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Article

Quantification of Gas-to-Particle Conversion Rates of Sulfur in the Terrestrial Atmosphere Using High-Sensitivity Measurements of Cosmogenic S 35

Mang Lin, Saman Biglari, and Mark H. Thiemens ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00047 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on July 4, 2017

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ACS Earth and Space Chemistry

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Quantification of Gas-to-Particle Conversion Rates of Sulfur in the

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Terrestrial Atmosphere Using High-Sensitivity Measurements of

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Cosmogenic 35S

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Mang Lin*, Saman Biglari and Mark H. Thiemens

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Department of Chemistry and Biochemistry, University of California San Diego, La Jolla,

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92093, California

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* Email: [email protected]

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Abstract

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The productions of sulfuric acid and sulfate aerosol (SO42-) from the oxidation of sulfur dioxide

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(SO2) are fundamental chemical processes in nature and play important roles in the sulfur cycles

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and climates of Earth and extraterrestrial bodies such as Venus and Europa. Numerous

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experimental and theoretical efforts have been made to understand kinetics and mechanisms of

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SO2 oxidation, but quantifying SO2-to-SO42- conversion rates in the terrestrial atmosphere

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remains a challenge due to varying sources for both SO2 and SO42-. Here we use high-sensitivity

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measurements of cosmogenic 35S (half-life = 87 days), a radiogenic isotope exclusively produced

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by cosmic rays in the atmosphere, in both SO2 and SO42- collected from the ambient atmosphere

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to assist in such quantification. The monthly SO2-to-SO42- conversion rates in the boundary layer

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over coastal California and the Tibetan Plateau are calculated using a steady-state 35S box model.

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A distinct seasonal variation of SO2-to-SO42- conversion rates with maximum in summer and

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minimum in winter is found in both regions. The rapid SO2-to-SO42- conversion rates in summer

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ACS Paragon Plus Environment


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(ranging from ~1 to ~2 d-1) not only provide an additional field-based constraint for reducing

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uncertainties in current chemistry transport and climate models, but also highlight the need for a

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better understanding of SO2 oxidation pathways in the chemically complex terrestrial

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atmosphere. Implications for future field missions, modeling and experimental investigations are

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discussed.

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Keywords: Sulfur cycle; sulfur dioxide; sulfate aerosol; kinetics; photochemical processes;

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California; Tibetan Plateau

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1. Introduction

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Airborne particles (or aerosols) in the terrestrial atmosphere have substantial influences on

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public health,1 visibility,2 ecosystem,3 weather4 and climate.5 They are emitted to the atmosphere

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directly (primary aerosols) or produced in the atmosphere via gas-to-particle conversion

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(secondary aerosols). Because secondary aerosols contribute significantly to the total aerosol

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burden,6 an accurate quantification of gas-to-particle conversion rate is crucial to estimate the

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budget of aerosols and evaluate their impacts.7 As a major component of aerosols, SO42-

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(including condensed-phase sulfuric acid and sulfate aerosol) is mainly oxidized from SO2 and

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closely associated with mortality8 and radiative forcing.9 Sulfur is an ubiquitous element in the

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Universe, and therefore SO2 and SO42- are also important components in some other planets (e.g.,

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Venus and Mars) and moons of Jupiter (e.g., Europa and Io) that profoundly influence climates

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and geological structures.10-13 A complete understanding of sulfur chemistry in the present-day

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terrestrial atmosphere can shed light on similar processes that occur in the Earth’s paleo-

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atmosphere and in the atmospheres of extraterrestrial bodies.

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In the terrestrial atmosphere, secondary SO42- is synthesized via a number of SO2 oxidation

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pathways, such as gaseous oxidation by OH radical or Criegee intermediates and aqueous

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oxidation by H2O2, O3, O2 (catalyzed by transition metal ions) or hypohalous acid (HOX; X = Cl

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or Br).14-15 Because of the complicated SO42- formation chemistry, laboratory kinetic

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experiments cannot accurately simulate processes occurring in nature. Models using laboratory

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data fail to fully explain observations in the ambient atmosphere, especially in the boundary

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layer.16-18 Early studies estimated the ambient SO2-to-SO42- conversion rate using in-situ

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measurements of SO2 and SO42- concentrations.19-22 There is a fundamental assumption in their

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calculations that atmospheric SO42- is exclusively produced from SO2 oxidation and not affected

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by other sources such as primary SO42- from crustal minerals and fossil fuel combustion.

