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. Oxidation flow reactors (OFRs) allow the concentration of a given atmosphericoxidant to be increased beyond ambient levels in order to study secondaryorganic aerosol (SOA) formation and aging over varying periods of equivalentaging by that oxidant. Previous studies have used these reactors to determinethe bulk OA mass and chemical evolution. To our knowledge, no OFR study hasfocused on the interpretation of the evolving aerosol size distributions. Inthis study, we use size-distribution measurements of the OFR and an aerosolmicrophysics model to learn about size-dependent processes in the OFR.Specifically, we use OFR exposures between 0.09 and 0.9 equivalent days of OHaging from the 2011 BEACHON-RoMBAS and GoAmazon2014/5 field campaigns. We usesimulations in the TOMAS (TwO-Moment Aerosol Sectional) microphysics boxmodel to constrain the following parameters in the OFR: (1) the rate constantof gas-phase functionalization reactions of organic compounds with OH,(2) the rate constant of gas-phase fragmentation reactions of organiccompounds with OH, (3) the reactive uptake coefficient for heterogeneousfragmentation reactions with OH, (4) the nucleation rate constants for threedifferent nucleation schemes, and (5) an effective accommodation coefficientthat accounts for possible particle diffusion limitations of particles largerthan 60 nm in diameter. We find the best model-to-measurement agreement when the accommodationcoefficient of the larger particles ( Dp  > 60 nm) was 0.1 orlower (with an accommodation coefficient of 1 for smaller particles), whichsuggests a diffusion limitation in the larger particles. When using theselow accommodation-coefficient values, the model agrees with measurementswhen using a published H2SO4 -organics nucleation mechanism andpreviously published values of rate constants for gas-phase oxidationreactions. Further, gas-phase fragmentation was found to have a significantimpact upon the size distribution, and including fragmentation was necessaryfor accurately simulating the distributions in the OFR. The model wasinsensitive to the value of the reactive uptake coefficient on these agingtimescales. Monoterpenes and isoprene could explain 24 %–95 % of theobserved change in total volume of aerosol in the OFR, with ambientsemivolatile and intermediate-volatility organic compounds (S/IVOCs)appearing to explain the remainder of the change in total volume. Theseresults provide support to the mass-based findings of previous OFR studies,give insight to important size-distribution dynamics in the OFR, and enablethe design of future OFR studies focused on new particle formation and/ormicrophysical processes.

atmospheric chemistry and physics

Constraining nucleation, condensation, and chemistry in oxidation flow reactors using size-distribution measurements and aerosol microphysical modeling