Impact of Wildfire Emissions on Chloride and Bromide Depletion in

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Impact of Wildfire Emissions on Chloride and Bromide Depletion in Marine Aerosol Particles Rachel Anne Braun, Hossein Dadashazar, Alexander B. MacDonald, Abdulamonam M. Aldhaif, Lindsay C. Maudlin, Ewan Crosbie, Mojtaba Azadi Aghdam, Ali Hossein Mardi, and Armin Sorooshian Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02039 • Publication Date (Web): 12 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017

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Environmental Science & Technology

Impact of Wildfire Emissions on Chloride and Bromide Depletion in Marine Aerosol Particles

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Rachel A. Braun1, Hossein Dadashazar1, Alexander B. MacDonald1, Abdulamonam M. Aldhaif1,

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Lindsay C. Maudlin2, Ewan Crosbie3,4, Mojtaba Azadi Aghdam1, Ali Hossein Mardi1, Armin

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Sorooshian1,5*

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USA

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Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ,

Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University,

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Raleigh, NC

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Dynamics Branch, Hampton, VA, USA

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Universities Space Research Association, Columbia, MD, USA

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Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ, USA

National Aeronautics and Space Administration Langley Research Center, Chemistry and

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*

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5858; Fax: (520) 621-6048; Postal Address: PO BOX 210011, Tucson, Arizona, 85721, USA)

Corresponding author: Armin Sorooshian (E-mail: [email protected]; Phone: (520) 626-

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Key words: Aerosol; sea salt; chloride depletion; bromide depletion; biomass burning

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ACS Paragon Plus Environment


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TOC/Abstract Art

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Abstract

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This work examines particulate chloride (Cl-) and bromide (Br-) depletion in marine aerosol

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particles influenced by wildfires at a coastal California site in the summers of 2013 and 2016.

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Chloride exhibited a dominant coarse mode due to sea salt influence, with substantially

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diminished concentrations during fire periods as compared to non-fire periods. Bromide

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exhibited a peak in the submicrometer range during fire and non-fire periods, with an additional

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supermicrometer peak in the latter periods. Chloride and Br- depletions were enhanced during

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fire periods as compared to non-fire periods. The highest observed %Cl- depletion occurred in

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the submicrometer range, with maximum values of 98.9% (0.32 – 0.56 µm) and 85.6% (0.56 – 1

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µm) during fire and non-fire periods, respectively. The highest %Br- depletion occurred in the

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supermicrometer range during fire and non-fire periods with peak depletion between 1.8 – 3.2

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µm (78.8% and 58.6%, respectively). When accounting for the neutralization of sulfate by

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ammonium, organic acid particles showed the greatest influence on Cl- depletion in the

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submicrometer range. These results have implications for aerosol hygroscopicity and radiative

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forcing in areas with wildfire influence owing to depletion effects on composition.

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1. Introduction

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One major source of atmospheric particles on a mass basis globally is marine sea salt

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emissions.1 As these particles age in the atmosphere, they undergo processes that affect their

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chemical composition and, consequently, their hygroscopic and radiative properties. An

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important mechanism by which sea salt particle composition is altered is via chloride (Cl-)

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depletion.2-7 The generalized form of Cl- depletion from sea salt particles by acidic species is

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given by:

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NaCl + HA →NaA + HCl(g)

(R1)

where HA denotes inorganic or organic acid species. Bromide (Br-) depletion also occurs in sea salt particles, releasing bromine gas, through

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reactions with different gaseous bromine species under acidic conditions.8-11 Such reactions

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involving Br- are especially important owing to subsequent effects on ozone depletion

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reactions.12 According to Sander et al.11, Br- depletion from sea salt particles would not occur in a

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manner analogous to R1 due to the markedly greater solubility of HBr as compared to HCl.