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Unfortunately, this assumption is not valid for most environments because of varying sources for

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both SO2 and SO42- (Figure 1a). To accurately quantify the ambient SO2-to-SO42- conversion rate

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using SO2 and SO42- concentrations, one has to consider the production/emission rates of all other

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sources, which is difficult to achieve.

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Cosmic ray spallation, in which a nucleus impacted by a high-energy particle shatters into

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many nuclei, is responsible for the production of some elements and isotopes. This process not

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only occur in deep space but also in the terrestrial atmosphere. Cosmogenic

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radiogenic isotopes spallogenically produced in the terrestrial atmosphere by the bombardment

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of

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stable sulfur,23-24 it exists in gas (SO2) and particulate (SO42-) phases simultaneously.

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Subsequently, the radiogenic sulfur may offer an actual clock to quantify the sulfur gas-to-

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particle conversion rate. A unique character of cosmogenic sulfur is that it is exclusively

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produced in the higher atmosphere and convert to radiogenic SO2 in ~1 s after production.23-24

40

35

S and

38

S are

Ar with high energy cosmic rays. Because cosmogenic sulfur behaves nearly identically to

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Given their short half-life (35S: ~87 d;

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primary SO42- sources (e.g. fossil fuel) and other sulfur-containing trace gases (e.g. biogenic

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dimethyl sulfide [DMS]) are negligible, which means that radiogenic SO42- is exclusively

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produced by the conversion of radiogenic SO2 in the atmosphere (Figure 1b). The single source

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of radiogenic SO2 and SO42- notably simplifies the quantification of SO2 and SO42- lifetimes.

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A pilot study measured

38

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S: ~2.8 h), concentrations of cosmogenic sulfur in

S in SO2 to estimate the lifetime of SO2.25 The result implied that

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SO2 oxidation rates in the ambient atmosphere might be much faster than previously thought.

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Because of the short half-life of 38S (~2.8 h) and large uncertainty in measurements, this method

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has not been further developed. The radionuclide

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possesses a half-life longer than 3 hours, and its half-life (~87 d) is of a proper time scale for

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understanding a variety of physical and chemical processes in nature, especially for quantifying

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the lifetime of SO2 and SO42- in the atmosphere. Turekian and Tanaka, the pioneers in such

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studies, first measured

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rate of SO2, with an assumption that the SO2-to-SO42- conversion rate is constant.24, 26 In a later

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study, with additional measurements of another cosmogenic isotope 7Be (half-life = ~53 d), the

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seasonal variation of SO2-to-SO42- conversion rate was determined,27 but the obtained values

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possess large uncertainties as a result of conventional low-sensitivity 35S measurements. Due to

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the difficulty of 35S measurements, similar studies had been scarce until 2010, when Brothers et

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al.28 developed an optimized

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sensitivity method,28 combined measurements of

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California and the Tibetan Plateau to understand atmospheric vertical mixing.28-30 However, the

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SO2-to-SO42- conversion rate was assumed to be constant in previous data interpretation, and

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chemical information recorded by cosmogenic 35S was not explored. In this study, we present a

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SO2 and

35

35

35

S is the only radiogenic sulfur isotope that

SO42- in aerosols and rain water to determine the deposition

S analytical method for atmospheric samples. Using the high-

4

35

SO2 and

35

SO42- were conducted at coastal


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new and comprehensive analysis to quantify ambient SO2-to-SO42- conversion rates at coastal

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California and the Tibetan Plateau using yearlong high-sensitivity 35S measurements.

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2. Experimental Methods

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2.1. Sample Collection and Cosmogenic 35S Analysis

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High-sensitivity measurements of cosmogenic

35

S from two sampling sites (Figure 2a and

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Figure S1) are used for quantifying SO2-to-SO42- conversion rates. Scripps Pier Shore Station

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(32.85°N, 117.28°W, 10 m above sea level) (hereafter referred to as Scripps) is located at the

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Scripps Institution of Oceanography in coastal southern California, which is affected by

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anthropogenic emissions from the highly-populated Los Angeles area and busy ship-traffic in the

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polluted marine boundary layer.31-32 Samples at this site were collected from June 2009 to July

