Direct Night-Time Ejection of Particle-Phase Reduced Biogenic Sulfur

Technol. , 2015, 49 (8), pp 4861–4867. DOI: 10.1021/es506177s. Publication Date (Web): April 2, 2015. Copyright © 2015 American Chemical Society. *...
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Direct Night-Time Ejection of Particle-Phase Reduced Biogenic Sulfur Compounds from the Ocean to the Atmosphere Cassandra J. Gaston,†,⊥ Hiroshi Furutani,‡,§ Sergio A. Guazzotti,‡,# Keith R. Coffee,‡,∇ Jinyoung Jung,§,○ Mitsuo Uematsu,§ and Kimberly A. Prather*,†,‡ †

Scripps Institution of Oceanography, and ‡Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla California 92093, United States § Atmosphere and Ocean Research Institute, University of Tokyo, Chiba 277-8564, Japan S Supporting Information *

ABSTRACT: The influence of oceanic biological activity on sea spray aerosol composition, clouds, and climate remains poorly understood. The emission of organic material and gaseous dimethyl sulfide (DMS) from the ocean represents welldocumented biogenic processes that influence particle chemistry in marine environments. However, the direct emission of particle-phase biogenic sulfur from the ocean remains largely unexplored. Here we present measurements of ocean-derived particles containing reduced sulfur, detected as elemental sulfur ions (e.g., 32S+, 64S2+), in seven different marine environments using real-time, single particle mass spectrometry; these particles have not been detected outside of the marine environment. These reduced sulfur compounds were associated with primary marine particle types and wind speeds typically between 5 and 10 m/s suggesting that these particles themselves are a primary emission. In studies with measurements of seawater properties, chlorophyll-a and atmospheric DMS concentrations were typically elevated in these same locations suggesting a biogenic source for these sulfur-containing particles. Interestingly, these sulfur-containing particles only appeared at night, likely due to rapid photochemical destruction during the daytime, and comprised up to ∼67% of the aerosol number fraction, particularly in the supermicrometer size range. These sulfur-containing particles were detected along the California coast, across the Pacific Ocean, and in the southern Indian Ocean suggesting that these particles represent a globally significant biogenic contribution to the marine aerosol burden. sulfide (H2S).16,17 Gaseous elemental sulfur has also been detected in the marine environment; however, it is unknown whether its source was biogenic or anthropogenic.18 In general, marine biological activity has been found to contribute sulfurcontaining compounds to the gas-phase, which can lead to the contribution of secondary sulfur-containing compounds to marine aerosols. However, the emission of biogenic sulfur compounds as primary marine particles has been largely unexplored. Here we present real-time, single-particle measurements during seven field campaigns conducted in different marine environments; these measurements revealed the presence of unique sulfur compounds in the particle-phase that were chemically distinct from sulfate and methanesulfonate and exhibited a strong diurnal profile. The atmospheric and biogeochemical implications of our findings are discussed.

1. INTRODUCTION Aerosols influence global climate directly by scattering and absorbing incoming solar radiation and indirectly by acting as cloud condensation and ice nuclei; both particle size and composition affect the radiative properties of aerosols.1,2 Primary sea spray aerosol, consisting of both sea salt and biologically produced organic material, is directly emitted from the ocean to the atmosphere via bursting bubbles generated from breaking waves.3−5 While biologically produced organic material can be directly ejected as a primary emission from the ocean, most biogenic sulfur found in marine aerosols is from secondary sources, generated from biogenic gases such as dimethyl sulfide (DMS).6−8 Once in the atmosphere, DMS is oxidized to form sulfur-containing compounds (e.g., sulfuric acid, methanesulfonic acid, etc.), which can either condense onto pre-existing particles or, under certain circumstances, nucleate new particles potentially impacting both particle chemistry and particle number concentrations in the marine environment.4,9−15 In addition to DMS, the biogenic marine sulfur cycle contributes reduced sulfur compounds in the gasphase to the atmosphere such as methanethiol (CH3SH), carbon disulfide (CS2), carbonyl sulfide (OCS), and hydrogen © 2015 American Chemical Society

Received: Revised: Accepted: Published: 4861

December 18, 2014 March 13, 2015 April 2, 2015 April 2, 2015 DOI: 10.1021/es506177s Environ. Sci. Technol. 2015, 49, 4861−4867

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Environmental Science & Technology

Figure 1. Percentages of S-type particles (colored dots) are shown as a function of cruise track (gray lines) for the INDOEX, ACE-Asia, CalCOFI, SORA, and CalNex field campaigns. The locations of the ground-based CIFEX and SIO Pier campaigns are also shown (bottom, right panel).

