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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. 

Atmospheric aerosol, particulate matter suspended in the air we breathe, exerts a strong impact on our health and the environment. Controlling the amount of particulate matter in air is difficult, as there are many ways particles can form by both natural and anthropogenic processes. We gain insight into the sources of particulate matter through chemical composition measurements. A substantial portion of atmospheric aerosol is organic, and this organic matter is exceedingly complex on a molecular scale, encompassing hundreds to thousands of individual compounds that distribute between the gas and particle phases. Because of this complexity, no single analytical technique is sufficient. However, mass spectrometry plays a crucial role owing to its combination of high sensitivity and molecular specificity. This review surveys the various ways mass spectrometry is used to characterize atmospheric organic aerosol at a molecular level, tracing these methods from inception to current practice, with emphasis on current and emerging areas of research. Both offline and online approaches are covered, and molecular measurements with them are discussed in the context of identifying sources and elucidating the underlying chemical mechanisms of particle formation. There is an ongoing need to improve existing techniques and develop new ones if we are to further advance our knowledge of how to mitigate the unwanted health and environmental impacts of particles. 

The ability of particle-phase chemistry to alter the molecular composition and enhance the growth rate of nanoparticles in the 2–100 nm diameter range is investigated through the use of a kinetic growth model. The molecular components included are sulfuric acid, ammonia, water, a non-volatile organic compound, and a semi-volatile organic compound. Molecular composition and growth rate are compared for particles that grow by partitioning alone vs. those that grow by a combination of partitioning and an accretion reaction in the particle phase between two organic molecules. Particle-phase chemistry causes a change in molecular composition that is particle diameter dependent, and when the reaction involves semi-volatile molecules, the particles grow faster than by partitioning alone. These effects are most pronounced for particles larger than about 20 nm in diameter. The modeling results provide a fundamental basis for understanding recent experimental measurements of the molecular composition of secondary organic aerosol showing that accretion reaction product formation increases linearly with increasing aerosol volume-to-surface-area. They also allow initial estimates of the reaction rate constants for these systems. For secondary aerosol produced by either OH oxidation of the cyclic dimethylsiloxane (D₅) or ozonolysis of β-pinene, oligomerization rate constants on the order of 10⁻³ to 10⁻¹ M⁻¹ s⁻¹ are needed to explain the experimental results. These values are consistent with previously measured rate constants for reactions of hydroperoxides and/or peroxyacids in the condensed phase.