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Aircraft measurements of total mercury and monomethyl mercury in summertime marine stratus cloud water from coastal California, USA Peter Weiss-Penzias, Armin Sorooshian, Kenneth Coale, Wesley Heim, Ewan Crosbie, Hossein Dadashazar, Alexander B. MacDonald, Zhen Wang, and Haflidi Jonsson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05395 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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Environmental Science & Technology

Aircraft measurements of total mercury and monomethyl mercury in summertime marine stratus cloud water from coastal California, USA

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AUTHORS: WEISS-PENZIAS, Peter 1*; SOROOSHIAN, Armin 2,3 ; COALE, Kenneth 4 ; HEIM, Wesley 4 ; CROSBIE, Ewan 5 ; DADASHAZAR, Hossein 2 ; MACDONALD, Alexander B. 2 ; WANG, Zhen 2 ; JONSSON, Haflidi 6

(1) Department of Microbiology and Environmental Toxicology, UC Santa Cruz, Santa Cruz, CA 95064, United States; (2) Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ 85721, United States; (3) Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ 85721, United States; (4) Moss Landing Marine Labs, Moss Landing, CA 95039, United States; (5) NASA Langley Research Center, Hampton, VA 23666, United States; (6) Naval Postgraduate School, Monterey, CA 93943, United States

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Corresponding author: Peter Weiss-Penzias (University of California, Santa Cruz, 1156 High St. Santa Cruz, CA 95064, [email protected])

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Abstract

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Water samples from marine stratus clouds were collected during 16 aircraft flights above the Pacific

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Ocean near the Central California coast during the summer of 2016. These samples were analyzed for

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total mercury (THg), monomethyl mercury (MMHg) and 32 other chemical species in addition to aerosol

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physical parameters. The mean concentrations of THg and MMHg in the cloud water samples were 9.2 ±

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6.0 ng L-1 (2.3-33.8 ng L-1) (N = 97) and 0.87 ± 0.66 ng L-1 (0.17-2.9 ng L-1) (N = 22), respectively. This

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corresponds to 9.5% (3-21%) MMHg as a fraction of THg. Low and high non-sea salt calcium ion (nss-

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Ca2+) concentrations in cloud water were used to classify flights as “marine” and “continental”,

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respectively. Mean [MMHg]marine was significantly higher (p>0.05) than [MMHg]continental consistent with

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an ocean source of dimethyl Hg (DMHg) to the atmosphere. Mean THg in cloud water was not

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significantly different between the two categories, indicating multiple emissions sources. Mean

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[THg]continental was correlated with pH, CO, NO3-, and NH4+, and other trace metals, whereas [THg]marine

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was correlated with MMHg and Na+. THg concentrations were negatively correlated with altitude,

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consistent with ocean and land emissions, coupled with removal at the cloud-top due to drizzle

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formation.

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1. Introduction

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Exchange between the ocean and atmosphere plays an important role in the transport and chemical

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cycling of mercury (Hg), a bioaccumulative neurotoxin. Atmospheric deposition of oxidized Hg (HgII)

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compounds is the primary source of Hg to the global oceans (1). Once in the ocean, HgII undergoes rapid

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transformations through biotic and abiotic reduction and biotic methylation processes whereby

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approximately 70% of the deposited Hg is reemitted to the atmosphere as Hg0 (10.0-14.7 Mmol a-1),

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with much lower amounts as dimethyl Hg (DMHg) (0.1 Mmol a-1) (1). In total, the oceans are a large

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source of Hg to the atmosphere, contributing about 40% of the total of all anthropogenic and natural

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sources (2).

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Gaseous elemental Hg in the atmospheric marine boundary layer (AMBL) is oxidized primarily by

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reactive halogen precursors liberated from sea-salt particles via chloride and bromide depletion

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reactions (3), leading to formation of gaseous HgII (4-6). Entrainment of air from the free troposphere,

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which can be enriched in HgII (7), can also be a source of HgII to the AMBL. Gaseous and particulate HgII is

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highly soluble and readily incorporates into cloud water via in-cloud scavenging (8). Off the coast of

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California during the summer where marine stratus clouds are common and coincide with wind-driven

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upwelling, oceanic emissions of Hg0 may be enhanced (9). Rapid uptake of HgII by cloud droplets and

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subsequent return to the sea surface through wet deposition is a poorly understood process.

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In the ocean, microorganisms from various phyla including iron and sulfate-reducing bacteria and

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methanogenic archaea, can methylate inorganic HgII based on the presence of hgcAB genes (10-12).

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Through complex and not fully understood mechanisms, concentrations of monomethyl Hg (MMHg) in

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the ocean are most closely linked with processes of organic carbon remineralization (13). Microbial

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production of MMHg can occur throughout the water column (14) and is offset by demethylation (biotic

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and abiotic) and particle scavenging, resulting in a typical water column maximum in MMHg



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concentrations between the subsurface chlorophyll maximum and the oxycline (15, 16). Dimethyl Hg

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(DMHg) has also been hypothesized to be bacterially mediated; however, direct experimental support

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for this assertion is missing (15). It is possible that DMHg is produced abiotically on mineral surfaces

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from MMHg (17). Where upwelling brings DMHg to the upper ocean (18, 19), and where the stability of

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DMHg in seawater is enhanced (20), DMHg emissions to the atmosphere may account for a significant

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fraction of total Hg emissions (21). In the atmosphere, fast reactions with chlorine and the hydroxyl

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radical (22, 23) likely convert DMHg to MMHg, which can be quickly scavenged by marine aerosols (24).