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2010. Nam Co Monitoring and Research Station for Multisphere Interactions (30.77°N, 90.98°E,

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4730 m above sea level) (hereafter referred to as Nam Co) is a relatively pristine background site

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located at the central Tibetan Plateau and influenced mainly by long-range transport of air

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pollutants from South Asia.33-34 Samples at this site were collected from November 2010 to

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December 2011 (except for January 2011 and the period of July-November 2011 due to

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operational issues). Total suspended particle (TSP) samples were collected on filter papers by

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high-volume aerosol samplers at an operation flow rate of ~1 m3 min-1, while ambient SO2 was

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trapped by KOH-impregnated backup filters installed in the same samplers. Glass-fiber and

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quartz filters (Whatman) were used in Scripps and Nam Co, respectively.29-30 Fractions of SO2

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adsorbed on filters that collect TSP were estimated as ~3% for both substrates.35 All samples

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were converted to aqueous SO42- in the University California San Diego and subjected to

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cosmogenic

35

S analysis using an ultra-low-level liquid scintillation counting spectrometer

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(Wallac 1220 Quantulus) technique developed by Brothers et al.28 The reported

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concentrations are corrected for blank and decay time. Detailed chemical analysis procedure can

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be found in the literature.28-30

S

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2.2. Quantification of SO2-to-Sulfate Conversion Rate Figure 1b illustrates the sources of

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SO2 and

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SO42- in the atmosphere. Because both

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SO2

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and 35SO42- are subject to dry and wet removal, the time-dependent variation of 35SO2 and 35SO42-

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in the boundary layer can be expressed as follows: ݀[ଷହ ܱܵଶ ] ‫[ ܨ‬ଷହ ܱܵଶ ] [ଷହ ܱܵଶ ] [ଷହ ܱܵଶ ] [ଷହ ܱܵଶ ] = ܲୈ + − − − − ݀‫ݐ‬ ‫ܪ‬ ߬௢௫ ߬ௌைଶି௪ ߬ௌைଶିௗ ߬ఒ

(1)

݀[ଷହ ܱܵସଶି ] ‫[ ܨ‬ଷହ ܱܵଶ ] [ଷହ ܱܵସଶି ] [ଷହ ܱܵସଶି ] [ଷହ ܱܵସଶି ] =݊× + − − − ݀‫ݐ‬ ߬௢௫ ߬ௌைସି௪ ߬ௌைସିௗ ߬ఒ ‫ܪ‬

(2)

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where [35SO2] and [35SO42-] represent 35S concentrations measured in SO2 and SO42- (unit: atoms

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m-3), respectively; PCR is the cosmogenic production rate of

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atoms m-3 d-1); F is the downward vertical flux of

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boundary layer (unit: atoms m-2 d-1) and H is the height of the boundary layer (unit: m); the

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coefficient n is the [35SO42-]/[35SO2] ratio in the free troposphere; τox is the oxidation lifetime of

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SO2 (unit: d); τSO2-w, τSO2-d are the wet and dry removal lifetimes of SO2, respectively, whereas

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τSO2-w, τSO2-d represent the wet and dry removal lifetimes of SO42-, respectively (unit: d); τλ is the

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decay lifetime of 35S (half-life / ln(2) = 126 d). In this equation system, it is assumed that 1) the

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boundary layer is well mixed and 35SO2 and 35SO42- concentrations are vertically uniform in the

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boundary layer; 2) horizontal mixing of 35SO2 and 35SO42- is neglected. Because the conversion

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of cosmic-ray-produced

35

S to

35

35

S in the boundary layer (unit:

SO2 from the free troposphere outside the

35

SO2 is of the order of ~1 s, the production rate of

6


35

SO2 in the

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boundary layer is limited by the production of 35S (PCR), while the production rate of

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controlled by the SO2-to-SO42- conversion ([35SO42-]/τox).

