Formation of Secondary Organic Aerosol from the Heterogeneous

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Formation of Secondary Organic Aerosol from the Heterogeneous Oxidation by Ozone of a Phytoplankton Culture Stephanie R. Schneider, Douglas Collins, Christopher Y. Lim, Linglan Zhu, and Jonathan Abbatt ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.9b00201 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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ACS Earth and Space Chemistry

Formation of Secondary Organic Aerosol from the Heterogeneous Oxidation by Ozone of a Phytoplankton Culture Stephanie R. Schneider1, Douglas B. Collins1,2, Christopher Y. Lim1, Linglan Zhu1, Jonathan P.D. Abbatt1,* 1Department

of Chemistry, University of Toronto, 80 St. George Street Toronto, ON, Canada. M5S 3H6

2Department

of Chemistry, Bucknell University, 1 Dent Drive, Lewisburg, PA 17837, USA

Keywords: Sea surface microlayer (SSML), heterogeneous oxidation, ozone, secondary organic aerosol (SOA), diatom, marine aerosol, particle growth

Abstract

The sea surface microlayer (SSML) is often present at the ocean interface and provides a unique environment for chemical reactions to occur. One such reaction is the heterogeneous oxidation of the SSML components with ozone, which is hypothesized to be an important source of volatile compounds that may participate in marine aerosol formation and growth. To better understand this source, a biologically relevant model SSML is constructed using axenic Thalassiosira pseudonana cultures. This model SSML is shown to be reasonably reproducible for repeated experiments with a biological system 1 ACS Paragon Plus Environment

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and offers considerably more chemical and morphological complexity than single-molecule SSML representations for trying to understand the impact of marine biological processes on the atmosphere. Using proton transfer reaction-mass spectrometry (PTR-MS), this study demonstrates that C7-C10 gasphase carbonyls arise from the oxidation of the model SSML with ozone. The ability of gas-phase products of ozone oxidation at the SSML to form aerosol particles was investigated with a scanning mobility particle analyzer (SMPS) to determine particle size and concentration of newly formed ultrafine aerosol particles. These particles are confirmed to be secondary organic aerosol (SOA) by analyzing their composition with an aerosol mass spectrometer (AMS), indicating that the source of the aerosol precursors is the organic material generated by the T. pseudonana cultures. The rates of SOA and carbonyl production are larger for 21-day-old cultures than for 7-day-old cultures, likely due to the release of organic material from cell lysis in the older cultures. By demonstrating that heterogeneous oxidation of the SSML forms SOA precursors that contribute to aerosol growth, this study emphasizes the importance of biological processes on the chemical reactions that can occur within the SSML.

Introduction The accumulation of compounds at the surface of ocean, called the sea surface microlayer (SSML), creates a unique environment at the air-sea interface.1 The SSML differs from the underlying seawater by concentrating materials such as lipids, carbohydrates, microorganisms, and proteins within the top 1-1000 µm of the ocean.2,3 The SSML is an important mediator of the interaction between the bulk aqueous phase of the ocean and the gaseous phase of the atmosphere. This mediation can occur by influencing air-sea gas exchange, changing sea spray aerosol composition, and through chemical reactions within the SSML that release volatile organic compounds (VOCs).1