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A comprehensive view of depletion reactions in sea salt particles requires size-dependent

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profiles of the participating species. Sulfate (SO42-) has been shown to more effectively substitute

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for Cl- in smaller marine particles as compared to nitrate (NO3-), which more actively contributes

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to this process in coarse particles.12-15 While inorganic acids (i.e., H2SO4, HNO3) have received

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more attention in playing the role of the acid in Cl-/Br- depletion reactions, they cannot fully

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account for global sea salt dechlorination.16 Consequently, attention has also been given to the

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role of organic acids in depletion reactions, especially methanesulfonic acid (MSA) and oxalic

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acid (C2), which have been shown to be more active in depletion reactions for the smallest

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marine particle sizes.15,17

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A mechanism of Cl-/Br- depletion that, while understudied, is potentially significant is

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that of interactions between sea salt and other species in biomass burning plumes. Wildfires are a

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major emissions source growing in importance, especially over the western United States, due to

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changing vegetation and climate.18-20 While some studies report Cl- depletion in aged smoke

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plumes from biomass burning21-24, other work has shown that, depending on the particle size,

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there can be a lack of chemical interaction between smoke and sea salt owing to the particle

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types being externally mixed.25 There are scarce reports of size-resolved chemical measurements

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where fire versus non-fire conditions can be contrasted to investigate the nature of Cl-/Br-

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depletion in marine particles.

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In this study, the impacts of inorganic and organic acids on Cl- and Br- depletion are

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examined using size-resolved aerosol measurements collected over two summer periods with

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wildfire influence at a coastal California site. This study characterizes (i) differences in Cl- and

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Br- concentrations and their respective depletion between non-fire and fire periods, and (ii)

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relative influences of inorganic and organic acids on observed Cl- depletion. Such a study is

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important due to ongoing field campaigns focused on wildfires that struggle to speciate and

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quantify both supermicrometer species and the size-resolved nature of refractory species such as

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sea salt. The results of this study have implications for the chemical, hygroscopic, and optical

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properties of aerosol particles in wildfires and in marine and coastal atmospheres.

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2. Materials and Methods

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2.1 Field Study Descriptions

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Aerosol measurements were conducted at a coastal site in Marina, California (36.7°N, 121.8°W) (Figure 1). Due to proximity to the coast, we assume a strong influence of marine

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aerosol. Some anthropogenic sources of aerosol may arise from the town of Marina (2016

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population estimate: ~21,700; US Census Bureau) and the larger city of Salinas (2016 population

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estimate: ~157,200; US Census Bureau) located ~12 km east of the sample site. Measurements

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were taken during two field campaigns: the Nucleation in California Experiment (NiCE) in 2013

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and the Fog and Stratocumulus Evolution (FASE) experiment in 2016. Sample sets from NiCE

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are referred to as N1-N10 and sample sets from FASE as F1-F7. A summary of sampling

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parameters and meteorological conditions for each of the 17 sample sets is provided in Table 1.

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While FASE sample sets represented full-day measurements, half of the NiCE sample sets

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(N1/N3/N5/N7/N9) represented daytime collection (06:00-21:00 local time) and the other half

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(N2/N4/N6/N8/N10) were nighttime sets (21:00-06:00). Average meteorological conditions for

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the sampling periods were calculated using data from the Monterey Peninsula station (KMRY in

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Figure 1; 36.6°N, 121.8°W) in the Mesowest database.26

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The fire-influenced sample sets are N7 and N8 from the NiCE campaign, and F3-F7 from the FASE campaign. The fires impacting N7 and N8 were located by the California–Oregon

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border and included the Big Windy, Whiskey Complex, and Douglas Complex forest fires

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(Figure 1). The F3-F7 sample sets were influenced by the Soberanes fire, which originated in

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Garrapta State Park, approximately 30 km southwest of the sampling location (Figure 1).

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Because of the proximity of the fire to the sample site during the FASE campaign, the area was

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blanketed in a cover of smoke; however, based on the distance to the fire and the average wind

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speeds measured at the KMRY weather station, fresh smoke emissions from the Soberanes fire

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are estimated to have reached the sample site in approximately 2.5 – 4.5 h. Fire sets were

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confirmed using a variety of methods as described by Maudlin et al.24, including visual and

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olfactory evidence, confirmation from aircraft aerosol data (i.e., number concentration and

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composition) near the sampling site, and enhancement in fire tracer species in the sample sets.

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2.2 MOUDI Measurements Size resolved aerosol measurements were collected using two micro-orifice uniform

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deposit impactors (MOUDI, MSP Corporation27). Aerodynamic cutpoint diameters for the

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MOUDI were 0.056, 0.1, 0.18, 0.32, 0.56, 1.0, 1.8, 3.2, 5.6, 10.0, and 18 µm. Teflon filters

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(PTFE membrane, 2 µm pore, 46.2 mm, Whatman) were used for aerosol collection. Each

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collected filter was divided in two equal parts, with one half archived and the other extracted

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with 10 mL of milli-Q water in sealed glass vials. The vials were sonicated at 30 °C for 20 min.

