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

Abstract

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.

Arctic marine secondary organic aerosol contributes significantly to summertime particle size distributions in the Canadian Arctic Archipelago

“…In general, particles with diameters of 50 nm or larger activate in our simulations, although observations from the Canadian Arctic Archipelago indicate that particles as small as 20 nm could activate in clean summertime atmospheric layers above 200 m altitude when low concentrations of larger particles (diameters greater than 100 nm) enable relatively high supersaturations (Leaitch et al, 2016). MSA that is produced by the DMS-OH-addition channel can contribute to condensational growth of existing particles (Chen et al, 2015;Hoffmann et al, 2016;Willis et al, 2016;Hodshire et al, 2019). In our simulations, MSA contributes to particle condensational growth, but not to particle nucleation.…”

A new aerosol flow reactor to study secondary organic aerosol

“…The chemical and physical transformations of SOA are often studied using atmospheric simulation chambers or oxidative flow reactors (e.g. Bloss et al, 2005;Cocker et al, 2001;Friedman and Farmer, 2018;Glowacki et al, 2007;Hamilton et al, 2011;Hodshire et al, 2018;Liu and Zeng, 2018;Rohrer et al, 2005;Witkowski et al, 2018). These techniques allow SOA formation and ageing to be investigated under controlled conditions.…”

“…These kinetic limitations are driven by particle viscosity, which is influenced by temperature (Koop et al, 2011;Zobrist et al, 2008), relative humidity (Bateman et al, 2015;Mikhailov et al, 2009) and the chemical composition of the particle (Bateman et al, 2015;DeRieux et al, 2018;Grieshop et al, 2007;Kidd et al, 2014;Koop et al, 2011;Perraud et al, 2012;Reid et al, 2018;Roldin et al, 2014;Rothfuss and Petters, 2017b;Vaden et al, 2011). Recent studies have suggested that particle viscosity is influenced by certain chemical components within the SOA, such as oligomers (Huang et al, 2018) and nitrate-containing species (Perraud et al, 2012).…”

The potential role of organics in new particle formation and initial growth in the remote tropical upper troposphere

“…where Si and Oi are i th moments of the simulated and observed size distributions, respectively (Hodshire et al, 2018).…”

“…While Westervelt et al (2014) suggested that the global-mean boundary layer CCN are not very sensitive to the number of particles formed in the UT due to the dampening effects of coagulation (i.e., more nucleation leads to faster coagulational losses), different choices of NPF mechanisms in models might alter the spatial and temporal pattern of NPF, and thus affect the spatial distribution and magnitude of CCN abundance. It is clear that accurate simulation of NPF and growth processes is essential to adequately represent particle size distributions and their spatial distribution in global models and improve predictions of aerosol-cloud-radiation effects (Hodshire et al, 2018;Williamson et al, 2019).…”