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Experimental studies using real SSML samples have shown that chemical reactions within the SSML can act as a source of VOCs to the atmosphere, through photochemistry or heterogeneous oxidation mechanisms.4–7 This conclusion is supported by a recent field study in the Canadian Arctic that related an increase in oxygenated VOCs to an enriched SSML, hypothesizing that photochemical or heterogeneous oxidation reactions within the SSML were the source.8 Understanding the formation routes of the VOCs is important because they could condense to contribute to aerosol growth or nucleation, as was suggested by a complimentary study noting an increase in the organic fraction of secondary aerosol from a marine source.9 An oxygenated organic fraction within these aerosols could make them efficient CCN at smaller sizes10, thus having an indirect climate effect. The exact source of VOCs that can act as secondary organic aerosol (SOA) precursors from the ocean is still not well understood. VOC emissions from a marine source needed to be included in a chemical transport model in order to grow particles to their observed size.11 Overall, the climate forcing of marine SOA remains very poorly constrained, primarily due to unknown SOA precursors and the mechanism behind their formation. To understand the chemical mechanisms of abiotic VOC production from the SSML in the laboratory, a model compound has been historically chosen to represent the SSML. Specifically, studies investigating photochemical reactions have used fatty acids or alcohols to represent the SSML. Monolayers of long chain fatty acids and/or alcohols have been shown to produce a variety of functionalized and oxidized products, including both saturated and unsaturated carbonyls.5–7,12–18 Most of these systems need photosensitizers, such as humic acid or 4-benzoyl benzoic acid, to initiate the reactions that form VOCs. Recently, dissolved organic matter (DOM) produced by a phytoplankton bloom was used as a photosensitizer for a fatty-acid monolayer to create a more authentic photosensitizing system, since the previous photosensitizers were terrestrially derived or not found in large amounts in the environment.14 The photochemical products are reactive in the atmosphere and contribute to SOA mass.19 The 3 ACS Paragon Plus Environment


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heterogeneous oxidation of the SSML with ozone has been suggested as another mechanism for release of VOCs to the atmosphere, although it has been less thoroughly studied. One preliminary study explored the heterogeneous oxidation of the SSML using linoleic acid to simulate the SSML, which produced gasphase carbonyls when exposed to ozone.4 This study also exposed a natural SSML to ozone, which produced a similar array of oxygenated VOCs. While using simple model SSML components is useful from a mechanistic standpoint, the SSML is a complex and changing environment that cannot be captured using only a few select molecules. As well, biological processes play a dominant role in setting its composition. A chemically complex system that is more representative of the SSML is needed to fully understand the changing conditions which could affect the mechanisms by which VOCs are released into the atmosphere.1,14,20 For example, the system should include transparent exopolymer particles (TEP), which are formed through the coagulation of negatively charged biogenic polysaccharides.21 TEP is just one example of a component that is recognized as being enriched in the SSML and not represented by simple SSML proxies. In this study we use Thalassiosira pseudonana, a common diatom, to generate organic material to simulate the biological complexity present at the SSML. T. pseudonana produces classes of surface-active compounds, that are similar to those found in the natural SSML.22 T. pseudonana is prevalent in many freshwater and saltwater ecosystems from tropical to temperate climates, making it representative of many different environments.23 The cultures are also easy to maintain as they grow readily at room temperature.24 Previous studies investigating abiotic mechanisms of VOC release have typically used model lipids to represent the SSML.4–6,14–16,18,19 T. pseudonana is a high lipid diatom, so the SSML it generates would be more lipid-like than other lower-lipid species, allowing easier comparison to previous studies.25–27 Like other diatoms, T. pseudonana lipids on average contain more than one carbon-carbon double bond, which are likely susceptible to heterogeneous ozonolysis.25 T. pseudonana is considered a 4 ACS Paragon Plus Environment

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model diatom for experimental studies as it was the first diatom to have its genome fully sequenced, which may result in multidisciplinary studies that could be used to understand our system more readily.28 For these reasons, we chose to use axenic T. pseudonana cultures to form a biologically relevant and reproducible SSML, which we will refer to as the model SSML. VOC formation from heterogeneous oxidation of the model SSML with ozone was monitored with proton-transfer-reaction mass spectrometry (PTR-MS). The concentration and size of particles that formed were monitored with a scanning mobility particle sizer (SMPS) and their composition was characterized with an aerosol mass spectrometer (AMS). The main goals of this study are to: (i) evaluate whether the model SSML forms VOCs from heterogeneous oxidation with ozone, (ii) determine if oxidation gives rise to the formation of SOA, and (iii) assess how VOC and SOA formation depend on the growth cycle of the phytoplankton cultures. Overall, these studies will increase our understanding of the influence of biological processes on VOC and SOA formation in marine environments.