55% and to 60% during CalNex using a heated inlet;25 during the SIO Pier 2009 and SORA campaigns, the RH was conditioned to 15−30% using one silica gel drier. The RH of the sampled aerosol was not conditioned during the CalCOFI field campaign. The standard nozzle inlet ATOFMS,26 which sizes and chemically analyzes particles in the 0.2−3 μm aerodynamic diameter size range, was used for each field campaign except for SORA, where an ATOFMS using an aerodynamic lens inlet was operated in the 0.1−2.0 μm aerodynamic diameter size range.27 The operating principles of the ATOFMS have been previously described.26,27 Briefly, atmospheric particles are pulled into a vacuum chamber though the sampling inlet. Particles enter the light scattering region of the instrument consisting of two continuous wave (532 nm) scattering lasers in which the time taken to traverse the laser beams is recorded giving the terminal velocity of the particle, which is used to calculate the aerodynamic diameter of the particle. The transit time is also used to fire a Q-switched Nd:YAG laser that desorbs and ionizes the particles simultaneously creating positive and negative ions, which are analyzed in a dual-polarity time-of-flight mass spectrometer. Mass spectra are imported into Matlab (The MathWorks, Inc.)

2. METHODS Shipboard measurements were conducted during the Indian Ocean Experiment (INDOEX) from January−March 1999,19 the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) from March−April 2001,20 the California Cooperative Oceanic Fisheries Investigation (CalCOFI) in November 2004,21 the South Pacific Ocean Research Activity (SORA) field campaign from January−March 2009 (http://www. jamstec.go.jp/j/jamstec_news/sora2009/index.html;14), and the CalNex field campaign from May−June 2010 (http:// www.esrl.noaa.gov/csd/calnex/; http://saga.pmel.noaa.gov/ data;22,23). Ground-based measurements were conducted during the Cloud Indirect Effects Experiment (CIFEX) at a coastal site in Trinidad Head, CA, in April 200424 and on the Scripps Institution of Oceanography (SIO) pier from August− October 2009. Additional experimental details can be found in Table S1 and the text of the Supporting Information (SI). Real-time, single-particle measurements were conducted using aerosol time-of-flight mass spectrometry (ATOFMS) during the seven different field campaigns. The relative humidity (RH) of the sampled aerosol during three of the studies (INDOEX, ACE-Asia, and CIFEX) was conditioned to 4862

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Environmental Science & Technology using a software toolkit, YAADA,28 and clustered together based on similarities in ion peaks and ion intensity using an adaptive-resonance neural network (ART-2a).29 Each ion peak assignment presented in this paper corresponds to the most likely ion produced at a given mass-to-charge (m/z). Size resolved chemical composition was monitored continuously and subsequently averaged in 1 h time bins for the data presented in this paper.