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To solve τox in the equation system, we utilize the measured monthly

35

35

SO2 and

SO42- is

35

SO42-

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concentrations. The SO2 and SO42- removal lifetime is adopted from the corresponding monthly

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values at the sampling sites simulated by the National Center for Atmospheric Research-

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Community Atmospheric Model (NCAR-CAM) in the Atmospheric Chemistry and Climate

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Model Intercomparison Project (ACCMIP).36 To evaluate the uncertainties, two versions (5.1

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and 3.5) and multiple-year data in the 21st century are used. A total of 10-year data (2000-2009)

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from the version 5.1 is used, while the version 3.5 provides 8-year data (2002-2009). It is noted

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that the removal lifetime depends on the height of the boundary layer (H), which is not directly

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measured in this study. The average height during summer measured by 9 sounding stations

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along the coastal California is ~800 m,37 while the annual average is reported to be ~500 m on

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the basis of radiosonde observations, meteorological reanalysis and climate models.38 The H

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value at the Tibetan Plateau is more variable, which is typically 1-2 km and can be developed to

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~4 km or several hundred meters in some extreme cases.39 At Nam Co, the 5-year average is

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reported to be ~1 km based on gridded meteorological data.40 Because H was not measured in

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this study, an arbitrarily imposed seasonal variation of H may introduce more uncertainties.

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Therefore, seasonal variability of H is not included in calculations but will be qualitatively

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discussed. We use 2 values for each station (Scripps: 500 and 800 m; Nam Co: 1000 and 3000

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m) to evaluate the uncertainty of calculated results. The average production rate of 35S in the box

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depends on the location of sampling site and the height of the boundary layer. The PCR for

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Scripps (H = 500 m: 1.6 atoms m-3 d-1; H = 800 m: 1.9 atoms m-3 d-1) and Nam Co (H = 1000 m:

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42.3 atoms m-3 d-1; H = 3000 m: 52.9 atoms m-3 d-1) are adopted from the calculation of Lal and

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Peters.41 The value of n is difficult to determine because 35SO2 and 35SO42- concentrations in the

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free troposphere have never been measured. In a 35S one-dimensional four-box model (consisting

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of the boundary layer, buffer layer, free troposphere and lower stratosphere) the [35SO42-]/[35SO2]

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ratio in the free troposphere is estimated to be 2.3 in California and 3.0 in East China.42-43

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Because of the relatively short chemical lifetime of SO2 in the boundary layer (compared to the

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free troposphere), the [35SO42-]/[35SO2] ratio in the boundary layer is usually larger (>3) than the

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free troposphere.30,

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boundary layer ( 0),

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solutions with τox < 0 are excluded. The total numbers of valid solutions for each month and

8


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station are summarized in Table S2. The calculated results using deposition data obtained from

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CAM 5.1 and 3.5 will be hereinafter simply referred to as CAM 5.1 and 3.5, respectively. One

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should keep in mind that only deposition data from CAM is used for calculations and the SO2-to-

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SO42- conversion rate reported in this study is not simulated by CAM. It is noted that a small

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fraction of

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impregnated backup filters. Because of the low volatility of sulfuric acid (boiling point: ∼330 °C;

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vapor pressure: 100%) exist in results

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from Scripps in winter (Figure 3). Because wet precipitation become more frequent in winter in

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coastal California,52 the uncertainties may be derived from a potential misrepresentation of SO2

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and SO42- wet deposition rates in CAM models at this region. A thorough discussion of the

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CAM-modeled SO2 and SO42- deposition data is beyond the scope of this study, but a previous

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study showed significant overestimations in ACCMIP-modeled SO42- wet deposition rates (more

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than twice than observed values) in coastal California,53 in part supporting our interpretation.

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Results using CAM 3.5 deposition data are ~24% lower than CAM 3.5 in Scripps but ~18%

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lower than CAM 5.1 in Nam Co (Figures 3 and S3). The discrepancy may be because CAM 5.1

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considers size-resolved deposition processes while CAM 3.5 does not.36

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The calculated SO2-to-SO42- conversion rates using different combinations of H and n values

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are summarized in Figures S4 and S5. Increases of H from 500 to 800 m (+60%) at Scripps lead

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to 26±11% and 30±28% decreases of SO2-to-SO42- conversion rates for CAM 5.1 and 3.5,

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respectively. Increases of n from 2 to 3 (+50%) at Scripps result in 55±19% and 62±21%

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decreases of SO2-to-SO42- conversion rates for CAM 5.1 and 3.5, respectively. The results show

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that the selection of ratio n significantly influences the quantification. Although no combined

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measurement of

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simulated by previous box-models (2.3 - 3)42-43 suggest that the uncertainties derived from this