Methods Phytoplankton Cultures. Axenic Thalassiosira pseudonana (CCMP1335) cultures were purchased from the Bigelow National Center for Marine Algae and Microbiota. The cultures were grown in 75 mL of L1 growth media with a 12-hour cool white fluorescent lamp cycle. The L1 growth media is a generalpurpose recipe which adds nitrogen, phosphorous, silica, trace vitamins and elements to seawater to grow algae. The cultures were tested for the absence of bacteria and fungi by sub-straining the cultures into an f/2 Freshwater Medium and Test Medium that were incubated for 3 weeks and compared to a positive control. Cell concentration was measured using a BD Facs Canto flow cytometer with an excitation wavelength of 488 nm and emission wavelength of 650 nm. The corresponding extinction was measured to quickly estimate cell concentrations with a 1 cm path length cuvette, using a UV-vis spectrometer 5 ACS Paragon Plus Environment


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(USB2000+, Ocean Optics) attached to a deuterium tungsten halogen light source (DT-Mini-2, Ocean Optics). SpectraSuite software (Ocean Optics) was used to collect and record data. The optical density at 500.43 nm was measured at different cell concentrations to form a calibration curve. The total organic carbon (TOC) was measured throughout the T. pseudonana lifetime with a TOC-LCPH/CPN Shimadzu instrument. Samples were centrifuged gently to remove cells to prevent clogging of the instrument, so TOC represents the concentration of total extracellular organic carbon. TEP Measurements. Sampling of the SSML was done with a glass microscope slide and a shower squeegee, to simulate the glass plate collection done in the field.1,29 TEP concentration was determined spectrophotometrically, as is commonly done for authentic SSML samples.30,31 The updated staining method of Bittar, Passow, Hamaraty, Bidle and Harvey (2018)31 using an Alcian blue solution (SigmaAldrich) was used to quantify TEP in the sample, with some minor changes. TEP was isolated using a filtering technique described by Thornton, Brooks and Chen (2016) that used 25 mm glass fiber filter (GF/C, Whatman) and polycarbonate filters (0.4 μm, Nuclepore, Whatman).32 The samples of SSML and bulk water were diluted by a factor of 5 and 2, respectively, to fit on the calibration curve. The polycarbonate filters were soaked in 3 mL of an 80% sulfuric acid extraction solution for 3 hours, and the absorbance was measured in a 0.5 cm cuvette. Procedural blanks were done using 1 mL of ultrapure water stained with 0.5 mL of Alcian Blue solution. All absorbance measurements were done in triplicate. Concentration calculations were done using equations given by Passow and Alldredge (2011) using the measured absorbance at 787 nm.30


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Figure 1. Schematic diagram of experimental set-up. MFC = mass flow controller, DMA = differential mobility analyzer, SMPS = scanning mobility particle sizer, TPOT = Toronto photo-oxidation tube, AMS = aerosol mass spectrometer, PTR = proton transfer reaction–mass spectrometer. Model SSML Oxidation. Cultures were transferred to a 250 mL Erlenmeyer flask (Figure 1) that was kept in the dark, to which ozone was introduced in a flow of 200 sccm of air. The flow of ozone was not bubbling through the culture but was introduced to the culture headspace, with a residence time of 75 seconds. Ozone was generated by passing 200 sccm of zero air over a UV lamp (254 nm Pen-Ray Lamp, UVP, Inc). An ozone concentration of 8.5 ppm in the flask was measured with a UV photometric ozone analyzer (Thermo Model 49i). The sample flow was mixed with 650 sccm of humidified air and introduced into the Toronto photo-oxidation tube (TPOT), which is described elsewhere,33,34 to give a final relative humidity of ~70%. Briefly, the TPOT is silcosteel-coated, with a volume of 3.2 L, to give a residence time of 225 seconds. One 23-cm-long 254 nm Hg lamp was used in the TPOT to photolyze ozone into OH through reaction with water. The diluted ozone concentration in the TPOT is 1.5 ppm. The OH concentration in the TPOT was measured with the reaction of toluene with OH. Small concentrations of toluene (>99.9%, Sigma-Aldrich) between 74-420 ppb were introduced to the TPOT with normal experimental flows. The decay of the toluene signal upon reaction with OH was monitored with the PTRMS. The second order rate constant of the reaction of toluene and OH used to calculate the final OH



