bk-1989-0393.ch022

Chapter 22

Sulfur Isotope Ratios Tracers of Non-Sea Salt Sulfate in the Remote Atmosphere 1

Julie A. Calhoun and Timothy S. Bates

2

Downloaded by UNIV OF MINNESOTA on May 27, 2018 | https://pubs.acs.org Publication Date: April 27, 1989 | doi: 10.1021/bk-1989-0393.ch022

1

Department of Chemistry, University of Washington, Seattle, WA 98195 Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, WA 98115 2

The atmospheric biogeochemical sulfur cycle is being significantly impacted by increasing anthropogenic sulfur emissions. The effect of these emissions on the concentration of sulfate aerosol particles in the remote marine atmosphere is difficult to assess due to uncertainties surrounding the relative contributions of natural and anthropogenic sources. Sulfur isotope ratios can be used to determine the relative magnitude of these sources in the remote atmosphere provided 1) the isotopic ratios of the potential sulfur sources are known, 2) the isotopic compositions of the various sources are different from one another, and 3) the isotopic changes that occur during transformations are thoroughly documented. In the text which follows, these aspects of sulfur isotope chemistry are addressed. Isotopic interpretation of sulfur sources to the remote atmosphere is severely limited by the absence of critical isotopic measurements, yet it appears that continental sulfur sources are isotopically distinguishable from seasalt or marine biogenic sulfur sources. Improved analytical techniques will soon provide the means to obtain the necessary data. Concerns about the environment have drawn considerable attention to anthropogenic sulfur emissions, particularly their affects in remote areas (1). On a global scale, anthropogenic sulfur emissions from the combustion of coal and oil are now equivalent to those from natural sources (2). However, the relative importance of anthropogenic sulfur in the remote atmosphere is unclear. Biogenic reduced sulfur compounds from the ocean, particularly dimethylsulfide (DMS) from phytoplankton (3-6). are the major natural source of sulfur to the remote marine atmosphere. Other natural sources include biogenic sulfur emissions from the continents, sea spray and volcanic emissions (2). Understanding the relative importance of these sources to the remote atmosphere is crucial for determining the environmental significance of anthropogenic sulfur emissions. The presence of non-seasalt sulfate particles in the remote marine atmosphere has important environmental consequences. As a result of their size and hydrophylicity, sulfate particles make good cloud condensation nuclei 0097-6156/89/0393-0367S06.00/0 « 1989 American Chemical Society

Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

368

BIOGENIC SULFUR IN THE ENVIRONMENT

(CCN). Changes in the concentrations of C C N may alter the cloud droplet concentration, the droplet surface reflectivity, the radiative properties of clouds (cloud albedo) (2), and hence, the earth's climate (8-10). This mechanism has been proposed for the remote atmosphere, where the radiative properties of clouds are theoretically predicted to be extremely sensitive to tne number of CCN present QQ). Additionally, these sulfate particles enhance the acidity of recipitation due to the formation of sulfuric acid after cloud water dissolution U ) . The importance of sulfate aerosol particles to both radiative climate and rainwater acidity illustrates the need to document the sources of sulfur to the remote atmosphere. One means to assess the relative contributions of the various sources of sulfur to the atmosphere is through the use of sulfur isotope ratios (12-16). Isotopic ratios may be used as source tracers if 1) the isotopic composition of the sources as they enter the atmosphere are known, 2) the isotopic compositions of the various sources are different from one another, and 3) the isotopic changes that occur during biological, physical and/or chemical transformations are understood. Presently, isotopic data for sulfur compounds in the remote atmosphere (Table I) are limited. However, collection and analytical techniques are now available to make isotopic measurements of the critical species. In the text that follows, various aspects of sulfur isotope chemistry will be discussed.