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value would not be larger than 50%. For Nam Co, increases of H from 1000 to 3000 m (+300%)

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only lead to 32±12% and 42±7% decreases of SO2-to-SO42- conversion rates for CAM 5.1 and

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3.5, respectively. Increases of n from 2 to 3 (+50%) at Nam Co only result in 13±5% and 12±3%

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decreases of SO2-to-SO42- conversion rates for CAM 5.1 and 3.5, respectively. The results

35

SO2 and

35

SO42- has been made in the free troposphere, the range of ratio n

12


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suggest that our estimation is more reliable in Tibetan Plateau. Because H is usually higher in

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summer than winter, if the seasonal variation of H was measured and considered in calculations,

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the accuracy of our results may be improved, but the ability to acquire H accurately was not

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available in this study. Nevertheless, the seasonal pattern of SO2-to-SO42- conversion rates

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revealed in this study is unlikely to be altered as our previous sensitivity tests show that

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enormous changes of H (>1500%) are required to explain the observed winter-summer

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differences (Figure 3), which is dynamically unlikely.

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The last potential source of uncertainty is the production rate of 35S, which was calculated by

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Lal and Peters half a century ago using direct field-based observations and empirical

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parameterizations.41 The vertical production profiles of other cosmogenic isotopes (e.g., 7Be,

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10

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atmospheric cascade and yield functions,54-56 but a new state-of-the-art estimation of

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production rates remains absent. Given the similarity of

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(cosmic ray spallation of argon in the atmosphere), we investigate the sensitivity of calculated

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SO2-to-SO42- conversion rates to the 35S production rate on the basis of 36Cl results. The recently

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calculated global average production rate of 36Cl is twice the early estimation by Lal and Peters,

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with an uncertainty of 15%.54-56 We assume the production ratio of

290

(0.8),41 and find that doubling the 35S production rate in our calculations only lead to 8±17% and

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29±12% increases in the SO2-to-SO42- conversion rates at Scripps and Nam Co, respectively.

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Because the 35S production rate at the top of the boundary layer is approximately twice the value

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used in this study, this sensitivity test also indicates that ignoring the vertical variation of

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production rates in the boundary layer would not significantly affect the quantification results.

Be,

14

C and

36

Cl) have been updated basing on more comprehensive calculations of the

35

S and

295

13


36

35

S

Cl production pathways

36

Cl to

35

S is a constant

35

S


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3.4. Possible Factors Responsible for Fast SO2-to-SO42- Conversion Rates in Summer

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The SO2-to-SO42- conversion rates in the chemically active boundary layer can be controlled

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by a number of factors (e.g., temperature, solar radiation, concentrations of oxidants). A seasonal

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pattern with summer maximum and winter minimum found in both northern mid-latitude sites

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suggest that temperature and solar radiation may be important factors affecting the SO2-to-SO42-

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conversion rates at the seasonal scale. It is noted that this seasonal variation is similar to the

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result from the pioneering study conducted by Turekian and Tanaka in New England, but their

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value in summer seems unrealistically large (7 d-1), probably due to large uncertainties in early

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35

S measurements and several unconstrained parameters in their calculations.27

305

It is well known that the production rate of OH radical is directly linked to solar radiation,

306

especially in our sampling sites that possess relatively high O3 levels and humidity in most

307

time.40, 57-58 It is possible that the enhanced SO2-to-SO42- conversion rates observed in summer

308

are due to a more important role of OH oxidation in SO42- production. This interpretation is

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supported by the measurement of the 17O anomaly in SO42- (an isotopic signature for quantifying

310

the relative contribution of oxidation pathways involved in SO42- formation processes) made in

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coastal California.59 The

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subsequently cannot provide oxidation pathway information for summer.60 It is noted that at

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typical atmospheric OH levels at northern mid-latitudes in summer (2.5×106 radicals cm-3) the

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lifetime of SO2 with respect to OH oxidation is ~1 week, which cannot fully explain the fast

315

SO2-to-SO42- conversion rates observed in summer (~1 d-1). This result is consistent with a

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previous field measurement at Mace Head showing that at least one gas phase oxidation process

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(apart from OH oxidation) is apparently missing in the marine boundary layer, which contributes

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~5 times more than the OH oxidation.17 The study suggested that the halogen oxide oxidations

17

O(SO42-) in the Tibetan Plateau is only measured in spring and

14


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are kinetically unlikely because of their slow reaction rates (at least 35 times slower).17 An

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oxidation by Criegee intermediates is possible but may be unlikely at present understanding

321

because of the low concentrations of precursors in the marine boundary layer (compared to

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forested environments).17, 61-62 Our results highlight again the importance of laboratory efforts to

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find out an oxidation mechanism accounting for the fast SO2-to-SO42- conversion rate observed

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in the terrestrial atmosphere.