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The aqueous extracts were subsequently analyzed with ion chromatography (IC; Thermo

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Scientific Dionex ICS – 2100 system) and with either inductively coupled plasma mass

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spectrometry (ICP-MS; Agilent 7700 Series) for NiCE samples or triple quadrupole inductively

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coupled plasma mass spectrometry (ICP-QQQ; Agilent 8800 Series) for FASE samples. All

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speices concentrations were used from IC except for sodium (Na+) and Br-, which were used

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from the ICP techniques due to improved data quality. Species concentrations of background

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filter samples were subtracted from true sample concentrations. For measurements of Cl-, Br-,

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and Na+ that were blank, their concentrations were replaced with one-half the limit of detection

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(LOD) for each species.28 LOD values for Cl- were 21 ppb for FASE and 10 ppb for NiCE and in

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the ppt range for Br- (31 ppt) and Na+ (87 ppt). Non-zero values for these three species were

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necessary for calculating finite Cl-/Br- depletion percentages as summarized below.

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2.3 Depletion and Concentration Calculations

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Chloride and Br- depletion percentages were calculated using the typical mass ratios of

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Cl- or Br- to Na+ in seawater. This assumes that the major source of these species is sea salt,

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which has been confirmed with past measurements in the study region.24,29 As denuders and gas-

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phase measurements were not available, sampling artifacts could not be quantified but have

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previously been shown to lead at times to excess Cl- depletion.30 Chloride depletion was

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quantified using the following equation:

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%Cl- depletion = (1.81×[Na+]- [Cl-])/(1.81×[Na+] )×100%

(R2)

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where [Na+] and [Cl-] are mass concentrations (µg m-3) and 1.81 is the typical mass ratio of

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Cl:Na in seawater.2,31 For any measurements where the Cl:Na ratio exceeded that of typical

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seawater, the %Cl- depletion was regarded as 0%. Bromide depletion was calculated in an

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analogous manner by substituting the typical mass ratio of Br:Na in seawater (0.0061576).32

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Non-sea salt sulfate (NSS-SO42-) was calculated as follows:

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NSS-SO42- = [SO42-] - 0.252×[Na+]

(R3)

where 0.252 is the typical mass ratio of SO42- to Na+ in seawater.33 Assuming that NSS-SO42- reacts with available ammonium (NH4+), a measure of the SO42- not associated with ammonium, excess-SO42- (EX-SO42-), can be calculated as follows: EX-SO42- = [NSS-SO42-] – MWsulfate/MWammonium×[NH4+]/2

(R4)

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where MW represents molecular weight.34 Dimethylamine (DMA) is the most abundant alkyl

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amine in the study region and can serve as a base, but it is ignored in this analysis as our past

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work in the region has shown that the DMA:NH4+ molar ratio is usually less than 0.04 across the

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size spectrum of the MOUDI.35

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To find the theoretical maximum amount of observed Cl- depletion that could be attributed to a specific species, the following equation was used:

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%Cl- depletion attributable to A = [A] × y × (MWchloride / MWA) / (1.81×[Na+] – [Cl-]) ×100%

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(R5)

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where [A] is the mass concentration (µg m-3) of the acidic species and y is the charge of the fully

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deprotonated conjugate base of A (e.g., y = 2 for H2SO4, y = 1 for HNO3).

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3. Results and Discussion

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3.1 Size-Resolved Chloride Results

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The mass size distribution profile of Cl- is well established in literature, with a

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predominant coarse mode of Cl- owing to sea salt.36-38 Figure 2 shows size-resolved results for

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both average Cl- concentrations and %Cl- depletion for fire and non-fire periods. Chloride

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concentrations in the supermicrometer range (≥ 1 µm) accounted for 77.9 – 99.1% and 94.9 –

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99.2% of the total Cl- concentrations integrated over all stages of the fire and non-fire sample

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sets, respectively, with substantially reduced concentrations during fire periods as compared to

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non-fire sets. The highest observed %Cl- depletion occurs in the submicrometer range (< 1 µm),

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consistent with past work,39 with maximum values of 98.9% (0.32 – 0.56 µm) during fire periods

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and 85.6% (0.56 – 1 µm) during non-fire periods. As will be shown, some of the species

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responsible for Cl- depletion (i.e., SO42-) peak in concentration at these same sizes in the

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accumulation mode.