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Soerensen et al. (21) calculated that 57% of DMHg emitted over the Arctic Ocean returns to the surface

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as MMHg through wet and dry deposition, thereby comprising the largest source of MMHg to the polar

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mixed layer. Wet and dry deposition of marine aerosols in the coastal High Arctic is also suspected to be

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the cause of elevated MMHg concentrations in lichen species that are eaten by caribou, a food source

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for humans (25).

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On the coast of California, MMHg concentrations in summertime advective fog were significantly

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elevated (8 times higher) over those found in rain water from the same region (26-29). Measurements

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of DMHg in surface waters of the California Current at the time of fog sampling also showed elevated

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concentrations of DMHg and a likely sea-air flux due to production in the water-column and the effects

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of mesoscale upwelling eddies (28). Recent work by Baya et al. (30) in the Arctic suggests there is a

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relationship between surface seawater DMHg and gaseous DMHg and MMHg concentrations in the

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AMBL. A direct link between ocean emissions of DMHg and MMHg in fog water on the California coast

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has been hypothesized (26), but precise knowledge of the gaseous and aqueous phase chemical

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processes of the Hg cycle in cloud droplets is lacking.

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In order to better understand the cycling of MMHg and THg in the aqueous phase of the AMBL, samples

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were collected on 16 research flights in marine stratus clouds between 35oN and 40oN by the Center for

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Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft. Over 60 additional 4 ACS Paragon Plus Environment

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chemical and physical parameters were measured simultaneously with speciated Hg measurements and

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these data are used to address the following: (Research Question A) are concentrations of MMHg and

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THg elevated in marine stratus cloud water from offshore locations relative to rain water?; and

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(Research Question B) can tracers of emissions from both the ocean and land be used to identify sources

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of MMHg and THg in cloud and fog water? Results of this study are used to address the knowledge gaps

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regarding both the lifetime of all species of Hg in the AMBL and the potential for wet deposition of

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MMHg and THg to enrich the sea surface.

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2. Materials and Methods

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2.1 Field study description

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Sixteen research flights were conducted with the CIRPAS Twin Otter as part of the Fog and

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Stratocumulus Evolution (FASE) Experiment between July 18 and August 12, 2016. Based out of Marina,

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CA, flights extended across a region over the Pacific Ocean from 40oN to 34.5oN and 121oW to 125.5oW.

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This region has been the subject of extensive studies of cloud aerosol interaction due to the persistent

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nature of stratocumulus clouds and strong aerosol perturbations stemming from ship emissions (31-34).

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Objectives of FASE were to investigate moisture and energy budgets, to understand the nature of

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stratocumulus cloud clearings (35), and characterize cloud water composition. As with earlier campaigns

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in the same region using the same aircraft (36-39), over 32 chemical species were analyzed in cloud

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water samples and related to other aerosol and environmental properties.

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The sampling protocol used here was guided by EPA method 1631 (40) and Parker and Bloom (41). The

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cloud water samples were obtained through cooperation with the FASE-2016 campaign, thus there were

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certain restrictions on sampling, such as materials used for the cloud water collector and the type of

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sampling containers. Cloud water samples were collected at distances between 10 and 150 km from the

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Central California coastline at altitudes between 150 and 750 m above the sea surface (Figure 1). The 5 ACS Paragon Plus Environment


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mean values for cloud base and cloud top heights were 155 ± 93 m and 489 ± 167 m, respectively, and

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the mean cloud depth was 331 ± 161 m. The mean liquid water content of clouds, as measured by a

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PVM-100A probe (42), was 0.31 ± 0.11 g m-3 (0.12 - 0.57 g m-3).

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2.2 Cloud water sampling description

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One hundred forty-six cloud water samples were obtained with a modified Mohnen slotted-rod cloud

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water collector (44) and 144 samples were collected with the NASA Langley Axial Cyclone Cloud Water

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Collector (AC3) probe. The Mohnen collector is made of Delrin with a PTFE Teflon ¼” tube leading to the

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collection bottle. The AC3 is made of aerospace-grade 6061-T6 aluminum and 316 stainless steel with

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PFA Teflon sample lines. Cloud water samples for speciated Hg analysis were taken primarily with the

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Mohnen collector, aliquotted on the aircraft into new 15 mL polypropylene tubes yielding between 2

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and 10 mL from each sampling segment. Samples were subsequently stored at a nominal 5oC until

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laboratory analysis. The sampling tubes were pretested and found to meet our typical MilliQ 18.2 MΩ

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water blank levels for THg and MMHg.

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When the Twin Otter was in cloud, the Mohnen slotted rod collector was inserted upwards through a

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port at the top of the aircraft surface. The AC3 probe centrifugally separated cloud droplets from the

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airstream and aerodynamically collected liquid at the rear of the probe, which was then pumped into

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the aircraft cabin for collection. Median liquid sample collection times were 16 minutes and 11 minutes,

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respectively, for the Mohnen and AC3 collectors. Field blanks were collected after cleaning for both

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collectors by spraying MilliQ water stored in a 2 L carboy filled before the flight, through the cloud water

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collectors, and collecting the rinse water in detachable bottles. Both probes were rinsed repeatedly with

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MilliQ water (no other solvents). The AC3 was covered with lab film overnight, while the Mohnen

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collector was stored in a custom-made cylindrical casing with lab film over its base that was exposed to


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air. MilliQ water rinse blank of these collectors produced suitably low THg and MMHg concentrations

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(0.97 ± 0.66 ng L-1 and 0.12 ± 0.03 ng L-1, respectively).