325

Since the ambient SO2-to-SO42- conversion rate incorporates both gas- and aqueous-phase

326

oxidations in a chemically active environment, a potentially enhanced aqueous-phase oxidation

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pathway in summer should be considered. The seasonal variations of the

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coastal California do not support an enhanced role of aqueous H2O2/O3 oxidation in summer.59

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Transition-metal-ion-catalyzed oxidation by O2 is also unlikely due to its slow reaction rate in

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summer.63 A recent study showed that 33-50% of SO42- in the marine boundary layer is produced

331

through the HOX oxidation.15 The concentration of HOX is linked to photochemistry as a clear

332

diurnal (with maximum at noon and undetectable amount at night) is observed.64 Because the

333

SO2 and SO42- samples from Scripps were collected at the marine boundary layer (Figure S1), the

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HOX oxidation may contribute, in part, to the enhanced SO2-to-SO42- conversion rates during

335

summer. It is noted that the reaction may occur predominantly on sea spray aerosols in Scripps

336

due to the cloudless sky through most of summer in coastal southern Califonia.65 Given the fast

337

HOX oxidation (k = ~2×109 M-1 s-1; [HOX]aq = ~9×10-13 M),15 the SO2-to-SO42- conversion rate

338

should be limited by the SO2 uptake rate. The net uptake coefficient γ (unitless) at Scripps in

339

summer is therefore estimated to be of the order of 10-2, consistent with the value measured in

340

laboratory using synthetic sea salt.66 The higher aerosol loading in summer (Figure S6) may

341

provide more available surface areas for facilitating this reaction and the condensation of vapor-

15


17

O anomaly in the


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Page 16 of 28

342

phase H2SO4.31 Our results thus provide additional isotopic evidences for the important role of

343

the HOX oxidation in the marine boundary layer. Reactive halogen species have been observed

344

at Great Salt Lake in Utah, which mixing ratios are comparable to the marine boundary layer.67-68

345

Given that Nam Co Lake is the second largest salt lake at the Tibetan Plateau (surface area:

346

~1900 km2) and our SO2 and SO42- samples were collected at the shore of this lake, it is possible

347

that the HOX oxidation also plays an important role at Nam Co, especially a higher aerosol

348

loading is also observed in this region in summer.69 In the future, a proper modeling of the

349

chemical interaction between SO2 and all possible oxidants can validate our interpretation.

350 351

4. Summary and Outlook

352

In this study, we use the combined measurement of ambient 35SO2 and 35SO42- to quantify the

353

SO2-to-SO42- conversion rate (and even the uptake coefficient of SO2) in the terrestrial

354

atmosphere. The unique advantage of this method is its simplicity as it eliminates requirements

355

of accurately quantifying emission/production rates from numerous sulfur sources (Figure 1) and

356

measuring

357

aerosol samples.27 Interestingly, results from two sampling sites (coastal California and the

358

Tibetan Plateau) reveal rapid SO2-to-SO42- conversion rates in summer, suggesting that some

359

unconventional oxidation pathways (e.g. the HOX oxidation) might be more important than

360

previously thought. Further combined efforts in field missions, modeling and laboratory studies

361

should be encouraged in the future to understand the rapid gas-to-particle conversion rate of

362

sulfur in the chemically complex terrestrial atmosphere.

35

S in precipitation samples and other cosmogenic isotopes (e.g. 7Be and

210

Pb) in

363

Comprehensive field works can significantly reduce uncertainties in estimations.