ruled out by the relative isotopic abundance pattern for the ion peaks m/z +64, +65, +66, and +68, which matches S2+ but not SO2+. Another unique feature of these particles is the lack of negative ion spectra on the majority of S-type particles indicating the presence of tightly bound particle-phase water, which suppresses negative ions produced by laser desorption/ ionization.31,32 While ATOFMS studies frequently detect oxidized sulfurcontaining compounds in the marine environment including sulfate, sulfite, hydroxymethanesulfonate, and methanesulfonate, these compounds are detected as negative ions at 97 HSO4−, 80SO3−, 111CH3SO4−, and 95CH3SO3−, respectively,12,33,34 and do not produce the positive elemental sulfur ions at 32S+ and 64S2+ described herein (see Figure S1 in the SI). It is thus likely that these elemental sulfur ions result from a reduced form of sulfur. To test this hypothesis, sulfurcontaining standards including amino acids, peptides, DMSP, elemental sulfur, and a solution of H2S(aq) were atomized to generate aerosols, which were chemically analyzed by ATOFMS. Representative spectra for each standard are included in SI, Figure S2. Only the elemental sulfur standard and the hydrogen sulfide solution, which precipitates elemental sulfur due to the reaction of H2S(aq) with dissolved oxygen in water, reproduced the elemental sulfur ion peaks (32S+ and 64 + S2 ) found in ambient spectra. This provides strong evidence that the S-type particles arise from compounds containing a reduced form of sulfur rather than oxidized sulfur compounds. Evidence for the Primary Formation of S-Type Particles. As described above, S-type particles describe particles with elemental sulfur ion peaks as the most intense ions in the mass spectrum. However, elemental sulfur ions were also internally mixed with other particle types including Mgtype particles, which contain intense 24Mg+ and/or 40Ca+ (Figure 2b), 23Na+ that is sometimes present, and organic carbon ion peaks. Because Mg-type particles have been shown to be produced in marine regions with elevated DMS and chlorophyll-a,30 internal mixtures of elemental sulfur ions and Mg-type particles suggest that sulfur-containing compounds responsible for the detected elemental sulfur ions are likely associated with marine biological material. Sulfur ions were also detected on sea salt particles (Figure 2c) likely due to the presence of internally mixed sea salts and biological material; such internal mixtures of sea salts and biological material have also been detected in previous ATOFMS studies of sea spray aerosol.5 Of note, a comparison of ion peak intensities of 23Na+ and 24Mg+ to 32S+ and 64S2+ on S-type particles, and Mg-type and sea salt particles containing elemental sulfur ions suggests that the sulfur-containing compounds producing these sulfur ions are enriched on the particle surface (see Figure S3 and text in the SI). The fact that sulfur ions were found exclusively on primary, ocean-derived particle types (e.g., sea salt, Mg-type particles), even though particles from other sources (e.g., soot, organic carbon, etc.) were present in the same air masses, suggests that the sulfur-containing compounds producing elemental sulfur ions in the ATOFMS were directly ejected from the ocean in the particle-phase as opposed to forming from a secondary source due to gas-to-particle partitioning. Further, during most of the studies, S-type particles were typically observed as wind speeds reached, on average, at least 5−10 m/s as shown in Figure S4. In fact, S-type particles, representing ∼12% of the detected particles by number during CIFEX, peaked in number concentration when a storm occurred that resulted in strong winds (up to 20 m/s) and

3. RESULTS AND DISCUSSION Characteristics of Sulfur-Containing Particles. Figure 1 shows the locations where unique sulfur-containing compounds, detected as elemental sulfur ions (m/z 32S+ and 64S2+), were measured by ATOFMS in addition to the percentage detected during the shipboard studies as a function of cruise track. It is important to note that particles containing these elemental sulfur ion peaks have never been detected outside of the marine environment. Particles containing intense elemental sulfur ions, herein referred to as S-type particles, were found to represent between ∼5−67% of the total particle number concentration detected by ATOFMS and 0.77 μg/m3 of the aerosol mass concentration, on average, and up to 4 μg/m3. The mass spectral characteristics of this particle type are shown in Figure 2a. S-type particles are characterized by a 64S2+ peak

Figure 2. Representative mass spectra of (a) an S-type particle containing intense elemental sulfur ions (32S+, 64S2+), (b) a Mg-type particle internally mixed with elemental sulfur ions, and (c) a sea salt particle internally mixed with elemental sulfur ions.

that is more intense than the 32S+ peak (see Figure 2a); additional peaks shown in Figure 2a include 30NO+, which could be due to the presence of organic nitrogen compounds, and less intense peaks from inorganic ions (23Na+, 24 Mg+, 39K+, 40 Ca+) and organic ions (12C+, 27C2H3+, 43C2H3O+) typical of ocean-derived particles measured by ATOFMS. 30 The possibility that the ion peak at m/z +64 is due to SO2+ is 4863

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Figure 3. Hourly time series of ATOFMS measurements of S-type particles (red line) and solar radiation (yellow filled line) in W/m2 during (a) INDOEX, (b) ACE-Asia, (c) CalCOFI 2004, (d) CIFEX, (e) SORA 2009, (f) Pier 2009, and (g) CalNex.

Figure 4. Measurements of S-type particles detected by ATOFMS (colored dots) and chlorophyll concentrations (colored gradient over the oceans) during the 99 day SORA 2009 cruise from Japan to South America. S-type particles are shown for data averaged over 4 h.