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concentration was 5.6 x 1012 cm3 molecule-1 s-1. The average OH concentration in the flow tube was 1.7 x 109 molecule cm3. The exposure of ozone (i.e. mixing ratio multiplied by time) to the culture is 177 ppb hours, which is equivalent to a 5.9-hour exposure to ambient ozone mixing ratios of 30 ppb. Thus, the exposure is environmentally relevant, although the mixing ratios are very much higher than those encountered in the atmosphere. Likewise, in the TPOT, the overall OH exposure is also atmospherically relevant, being equivalent to 106 hours of exposure to ambient OH concentrations of 1 x 106 molecules/cm3 (0.04 ppt at STP). Aerosol Measurement. Aerosol particle size distributions were measured with a scanning mobility particle sizer (SMPS; TSI, Inc. model 3080/3787) using a sample flow rate of 0.3 L/min and a sheath flow rate of 3.0 L/min. Each sample was analyzed over a period of 45 minutes, with repetitive scans two minutes long between electrical mobility diameters (dm) of 14-500 nm. All particles were assumed to be spherical. Gas-Phase Analysis. The flow coming from the culture flask (Figure 1) was sampled at times with a quadrupole proton transfer reaction mass spectrometer (PTR-MS; Ionicon Analytik GmbH, Innsbruck Austria) instead of proceeding into the TPOT. As carbonyls are the expected products of ozonolysis reactions, the mass-to-charge ratios of C1-C10 carbonyls were monitored under selected ion monitoring mode at nominal masses: m/z 31, 45, 59, 73, 87, 101, 115, 129, 143 and 157. We note that nominal masses are relatively accurate markers for carbonyl compounds, with few interferences from other compounds.35 A control experiment that flowed clean air over the cultures instead of ozone was performed to measure any primary biological emissions present at those m/z values. Experiments performed in the different conditions (air vs ozone) were performed on different cultures on different days. PTR-MS signals for each nominal m/z value were averaged for the 10 minutes immediately after culture introduction to the flow 8 ACS Paragon Plus Environment

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system. The difference in signal for experiments with only the L1 growth media and with the T. pseudonana cultures in air and ozone was calculated. A Student’s two tailed t-test was performed to compare the difference in carbonyl signal for ozone and zero air conditions to determine if ozone is responsible for the release of carbonyls. Aerosol Composition. Separate experiments from the SMPS experiments measured particle composition. Particle composition was measured with a high-resolution aerosol mass spectrometer (HRAMS; Aerodyne Research Inc.). The particles formed are small (median diameter = 20 nm) and outside the range of particle size detected by the AMS, i.e. the aerodynamic lens of the AMS has poor transmission efficiency for particles smaller than ~70 nm. Therefore, ammonium nitrate was used as a seed particle to increase the particle size and AMS detection efficiency.36 A schematic representation of the seed particle generation is presented in the gray box in Figure 1. To reduce the concentration of trace organics in the ammonium nitrate solution that was atomized to form the seeds, the ammonium nitrate solute was washed 3 times with 300 mL of dichloromethane in a separatory funnel, then gently heated to remove the residual organic solvent. Ammonium nitrate particles were formed using a TSI constant output atomizer with a flow of 3 LPM, with ammonium nitrate solutions of 0.25 or 0.5 M. 2.7 LPM of the flow was pulled through a filter with a mass flow controller and pump, leaving 300 sccm of aerosol flow to go through a differential mobility analyzer (DMA; TSI, Inc. model 3080). Particle sizes were selected at 100 or 150 nm, with a high sheath flow of 18 L/min that narrows the transfer function to select fewer particles. The monodisperse aerosol flow was introduced to the culture flask in addition to the 200 sccm ozone flow to give a combined flow of 500 sccm. The ozone concentration in the flask was therefore reduced to 3.4 ppm, with a residence time of 30 seconds. The AMS sampled directly from the culture flask, with a vaporizer temperature of 600 C. Data were analyzed using the standard AMS analysis toolkits (Squirrel



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1.61 and Pika 1.21). The average mass signals of sulphate, nitrate and organics measured by the AMS are presented in Figure S1.

