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Characterization of Natural and Affected Environments

Unexpected contributions of sea spray and lake spray aerosol to inland particulate matter Nathaniel W. May, Matthew J Gunsch, Nicole Olson, Amy Lynne Bondy, Rachel M Kirpes, Steven Bertman, Swarup China, Alexander Laskin, Philip K. Hopke, Andrew P Ault, and Kerri A. Pratt Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00254 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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Environmental Science & Technology Letters

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Unexpected contributions of sea spray and lake spray aerosol to inland particulate matter

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Nathaniel W. May1, Matthew J. Gunsch1, Nicole E. Olson1, Amy L. Bondy1, Rachel M. Kirpes1, Steven B. Bertman2, Swarup China3, Alexander Laskin3ǂ, Philip K. Hopke4,5, Andrew P. Ault1,6*, Kerri A. Pratt1,7*

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Department of Chemistry, University of Michigan, Ann Arbor, MI 48109 Institute of the Environment and Sustainability, Western Michigan University, Kalamazoo, MI 49008 3 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99354 4 Department of Public Health Sciences, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642 5 Center for Air Resources, Engineering and Science, Clarkson University, Potsdam, NY 13699 6 Department of Environmental Health Sciences, University of Michigan, Ann Arbor, MI 48109 7 Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109 ǂ Current: Department of Chemistry, Purdue University, West Lafayette, IN 47907 * Corresponding authors: Kerri Pratt ([email protected]), Andrew Ault ([email protected] ) 2

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TOC Figure

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Abstract Sea spray aerosol (SSA) and lake spray aerosol (LSA) from wave breaking contribute to

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particulate matter (PM) in coastal regions near oceans and freshwater lakes, respectively.

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However, SSA and LSA contributions to atmospheric aerosol populations in inland regions are

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poorly understood due to difficulties differentiating them from other inland sources when using

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bulk particle measurements. Herein, we show that SSA and LSA episodically contribute to

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atmospheric aerosol populations at a rural site in northern Michigan >700 km and >25 km from

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the nearest seawater and Great Lakes sources, respectively. During July 2014, individual SSA

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and LSA particles were identified by single particle mass spectrometry and electron microscopy,

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then combined with air mass trajectory analysis for source apportionment. SSA comprised up to

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33% and 20% of PM mass (0.5 – 2.0 µm) during two multiday transport events, respectively,

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from Hudson Bay, and a 3% average background outside these periods. LSA transported from

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Lake Michigan reached a maximum of 7% of PM mass (0.5 – 2 µm) during a daylong high wind

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event, and contributed a 3% average background during the remainder of the study. The

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observation of SSA and LSA transported inland motivates further studies of the impacts of wave

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breaking particles on cloud formation and air quality at inland locations far from marine and

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freshwater sources.



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Introduction Sea spray aerosol (SSA) produced from oceanic wave breaking impact climate by

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scattering solar radiation, acting as cloud condensation nuclei (CCN) and ice nucleating particles

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(INP), and serving as a source of reactive halogen gases.1,

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marine and coastal locations, where it dominates particle mass concentrations.3, 4 Remote and

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rural inland regions often have fewer local particulate matter (PM) sources than urban or coastal

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regions, such that, under certain meteorological conditions, transported particles from distant

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sources contribute significantly to PM mass.5-8 Bulk PM measurements have demonstrated that

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SSA undergoes long-range transport to inland regions 100 – 1,100 km from the ocean.9-17

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However, these methods rely on Na+, Mg2+, and Cl- ratios, which can lead to an underestimation

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of inland SSA concentrations due to contributions from other particle sources (e.g. dust)18, 19 and

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changes in initial source ratios due to multiphase reactions causing Cl- depletion.6, 20

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SSA is primarily observed over

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The atmospheric abundance and climate impacts of lake spray aerosol (LSA) produced

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from freshwater wave breaking are more uncertain.21-24 Ambient identification of LSA is