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reactivity toward the night-time NO3 radical. We note below that one possible source of the detected sulfur compounds is dissolved sulfides including H2S(aq). Atmospheric H2S is known to react rapidly with OH and very slowly with the NO3 radical leading to a diurnal profile peaking at night.16,37 If H2S is indeed responsible for the observed S-type particles, the different reactivity of H2S with OH and NO3 radicals would also explain the observed diurnal profile of the S-type particles. Further studies are being conducted to address the night-time production mechanism leading to these unique S-type particles. Possible Sources of S-Type Particles. S-type particles could result from elemental sulfur produced by marine bacteria, proteins and peptides containing disulfide bridges produced by marine bacteria and viruses, and intact or fragments of microorganisms. Another possibility is that S-type particles are derived from reactions involving H2S dissolved in seawater. Once formed in the surface ocean, H2S(aq) dissociates to form HS−(aq) and S2−(aq); the sum of all three compounds are referred to as total dissolved sulfides (TDS).38 TDS could be ejected with primary sea spray aerosol and produce the observed sulfur ions. TDS is also taken up by marine bacteria and subjected to sulfur oxidation leading to the formation of particulate elemental sulfur globules.39,40 These globules are composed of polysulfides consisting of long chain elemental sulfur ending in carbon-containing moieties (R−Sn−R) that can reach supermicrometer sizes (e.g., greater than 1 μm).39−42 The scaled size distribution of S-type particles detected by ATOFMS peaks at ∼1.5 μm aerodynamic diameter (see Figure S6 in the SI). Notably, Mouri et al. [1995] also detected giant sulfur particles (1−1.5 μm radii) during a cruise in the equatorial Pacific; however, since these particles were detected off-line and the speciation of the sulfur detected was not obtained, the source and composition of these particles remain unknown.43 The size range of S-type particles is consistent with previous measurements of polysulfides produced by sulfur oxidizing bacteria, which are in the 1−3 μm diameter size range. This size range is also consistent with whole marine bacteria, which are typically ∼0.3−1.0 μm in diameter,44 suggesting that this particle type could be reduced sulfur compounds associated with bioaerosols. This potential biological source of S-type particles is also consistent with the night-time detection of sulfur-containing compounds in terrestrial environments that are chemically distinct from sulfate and are associated with bioaerosols.45 Associations of elemental sulfur ions with Mgtype particles, which have been linked with elevated marine biological activity and marine bacteria,30,46 provide further evidence that S-type particles represent biologically produced compounds. Atmospheric and Biogeochemical Implications. Sulfur cycling in the marine atmosphere has been shown to occur mainly through the contribution of secondary sulfate and methanesulfonate to marine aerosols;9−11 however, the results presented herein provide evidence for the direct ejection of reduced sulfur in the particle-phase to the marine atmosphere at night. As highlighted in Figures 1, 3, and 4, S-type particles have been observed off the California coast (CalCOFI, CIFEX, SIO Pier, and CalNex), across the northern Pacific Ocean (ACE-Asia, SORA), in the southern Pacific Ocean (SORA), and in the southern Indian Ocean (INDOEX) highlighting the global significance of these reduced sulfur-containing compounds. Particles containing reduced sulfur-containing compounds showed a strong diurnal cycle with concentrations peaking at night likely due to photolysis or rapid oxidation by

rain, which scavenged background particle types and increased the production of fresh sea spray aerosol30,35,36 (see SI, Figure S5). It is noted, however, that a strong correlation between Stype particles and wind speed is not expected. Unlike abiotic marine particle types, such as sea salt particles, S-type particles are also reactive during the day and the production of this type is likely controlled by biological processes, as discussed later. Thus, additional loss and production processes are controlling the observation of this particle type. Linking Marine Biological Activity and the Detection of S-Type Particles. As shown in Figures 1 and 3, which show the temporal profile of S-type particles during each field campaign, and SI, Table S1, S-type particles have primarily been observed during studies in the California Current, which is a highly productive upwelling environment. In fact, during CalNex, a red tide bloom of L. polyedrum, an organism found to contribute marine biogenic sulfur, such as sulfate and methanesulfonic acid, to the particle-phase,12 was observed suggesting a possible link between enhanced levels of DMS producers and the observed S-type particles.23 Additional evidence for this link is shown in SI, Figure S4, which generally shows elevated proxies for biological activity (e.g., chlorophyll-a and atmospheric DMS) concurrent with the detection of S-type particles. In addition to these proxies, biogenic Mg-type particles,30 including Mg-type particles internally mixed with elemental sulfur ions, were observed when S-type particles were present during every study further supporting the hypothesis that S-type particles represent a marine biogenic particle type. The strongest evidence for S-type particles representing a marine biogenic particle type occurred during INDOEX; S-type particles were observed at the southern-most point of the cruise (see Figure 1 and SI, Figure S4a) where only ocean-derived particles were observed and proxies for biological activity were also elevated.30 S-type particles were correlated with atmospheric DMS concentrations (R2 = 0.34); however, we do note that the association with atmospheric DMS only explains some of the variance and other factors likely play a role in the observation of this particle type. Lastly, evidence for the link between biological activity and S-type particles was also observed on a global scale across the northern and southern Pacific Ocean during the SORA campaign. Figure 4 shows Stype particles detected during SORA as a function of the cruise track and chlorophyll-a concentrations as measured by the MODIS satellite. Elevated concentrations of MSA were detected in marine aerosols during this field campaign at high latitudes coincident with the detection of high number concentrations of S-type particles.14 Further, S-type particles were primarily detected in regions of elevated chlorophyll-a concentrations such as eastern boundary currents where upwelling occurs (e.g., eastern tropical Pacific), again highlighting the link between this unique particle type and biological activity. Diurnal Profiles of S-Type Particles. The observed diurnal behavior of this particle type, shown in Figure 3, provides additional constraints on the sources and formation mechanisms of S-type particles. The percentage of detected Stype particles typically exhibits a nocturnal maximum that decreases either before sunrise or ∼1−2 h after solar radiation rises (Figure 3). One potential explanation could be diurnal changes in biological processes. However, the more likely explanation for the diurnal profile of this particle type is either photolysis or rapid oxidation of the sulfur-containing compounds by the OH radical during the day with little 4865