P

Background Review of Isotope Concepts. The average relative concentration of the two most abundant stable isotopes of sulfur, S and ^ S , are 95.0% and 4.2%, respectively. Ratios of these two isotopes are measured with specialized isotope ratio mass spectrometers, normalized to standard Canyon Diablo meteoritic sulfur, and expressed as a delta value (S^S) according to the relation: 3 2

32

(^S/ S)sample «34 = [

1](1000)

S

(1)

(^S/^SJstandard 34

The unit for the calculated 6 S value is o/oo, which is expressed "per mil". The mass difference between the two isotopes of sulfur can alter the isotope ratio of a system during chemical and physical transformations (17-21) a process referred to as "fractionation." Quite simply, the zero point energy of a molecule, and hence its rotational and vibrational energy, is mass dependent (22). Therefore, molecules containing different isotopic masses will have different reactivities and fractionation will occur during mass dependent transformations. The degree to which the isotope ratio is fractionated differs for individual processes. The per mil isotope difference between the 6 S value for the reactant, R, and the product. P, at any instant in time is termed discrimination, D, where D = ^ ^ ( R ) - 6 S(P). By convention, if the lighter isotope reacts faster resulting in an isotopically lighter product, D is positive (14). Reactions may exhibit kinetic isotope effects as a result of the lighter isotope reacting faster. To illustrate this type of isotope effect, consider the oxidation of methane in the atmosphere (22). The oxidation is initiated by a reaction with the hydroxyl free radical (OH) in which O H irreversibly abstracts a hydrogen atom from the carbon. Methane molecules containing the lighter carbon isotope react faster, and as a consequence, the 6 C value of the product is lower. Although kinetic effects typically lead to positive discriminations, the discrimination from equilibrium interactions may be either positive or negative 34

34

13


22.

CALHOUN AND BATES

Sulfur Isotope Ratios

369

Table I. 6^S Values for Compounds Potentially Involved in the Production of Remote Atmospheric Sulfate «34 (in o/oo)

Reference

Seasalt Sulfate Seawater Atmospheric

+21 +21

Rees et al. (25) Luecke et al. (51)

DimethylsulfonioProprionate (DMSP)

+ 19.8

Fry and Andreae (pers. commun.)*

Dimethylsulfide Seawater Atmospheric

none none

Hydrogen Sulfide Terrestrial Biogenic Geothermal

-32 to-6


Compound

S

+5

+8.5 Sulfur Dioxide Volcanic Anthropogenic Power riant Plume Non-urban N.E. US Coal fired Power Plant Oil fired Power Plant Terrestrial Biogenic

-10 to +10 {-lto+6} Oto +6 -1.1 to +2.3 +5 + 1.3 to +3.6 -34 to-35

Atmospheric Aerosol Non-Seasalt Sulfate Non-Remote Marine { -10 to +13 } San Francisco Bay -10 to +12 N.W.Atlantic +7 to +13 HBEF, non-urban US +1 to + 4 Miami, Florida +1 to + 2 Mauna Loa Observatory + 4 to + 6 Rainwater Sulfate Non-remote Marine Atlantic Ocean Pacific Ocean New Zealand Coast New Zealand Coast Continental Non-industrial Japan Industrial Japan Pisa, Italy N. Amenca/Scandinavia Great Salt Lakes

Krouse et al. (61) Robinson et al. (56) Spedding et al. (55) Nielsen (54) Holtetal.(52) Saltzman et al. (21) Newman et al. (52) Newman et al. (5Q) Krouse et al. (61)

Ludwig(62)** Gravenhorst(62)*** Saltzman et al. (24) Saltzman (pers. commun.) Zoller & Kelly (pers. commun.)

