Constraining nucleation, condensation, and chemistry in oxidation flow reactors using size-distribution measurements and aerosol microphysical modeling
Authors: Hodshire Anna L. , Palm Brett B. , Alexander M. Lizabeth , Bian Qijing , Campuzano-Jost Pedro , Cross Eben S. , Day Douglas A. , de Sá Suzane S. , Guenther Alex B. , Hansel Armin , Hunter James F. , Jud Werner , Karl Thomas , Kim Saewung , Kroll Jesse H. , Park Jeong-Hoo , Peng Zhe , Seco Roger , Smith James N. , Jimenez Jose L. , Pierce Jeffrey R. 
Published: August 2018

Abstract. Oxidation flow reactors (OFRs) allow the concentration of a given atmospheric
oxidant to be increased beyond ambient levels in order to study secondary
organic aerosol (SOA) formation and aging over varying periods of equivalent
aging by that oxidant. Previous studies have used these reactors to determine
the bulk OA mass and chemical evolution. To our knowledge, no OFR study has
focused on the interpretation of the evolving aerosol size distributions. In
this study, we use size-distribution measurements of the OFR and an aerosol
microphysics model to learn about size-dependent processes in the OFR.
Specifically, we use OFR exposures between 0.09 and 0.9 equivalent days of OH
aging from the 2011 BEACHON-RoMBAS and GoAmazon2014/5 field campaigns. We use
simulations in the TOMAS (TwO-Moment Aerosol Sectional) microphysics box
model to constrain the following parameters in the OFR: (1) the rate constant
of gas-phase functionalization reactions of organic compounds with OH,
(2) the rate constant of gas-phase fragmentation reactions of organic
compounds with OH, (3) the reactive uptake coefficient for heterogeneous
fragmentation reactions with OH, (4) the nucleation rate constants for three
different nucleation schemes, and (5) an effective accommodation coefficient
that accounts for possible particle diffusion limitations of particles larger
than 60 nm in diameter. We find the best model-to-measurement agreement when the accommodation
coefficient of the larger particles (Dp > 60 nm) was 0.1 or
lower (with an accommodation coefficient of 1 for smaller particles), which
suggests a diffusion limitation in the larger particles. When using these
low accommodation-coefficient values, the model agrees with measurements
when using a published H2SO4-organics nucleation mechanism and
previously published values of rate constants for gas-phase oxidation
reactions. Further, gas-phase fragmentation was found to have a significant
impact upon the size distribution, and including fragmentation was necessary
for accurately simulating the distributions in the OFR. The model was
insensitive to the value of the reactive uptake coefficient on these aging
timescales. Monoterpenes and isoprene could explain 24 %–95 % of the
observed change in total volume of aerosol in the OFR, with ambient
semivolatile and intermediate-volatility organic compounds (S/IVOCs)
appearing to explain the remainder of the change in total volume. These
results provide support to the mass-based findings of previous OFR studies,
give insight to important size-distribution dynamics in the OFR, and enable
the design of future OFR studies focused on new particle formation and/or
microphysical processes.