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restricted to one aircraft study over portions of northern Lake Michigan and Lake Huron12 and

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one ground-based coastal Lake Michigan study.22 However, previous identification of calcium-

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containing particles in clouds over the Great Lakes region25, 26 suggests participation of calcium-

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rich LSA22 in cloud droplet formation. LSA may also have potential climate27-29 and health30-32

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implications due to the incorporation of organic and biological material from harmful algal

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blooms (HABs).33 Identification and quantification of LSA in the atmosphere has been limited as

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chemical characterization of LSA has only occurred recently,22, 33 and differentiating LSA from

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other sources of calcium-containing particles using bulk analytical measurements can be

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difficult.7, 18 Herein, we show periodic contributions of transported SSA and LSA to ambient PM


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in northern Michigan during July 2014, motivating further study of the impacts of wave breaking

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particles on inland atmospheric composition, climate processes, and air quality.

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Methods Aerosol Measurements

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Atmospheric measurements were conducted from July 13-24, 2014 at the University of

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Michigan Biological Station (UMBS) near Pellston, MI, >700 km from the nearest seawater

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source (Hudson Bay) and >25 km from Great Lakes sources (Lake Michigan = 25 km; Lake

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Superior = 100 km). Instrumentation was located within a laboratory at the base of a 30 m tall

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tower (45°33'31"N, 84°42'52"W) and individually connected by insulated 0.79-cm I.D. copper

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tubing to a manifold that sampled air at ambient relative humidity (RH) from 34 m above ground

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level (AGL) through insulated 1.09-cm I.D. copper tubing at a flow rate of 9.25 L min-1

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(laminar) with a residence time of 15 s,34 as described in detail by Gunsch et al.35 An aerosol

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time-of-flight mass spectrometer (ATOFMS, TSI 3800)36,

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composition of 11,430 individual atmospheric particles ranging from 0.5 – 2 µm (vacuum

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aerodynamic diameter, dva) in real-time, with particle losses in the sampling inlet tubing

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calculated to be 1 µm (Figure 1c), consistent with previous SSA observations.4, 52, 53 Consistent

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with the ATOFMS results, CCSEM-EDX also identified SSA particles based on elemental

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composition similar to seawater49, 54, 55 following chloride depletion.6, 50, 56 The EDX spectrum

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and elemental map of a representative individual SSA particle are shown in Figures 1d and S1,

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respectively, and are consistent with previous observations of the distribution of major elements

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in SSA.54,

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consistent with seawater (0.11),57 and the average SSA particle Ca/Na (0.04 ± 0.06) and K/Na

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(0.04 ± 0.06) elemental mole ratios were also within error of seawater (both 0.02)57 (Figure 1e).

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Overall, 35% and 65% of individual SSA particles, by number (measured by CCSEM-EDX),

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were aged (Cl/Na mole ratios < 0.1) or partially aged (1 ≥ Cl/Na ≥ 0.1),6 respectively (Figure

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S3). No SSA particles were observed without at least some chloride depletion. The capability of

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both single particle measurement techniques (ATOFMS and CCSEM-EDX) to identify

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individual SSA, even after Cl depletion, and differentiate between multiple sources contributing

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to Na and Mg,

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ratios,20 which hinder accurate identification and quantification of inland SSA.

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The SSA mass concentration mode identified by

The average SSA single particle Mg/Na elemental mole ratio (0.11 ± 0.07) was

6, 7, 51

provides an advantage over bulk methods relying on average elemental


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The PSCF trajectory domain of the observed SSA extended north and northeast of the

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UMBS sampling site to Hudson Bay (Figure 2), where wind speeds capable of producing SSA

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(> 3 m s−1)52,

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nearest point of Hudson Bay is over 700 km away from UMBS, the SSA particles underwent

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long-range transport (>48 h), providing time for chloride depletion51 by reaction with HNO3 and