364

Measurements of 35SO2 and 35SO42- in the free troposphere are particularly crucial because their

16


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35

365

concentrations greatly influence the budget of

S in the boundary layer, especially at low-

366

altitude regions due to the extremely low in-situ production rate of cosmogenic 35S. Field-based

367

observations of the boundary layer height and deposition rates can also improve the accuracy of

368

quantification. A recent study underlines the importance of incorporating 35S in global chemical

369

transport models to understand the stratosphere-troposphere exchange.58 Such simulations have

370

been done using cosmogenic 7Be, which are found to be technically simple and computational

371

low-cost.70 The additional advantage of 35S is its ability to quantify the SO2-to-SO42- conversion

372

rate as demonstrated in this study. Due to the single source of 35SO42-, a comparison of simulated

373

and observed 35S concentrations is a more reasonable and unambiguous way to constrain the SO2

374

oxidation rate in models. Since the physical and chemical properties of 35S are nearly identical to

375

stable S, such model development should be straightforward if the quantification of 3-

376

dimensional 35S production rates is improved.

377

Cosmogenic 35S is of high potential in laboratory studies. Although the 35S-labeled technique

378

has been used in laboratory experiments to understand a wide variety of biochemical and

379

biogeochemical processes for decades,71-73 it is not applied to investigate kinetics and

380

photochemical processes of sulfur compounds in the terrestrial and extraterrestrial atmospheres.

381

Due to the low-energy decay of

382

least 2 h and a high-temporal-resolution in-situ measurement is therefore impossible at present.28,

383

74

384

(~2×106 molecules; 10σ above the background counting rate).28, 74 A pilot study highlights that

385

this feature is particularly crucial to accurately quantify the slow uptake of SO2 onto some

386

minerals such as SiO2.75 The 35S-labeled technique will also allow comprehensive understanding

387

the mechanistic nature of complicated reactions in a system containing various sulfur-containing

Nevertheless, the advantage of

35

S (EMax = 167 keV), an accurate counting of

35

35

S requires at

S-labeled technique is its extremely low quantification limit

17



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Page 18 of 28

388

compounds such as the Criegee intermediate reactions with H2O, SO2 and DMS,61 and formation

389

of organosulfates (an important component in secondary organic aerosol in the terrestrial

390

atmosphere)76-78 and elemental sulfur aerosols (a dominant constituent in the atmospheres of

391

Venus, early Earth and Mars).79

392 393

Supporting Information

394

Pictures of sampling sites (Figure S1); a brief discussion of calculated downward fluxes of 35SO2

395

from the free troposphere (F) (Text S1 and Figure S2); detailed results of sensitivity tests

396

(Figures S3, S4 and S5; Tables S1 and S2); a time series of aerosol concentrations in San Diego

397

during 2009-2010 (Figure S6)

398 399

Acknowledgements

400

This paper is dedicated to the late Karl K. Turekian for his pioneering work and friendship. We

401

thank Lin Su for her guidance in obtaining CAM data, and Jiayue Huang for beneficial scientific

402

discussions on interpreting deposition data. Mang Lin acknowledges a fellowship from the

403

Guangzhou Elite Project (JY201303).