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With respect to polypropylene as a sampling material, it is not a recommended due to the permeability

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of Hg0. However, since samples were analyzed quickly after collection, the error due to diffusion through

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the container walls was minimized. EPA method 1631 (40) allows for using polyethylene and/or

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polypropylene labware for digestion and other purposes as long as the time of sample exposure to these

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materials is short.

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2.3 Cloud water measurements in the laboratory

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Samples and blanks for speciated Hg analysis were acidified to 0.4% with 12M HCl (Fisher Sci., Trace

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Metal Grade) within 48 hours of collection. In the lab, at least 2 mL of samples from consecutive flight

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segments were combined to make at least 20 mL of a composite sample for MMHg analysis. The fewest

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number of segments were combined to achieve the target volume while still leaving enough volume for

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THg analysis. A total of 22 samples were analyzed for MMHg using Method 1630 (45), which involved

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125oC distillation of the acidified sample, followed by ethylation with sodium tetraethylborate, purge

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and trap onto a Tenax solid adsorbent, gas chromatography (GC) separation and detection by cold vapor

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atomic fluorescence spectroscopy (CVAFS) (Tekran 2500). The mean distillation recovery of a 50 pg

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MilliQ water spike of MMHg was 75 ± 9%, and this value was used to correct the sample concentrations

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of MMHg. The mean MMHg concentration of the field blanks was 0.12 ± 0.03 ng L-1, and the method

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detection limit, defined as three times the σ of the field blanks, was 0.10 ng L-1. The field blanks were

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within the uncertainty of the stated method detection limit, and thus field blank correction of

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environmental samples was not done.

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Within four days, the remaining sample (2 to 5 mL) was put in a 50 mL polypropylene tube, diluted to 30

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mL with MilliQ water, and 0.30 mL of BrCl stock solution was added to samples to make a 1% solution. 7 ACS Paragon Plus Environment


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Samples were shaken vigorously and allowed to sit for at least 30 minutes. NH2OH was then added to

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make a 0.1% solution. We assumed that the DOC concentrations in cloud water were low so that wall

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losses of Hg were not significant (41). The resulting samples were loaded into an automated aqueous

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THg analyzer (Tekran 2600), which reacts the sample with SnCl2, and purges off the resulting Hg0 into a

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CFAVS detector. This instrument was calibrated with a primary HgCl2 standard (Fisher Scientific). The

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mean THg concentration of the field blanks was 0.97 ± 0.66 ng L-1 and since three times the standard

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deviation was greater than the field blank THg concentration, cloud water sample THg concentrations

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were not blank-adjusted.

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Measurements of pH were conducted using a Thermo Scientific Orion 8103BNUWP Ross Ultra Semi-

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Micro pH probe calibrated with pH 4.01 and pH 7.00 buffer solutions. A 0.5 mL fraction of each cloud

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water sample was analyzed with ion chromatography (IC; Thermo Scientific Dionex ICS-2100 anion

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system) for major inorganic and organic acid anions (46). Specific organic acids examined included

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oxalate, malonate, maleate, acetate, formate, pyruvate, lactate, propionate, glyoxylate, and

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methanesulfonate (MSA). The concentrations of these (except for MSA) were combined and reported as

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dissolved organic carbon (DOC). Another fraction of each cloud water sample was analyzed with triple

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quadrupole inductively coupled plasma mass spectrometry (ICP-QQQ; Agilent 8800 Series).

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Cloud water organic ions and trace element concentrations (not Hg) were converted to air equivalent

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concentrations by multiplication with the average liquid water content (LWC) measured during the

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collection of individual cloud water samples. Mass ratios of 0.383 and 0.370 (47) were used to calculate

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non-sea salt Ca2+ (nss-Ca2+) and non-sea salt K+ (nss-K+) concentrations, respectively, according to the

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following relationship: [nss-Ca2+ or K+] = [Ca2+ or K+] – mass ratio * [Na+].

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Sub- and above-cloud particle number concentrations were obtained with a condensation particle

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counter (CPC 3010; TSI Inc.). The CPC diameter range is >10 nm. Gas phase carbon monoxide (CO)


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measurements were made at 1 Hz frequency using off-axis integrated-cavity output spectroscopy (Los

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Gatos Research). All aircraft data are freely accessible from the Figshare database (48).

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Seventy-two hour air mass back trajectories were generated using the NOAA Hybrid Single-Particle

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Lagrangian Integrated Trajectory (HYSPLIT) model (49, 50) ending at the location and altitude of the

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average point for each sample. HYSPLIT was run using the Global Data Assimilation System data with the

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“Model vertical velocity” method.

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Maps were constructed using ArcGIS Desktop 10.4.1. Map layers of the coastline were from the NOS80K

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Medium Resolution Digital Vector U.S. Shoreline shapefile from the US Geological Survey

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(https://pubs.usgs.gov/of/2003/of03-221/data/basemaps/usa/nos80k.htm).