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Overall %Cl- depletion, for concentrations integrated over all MOUDI stages, is greater

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during fire periods (79.2 ± 15.7%) than during non-fire periods (24.9 ± 23.1%). This is consistent

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with other studies that have shown more polluted air masses typically exhibit higher depletion

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than cleaner marine air masses.5 During fire periods, %Cl- depletion decreases with increasing

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particle size in the supermicrometer range (74.2% to 54.5% from aerodynamic cutpoint

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diameters of 1 µm to 10 µm), consistent with previous results.2,14,15,17,31,34 In contrast, during

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non-fire periods, %Cl- depletion remains lower with less variability in the supermicrometer

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stages (11.3 – 23.2%).

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3.2 Size-Resolved Bromide Results Previous studies have shown that size distribution profiles are more varied for Br- than for

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Cl-, with either a unimodal profile exhibiting a peak in the submicrometer range,40-42 or a

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bimodal profile with submicrometer and supermicrometer modes.43-45 In this study, Br- exhibited

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a bimodal size distribution during non-fire periods (Figure 3). In all of the fire sample sets,

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except N7, the coarse mode Br- concentration was considerably diminished. For both fire and

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non-fire periods, Br- concentration peaks between 0.32 – 0.56 µm; however, during non-fire

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periods, a second, larger peak is observed between 3.2 – 5.6 µm, similar to Cl-. A study from an

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eastern Mediterranean site also reported a bimodal Br- profile and attributed the submicrometer

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and supermicrometer modes to anthropogenic emissions and sea salt, respectively.45 In this work,

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all Br- was assumed to have sea salt origins; however, if the MOUDI samples were indeed

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impacted by non-sea salt Br- sources, values reported in this study would represent an

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underestimation of the actual %Br- depletion from sea salt particles.

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Similar to Cl-, %Br- depletion is greater during fire periods (55.2 ± 34.5%) as compared

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to non-fire periods (25.7 ± 24.4%) for all MOUDI stages. The highest %Br- depletion occurred in

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the supermicrometer range during fire and non-fire periods with peak %Br- depletion between

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1.8 – 3.2 µm (78.8% and 58.6%, respectively). This is in contrast to Cl- where the highest %

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depletion was between 0.32 – 0.56 µm and 0.56 – 1 µm for fire and non-fire periods,

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respectively. This difference between the size-resolved depletion of Br- and Cl- is suggestive of

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different loss mechanisms, as described in Section 1. Measurements over the Southern Ocean at

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Cape Grimm revealed that Cl- and Br- deficits in aerosol were highest around a diameter of 0.3

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µm and that the Br- depletion was greater than Cl- depletion;46 however, our results showed

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greater Cl- depletion than Br- depletion and that Br- depletion peaked at larger sizes (and smaller

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ones during fire periods).

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3.3 Variability between NiCE and FASE

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Figure S1 (Supplementary Information) summarizes Cl- and Br- mass size distributions

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for all sample sets from both NiCE and FASE, while Figure 4 shows results for submicrometer

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and supermicrometer stages. Two of the non-fire sample sets exhibited no Cl- depletion (N1 and

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N3). No Br- depletion was observed in four sample sets during non-fire periods (N4, N5, N6,

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N10) and one during a nighttime fire period (N8). All sample sets from FASE exhibited Br- and

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Cl- depletion.

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On average, the biomass burning plumes had a longer distance to travel to the sampling

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site (Figure 1) and thus had longer aging times associated with them during the NiCE campaign

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(34 – 70 h) as compared to FASE (< 4.5 h). The definition of “aged smoke” varies widely in the

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literature; for example, “aged smoke” has been defined as having aged 40 min,25 1.5 hrs,22 and

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2.5-4.5 hrs23 from the source. Therefore, our measurements from FASE would be considered

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“aged” by these standards. The results of this study suggest that marine aerosol with closer

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proximity to fires (less aging) exhibit greater depletion. More specifically, during fire periods,

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the average %Br- depletion across all stages for NiCE versus FASE was 11.0% and 72.9%,

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respectively, while the average %Cl- depletion values were 63.5% and 85.5%, respectively.