404 405

References

406 407 408 409 410 411 412 413 414 415 416

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and Tibetan Plateau: observed characteristics of aerosol mass loading. Atmos Chem Phys 2017, 17 (1), 449-463. 70. Liu, H. Y.; Considine, D. B.; Horowitz, L. W.; Crawford, J. H.; Rodriguez, J. M.; Strahan, S. E.; Damon, M. R.; Steenrod, S. D.; Xu, X. J.; Kouatchou, J.; Carouge, C.; Yantosca, R. M., Using beryllium-7 to assess cross-tropopause transport in global models. Atmos Chem Phys 2016, 16 (7), 4641-4659. 71. Hwang, C.; Sinskey, A. J.; Lodish, H. F., Oxidized Redox State of Glutathione in the Endoplasmic-Reticulum. Science 1992, 257 (5076), 1496-1502. 72. Thodeandersen, S.; Jorgensen, B. B., Sulfate Reduction and the Formation of S-35Labeled Fes, Fes2, and S0 in Coastal Marine-Sediments. Limnol Oceanogr 1989, 34 (5), 793806. 73. Haase, A. T.; Gantz, D.; Eble, B.; Walker, D.; Stowring, L.; Ventura, P.; Blum, H.; Wietgrefe, S.; Zupancic, M.; Tourtellotte, W.; Gibbs, C. J.; Norrby, E.; Rozenblatt, S., NaturalHistory of Restricted Synthesis and Expression of Measles-Virus Genes in Subacute Sclerosing Panencephalitis. P Natl Acad Sci USA 1985, 82 (9), 3020-3024. 74. Lin, M.; Wang, K.; Kang, S.; Thiemens, M. H., Simple Method for High-Sensitivity Determination of Cosmogenic 35S in Snow and Water Samples Collected from Remote Regions. Anal Chem 2017, 89 (7), 4116-4123. 75. Hill-Falkenthal, J., Investigations of Atmospheric Sulfur Cycle Oxidation Using Stable and Radioactive Isotopes. University of California, San Diego: 2014. 76. Froyd, K. D.; Murphy, S. M.; Murphy, D. M.; de Gouw, J. A.; Eddingsaas, N. C.; Wennberg, P. O., Contribution of isoprene-derived organosulfates to free tropospheric aerosol mass. P Natl Acad Sci USA 2010, 107 (50), 21360-21365. 77. Surratt, J. D.; Gomez-Gonzalez, Y.; Chan, A. W. H.; Vermeylen, R.; Shahgholi, M.; Kleindienst, T. E.; Edney, E. O.; Offenberg, J. H.; Lewandowski, M.; Jaoui, M.; Maenhaut, W.; Claeys, M.; Flagan, R. C.; Seinfeld, J. H., Organosulfate formation in biogenic secondary organic aerosol. J Phys Chem A 2008, 112 (36), 8345-8378. 78. Riva, M.; Tomaz, S.; Cui, T. Q.; Lin, Y. H.; Perraudin, E.; Gold, A.; Stone, E. A.; Villenave, E.; Surratt, J. D., Evidence for an Unrecognized Secondary Anthropogenic Source of Organosulfates and Sulfonates: Gas-Phase Oxidation of Polycyclic Aromatic Hydrocarbons in the Presence of Sulfate Aerosol. Environ Sci Technol 2015, 49 (11), 6654-6664. 79. Kumar, M.; Francisco, J. S., Elemental sulfur aerosol-forming mechanism. P Natl Acad Sci USA 2017, 114 (5), 864-869.

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Figure Captions :

657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672

Figure 1 Schematic graph showing main sources of (a) stable and (b) cosmogenic sulfate in the troposphere. DMS, MSA and DMDS stand for dimethyl sulfide (CH3SCH3), methane sulfonic acid (CH3SO3H) and dimethyl disulfide (CH3SSCH3), respectively. The shaded circles are particle-phase while others are gas-phase. Figure 2 (a) Map showing locations of Scripps and Nam Co. The topographic base map is obtained from the National Centers for Environmental Information of the National Oceanic and Atmospheric Administration (https://www.ngdc.noaa.gov/mgg/global/). (b) Monthly averaged concentrations of 35SO2 (black) and 35SO42- in TSP (orange) and (c) ratios of [35SO2]/([35SO2]+[35SO42-]) measured in Scripps and Nam Co. Figure 3 Time series of monthly SO2-to-SO42- conversion rates in (a) Scripps and (b) Nam Co calculated using deposition data from CAM 5.1 (red) and 3.5 (blue). The error bar represents one standard deviation. The dotted curves represent the fitted second-order polynomial regression trendlines.

25



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Figure 1 Schematic graph showing main sources of (a) stable and (b) cosmogenic sulfate in the troposphere. DMS, MSA and DMDS stand for dimethyl sulfide (CH3SCH3), methane sulfonic acid (CH3SO3H) and dimethyl disulfide (CH3SSCH3), respectively. The shaded circles are particle-phase while others are gas-phase.


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(A)

(B)

(C)

Figure 2 (a) Map showing locations of Scripps and Nam Co. The topographic base map is obtained from the National Centers for Environmental Information of the National Oceanic and Atmospheric Administration (https://www.ngdc.noaa.gov/mgg/global/). (b) Monthly averaged concentrations of 35SO2 (black) and 35SO42- in TSP (orange) and (c) ratios of [35SO2]/([35SO2]+[35SO42-]) measured in Scripps and Nam Co.



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(A)

(B)

Figure 3 Time series of monthly SO2-to-SO42- conversion rates in (a) Scripps and (b) Nam Co calculated using deposition data from CAM 5.1 (red) and 3.5 (blue). The error bar represents one standard deviation. The dotted curves represent the fitted second-order polynomial regression trendlines.


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