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Data reduction was done using Microsoft Excel and Access, and statistical analyses were done using

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Origin 2017 (OriginLabs).

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3. Results and Discussion

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3.1 Overview of THg, MMHg, and other parameters measured on research flights

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Mean (± 1σ), maximum and minimum concentrations, and number of observations of THg and MMHg

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from all cloud water samples were 9.2 ± 6.0 ng L-1 (2.3-33.8 ng L-1) (N = 97) and 0.87 ± 0.66 ng L-1 (0.17-

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2.9 ng L-1) (N = 22), respectively. The 22 composite samples that were analyzed for MMHg had a mean

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%MMHg compared to THg of 9.5%. For MMHg, the location of sample collection shown in Figure 1 is the

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mean latitude and longitude of the composite sample.

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Table 1 compares the mean THg and MMHg concentrations in cloud water with speciated Hg

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measurements from the literature on sample types from California and other regions – mountain-top

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cloud water, fog water (coastal and inland), rain water (coastal and inland), AMBL air, free-tropospheric

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air, and high elevation snow. Total Hg concentrations observed in cloud water from this work are on the 9 ACS Paragon Plus Environment


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low end of the range of those seen previously in coastal fog water but within the range seen in other

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atmospheric waters. Total Hg in cloud water from this work converted to air concentration units gives a

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value of 2.94 pg m-3, assuming a mean observed liquid water content of 0.31 g m-3. This compares

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closely to the concentration of HgII(g) + particulate HgII (HgII(p)) observed in the marine boundary layer

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(Table 1), which partition into cloud water due to their high water solubilities.

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Cloud water MMHg concentrations and %MMHg from this work are on the high end of those found in

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coastal fog water (0.6-3.3 ng L-1, and 2.0-13.3% MMHg) (Table 1). Inland fog as well as coastal and inland

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rain have relatively lower MMHg concentrations and %MMHg. This supports the notion that volatile

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methylated Hg is emitted from the ocean and accumulates into marine stratus and fog water, thereby

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sustaining the potential for MMHg to wet deposit to land and to the ocean surface through drizzle.

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Air concentrations of Hg0 from the AMBL and the free troposphere are relatively uniform with altitude

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(59), however, concentrations of reactive gaseous HgII generally increase with altitude due to

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photooxidation of Hg0 in the free troposphere (57) and lack of scavenging. Interestingly, the presence of

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MMHg in high elevation snow (58) could represent an unknown free tropospheric source of MMHg in

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wet deposition, but this source is likely to be small given the low %MMHg in snow.

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A selected set of ancillary chemical and physical measurements for research flights 2-16 are shown in

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Table 2 and the means across all flights are shown in Table 3. Cloud water samples were acidic with pH

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varying between 4.23 and 5.53. Sodium ion concentrations varied considerably between 0.03 and 11.99

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µg m-3. Concentrations of various trace metals (K, Ca, Fe, Zn, Mn) also varied by an order of magnitude

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between flights. The impact of the Soberanes Fire was evident not only by visual and olfactory

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observations during specific flights, but also by enhanced CPC particle number concentrations, CO

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concentrations, and pH values (i.e., flights 4 and 10).

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3.2 Segregation of data into “marine” and “continental” categories 10 ACS Paragon Plus Environment

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Flights were classified as either “marine” or “continental” based on if the flight mean [nss-Ca2+] was

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greater than the all flight mean [nss-Ca2+] (94 ng m-3). Calcium has been used previously as a tracer of

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dust in ice cores (60). Calcium in excess of that contributed from sea salt showed clear enhancements on

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Flights 4, 10, 15-1 and 16 (Table 2). A linear fit between nss-Ca2+ and Na+ concentrations in cloud water

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samples from “marine” classified fights yielded a slope of 0.039, which compares closely with the

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Ca2+:Na+ mass ratio in sea water of 0.38 (Figure 2A). The cloud water samples classified as “continental”

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show clear enhancements in Ca2+ in excess of that contributed by sea salt (Figure 2A). However, it

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should be noted that in the AMBL at the land-sea interface, this simple classification method cannot

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exclude the mixing of continental sources at some point during flight sampling.

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Mean values of chemical and physical parameters were determined based on this flight classification

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scheme (Table 3). Many chemical species had significantly higher concentrations during “continental”-

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influenced flights, including tracers of urban and industrial emissions like CO, nss-SO42-, NO3-, V, and Mn.

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Tracers of soil minerals (Fe, Ca and Sr) and biomass burning (K, DOC) (61) were also significantly

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enhanced in the “continental” samples, as were NH4+ concentrations. Flights 4 and 10 were influenced

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by biomass burning and displayed elevated pH values and elevated concentrations of CO, DOC, and Mn.

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In contrast, flight 16 was influenced by air transport from Southern California (Figure S-1) and contained

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elevated Al, Fe, Mn, and Zn concentrations. However, CO and particulate concentrations were not

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enhanced on this flight, indicating a complex mix of sources that was not easily identifiable.

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Overall, “marine” classified flights had higher concentrations of Na+ and MSA (Table 3), and flights 3, 6,

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and 9 were notable with elevated concentrations of both species (Table 2). While Na+ is directly emitted

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from the ocean surface, MSA is produced from its precursor, dimethylsulfide (DMS), which is emitted

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from the ocean before undergoing gas-to-particle conversion.



