Impacts of Sulfate Seed Acidity and Water Content on Isoprene

Oct 13, 2015 - In addition, the dryer may scrub out semivolatile material that forms in the particle upon reaction in the aqueous phase. We note that ...
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Impacts of Sulfate Seed Acidity and Water Content on Isoprene Secondary Organic Aerosol Formation Jenny P. S. Wong,* Alex K. Y. Lee, and Jonathan P. D. Abbatt Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6 S Supporting Information *

ABSTRACT: The effects of particle-phase water and the acidity of preexisting sulfate seed particles on the formation of isoprene secondary organic aerosol (SOA) was investigated. SOA was generated from the photo-oxidation of isoprene in a flow tube reactor at 70% relative humidity (RH) and room temperature in the presence of three different sulfate seeds (effloresced and deliquesced ammonium sulfate and ammonium bisulfate) under low NOx conditions. High OH exposure conditions lead to little isoprene epoxydiol (IEPOX) SOA being generated. The primary result is that particle-phase water had the largest effect on the amount of SOA formed, with 60% more SOA formation occurring with deliquesced ammonium sulfate seeds as compared to that on effloresced ones. The additional organic material was highly oxidized. Although the amount of SOA formed did not exhibit a dependence on the range of seed particle acidity examined, perhaps because of the low amount of IEPOX SOA, the levels of high-molecular-weight material increased with acidity. While the uptake of organics was partially reversible under drying, the results nevertheless indicate that particle-phase water enhanced the amount of organic aerosol material formed and that the RH cycling of sulfate particles may mediate the extent of isoprene SOA formation in the atmosphere.

1. INTRODUCTION Isoprene is the most abundant nonmethane hydrocarbon emitted to the atmosphere, and its oxidation is estimated to be the largest source of secondary organic aerosol (SOA).1,2 Despite the importance of SOA on climate and human health, there is a lack of understanding on its atmospheric formation and evolution pathways.1 Traditionally, SOA formation is thought to occur via the partitioning of semivolatile organics from gas-phase photochemistry. Current atmospheric models, based on this volatility-driven partitioning theory, underestimate observed SOA mass, suggesting unidentified formation mechanisms.2 Previous studies have shown that water-soluble isoprene photooxidation products, even if highly volatile, can readily partition to particle-phase water where subsequent condensedphase reactions can lead to the formation of SOA.3 In particular, the formation of isoprene epoxydiols (IEPOX) from isoprene photo-oxidation under low NOx conditions and their reactive uptake to the particle phase have been proposed as key processes for isoprene SOA formation.4,5 Studies have shown that the reactive uptake of IEPOX is enhanced under acidic seed particle conditions, leading to substantial SOA formation via acid-catalyzed nucleophilic addition to the epoxide ring.6−8 However, ambient studies have observed weak correlations between particle acidity and IEPOX-derived SOA,9−12 suggesting that other factors may also affect SOA formation. Given that acidic seed particles are hygroscopic, even at the low-RH conditions used by most isoprene SOA © 2015 American Chemical Society

studies, the relative importance of seed particle acidity and particle-phase water on the formation of isoprene SOA remains uncertain. Recent laboratory studies have focused on elucidating the effects of particle acidity and particle-phase water on the reactive uptake of IEPOX.13,14 Collectively, these studies have shown that particle acidity is the dominant factor affecting the reactive uptake rate of IEPOX for particles with pH values 3), the availability of efficient nucleophiles (e.g., NH3) is key for the reactive uptake process to occur. Recently, Liu et al., quantified that the uptake of IEPOX onto acidic sulfate seed particles under low-RH conditions accounts for 50% of the SOA mass formed.15 Although the reactive-uptake process of IEPOX and its contribution to the SOA burden are becoming better understood, it is important to note that other isoprene oxidation products are water-soluble (e.g., carbonyls) and that the effects of acidity and particle-phase water on such oxidation products remain unknown. In this paper, we present laboratory measurements of SOA formation from the photo-oxidation of isoprene under high RH (70%) conditions onto various seed particles in a flow-tube reactor: effloresced and deliquesced ammonium sulfate as well Received: Revised: Accepted: Published: 13215

June 1, 2015 October 12, 2015 October 13, 2015 October 13, 2015 DOI: 10.1021/acs.est.5b02686 Environ. Sci. Technol. 2015, 49, 13215−13221

Article

Environmental Science & Technology

Figure 1. (a) Schematic of the experimental setup. For each corresponding region of the setup, the physical state of the particles is illustrated in (b) for sulfate seed particles, effloresced AS is represented by the red hexagon, and deliquesced AS is represented as the small red circles. Abbreviations are described in the main text.