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We postulate that some combination of the following factors may be responsible for this

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observed behavior: (i) different fuel types were present during NiCE (“Timber; grass and shrub

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models”, https://inciweb.nwcg.gov/incident/3562/) and FASE (“chaparral, tall grass, and

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timber”, https://inciweb.nwcg.gov/incident/4888/), which affects the composition of the emitted

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aerosol; (ii) potential increases in anthropogenic (including agricultural) emissions three years

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later during FASE could contribute to higher depletion; for example, Monterey County, CA,

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where the sample site was located, had a 4.9% increase in population between 2010 and 2016

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(US Census Bureau); (iii) because of the proximity of the Soberanes fire to the sampling site

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during FASE, high concentrations of gaseous species emitted from the fire may have led to

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increased Cl- and Br- depletion through heterogeneous reactions,47 while similar emissions

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during the NiCE fire may have been diluted during the travel time to the sampling site; and (iv)

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rapid decreases in gaseous NH3 concentrations occur in the first 2.5 – 12 hrs of smoke plume

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aging,48,49 due in part to particulate NH4+ formation.50 While gas measurements are not available

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for the current study, the measured particulate NH4+ is much higher on average during fire

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periods for NiCE (0.94 µg m-3) as opposed to FASE (0.18 µg m-3). We postulate that the lower

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amounts of particulate NH4+ during FASE allowed for greater Cl- depletion by SO42-, as

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evidenced by the amount of EX-SO42- available for reaction. During the FASE campaign, three

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of the five fire sets exhibited non-zero amounts of total EX-SO42-, while during NiCE, neither of

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the two fire sets contained any overall EX-SO42- (i.e., all measured SO42- could be neutralized by

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available NH4+).

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3.4 Concentrations of Acidic Species Figure 5 shows size distributions for acidic particle species from all sample sets during fire and non–fire periods. In addition to SO42- and NO3-, deprotonated organic anions (hereafter 12


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referred to as organic acids or organics) are also examined owing to past work showing their

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potential importance for depletion reactions.15,17,31,51 The category ‘organics’ is comprised of the

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sum of organic acid species, including formate, acetate, maleate, oxalate (C2), succinate (C4),

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glutarate (C5), and methanesulfonate (MSA). Malonate (C3) was not included in this analysis

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due to insufficient measurements during the FASE campaign. Figure S2 (Supplementary

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Information) provides mass size distributions for the organic acid species during both fire and

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non-fire periods.

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The dominant species in the submicrometer mode is NSS-SO42-, which exhibits

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maximum concentrations between 0.32 – 0.56 µm during both fire (0.30 ± 0.13 µg m-3) and non-

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fire periods (0.35 ± 0.11 µg m-3). The organics followed the mass size distribution profile of

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NSS-SO42- with maximum concentrations in the same size range, 0.32 – 0.56 µm (0.14 ± 0.07 µg

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m-3 and 0.09 ± 0.03 µg m-3 during fire and non-fire periods, respectively). Among the organics,

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C2, MSA, and acetate were the most abundant species, with a sharp enhancement of C2 in both

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the sub- and supermicrometer ranges in fire periods as compared to non-fire periods (Figure S2).

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Nitrate is the dominant species above 1 µm, especially between 1 – 3.2 µm, consistent with past

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work in the study region.52

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The amount of NSS-SO42- available for reaction decreases substantially when accounting

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for neutralization by NH4+. The total SO42- available for reaction over all stages in each sample

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set decreased from an average of 0.76 µg m-3 (NSS-SO42-) to 0.10 µg m-3 (EX-SO42-) during fire

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periods, while in non-fire periods the reduction was from 0.94 µg m-3 to 0.04 µg m-3.