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N2O5. As shown by Gunsch et al.,35 this air mass was also impacted by Canadian wildfire smoke,

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the long-range transport of which is associated with elevated NOx levels.59 SSA contributions to

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particle mass concentrations were largest during two separate periods of Hudson Bay influenced

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air masses (Figure 3), as determined by 72 h HYSPLIT backward air mass trajectories arriving

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at UMBS at 100 m AGL. During these two periods (7/15/2014 12:00 - 7/18/2014 0:00 EST &

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7/23/2014 0:00 - 7/25/2014 0:00 EST), SSA comprised 20 ± 10% (0.06 ± 0.04 µg m-3) and 15 ±

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6% (0.09 ± 0.04 µg m-3) of 0.5 – 2 µm particle mass on average, corresponding to number

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concentrations of 0.03 ± 0.02 and 0.06 ± 0.02 particles cm-3, respectively (Figure 3). Elevated

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wind speeds capable of producing SSA52, 58 (5 ± 2 m s−1 and 5 ± 2 m s−1, respectively) were

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present over Hudson Bay when these two air masses crossed over the water (Figure 3). Outside

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of these periods, SSA mass contributions were minor (average 3 ± 3%; 0.03 ± 0.02 µg m-3),

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although number concentrations were similar (0.03 ± 0.02 particles cm-3). Air mass PSCF

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analysis, in combination with ATOFMS and CCSEM-EDX data, suggests that SSA produced

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over Hudson Bay contributed to atmospheric particle mass concentrations in the upper

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Midwestern United States. The results presented here represent the furthest inland quantification

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of SSA particle mass contributions by single particle analysis6 and support previous bulk particle

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measurements that suggested long-range transport of SSA >500 km inland.9-12

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Identification and Quantitation of Lake Spray Aerosol

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were present for 78% of the July 2014 sampling period (Figure S4). As the



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LSA were identified by ATOFMS based on individual particle dual-polarity mass spectra

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consistent with previous LSA studies,22 as well as LSA generated in the laboratory from

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freshwater collected from both Lake Michigan and Lake Superior during this study (Figure S5).

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Calcium is the highest concentration cation in freshwater from the calcareous Great Lakes45 and

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the highest concentration cation on average in global freshwater.60 Both the ambient and

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laboratory-generated LSA were characterized by m/z +40 (Ca+) as the highest intensity positive

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ion (Figures 1b & S5). Minor inorganic ion peaks included m/z +23 (Na+), +24 (Mg+), +39 (K+),

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and +56 (CaO+) (Figures 1b & S5). The significant presence of organic nitrogen (m/z -26 (CN-))

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is consistent with mass spectra of LSA generated in the laboratory in this study, and previously,

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from Lake Michigan freshwater.22, 33 The laboratory-generated LSA mass spectra also showed a

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minor contribution from a nitrate marker at m/z -46 (NO2-). However, the average relative peak

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area ratio of m/z -46 (NO2-) / m/z -26 (CN-) was lower in the mass spectra of individual LSA

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generated in the laboratory from Lake Michigan (1.0 ± 0.4) and Lake Superior (0.2 ± 0.2)

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freshwater (Figure S5), compared to the ambient LSA mass spectral ratio (11 ± 5) (Figure 1b).

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Similar to SSA, the enriched presence of nitrate markers, m/z -46 (NO2-) and -62 (NO3-),50

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suggests atmospheric processing of ambient LSA particles during transport, as shown in

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laboratory studies of calcite.61 This result suggests the ambient LSA had undergone multiphase

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reactions, with the formation of nitrate increasing its ratio relative to organic nitrogen, during

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transport inland. Similar to calcite dust aging,62 LSA aging could impact particle optical

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properties and cloud activation efficiencies. Since soil in the region is rich in Fe,63 the observed

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LSA was differentiated from mineral dust22 based on the lack of iron (m/z +54 (Fe+)18) in the

