The Dynamics and Kinetics of Water Interactions with a Condensed

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The Dynamics and Kinetics of Water Interactions with a Condensed Nopinone Surface Sofia M. Johansson, Xiangrui Kong, Erik S Thomson, Mattias Hallquist, and Jan B. C. Pettersson J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b06263 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 13, 2017

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The Journal of Physical Chemistry

Manuscript for Journal of Physical Chemistry A, revised

The Dynamics and Kinetics of Water Interactions with a Condensed Nopinone Surface

Sofia M. Johansson, Xiangrui Kong, Erik S. Thomson, Mattias Hallquist, and Jan B. C. Pettersson* Department of Chemistry and Molecular Biology, Atmospheric Science, University of Gothenburg, SE-412 96 Gothenburg, Sweden

*

To whom correspondence should be addressed. E-mail: [email protected]; Tel. +46 31 7869072.

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Abstract Water and organic molecules are omnipresent in the environment and their interactions are of central importance in many Earth system processes. Here we investigate molecular-level interactions between water and a nopinone surface using an environmental molecular beam (EMB) technique. Nopinone is a major reaction product formed during oxidation of β-pinene, a prominent compound emitted by coniferous trees, which has been found in both the gas and particle phases of atmospheric aerosol. The EMB method enables detailed studies of the dynamics and kinetics of D2O molecules interacting with a solid nopinone surface at 202 K. Hyperthermal collisions between water and nopinone result in efficient trapping of water molecules, with a small fraction that scatter inelastically after losing 60−80 % of their incident kinetic energy. While the majority of the trapped molecules rapidly desorb with a time constant τ < 10 µs, a substantial fraction (0.32 ± 0.09) form strong bonds with the nopinone surface and remain in the condensed phase for milliseconds or longer. The interactions between water and nopinone are compared to results for recently studied wateralcohol and water-acetic acid systems, which display similar collision dynamics but differ with respect to the kinetics of accommodated water. The results contribute to an emerging surface science-based view and molecular level description of organic aerosols in the atmosphere.


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1. Introduction The atmosphere is a complex mixture of interacting gases, aerosols and clouds, and this multiphase system plays a central role for the climate on Earth. Phenomena taking place at the interface between different phases are of particular interest because of their effects on the chemistry and physics of the atmosphere.1 Surface processes on atmospheric particles influence the distribution of reactive gases and thereby affect several phenomena including photochemical smog, new particle formation, removal of reactive greenhouse gases, and ozone depletion. Uptake of water and other trace gases modifies the physicochemical properties of particles including their size, morphology, optical properties, and ability to nucleate cloud droplets and ice. Here, we focus on the molecular-level description of these surface processes, and in particular the behavior of water and organic material at the interface.

Organic compounds are of interest due to their abundance and because their properties can evolve due to atmospheric processing like oxidation that results in the formation of secondary organic aerosols (SOA) and coatings on existing particles.2 We know that organic surface films can modify the critical supersaturation required for cloud droplet activation, for example by reducing the droplet surface tension.3 Organic compounds may also present a barrier to gas–aerosol mass transport that reduces uptake of water and other trace gases.4 In addition, the surface properties of the particle affect its heterogeneous chemistry and its ability to nucleate ice.5,6 However, we currently lack sufficient knowledge to make reliable predictions for most of these processes in ambient aerosols. This is partly due to the complexity of the organic fraction that potentially involves thousands of compounds, and to missing data on e.g., the phase behavior and surface-bulk partitioning in both model systems and naturally occurring organics.



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Detailed conditions at gas-liquid and gas-solid organic interfaces of atmospheric interest are particularly unclear and a largely unexplored research area. Studies have highlighted the surface structure and dynamic nature of organic surface layers using experimental techniques like nonlinear Raman spectroscopy, sum frequency generation, and neutron scattering.7-9 Molecular dynamics simulations have been used to study the effect of organic coatings on water uptake by aqueous droplets, and recently to investigate accommodation of organic compounds on their respective condensed phases.10-12 A few experimental molecular beam studies have been carried out including investigations of the effect of adsorbed alcohols on water uptake by sulfuric acid, and studies of water uptake on volatile organic phases including alcohols and carboxylic acids.13-15

The emergence of surface sensitive experimental methods operating at elevated pressures provides new opportunities for studies of the unique conditions at atmospheric interfaces. One of these methods is the environmental molecular beam (EMB) technique, which has recently been developed to reach operating pressures above 1 Pa.16 Here the EMB method is here employed to characterize water interactions with condensed nopinone, which is formed during atmospheric oxidation of β-pinene.17-22 The compound β-pinene is emitted from coniferous vegetation, and the molecule contains a double bond that makes it relatively reactive such that it is rapidly oxidized in the presence of O3, NO3 and/or OH. Nopinone is a major reaction product from β-pinene oxidation with reported yields of between 0.15 and 0.79.18-20,23 The molecule has been observed in both the gas phase and in the condensed SOA phase,24-27 which is surprising considering that pure nopinone has a relatively low evaporation enthalpy.27 The nonideality of nopinone has been suggested to be due to particle phase processes including ketone hydrate and dimer formation.22 Uptake of nopinone on liquid water has been studied using a wetted-wall reactor coupled with chemical ionization mass