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3.5 Chloride Depletion by Acidic Species

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Figure 6 shows the theoretical percentage of observed Cl- depletion attributable to

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specific acidic species. The maximum percentage attributable to each species was capped at

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100.0%; therefore, in some instances the percentage attributable to NSS-SO42- + NO3- was less

271

than the sum of the parts. For instance, during F2 in the 1.8 - 3.2 µm range, NSS-SO42- could

272

account for 56.3% of the observed Cl- depletion and NO3- could account for 45.4%; however, the

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total depletion attributable to NSS-SO42- + NO3- was capped at 100.0%. Because of the different

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mechanisms and various pathways available for Br- depletion, we have chosen to instead focus

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our discussion on the role of acidic species in Cl- depletion given by the reaction R1. Non-sea

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salt SO4- + NO3- contributed the most to the observed Cl- depletion in the submicrometer range,

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peaking at an average of 93.2% of Cl- depletion during fire periods between 0.56 – 1 µm and

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100.0% (on average) during non-fire periods between 0.32 – 0.56 µm. However, after accounting

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for neutralization of NSS-SO42- by NH4+, the contribution from EX-SO42-+ NO3- drops to 40.0%

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between 0.56 - 1 µm during fires and 20.1% between 0.32 – 0.56 µm during non-fire periods. In

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this case, organics can account for a higher percentage of the observed Cl- depletion (55.6%

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during fire periods between 0.56 – 1 µm and 46.3% during non-fire periods between 0.32 – 0.56

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µm).

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At the smallest (< 0.1 µm) and largest sizes (> 3.2 µm), organics account for a higher

285

average %Cl- depletion than NSS-SO42- + NO3- during fires. For example, during fires, organics

286

can account for on average 19.9% of the observed Cl- depletion between 0.056 – 0.1 µm

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(compared to 17.1% for NSS-SO42- + NO3-) and 19.1% between 10 – 18 µm (compared to only

288

0.7% for NSS-SO42- + NO3-).

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Based on measurements at a coastal site in Hong Kong, Zhuang et al.34 attributed the

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majority of observed Cl- depletion in particles larger than 1.8 µm to NO3- and NSS-SO42-, but

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showed that these two species could not account for 16-25% of the observed Cl- depletion at

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certain measurement times. Our results during non-fire periods showed that on average, 86.2% of

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the observed Cl- depletion could be attributed to NO3- and NSS-SO42- between 1.8 – 3.2 µm, with

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%Cl- depletion attributable to these two species decreasing for larger sizes. Measurement studies

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in southern Africa showed that Cl- was largely unreacted in supermicrometer particles, whereas

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excess sulfur was linked to nearly full Cl- depletion in smaller particles.25,53

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Table 2 shows the average %Cl- depletion attributable to each acid species, including

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total organics, in the submicrometer and supermicrometer ranges during both fire and non-fire

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periods. The values given in Table 2 represent theoretical maximum values for %Cl- depletion

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attributable to each species. The total of all organics is less than the sum of each organic acid

301

individual species because %Cl- depletion contributions were capped at 100.0% for each

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individual sample set.

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Of the organic acid species, C2 exhibited the highest potential contribution to observed

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Cl- depletion in the submicrometer (11.5%) and supermicrometer (12.8%) ranges out of all of the

305

observed organic acids during fire periods. At a site near the Arctic Ocean, Kerminen et al.17

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found that Cl- depletion in smaller sea salt particles (0.94 – 1.8 µm) could be significantly

307

impacted by MSA and dicarboxylic acids; in particular, oxalate alone was found to account for

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7-22% of the observed Cl- loss in the given size range. During non-fire periods in the current

309

study, MSA could account for 24.2% (on average) of the observed Cl- depletion in the

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submicrometer range.

311

NSS-SO42- was the highest overall contributor to submicrometer Cl- depletion during fires

312

(65.2%); however, the impact of SO42- decreased substantially when accounting for

313

neutralization by NH4+, such that the contribution of EX-SO42- to observed Cl- depletion during

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fires in the submicrometer range was only 10.8%, compared to 39.2% for all organics and 4.6%

315

for NO3-. While NSS-SO42- could account for 91.5% of the observed Cl- depletion in the

316

submicrometer range during non-fire periods, EX-SO42- could only account for 5.3% and NO3-

317

could only account for 19.3%, compared to 61.4% for all organics. Therefore, the influence of

318

organics is especially important in the submicrometer range after accounting for neutralization of

319

SO42- by NH4+.

320

Measurements on the eastern United States coast have shown that NO3- can account for

321

almost all of Cl- depletion in most coarse mode particles, with SO42- and organic acids

322

contributing, at a maximum, 33% and 34.6%, respectively.31 As shown in Table 2, our findings

323

show that during non-fire periods, NO3- can account for approximately 61.2% of the observed Cl-

324

in the supermicrometer range, with organics and NSS-SO4 contributing on average 45.9% and

325

50.1% in that range, respectively.