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Monomethyl Hg concentrations in samples from “marine” flights were significantly higher compared to

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those in “continental” flights (1.04 ± 0.68 vs 0.42 ± 0.31 ng L-1). This outcome provides an answer to

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Research Question (A), and is a notable result given that all other chemical species were either

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significantly elevated in samples from “continental” flights or there was no significant difference

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compared to “marine” flights. Furthermore, the spatial distribution of MMHg concentration in cloud

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water displayed a north-south gradient (Figure 1) with the highest concentrations observed north of the

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San Francisco Bay (flight 5) and the lowest occurring farther south (flight 16), off the coast of Big Sur.

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Upwelling during the aircraft campaign is indicated by modeled sea surface temperature (SST) shown on

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three selected days (Supporting information, Figure S-2). The area to the north of the San Francisco Bay,

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close to the coastline, displayed consistently lower SST than the more southerly regions. This is roughly

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consistent with the spatial pattern of MMHg concentrations in cloud water and suggests a source to the

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atmosphere of MMHg precursors from upwelled ocean water (Figure 1).

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MMHg and THg concentrations in composite cloud water samples from “marine” flights were

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significantly positively correlated (r2 = 0.31, slope = 6%, p < 0.05), whereas no correlation was observed

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in samples from “continental” flights. MMHg was negatively correlated to nss-Ca2+ concentrations,

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although the relationship was not significant. MMHg was not significantly correlated with any other

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parameters besides THg, presumably due to small sample size and wide spatial coverage of those

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samples.

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In contrast with MMHg, THg concentrations were not significantly different in samples from “marine”

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compared to “continental” flights, indicating both land and oceanic sources were contributing (see

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following discussion).

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3.3 Inferring sources of THg from interspecies correlations


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Cloud water samples from flights classified as “continental” exhibited THg concentrations that were

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significantly positively correlated with nss-Ca2+, CO, DOC, pH, Mn, V, Zn, As, Cd, Ag, and total particle

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concentration (Table 4, Figure 2B, 2C, 2D). Some of these positive correlations were driven by cloud

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water samples influenced by biomass burning plumes from the Soberanes Fire during flights 4 and 10

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(Figure 3). Note the significant positive correlations of CO and THg with particle concentration in this

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subset of samples.

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Total Hg in cloud water samples from flights classified as “marine”, displayed significant positive

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correlations with Na+, Cl-, and SO42- (Table 4). Note that some samples with high THg concentrations

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were associated with very low nss-Ca2+ concentrations (Figure 2B) and very high Na+ concentrations

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(Figure 2E).

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An inverse relationship was observed between THg and altitude (significant for both “marine” and

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“continental” categories) and between MMHg and altitude (not significant for either category) (Figure

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4). An observed decay in concentration with altitude is consistent with chemical species that are emitted

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from the ocean surface, scavenged by cloud droplets at the base of the cloud, and removed higher up in

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the cloud due to coalescence of small drops into larger drizzle drops (62).

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Thus, in addressing Research Question (B), we can conclude that while marine source regimes can be

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identified for MMHg, and both marine and continental source regimes can be identified for THg, specific

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sources (besides biomass burning) could not be determined due to low cloud water sampling frequency

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and wide spatial coverage of that sampling.

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3.4 Cycling of Hg species in coastal marine stratus cloud water

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The results from this aircraft campaign allow for a better understanding of the broader Hg cycle at the

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air-sea interface. Figure 5 outlines what we suggest are the key processes involved. DMHg emitted from

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the ocean can be taken up directly by a cloud drop, although with its low water solubility, oxidation of 13 ACS Paragon Plus Environment


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DMHg by the chlorine radical and uptake of the more soluble (larger Henry’s Law constant) MMHg(g)

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may be a more important pathway. Once in the cloud drop, DMHg can demethylate under acidic

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conditions and due to photolysis. MMHg in the cloud drop can also demethylate in the presence of the

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hydroxyl radical and through photolysis. From preliminary box-modeling results, we expect that the

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lifetime of MMHg in the cloud drop is on the order of several hours. Outside the cloud drop, gaseous Hg0

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is oxidized to HgII with bromine and chlorine radicals through sea salt aerosol activation. HgII gas and

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particulate species are highly water soluble and are quickly scavenged by cloud drops. In the AMBL, the

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lifetime against oxidation for Hg0 should be rapid (on the order of one day) in order for HgII

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concentrations in cloud water to be enhanced above ocean regions where Hg0 is fluxing from the ocean

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to the atmosphere. Once in the cloud drop, HgII may undergo abiotic formation of MMHg in a reaction

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with acetate ion. However, our data produced no relationship between acetate ion and MMHg

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concentrations, casting uncertainty about whether this reaction is relevant in the AMBL.