Following size selection, the monodispersed particle flow, controlled by a three-way valve, was alternatively mixed with a humidified N2/O2 flow (1550 sccm) in the conditioning tube (RH 82%) or in a bypass tube (RH < 30%). AS particles mixed with the humidified flow at this point deliquesce in the conditioning tube (residence time of approximately 1 min.); otherwise, they remain effloresced in the bypass tube. The resulting deliquesced AS particle flow was combined with a dry flow containing ozone (150 sccm, generated by passing O2 through a mercury lamp) in a mixing volume. For the effloresced AS particle flow, the same dry ozone flow was first mixed with the 1550 sccm N2/O2 humidified flow (see above), which was then added to the particle flow in the mixing volume. It is important to note that (i) the RH of the latter particle flow never exceeds 80%, and so the AS particles remain fully effloresced, and that (ii) within the mixing flow, the relative humidities are the same for both configurations, but the phase of the AS particles is not. Control experiments were conducted to ensure that the AS particles deliquesced in the conditioning tube. The particle-size distributions were measured from the outflow of the mixing volume using a scanning-mobility particle sizer (SMPS, TSI3075) whose sheath flow was conditioned to the same RH conditions as the mixing volume for several hours. For effloresced AS particles size-selected at a mobility diameter of 100 nm, exposure to RH 82% in the conditioning tube resulted in an increase of particle diameter to 120 nm, indicating the AS particles have deliquesced. For particles that flowed through the bypass tube, no change in particle mean diameter was observed, indicating that the particles remain effloresced. The observed increase in particle diameter due to deliquescence is consistent with that expected from the hygroscopic behavior of AS.16 For experiments with acidic sulfate particles, the particles only flowed through the conditioning tube. For the remainder of this paper, deliquesced AS will be referred to as “wet AS”, effloresced AS as “dry AS,” and acidic sulfate as “wet ABS” for simplicity. These sulfate seed particles were chosen because they are ubiquitous in the atmosphere and are commonly used seed particles in previous isoprene SOA and IEPOX studies.

as acidified sulfate particles. In particular, by operating at constant relative humidity, experimental issues related to varying gas-phase chemistry or wall-loss rates are minimized. By taking advantage of the hysteresis in the phase of ammonium sulfate aerosol particles as a function of increasing and decreasing relative humidity, we can operate with seeds that have the same chemical composition but different phases. Overall, this study aims to (i) isolate the roles of particle-phase water and acidity on the nature of isoprene SOA formation and (ii) investigate the reversibility of the uptake processes leading to SOA formation that occurs upon changes in relative humidity.

2. EXPERIMENTAL SECTION A schematic of the experimental setup used to investigate seed particle effects on isoprene SOA formation is shown in Figure 1. Key components of the setup are described in detail below. We emphasize that the overall experimental requirements to achieve our scientific goals are (i) to have control over the phase of the ammonium sulfate seeds, i.e., whether the particles are deliquesced or effloresced, and (ii) for the oxidation chemistry that forms the SOA to be conducted at the same relative humidity independent of seed phase or composition. 2.1. Seed-Particle Generation and Phase Characterization. Sulfate seed particles were generated by atomizing aqueous sulfate solutions with a constant output atomizer (TSI 3076) with N2 as carrier gas. For ammonium sulfate seed particles, a dilute solution (0.12 mM) of ammonium sulfate (AS, Sigma-Aldrich) was used. For acidified sulfate seed particles, the above AS solution was acidified using sulfuric acid (Fisher Scientific), resulting in a 1:1 ammonium-to-sulfate molar ratio. After atomization, 300 sccm of the atomizer output was dried using a silica-gel diffusion dryer, which reduced the RH to 100 was observed for wet ABS (Figure 3 insert). Given the hard ionization technique used in the AMS, organic fragments with m/z > 100 arise from the fragmentation of compounds of even larger molecular weight. We speculate that the high-molecularweight (HMW) species were formed via acid-catalyzed condensed-phase reactions. These observations are consistent with previous studies where high-molecular-weight species (i.e., esters and organosulfates) have been previously identified in isoprene SOA on acidic seed, where their contributions increase with acidity.6,38 The reversibility of the uptake process leading to isoprene SOA formation on wet ABS was also investigated, and the 13219

DOI: 10.1021/acs.est.5b02686 Environ. Sci. Technol. 2015, 49, 13215−13221

Article

Environmental Science & Technology flow tube. Because of this, the distribution of gas-phase isoprene oxidation products probably resembles that which was somewhat removed from isoprene sources, i.e., after IEPOX has oxidized away. We also note that the presence of pre-existing organics on sulfate particles may affect the role of particle-phase water and acidity on isoprene SOA formation. Gaston et al. observed that the uptake of IEPOX on mixed sulfate−polyethylene glycol particles was suppressed even for acidic sulfate seeds at high RH conditions.14 Given the complexity of seed-particle effects on just one SOA precursor, further investigations using a wide range of seed-particle composition, SOA precursors, and conditions (i.e., RH, effects of UV light) are warranted for understanding how seed particles mediate SOA formation in the atmosphere.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b02686. Sulfate-normalized organic spectra for wet and dry AS; organic spectra (normalized by total organic mass) for wet AS and ABS; spectral difference of organics (normalized by total organic mass) for wet AS, dry AS, and wet ABS illustrating changes due to water evaporation. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 416-946-7358; E-mail: [email protected]. ca. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Natural Sciences and Engineering Research Council for funding. REFERENCES

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