Results and Discussion

Figure 2. Cell concentration (left, black circles) and TOC (right, green squares) changes with culture age Sea Surface Microlayer Model. To characterize the T. pseudonana cultures grown for these experiments, the cell concentration and the total organic carbon (TOC) were measured throughout the T. pseudonana life cycle (Figure 2). The exponential growth phase lasts for the first week, after which the cell concentration remains steady in the stationary phase for the next 20 days. Experiments were done after the exponential growth phase (~7 days old) to allow the accumulation of material at the surface of the culture, and near the end of the stationary phase (~21 days old) to capture the effect of bloom age on the production of organic material. The TOC throughout the T. pseudonana culture growth did not follow the same trend as the cell concentration. Since the samples were gently centrifuged prior to analysis, carbon within the T. pseudonana cells was not included within measured TOC. As the cultures reach the 10 ACS Paragon Plus Environment

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end of the exponential growth phase, cell death triggers the release of the material held within the cell, which is then measured as TOC. Organic carbon released by lysing cells may be in the form of watersoluble organic compounds or colloidal material. Many studies have looked at the composition of lipids held within the T. pseudonana cell in different growth conditions.25,37–39 This material is released from the cells into the bulk aqueous solution during cell lysis and death, well-known within the biofuel industry.40,41 We note that lipids are not considered to be the primary exudates of live T. pseudonana cells, with amino acids and polysaccharides making up the majority of the exudate.42 TEP is an important aspect of the SSML21,43 and is not typically included in chemical models of the SSML. We wanted to verify that TEP was being formed in our model SSML. This TEP concentration measured in the T. pseudonana cultures is higher compared to what has been measured in the natural SSML using the same methods.21 The absolute concentration of TEP is increased in the 21-day-old samples vs the 7-day-old samples. Oxidation of the culture with ozone does not change the TEP concentration or enrichment.



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Figure 3. TEP measurements for different model SSML. Error bars represent the one sigma precision uncertainties for 3 culture replicates.

Gas-phase analysis. PTR-MS was used to identify volatile products arising from ozone exposure of the SSML. A PTR full scan was initially performed (Figure S3 and S4) to observe the products released in air from the T. pseudonana cultures directly, and then with ozone. The major products observed were at m/z 45 and 59, which correspond to the C2 and C3 carbonyls. As well, both DMS, isoprene and other small molecules were observed in full scan mode. Table S1 presents a calibration of DMS, isoprene and the C2 and C3 carbonyls using previously published sensitivities for our instrument. It was hypothesized that ozonolysis of unsaturated fatty acids that are present in the SSML would give rise to the formation of carbonyls, particularly aldehydes, which are common products from ozonolysis of long-chain unsaturated acids. If these molecules are sufficiently large, they could be involved in the formation of SOA. Specifically, the C1-C10 carbonyls were monitored using selected ion monitoring.


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Figure 4: PTR-MS signal of the nominal C1-C10 carbonyl mass-to-charge ratio with and without ozone over the cultures. The error bars represent one sigma precision uncertainties from 7 replicates of different cultures. Carbonyl signals from the model SSML were compared to those of the L1 growth media, both with and without ozone. This experiment was repeated without ozone to vet the possibility of primary biological emissions of these carbonyls by T. pseudonana, which have been previously studied with the PTR-MS.44 This study used the PTR-MS to observe the trends of biological emissions of T. pseudonana to show correlation of emissions with light and different growth stages of the cultures, which we controlled for by using similarly aged cultures in the dark. Figure 4 shows the difference in carbonyl signal between the culture and the growth media with and without ozone, normalized to the L1 growth media signal. For almost all experiments the quantity on the y-axis is positive, indicating that there is more signal arising from the culture than from the L1 medium. The C1, C5, C7-10 carbonyl signals increased with statistical significance (p

Formation of Secondary Organic Aerosol from the Heterogeneous

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