total of 0.05 -0.1 wt%. Single component salt systems included ammonium bisulfate (99.9%, Alfa Aesar), sodium chloride (>99.9%, VWR Chemicals), potassium chloride (>99.9%, DOT Scientific) and sodium acetate (99%, Mallinckrodt Chemicals). All such were prepared at the Pennsylvania State University using the sample generation methods described in the Methods section using carbon/copper substrates. SEM samples of both ammonium sulfate and ammonium sulfate/2-methylglutaric acid particles were prepared at the Pennsylvania State University in the same manner as TEM samples and using continuous carbon/copper TEM substrates. SEM images were obtained using an Apreo SEM (Thermo Fisher).Additionally, a series of measurements were taken over the course of several weeks to test the volatility of the shell layer of coated particles. Particles composed of 0.1 wt% succinic acid and ammonium sulfate in a 1:1 ratio were produced via the Pennsylvania State University setup described in the Methods section where the solution was aerosolized, rapidly dried using a diffusion drier, size selected, and impacted onto carbon-coated copper TEM grids. Additionally, ammonium sulfate seed particles with secondary organic coatings were generated at the Pennsylvania State University by adding dry ammonium sulfate particles into a 1 m 3 Tedlar bag (Welch Fluorocarbon) which were allowed to equilibrate followed by 150 ppb of a-pinene and 200 ppb ozone from a corona discharge tube ozone generator (Poseidon 200, Ozotech Inc.). These

The effects of composition, size, and phase state on ultrafine seed particle growth by α-pinene ozonolysis were determined from diameter growth measurements after a fixed reaction time in a flow tube reactor. Modeling time-dependent particle growth under a given set of conditions allowed the reaction growth factor (GF) to be determined, which is defined as the fraction of α-pinene molecules that react to give a product that grows the particles. Growth factors were compared for initial seed particle diameters of 40, 60, and 80 nm that were composed of freshly formed α-pinene secondary organic aerosol (SOA), effloresced ammonium sulfate, and deliquesced ammonium sulfate. Overall, SOA seed particles gave the lowest growth factors. Effloresced ammonium sulfate particles gave somewhat higher growth factors and showed a slight dependence on relative humidity. Deliquesced ammonium sulfate particles gave the highest growth factors. Seed particle-size dependencies suggested that both surface-and volume-limited reactions may contribute to growth. Overall, the growth factors were found to vary by more than 4x across the reaction conditions studied. The results highlight the crucial role that seed particle characteristics play in determining particle growth rates in a size range relevant to formation of cloud condensation nuclei.

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.

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 alpha-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 aerosol kinetics. The goal of this study is to investigate how to best represent complex growth kinetics of ultrafine particles within a flow tube reactor when the chemical processes causing particle growth are unknown. In a typical flow tube experiment, one measures the inlet and outlet particle size distributions to give a time-averaged measure of growth, which may be difficult to interpret if the growth kinetics change as particles transit through the flow tube. In this work, we simulate particle growth for secondary organic aerosol (SOA) formation that incorporates both surface- and volume-limited chemical processes to illustrate how complex growth kinetics inside a flow tube can arise. We then develop and assess a method to account for complex growth kinetics when the chemical processes driving the kinetics are not known. Diameter growth of particles is represented by a growth factor (GF), defined as the fraction of products from oxidation of the volatile organic compound (VOC) precursors that grow particles during a specific time period. Defined in this way, GF is the sum of all non-volatile products that condensationally grow particles plus a portion of semi-volatile molecules that react on or in the particle to give non-volatile products that remain in the particle over the investigated time frame. With respect to flow tube measurements, GF is independent of wall loss and condensation sink, which influence particle growth kinetics and can vary from experiment to experiment. GF is shown to change as a function of time within the flow tube and is sensitive to factors that affect growth such as gas-phase mixing ratios of the precursors and the presence of aerosol liquid water (ALW) on the surface or in the volume of the particle. A method to calculate GF from the outlet-minus-inlet particle diameter change in a flow tube experiment is presented and shown to accurately match GFs from simulations of SOA formation.