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The key importance for the accumulation of MMHg in cloud drops, is that drizzle from marine stratus

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clouds leads to wet deposition of MMHg to the surface of the ocean and land. As was calculated for the

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Arctic Ocean, deposition of MMHg from the decomposition of DMHg in the atmosphere is the largest

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source of MMHg to the polar mixed layer (21). Methylated Hg in the photic zone of the ocean undergoes

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photodemethylation (15), however, wet deposition can act to continually replenish the supply of

302

MMHg. This raises the possibility that drizzle from marine stratus clouds may be a significant source of

303

MMHg to the surface waters of the California Current and may explain the observed enhanced

304

concentrations found in the microlayer (28). On land, marine stratus clouds lead to fog drip as the cloud

305

intercepts vegetation. In coastal California and similar environments, fog drip is a significant contributor

306

to the hydrologic cycle (67) during the dry summer months. MMHg flux calculations for fog water

307

deposition at coastal sites indicate that fog water can be the dominant source of MMHg to a terrestrial

308

food web that would be otherwise removed from aquatic sources of MMHg (26). The cloud water 14 ACS Paragon Plus Environment

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309

samples from FASE indicate that elevated MMHg concentrations were distributed across a broad area of

310

the coastal ocean, following the area with the most wind-driven upwelling. Depending on the chemical

311

lifetime of MMHg in cloud water, the source of MMHg from the ocean may have an impactful reach

312

onto land and contribute to bioaccumulation of Hg in terrestrial organisms (68).

313

Acknowledgements

314

The authors thank Jerry Lin for kinetic and solubility data on atmospheric Hg species. The work was

315

supported by the National Science Foundation: OCE-1333738. Aircraft measurements were supported

316

by Office of Naval Research grants N00014-10-1-0811 and N00014-16-1-2567.

317 318

Supporting Information Available: Figure S1 summarizes air mass back trajectory ensembles for each

319

flight. Figure S2 shows spatial maps of sea surface temperature for three selected flights. This

320

information is available free of charge via the Internet at http://pubs.acs.org.

321 322



323

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582 583 584 585


Table 1: Hg species measured in samples of marine stratus cloud water (this work), fog water (coastal and inland), rain water (coastal and inland), and marine boundary layer air, from the Central California region. Also shown are Hg species measurements in mountain-top cloud water, mountain-top air and high elevation snow at select locations. Sample type Marine Stratus Cloud Water MountainTop Cloud Water Coastal Fog Water Inland Fog Water Coastal Rain Water Inland Rain Water Air – Marine Boundary Layer Air – Free Troposphere Influenced Snow – High Elevation

0

Sample location

THg -1 (ng L )

MMHg -1 (ng L )

%MMHg

Pacific Ocean, Central Coast CA

9.18 ± 5.98

0.87 ± 0.66

9.5

Mt. Luilin, Tawain Mt. Mansfield, Vermont

9.6 24.8

CA Central and North Coast (2014-2015) Fresno, CA (2003) Atwater, CA (2016)

Hg -3 (ng m )

II

Hg (g) -3 (pg m )

II

Hg (p) -3 (pg m )

Ref. This work

51, 52

27.6 ± 25.8

1.6 ± 1.9

5.8

27

11.0 24.0 ± 10.5

0.5 0.18 ± 0.09

4.5 0.8

53, 28

Santa Cruz, CA (2007)

6.4 ± 4.2

0.2 ± 0.1

3.1

29

THg: NADP-MDN site CA75 (2014-2016) MMHg: Great Lakes (1997-2003)

16.2 ± 37.1

0.15 ± 0.05

0.9

54, 55

NADP AMNet Site CA48

1.50 ± 0.2

0.8 ± 1.2

3.3 ± 3.3

56

Cascade Mtns. Central Oregon

1.54 ± 0.18

39 ± 193

4.4 ± 100

57

French Alps

18.6 ± 39.7

0.037 ± 0.081

0.2

586


58


587 588

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Table 2: Mean (first number) and standard deviations (second number) for THg, MMHg and other selected parameters for each flight number during FASE-2016. Dates (m/d) are in 2016. Unshaded flights were categorized as “marine,” and shaded flights are categorized as “continental.” Flight # (Date) 2 (7/22) 3 (7/25) 4 (7/26) 5 (7/27) 6 (7/29) 9 (8/3) 10 (8/4) 11 (8/5) 13 (8/9) 14 (8/11) 15-1 (8/12) 15-2 (8/12) 16 (8/13) Flight # 2 3 4 5 6 9 10 11 13 14 15-1 15-2 16 Flight # 2 3 4 5 6 9 10 11 13 14 15-1 15-2 16

THg (ng L-1) 5.14 3.66 9.24 8.22 20.58 3.44 18.41 4.30 9.29 2.72 26.43 2.77 9.31 2.14 6.10 1.78 5.26 2.01 8.58 2.15 8.89 2.50 6.15 1.75 6.81 2.33 Ca2+ (ng m-3) 56 3 168 99 476 189 68 32 98 104 474 321 526 694 70 74 48 48 40 40 762 1480 38 48 126 70 Altitude (m) 208.3 14.6 518.0 81.0 104.3 7.5 187.1 10.5 318.3 84.0 356.0 59.2 459.3 57.2 636.5 89.2 432.5 103.9 397.7 67.1 330.0 158.3 262.1 60.0 362.9 48.8

MMHg (ng L-1) 0.43 0.06

2.38 0.70 0.43 1.29 0.24 0.06 0.94 0.48 0.91 1.17 0.14 0.81 0.02 1.09 0.64 0.20 0.04 nss-Ca2+ (ng m-3) 12 7 25 9 474 31 30 19 12 6 30 30 364 51 16 9 26 10 19 10 753 1470 34 48 125 55 Tamb (oC) 11.15 0.05 11.52 0.34 9.87 0.37 10.52 0.53 11.07 0.80 11.02 0.32 11.20 0.27 9.16 0.46 10.66 0.49 9.98 0.33 10.39 0.09 10.45 0.59 11.15 0.22