326 327

4. Implications of Depletion Processes

328

As wildfire research is growing in importance, the results of this work highlight the need

329

for considering how aerosol properties can be modified as a result of acidic species depleting Cl-

330

and Br- in sea salt particles. An interesting feature of the data is that there is an enhancement in

331

Br- depletion at the very smallest particles sizes as compared to larger sizes within the

332

submicrometer range. Br- depletion decreases gradually from 67.3% to 21.2% during fire periods

333

and 17.9% to 2.4% during non-fire periods for increasing aerodynamic cutpoint diameters from

334

0.056 to 0.32 µm. Since the critical activation diameters of cloud condensation nuclei (CCN) in

335

the study region have been reported to be exactly between the lower and upper bound diameters

336

of the smallest MOUDI stage based on airborne measurements,54,55 alterations in hygroscopicity

16


Page 16 of 35

Page 17 of 35


337

owing to Cl- and Br- depletion are of critical importance in determining whether a particle will

338

activate into a droplet.

339

Composition changes due to depletion reactions affect water-uptake properties of aerosol

340

in the sub-saturated regime, including their deliquescence relative humidity (RH) and RH-

341

dependent growth factor.56-58 Peng et al.59 found that the hygroscopic growth factor of pure 100

342

nm NaCl particles at 80% RH is reduced from approximately 2 to between 1.06 – 1.57 when it is

343

mixed with oxalic acid at dry mass ratios of NaCl:oxalic acid between 1:3 and 3:1. Boreddy and

344

Kawamura60 suggested that an observed difference in observed growth factor at 90% RH

345

between 2.1 in sea salt particles and an average of 1.76 in water-soluble matter extracted from

346

coastal aerosols could partially be attributed to Cl- depletion processes. Drozd et al.61 found the

347

single parameter representation of CCN (κ) to vary between 1.05 for NaCl and 0.68 for NaCl

348

particles after deposition of oxalic acid. Reduced aerosol liquid water results in a smaller

349

particle size, which impacts radiative properties, increases visibility, and results in a lower

350

efficiency for promoting heterogeneous chemistry for the production of species such as

351

secondary organic aerosol (SOA).62-65 It is cautioned though that very close to fire sources, the

352

enhancement in particle emissions would overwhelm and offset changes in aerosol effects, such

353

as visibility, owing to depletion reactions.

354

The results of this study motivate increased attention to depletion reactions, especially as

355

a function of plume age, to improve model treatment of biomass burning impacts on atmospheric

356

chemistry. A more comprehensive approach including gas-phase measurements simultaneous

357

with aerosol ion composition is recommended, especially coupled with aerosol thermodynamic

358

modeling.

359

17



360

Acknowledgements: This work was funded by Office of Naval Research grants N00014-10-1-

361

0811 and N00014-16-1-2567. We gratefully acknowledge the support of Dr. Shane Snyder’s

362

Laboratory at the University of Arizona, who is supported in part by Agilent Technologies.

Page 18 of 35

363 364

Supporting Information Available: Figure S1 summarizes mass size distributions of Cl- and Br-

365

for all MOUDI sets shown in Table 1. Figure S2 summarizes the mass size distributions of

366

organic acid species. This information is available free of charge via the Internet at

367

http://pubs.acs.org.

368 369

References

370

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3613-3628.

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559 560 561 562

563 564

27



565

Table 1. Operational details and average meteorological parameters associated with each

566

MOUDI set collected during NiCE (N1 – N10) and FASE (F1 – F7). Each odd sample set

567

number from NiCE represents daytime collection (06:00-21:00 local time) and the even number

568

sets are from nighttime (21:00-06:00). (RH: relative humidity, WS: wind speed, WD: wind

569

direction, T: temperature). The inlet air was not conditioned in any way. Sample Set N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 F1 F2 F3 F4 F5 F6 F7

Start Date 7/3/2013 7/3/2013 7/10/2013 7/10/2013 7/17/2013 7/17/2013 7/24/2013 7/24/2013 7/31/2013 7/31/2013 7/15/2016 7/19/2016 7/25/2016 7/28/2016 8/1/2016 8/5/2016 8/9/2016

