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Optical Properties of Secondary Organic Aerosol from Cis-3-Hexenol and Cis-3-Hexenyl Acetate: Effect of Chemical Composition, Humidity and Phase Rebecca Harvey, Adam P. Bateman, Shashank Jain, Yong Jie Li, Scot T. Martin, and Giuseppe A Petrucci Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00625 • Publication Date (Web): 13 Apr 2016 Downloaded from http://pubs.acs.org on April 15, 2016
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Optical Properties of Secondary Organic Aerosol from Cis-3-Hexenol and Cis-3-Hexenyl Acetate:
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Effect of Chemical Composition, Humidity and Phase
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Rebecca M. Harvey, ‡ Adam P. Bateman,@ Shashank Jain,‡ Yong Jie Li,@† Scot Martin,@,§ and Giuseppe
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A. Petrucci‡*
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‡
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@
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USA
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§
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Department of Chemistry, University of Vermont, Burlington, Vermont 05405, USA School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138,
Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138,
USA
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Keywords: green leaf volatiles, cis-3-hexenol, cis-3-hexenyl acetate, ozonolysis, secondary organic
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aerosol, mass absorption coefficient, mass scatter coefficient, optical property, viscosity, bounce factor,
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visual range
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Abstract: Atmospheric aerosols play an important role in Earth’s radiative balance directly, by scattering
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and absorbing radiation, and indirectly by acting as cloud condensation nuclei (CCN).
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aerosol is dominated by secondary organic aerosol (SOA) formed by the oxidation of biogenic volatile
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organic compounds (BVOCs). Green leaf volatiles (GLVs) are a class of BVOCs that contribute to SOA,
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yet their role in the Earth’s radiative budget is poorly understood. In this work we measured the
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scattering efficiency (at 450 nm, 525 nm and 635 nm), absorption efficiency (between 190-900 nm),
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particle phase, bulk chemical properties (O:C, H:C) and molecular-level composition of SOA formed
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from the ozonolysis of two GLVs; cis-3-hexenol (HXL) and cis-3-hexenyl acetate (CHA). Both HXL and
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CHA produced SOA that was weakly absorbing, yet CHA-SOA was a more efficient absorber than HXL-
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SOA. The scatter efficiency of SOA from both systems was wavelength-dependent, with the stronger
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dependence exhibited by HXL-SOA, likely due to differences in particle size. HXL-SOA formed under
Atmospheric
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both dry (10% RH) and wet (70% RH) conditions had the same bulk chemical properties (O:C), yet
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significantly different optical properties, which was attributed to differences in molecular-level
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composition. We have found that SOA derived from green leaf volatiles has the potential to affect the
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Earth’s radiative budget, and also that bulk chemical properties can be insufficient to predict SOA optical
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properties.
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1.0 Introduction
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The absorption and scatter of light by atmospheric aerosols play an important role in Earth’s
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radiative balance. However, the magnitude of absorption and scatter has proven difficult to pin down,
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largely due to challenges associated with measuring aerosol optical properties, which are size,
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composition and wavelength dependent. Organic aerosol (OA) contributes 20-90% of the total mass of
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atmospheric aerosol1, 2 and 70-90% of OA is secondary in nature (SOA), being formed by the oxidation of
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volatile organic compounds (VOCs) in the atmosphere.3 There is growing evidence to suggest that SOA
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contributes to light attenuation (by both absorbance and scatter) in the atmosphere and may play an
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important role in the radiative budget.4-12 Green leaf volatiles (GLVs) are a class of wound-induced
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VOCs emitted by several plant species. The most dominant and ozone-reactive GLVs include cis-3-
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hexenyl acetate (CHA) and cis-3-hexenol (HXL), which have been shown to react with ozone to produce
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SOA at comparable levels to predominant biogenic SOA sources, yet the optical properties of these SOA
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are not understood.13
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The optical properties of SOA can be characterized by the mass absorbance/scatter coefficient
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(MAC, MSC), which describe the mass-normalized contribution of light absorbance/scatter to the total
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extinction; and the Angstrom Scatter Exponent (ASE), which describes the wavelength dependence of
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scatter.7, 14-17 These optical properties are closely related to particle size, phase and chemical composition,
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which are themselves dependent on the parent VOC, oxidant (ozone, NOx, OH etc.) and environmental
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conditions (i.e. temperature, humidity). Over long time scales, particle-phase reactions and chemical
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aging likely also play a role in determining optical properties.18 Bulk chemical properties such as the
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oxygen-to-carbon ratio (O:C) of SOA have been used to predict or rationalize SOA optical properties.2, 4, 9, 2 ACS Paragon Plus Environment
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18-22
For example, a greater O:C has been shown to correlate to greater light absorption.17, 23, 24 However,
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recent studies have also shown no correlation between O:C and optical properties, or even the opposite
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trend. 18, 25
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In this work, we measure the optical properties and phase state of SOA as a function of chemical
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system (CHA vs HXL) and environmental conditions (relative humidity, RH). We further demonstrate
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that a molecular-level understanding of SOA chemical composition is important to understand its optical
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properties and predict its role in radiative forcing.
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2.0 Experimental
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All experiments were performed in the Harvard Environmental Chamber (HEC) operated under
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dark conditions and in continuous-flow mode.26, 27 The HEC consists of a 4.7 m3 PFA Teflon bag housed
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in a temperature and humidity-controlled room. Dry, clean, hydrocarbon-free air was produced with a
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pure air generator (Aadco 737). A syringe pump was used to continuously introduce standards of CHA or
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HXL (both >98 %, Sigma-Aldrich) into a secondary chamber. A constant flow of zero air was used to
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sweep volatilized compounds from the secondary chamber into the HEC. Neither seed particles nor OH
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scavengers were used in these experiments. Ozone was produced by passing air around an ultraviolet
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lamp (Jelight 600) and then monitored in the chamber using a Teledyne 400 E. Humidity was controlled
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by passing zero air through a water bubbler (18 MΩ cm) followed by a HEPA filter and into the chamber,
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where it was monitored with a Rotronics humidity sensor (Hygroclip SC05). Bulk chemical properties
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(O:C, H:C) were measured using an Aerodyne high-resolution time of flight mass spectrometer (HR-ToF-
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AMS),28, 29 according to Chen et al. (2011).30 Molecular-level chemical analysis of SOA was performed
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using Near-IR Laser Desorption/Ionization Aerosol Mass Spectrometry (NIR-LDIAMS).31,
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description of NIR-LDI-AMS can be found in the supplemental information.
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A brief
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An integrating sphere UV-Vis spectrometer (IS-UV-Vis, Shimadzu UV-2450 with ISR 2200) was
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used to measure the wavelength-dependent light absorption of SOA.9 SOA was collected on 25 mm
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quartz filters (Pall Tissuquartz PN 7200) at 4.95 ± 0.05 L min-1 for a known duration of time (See Table
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S1). Filters were individually wrapped in aluminum foil and stored in a desiccator at room temperature 3 ACS Paragon Plus Environment
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until analysis (