CO (ppbv) 81.2 0.2 77.7 1.8 152.1 38.5 69.1 5.6 66.1 1.8 75.7 1.8 105.1 26.4 77.8 8.4 83.5 15.3 69.4 2.4 72.6 5.3 69.9 3.4 79.0 2.8 Al (ng m-3)

DOC (ug m-3) 0.22 0.06 0.50 0.17 0.62 0.19 0.12 0.07 0.18 0.12 0.25 0.10 0.42 0.29 0.11 0.06 0.19 0.09 0.11 0.03 0.09 0.01 0.10 0.05 0.14 0.09 Fe (ng m-3)

12.7 9.6 30.6 22.8 20.2 12.2 23.7 17.0 45.1 8.0 50.8 46.0 10.1 9.4 10.0 6.8 10.3 9.3 21.0 6.1 13.3 10.9 54.5 26.3 U wind (m s-1) 4.29 0.06 4.66 1.06 1.15 1.05 4.05 0.25 2.47 1.30 3.87 0.84 5.52 1.19 6.42 1.73 4.03 0.98 1.76 0.89 -0.32 0.52 0.85 0.92 2.16 0.57

6.2 2.2 13.2 12.9 5.2 2.6 6.8 7.0 12.3 8.1 25.7 19.6 10.0 12.5 3.2 1.9 6.6 5.5 6.4 3.1 3.8 3.9 34.9 20.7 V wind (m s-1) -6.79 0.10 -11.65 3.00 -1.83 0.65 -6.25 2.43 -8.76 4.24 -2.30 1.77 -6.70 1.34 -9.07 4.71 -4.18 1.69 2.36 1.76 2.53 1.89 -1.18 3.05 -0.20 0.58


pH 4.65 0.22 4.23 0.05 5.53 0.74 4.59 0.26 4.32 0.21 4.55 0.14 4.93 0.86 4.83 0.22 4.67 0.22 4.75 0.35 5.01 1.42 4.66 0.64 4.44 0.33 V (ng m-3)

Na+ (ug m-3) 1.18 0.21 3.86 2.50 0.07 0.04 1.03 0.59 2.33 2.69 11.99 7.86 3.47 2.84 1.46 1.88 0.60 0.33 0.59 0.36 0.23 0.30 0.12 0.06 0.03 0.03 Zn (ng m-3)

MSA (ug m-3) 0.10 0.03 0.14 0.05 0.10 0.02 0.12 0.08 0.18 0.17 0.15 0.06 0.15 0.06 0.06 0.06 0.04 0.02 0.04 0.02 0.05 0.03 0.05 0.05 0.05 0.03 Mn (ng m-3)

0.08 0.09 0.03 0.07 0.15 0.19 0.06 0.03 0.05 0.03 0.02 0.13

9.93 12.73 1.84 1.24 0.90 0.54 1.59 0.97 1.66 1.08 2.54 1.76 0.68 0.55 1.01 1.33 0.43 0.14 0.74 0.18 0.71 0.83 1.22 0.87 CPC (cm-3) 675.4 102.3 269.5 59.5 981.3 211.3 234.5 52.5 304.1 102.4 556.4 69.7 339.8 182.2 477.1 123.1 402.9 169.8 476.5 60.2 493.3 137.2 371.8 83.0 228.4 98.6

0.52 0.18 9.02 3.02 0.85 0.65 0.95 0.73 1.40 0.59 7.82 8.00 0.64 0.53 0.72 0.97 0.94 0.48 0.87 0.38 0.50 0.42 2.34 1.42 LWC (g m-3) 0.27 0.01 0.40 0.11 0.19 0.01 0.23 0.02 0.37 0.10 0.30 0.09 0.36 0.11 0.47 0.16 0.29 0.10 0.30 0.07 0.37 0.22 0.27 0.05 0.34 0.05

0.02 0.04 0.01 0.05 0.03 0.14 0.07 0.02 0.02 0.01 0.01 0.07 SST (oC)

11.47 11.58 10.85 10.89 10.83 11.46 11.74 9.40 11.23 10.41 10.97 11.01 11.43

0.17 0.66 0.45 0.79 1.07 0.90 0.72 0.84 0.71 0.61 0.89 0.94 0.59

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Table 3: Mean and standard deviations for THg, MMHg, and other selected parameters for all FASE flights, and for flights categorized as having had influence from “marine” or “continental” air masses. A starred number indicates it is the significantly higher (p < 0.05) value between the “marine” and “continental” categories. All -1