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ambient mass spectra (Figure 1b) and EDX spectra (Figure 1d). In addition, the ambient LSA

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mass distribution identified by ATOFMS peaked near 0.7 µm da (Figure 1c), consistent with


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previous measurements of laboratory generated LSA (mass distribution mode of 0.75 µm)22 and

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distinct from mineral dust (mass distribution mode >1 µm).64

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CCSEM-EDX analysis further confirmed the identification of LSA particles with

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elemental composition consistent with Lake Michigan and/or Lake Superior freshwater sources

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(Figure 1e). As observed previously on the shore of Lake Michigan by Axson et al.,22 the LSA

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EDX spectra are defined by calcium with abundant carbon and oxygen (Figure 1d), due to the

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presence of carbonate in Great Lakes freshwater.45 Ca, Mg, Na, K, C, and O were evenly

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distributed across the individual ambient and laboratory generated LSA particles (Figure S1).

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The average ambient LSA Mg/Na mole ratio (1.8 ± 0.5) is consistent with that of Lake Michigan

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(1.72) and Lake Superior (1.88) freshwater.45 Similarly, the average ambient LSA K/Na and

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Ca/Na mole ratios (0.2 ± 0.2; 3 ± 3) were both consistent with the corresponding ratios in Lake

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Michigan (0.13; 3.31) and Lake Superior (0.21; 5.48) freshwater, respectively.45 Both SSA and

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LSA in the 0.5 – 2 µm diameter size range measured were predominantly inorganic, consistent

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with previous measurements.22,

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origin as jet drops that contain less organic material than particles 3.5 m s−1)22, 23 were present for 68% and 51% of this period for each lake,

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The lack of significant organic content may be due to their

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respectively (Figure S6). The highest contributions of LSA to 0.5 – 2 µm PM mass

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concentrations (6 ± 1%; 0.2 ± 0.1 µg m-3) and number concentrations (0.5 ± 0.3 particles cm-3)

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occurred during a period (7/22/2014 0:00 – 7/23/2014 0:00 EST) of air transport over Lake

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Michigan (Figure 4) when elevated wind speeds (6 ± 2 m s−1) and wave heights (1.0 ± 0.5 m)

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capable of producing LSA22, 23 were present across Lake Michigan (Figure 4). Since this period

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was also associated with elevated mass concentrations of urban pollution aerosols from

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Milwaukee and Chicago,35 the increase in LSA mass fractions, in comparison to other periods,

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was not proportional to the increase in LSA mass concentrations. During the remainder of the

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UMBS study (92% of the sampling time), the constant presence of lake-influenced air masses

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and elevated wind speeds resulted in a consistent background LSA contribution to 0.5 – 2 µm

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PM mass concentrations (average 3 ± 1%; 0.02 ± 0.01 µg m-3) (Figure 4) and number

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concentrations (0.03 ± 0.03 particles cm-3).

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Inland contributions of LSA were thus determined for the first time, by single particle

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chemical analysis (ATOFMS and CCSEM-EDX) (Figure 4). The identification and

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quantification of inland LSA and SSA, and their differentiation from mineral dust, in this study

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further supports the previously demonstrated advantage of single particle measurements

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techniques to overcome complications in the use of ion ratios in bulk methods to accurately

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identify inland contributions of wave breaking particles.6,

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identification of particles similar in composition to LSA in clouds over the Great Lakes region25,

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26

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study of the roles of LSA and SSA on inland environments. In particular, the impacts of inland

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LSA and SSA, which reached 0.5 – 2 µm PM number concentrations of 0.5 ± 0.3 particles cm-3

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and 0.06 ± 0.02 particles cm-3, respectively, on atmospheric composition and cloud formation

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Combined with previous

and the known participation of SSA in cloud formation,65 the results herein motivate future


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may be important in the rural northern Great Lakes region24 where low particle number

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concentrations (

Unexpected contributions of sea spray and lake ... - ACS Publications

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