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Japan, for the funding Grant-in-Aid for Scientific Research in Priority Areas “Western Pacific Air−Sea Interaction Study (WPASS)” Grant 18067009. The authors acknowledge the National Science Foundation Grant 0296170, National Science Foundation Grant 1038028, the California Air Resources Board (CARB), and NOAA for the funding to conduct this research.

the OH radical of this particle type during the day. This particle type was also typically observed when wind speeds were 5−10 m/s and indicators of biological activity in the ocean were elevated (e.g, chlorophyll-a, atmospheric DMS, Mg-type particles); however, as noted, the observation of this particle type depends on a combination of meteorological and biological factors in addition to photochemical sinks. These observations provide evidence that S-type particles likely form from biologically derived compounds including polysulfides, sulfur-containing proteins and peptides, and primary biological material such as marine bacteria. The sulfur-containing particles described herein could influence not only marine aerosol chemistry, but also gas-phase chemistry in the marine atmosphere. For example, if the reduced, particulate sulfur described herein is elemental sulfur, this could contribute to the gaseous elemental sulfur previously observed in the ambient marine atmosphere.18 The S-type particles described herein provide evidence for the direct emission of reduced, particulate biogenic sulfur to the marine atmosphere.





ASSOCIATED CONTENT

S Supporting Information *

A description and details of the field campaigns discussed in this manuscript; spectra of sulfur-containing standards; comparison of ion intensities of sulfur ions for Mg- and Nacontaining particles; temporal trends of particle types detected by ATOFMS, meteorological conditions, and proxies for biological activity in the ocean; temporal trends of S-type particles, sea salts, and Mg-type particles during a rain event during CIFEX; and scaled size distributions of S-type particles and sea salt particles. This material is available free of charge via the Internet at http://pubs.acs.org.



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AUTHOR INFORMATION

Corresponding Author

*Phone: 858-822-5312; fax: 858-534-7042; e-mail: kprather@ ucsd.edu. Present Addresses ⊥

Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA. # Thermo Fisher Scientific, 355 River Oaks Parkway, San Jose, CA, 95134, USA. ∇ Lawrence Livermore National Laboratory, Livermore, CA, 94550, USA. ○ Korea Polar Research Institute, Incheon 406-840, Korea. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS PNNL and the ARCS fellowship are acknowledged for support of C.J.G. K. Suski, J. Cahill, and D. Collins are thanked for help with measurements during the SIO pier field campaign. C. McDonald is acknowledged for use of the SIO pier for measurements. M. Spencer and J. Holecek are thanked for carrying out measurements during the CIFEX field campaign. M. Dall’Osto is acknowledged for assisting with measurements during the CalCOFI field campaign. D. Sodeman is thanked for assisting with measurements during ACE-Asia. T.S. Bates, P.K. Quinn, D. Hamilton, and D. Coffman are acknowledged for assistance during the ACE-Asia, INDOEX, and CalNex field measurements. H.F. and M.U. acknowledge the Ministry of Education, Culture, Sports, Science and Technology (MEXT), 4866

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Environmental Science & Technology

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DOI: 10.1021/es506177s Environ. Sci. Technol. 2015, 49, 4861−4867