Page 28 of 35

End Date Fire/Non-Fire Total Hours Flow Rate (LPM) RH (%) WS (m/s) WD(°) T(°C) 7/9/2013 Non-Fire 93.75 27.60 75 3 244 16 7/10/2013 Non-Fire 63.00 26.35 87 1 176 14 7/16/2013 Non-Fire 104.50 27.60 78 3 253 15 7/17/2013 Non-Fire 63.00 26.35 88 1 210 13 7/24/2013 Non-Fire 102.50 27.60 82 3 259 15 7/24/2013 Non-Fire 63.00 26.35 93 1 226 13 7/31/2013 Fire 94.25 27.60 77 3 258 16 7/31/2013 Fire 63.00 26.35 89 1 211 14 8/9/2013 Non-Fire 131.00 27.60 79 3 253 15 8/9/2013 Non-Fire 81.00 26.35 89 2 219 14 7/19/2016 Non-Fire 94.02 28.57 82 3 213 14 7/25/2016 Non-Fire 138.92 28.60 76 3 225 15 7/28/2016 Fire 69.00 28.98 93 2 152 12 8/1/2016 Fire 96.70 28.88 81 3 312 15 8/5/2016 Fire 94.60 29.27 83 3 234 16 8/9/2016 Fire 95.53 29.12 83 3 235 14 8/12/2016 Fire 77.83 28.61 84 3 185 15

570 571 572 573

28


Page 29 of 35


574

Table 2. Average contribution of each acidic species to observed %Cl- depletion in the

575

submicrometer and supermicrometer ranges. Values represent the theoretical maximum that each

576

species could contribute to %Cl- depletion. (NSS-SO42- : non-sea-salt sulfate, EX-SO42- : excess

577

sulfate, All organics: sum of organic species listed in the table)

Acid Formate Acetate Maleate MSA C5 C4 C2 All organics NSS-SO 4 EX-SO4 NO 3 578

-

2-

2-

< 1 µm ≥ 1 µm < 1 µm ≥ 1 µm < 1 µm ≥ 1 µm < 1 µm ≥ 1 µm < 1 µm ≥ 1 µm < 1 µm ≥ 1 µm < 1 µm ≥ 1 µm < 1 µm ≥ 1 µm < 1 µm ≥ 1 µm < 1 µm ≥ 1 µm < 1 µm ≥ 1 µm

% observed Cl depletion attributable to each acid Fire Non-Fire 4.7% 13.2% 6.8% 8.0% 6.4% 19.5% 7.4% 10.1% 3.6% 5.1% 0.9% 0.0% 8.4% 24.2% 1.6% 3.9% 1.8% 4.5% 0.8% 2.1% 2.8% 4.9% 2.4% 4.7% 11.5% 14.4% 12.8% 20.0% 39.2% 61.4% 26.8% 45.9% 65.2% 91.5% 19.5% 50.1% 10.8% 5.3% 3.6% 33.3% 4.6% 19.3% 24.7% 61.2%

29



579 580

Figure 1. Map showing the MOUDI sampling site in Marina, the location of fires during NiCE

581

(Douglas Complex/Big Windy/Whisky Complex Fires) and FASE (Soberanes Fire), and a

582

meteorological station (KMRY) located by the sampling site.

583

30


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584 585

Figure 2. Average size distributions for Cl- and %Cl- depletion during fire and non-fire periods.

586

Error bars for %Cl- depletion represent one standard deviation.

31



587 588

Figure 3. Average size distributions for Br- and %Br- depletion during fire and non-fire periods.

589

Error bars for %Br- depletion represent one standard deviation.

590

32


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591 592

Figure 4. Concentrations of Br- and Cl- for all sample sets, divided into submicrometer and

593

supermicrometer ranges. Total % depletion for each species is shown. Sets N1-F2 and N7-F7 are

594

non-fire and fire periods, respectively.

595

33



596 597 598

Figure 5. Average mass size distributions for acidic particle species in units of µg m-3. (NSS-

599

SO42- : non-sea-salt sulfate, EX-SO42- : excess sulfate).

600 601

34


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602 603

Figure 6. Theoretical %Cl- depletion by size for each acidic aerosol species. (NSS-SO42- : non-

604

sea-salt sulfate, EX-SO42- : excess sulfate).

605 606

35


Impact of Wildfire Emissions on Chloride and Bromide Depletion in

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