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spectrometry.28 A Henry’s Law coefficient H = 195 ± 60 M atm-1 was determined at 296 K, which suggests that condensation or particle formation may compete with other nopinone sinks in the atmosphere.28

Here, we investigate the detailed dynamics and kinetics of water interactions with condensed nopinone in order to contribute to an improved molecular level understanding of organic aerosols. The reported EMB results include angularly resolved measurements of water desorbing and scattering from the organic surface. These are used to determine accommodation coefficients and to elucidate the outcomes of the interactions. The results are compared with data from similar systems, and suggestions for further studies are discussed.

2. Experimental A) Environmental Molecular Beam experiments The EMB method has been described in detail elsewhere,16 and is only briefly presented here. The apparatus consists of a multi-chambered high vacuum system and the main components are schematically displayed in Figure 1. A gas flow is introduced into the high vacuum region through a pulsed molecular beam source. The gas consists of a He:D2O mixture with a total pressure of 1.2×105 Pa and a D2O pressure of 2.3×103 Pa, which results in a kinetic energy of 0.28 ± 0.01 eV for D2O after expansion into vacuum. Deuterium oxide (D2O 99.9 %, Sigma Chemical Co.) is used instead of H2O in order to increase the signal to noise ratio by discriminating beam molecules from H2O molecules in the background gas. Part of the gas flow is selected using a skimmer followed by a mechanical chopper rotating with a frequency of 120 Hz to form a beam consisting of well-defined low-density gas pulses. Within the main ultra-high vacuum (UHV) chamber a separate environmental chamber contains the sample



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surface, at which the beam is aimed. The beam enters the environmental chamber through an opening consisting of a 10×10 mm2 grating and collides with a 4×4 mm2 surface substrate (highly oriented pyrolytic graphite, grade ZYB, Advanced Ceramics Corp.) situated 1.6 mm behind the grating. The molecular flux from the surface re-enters the main vacuum chamber through the same grating and is detected with a rotatable quadrupole mass spectrometer (QMS). The beam is directed towards the surface with an incident angle of 43° to the surface normal direction, and the total distance travelled by incident and reflected beam molecules within the environmental chamber is approximately 4.4 mm. The relatively short path reduces effects of collisions between beam molecules and background gas when the environmental chamber is filled with gas during experiments.

During the experiments nopinone is introduced into the environmental chamber through a leak valve and the compound is allowed to condense on the temperature controlled graphite substrate forming a nopinone layer with an approximate thickness of 1 µm. The layer thickness is monitored via the interference pattern produced by the adlayer from the surface reflections of a red laser (670 nm, 860 µW).29 The short pulses of D2O molecules are directed onto the nopinone surface and the QMS is employed to detect the intensity of molecules leaving the surface over a 10 ms timescale after beam impact. The experiments described herein utilized an experimental surface temperature of 202 K, while between measurements the surface was cleaned by increasing the temperature to 600 K.

B) Analysis The time-resolved flux of D2O molecules reflected from the nopinone surface is described by the sum of two distributions representing inelastic scattering (IS) and thermal desorption (TD) of molecules. The quantification of the separate components involves a nonlinear least-square


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fitting of the convolution of the IS, TD, and incident beam distributions to the total flux. The velocity distribution generated from inelastically scattered molecules is assumed to have a common form30 expressed as,

= −

#

" $,

(1)

where Ci is a scaling factor, is the velocity, ̅ is the average velocity, kB is Boltzmann’s

constant, m is the mass of a water molecule, and TIS is related to the velocity spread of the IS distribution. The thermal desorption of D2O molecules from the nopinone surface is captured using two distributions. The first describes the velocity distribution of thermally desorbed molecules,

&'( = ) −

*+,-

#

" $.

(2)

While the second relates desorption intensities to the residence time of trapped molecules, &'# = . / .

(3)

Here ) and . are scaling factors, 01234 is the surface temperature, k is the desorption rate

coefficient, and t is the time. The total TD intensity is generated by convoluting the two functions described in Eq. 2 and 3.



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To obtain the absolute trapping probability 5&' for D2O on nopinone the intensity of

678 6769 thermally desorbed molecules from the nopinone surface &' is compared with

desorption from bare graphite &'

:3;8

The Dynamics and Kinetics of Water Interactions with a Condensed

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