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Auxiliary Material for
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Multiphase OH Oxidation Kinetics of Organic Aerosol:
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The Role of Particle Phase State and Relative Humidity
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J. H. Slade1 and D. A. Knopf1*
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Institute for Terrestrial and Planetary Atmospheres/School of Marine and Atmospheric
Sciences, Stony Brook University, Stony Brook, NY 11794, USA
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Correspondence to: Daniel.Knopf@StonyBrook.edu
Geophysical Research Letters, 2014
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1. Observed Loss Rate Dependence (k1) on Relative Humidity
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As shown in Eq. (3) in the manuscript, dL depends on both the diffusion coefficient of
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OH, DOH, in levoglucosan, and the observed pseudo-first order loss rate of condensed-phase OH,
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𝑘1𝐵 = k2[LEV]B [Abbatt et al., 2012], which both vary as a function of RH. k2 is the second-
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order rate constant derived from γ at a specific RH (see Table 1 in article), which is also a
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function of the pseudo-first order loss rate of gas-phase OH, k1. [LEV]B is the bulk concentration
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of levoglucosan, approximated as 6.27x1021 molecule cm-3, which assumes a densely packed
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system formed exclusively by LEV molecules (molecular volume of 1.59x10-22 cm3) [Arangio et
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al., 2015] Therefore, to plot a smooth dL as a function of RH as shown in Fig. 1(a) requires a
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parameterization for 𝑘1𝐵 as a function of RH. Figure S4 shows derived 𝑘1𝐵 as a function of RH.
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Based on the scatter in the data, we assume that 𝑘1𝐵 depends linearly on RH with 𝑘1𝐵 = 4x107RH
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+ 109. Note, however, that 𝑘1𝐵 and also by extension k1 varies with particle surface area density,
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Sa, as shown in Fig. S3, and thus the linear function does not account for changes in 𝑘1𝐵 or k1 due
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to changes in Sa. For example, the large scatter in k1_cond at RH=0% as shown in Fig. S4 is due to
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the wide range in applied Sa. A sensitivity analysis on the dependence of k1 as a function of RH
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at fixed Sa may resolve this bias, but is beyond the scope of this study.
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2. Langmuir-Hinshelwood Description Accounting for Co-Adsorption of H2O
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The modeled γ as shown in Fig. 1(b) in the manuscript assumes OH undergoes a
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Langmuir-Hinshelwood reactive uptake mechanism with 4-methyl-5-nitrocatechol (MNC) and
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that H2O partially inhibits reactive uptake of OH by MNC by competing for surface reactive sites
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and is based on the following equation [Pöschl et al., 2001]
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𝛾=𝜎
4𝑘s [Org]𝐾OH
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OH 𝜔OH (1+𝐾OH [OH]+𝐾H2 O [H2 O])
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where ks is the surface reaction rate constant between OH and the organic, [Org] is the surface
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concentration of the organic, i.e. MNC, estimated to be 2×1014 molecule cm-2 [Slade and Knopf,
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2013], KOH is the Langmuir adsorption constant of OH taken as 3.96×10-10 cm3 molecule-1 as
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derived from OH adsorption on nitroguaiacol, an isomer of MNC [Slade and Knopf, 2013], σOH
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is the surface area occupied by one adsorbed OH molecule approximated as 1.08×10-15 cm2
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molecule-1 [Slade and Knopf, 2013], and [OH] and [H2O] are the gas phase concentrations of OH
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and H2O, respectively. [H2O] was determined from the RH measurements in the flow reactor and
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the vapor pressure of water at T=293 K. Both ks and KH2O are free fitting parameters, which were
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determined by fitting the data in Fig. 1(b) to the above equation applying a non-linear least
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squares fit using a Levenberg-Marquardt algorithm. The best fit parameters to ks and KH2O are
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4.29(±0.70)×10-17 cm2 molecule-1 s-1 and 2.01(±1.07)×10-17 cm3 molecule-1, respectively. The
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derived ks is in agreement with ks of 8.38(±8.44)×10-17 cm2 molecule-1 s-1 derived from the OH
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surface reaction with nitroguaiacol [Slade and Knopf, 2013]. Excluded in this function are the
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possible influences of other co-adsorbates that may suppress OH reactive uptake. For example,
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recombination of OH to H2O2 in the adsorbed water layer [Hanson et al., 1992] may limit OH
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reactive uptake by the organic. Increasing RH would result in more monolayers of water on the
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surface of the particle, thus extending the time for OH to diffuse through the water layers to react
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with the organic, enabling greater production of H2O2. Since H2O2 is less reactive than OH with
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organics [Lai et al., 2014], less of the organic is degraded and the additional presence of H2O2
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may compete as well with OH for surface reactive sites. Clearly, more systematic studies are
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needed to properly assess the roles of other possible co-adsorbates that may also limit the
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reactive uptake of OH.
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References
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