THg (ng L ) -1 MMHg (ng L ) CO (ppbv) -3 DOC (µg m ) pH + -3 Na (µg m ) -3 Cl (µg m ) 2-3 SO4 (µg m ) 2-3 nss-SO4 (µg m ) -3 NO3 (µg m ) + -3 NH4 (µg m ) -3 MSA (µg m ) + -3 K (ng m ) + -3 nss-K (ng m ) 2+ -3 Ca (ng m ) 2+ -3 nss-Ca (ng m ) -3) Al (ng m -3 Fe (ng m ) -3 V (ng m ) -3 Zn (ng m ) -3 Mn (ng m ) -3 Cu (ng m ) -3 As (ng m ) -3 Sr (ng m ) -3 Cd (ng m ) -3 Ag (pg m ) Altitude (m) o Tamb ( C) -1 U wind (m s ) -1 V wind (m s ) o SST ( C) -3 CPC (cm ) -3 LWC (g m ) re drop (µm)

mean 9.18 0.87 81.2 0.22 4.66 1.89 3.34 0.965 0.500 0.391 0.076 0.095 78 6.9 181 93.8 22.0 10.2 0.072 2.51 1.94 0.50 0.040 2.59 0.43 1.39 379.6 10.6 3.28 -4.64 11.0 409.4 0.327 9.31

σ 5.98 0.66 23.2 0.20 0.55 3.29 5.85 1.157 0.454 0.494 0.105 0.083 140 53.4 382 353 22.8 13.0 0.068 5.85 3.50 0.99 0.028 4.88 0.48 1.64 151.4 0.8 2.11 5.23 1.0 203.2 0.116 1.18

Marine mean σ 9.17 6.18 1.04* 0.68 74.1 8.6 0.21 0.17 4.56 0.31 2.12 2.30 3.77 6.36 0.977 1.184 0.451 0.371 0.280 0.240 0.057 0.054 0.097 0.089 73 136 -7.2 8.3 98.4 141 17.2 22.3 15.9 12.8 6.3 6.0 0.056 0.044 2.80 6.74 0.76 0.60 0.48 0.91 0.039 0.023 1.84 2.84 0.48 0.52 1.32 1.74 395.3 153.0 10.60 0.85 3.49 1.97 -5.37 5.57 10.89 1.01 397.7 150.5 0.33 0.12 9.28 1.10

Continental mean σ 9.24 5.34 0.42 0.31 99.9* 36.0 0.31* 0.28 4.98 0.89 1.25 3.57 2.17 4.02 0.932 1.099 0.629* 0.613 0.691* 0.689 0.126* 0.173 0.087 0.065 92 151 45* 92 404* 654 356* 616 39.7 33.9 21.3* 19.9 0.119* 0.100 1.70 1.37 5.27* 5.64 0.55 1.20 0.045 0.038 4.60* 7.92 0.29 0.32 1.53 1.41 338.2 141.3 10.74 0.61 2.75 2.40 -2.71 3.64 11.26 0.75 440.2 302.2 0.32 0.11 9.41 1.38

593



594 595 596

Table 4: Pearson’s correlation coefficients (R) from linear regressions with the parameter indicated and the THg sample concentration. Bold numbers indicate significant correlations based on ANOVA tests with a criteria of p < 0.05.

All CO DOC pH Na+ ClSO42nss-SO42NO3NH4+ MSA K+ Ca2+ nss-Ca2+ Al Fe V Zn Mn Cu As Sr Cd Ag Altitude Tamb U wind V wind SST CPC LWC re drop

0.17 0.21 0.15 0.43 0.43 0.39 0.19 0.20 0.34 0.39 0.48 0.54 0.34 0.23 0.08 0.18 0.23 0.33 0.01 0.06 0.51 0.07 -0.19 -0.36 0.04 -0.04 -0.01 0.06 0.20 -0.27 -0.34

Marine Continental -0.11 0.04 -0.17 0.49 0.49 0.43 0.20 0.30 0.24 0.23 0.49 0.49 0.17 0.28 0.05 0.18 0.23 0.19 -0.02 0.07 0.44 0.04 -0.21 -0.31 0.11 -0.07 0.02 0.06 0.00 -0.22 -0.26

0.79 0.87 0.74 0.08 0.08 0.13 0.16 0.69 0.81 0.52 0.45 0.87 0.83 -0.18 0.26 0.82 0.71 0.37 0.20 0.44 0.03 0.77 0.85 -0.57 -0.38 0.04 -0.18 0.04 0.77 -0.51 -0.69

597


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Figure 1: Maps showing locations of cloud water samples collected by the CIRPAS Twin Otter (flight numbers shown) during the FASE-2016 campaign. Marker sizes are proportional to concentrations of A) THg (made on single samples within each flight number shown) and B) MMHg (made on composite samples). The general location of the Soberanes forest fire is shown by the black star.

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604 605 606 607 608

Figure 2: Linear relationships among species measured in cloud water samples classified as “marine” or “continental”. Samples were classified as “continental” if the mean flight [nss-Ca2+] exceeded the mean [nss-Ca2+], otherwise it was considered as “marine”. A) Ca2+ vs. Na+, B) THg vs. nss-Ca2+, C) THg vs. pH, D) THg vs. CO, E) THg vs. Na+, F) MMHg vs. THg. Regression statistics are for the category indicated; the category not indicated had regressions that were not significant (p > 0.05).

609

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Figure 3: Data from flights 4 and 10, which were impacted by smoke from the Soberanes forest fire. The particle concentration measurement is from the CPC (> 10 nm).

614 615



616 617

Figure 4: Log of MMHg and log of THg versus aircraft altitude in marine and continental segregated samples with linear regressions shown.

618 619


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Figure 5: Hypothesized major processes involved in the cycling of inorganic and methylated Hg between the coastal ocean and marine stratus cloud drops.

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