+ 12 to+15 +9 to +16 + 16 -1.5 to+19

Chukhrov etal.(6Q) Chukhrov et al. (6Q) Spedding et al. (55) Mizutam et al. (66;

+ 14 +6 -3 to +7 +2 +2 to +8

Jensen et al. (M) Jensen et al. (£4) Cortecci et al. (65) Ostland (62) Nriagu et al. (63)

6**S value for seawater sulfate was 20.1 o/oo] ** Unusually low 634s values were reported for seawater sulfate, ranging from +9.7 to 17 teglecting value of -12 o/oo]


370

BIOGENIC SULFUR IN THE ENVIRONMENT

and they are often larger than those from kinetic reactions (22). A general rule for determining the sign of the discrimination is that heavier isotopes tend to concentrate in the more strongly bonded molecules (22). It should be noted, however, that exceptions to this rule exist for some species and that fractionation may vary with temperature. Consider, the equilibrium governed dissolution of sulfur dioxide (SO2) in cloud droplets or on the surface of wet aerosol particles: S 0 (gas) S 0 • H 0 (dissolved)

(2A)

S0

(2B)


2

2

2

• H 0 (dissolved) HS0 -(aq) + H+(aq)

2

2

3

which is followed by a fairly rapid aqueous phase (aq) oxidation, 2

HS0 -(aq)-->S0 - (aq) + H 3

+

(2C)

4

In this case, both equilibrium (reactions 2A,2B) and kinetic isotope effects (reaction 2C) are expected. A large negative discrimination has been observed tor reaction (2B), with the heavier isotope concentrating in the HSO3- (24). Kinetic isotope effects for reaction (2C), however, would produce sulfate which is isotopically lighter than the HSO3-. The equilibrium effects are probably larger than the kinetic effects (24), making the overall discrimination for the aqueous phase oxidation of S 0 negative. 2

Mass Spectral Techniques. Samples for isotope ratio analysis are typically converted to sulfates or sulfides, then to S6 (g) for analysis on a mass spectrometer (MS). The precision of the S 0 measurement is commonly reported as 0.1 to 0.2 0/00 (16.24). yet systematic errors of 1 0/00 or larger may result from 1) memory effects due to adsorption of S 0 on the walls of the MS, and 2) secondary isotope effects due to the existence of two stable isotopes of oxygen, 0 and 0 (25). Both of these errors can be eliminated by using SF rather than S 0 as the analyte in the MS (25.26). However, existing sulfur fluorination procedures are relatively dangerous and tedious, making the S F method less desirable as a routine environmental technique (2fi). Another procedure for sulfur isotope measurements has been developed where samples are converted to solid arsenic sulfide, AS0S3 (s), and measured by thermal ionization mass spectrometry (TIMS) (27). This technique offers several advantages over the gaseous methods in that both memory and isotope effects are eliminated, and the chemical procedure is simpler. A precision of 1 0/00, and the capability of making measurements on small samples, makes the TIMS technique competitive with gas phase MS techniques. 2

2

2

1 6

1 8

6

2

6

Sources of Sulfur to the Remote Atmosphere Biogenic Sulfur Emissions from the Ocean. The ocean is a source of many reduced sulfur compounds to the atmosphere. These include dimethylsulfide (DMS) (2.4.5). carbon disulfide (CS ) (2fi), hydrogen sulfide (H S) (22), carbonyl sulfide (OCS) (30.31). and methyl mercaptan (CH SH) (£). The oxidation of DMS leads to sulfate formation. C S and OCS are relatively unreactive in the troposphere and are transported to the stratosphere where they undergo photochemical oxidation (22). Marine H S and C H 3 S H probably contribute to sulfate formation over the remote oceans, yet the sea-air transfer of these compounds is only a few percent that of DMS (2). DMS in the ocean is produced during assimilatory sulfate reduction (ASR) by phytoplankton (Figure 1) (3.33). This involves the uptake of oceanic sulfate 2

2

3

2

2


22. CALHOUN AND BATES

Sulfur Isotope Ratios

371


ATMOSPHERIC OXIDATION

METHANE SULFONATE t

'II REMOTE ATMOSPHERE

D =? DMS(g) (small) S02 ) (g

I

D = -4t©0%

bk-1989-0393.ch022

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