JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D3, PAGES 6911-6918, MARCH 20, 1996



# Atmospheric

sulfur cycling

in the tropical Pacific marine boundary layer (12øS,135øW):

A comparison

of field data and model results 2. Sulfur dioxide

S. A. Yvon 1 and E. S. Saltzman

Rosenstiel

School of Marine

and Atmospheric

Science, University

of Miami,Miami, Florida



### Abstract. The atmospheric chemistry.

of sulfur dioxide over thetropical

South Pacific Ocean is investigated

by usingresults

from field measurements and numerical

models. Simultaneous real 

### time measurements

of sulfur dioxide and its biogenic

precursor

dimethylsulfide

weremade

at 12øS, 135øW for a 6-day

period from March 3 through

March 9, 1992. The meanSO2 and DMS 

## mole fractions were 71 _+ 56 pmol mol 4 (lc0and 453 _+ 93 pmol mo1-1 (lc 0 respectively.

These concentrations

are compared

to those predicted

by a time-dependent

photochemical

box model

of 

### the marine

boundary

layer. Modelestimates

of the yieldof SO2

fromDMS oxidation

range from 27% to 54%. Even

with lowyields,

DMS is the dominant

source of SO• in this region. Estimates

of vertical

entrainment

velocities

based onthetropospheric

ozone budget suggest that vertical entrainment is a minor source of SO:. The relative rates of various loss mechanisms for 

### SO: are dry deposition

to the sea

surface (58%), in-cloud

oxidation

(9%), OH oxidation

(5%), and uptake

by sea-salt

aerosols (28%).

Introduction

Dimethylsulfide (DMS) is ubiquitous in the marine 

### atmosphere.It is produced in the surface

ocean by certain 

### species of phytoplankton

and is emitted to the atmosphere through air/seaexchange

processes.

In the marine boundary layer, DMS is oxidized

by OH radicals

to produce

sulfuric acid, 

### methanesulfonic

acid (MSA), and possibly dimethylsulfoxide (DMSO) and dimethylsulfone (DMSO2) [I/in et al., 1990a, b, 

### andreferences

therein]. The relative

yields of these products in 

### the remote

marine boundary

layeris not well established owing to the difficulties

in creating

a laboratory

environment

that is suitably similar to that found

over the remote

oceans. 

### Sulfurdioxide(SO2) is an intermediate in the production

of 

### sulfuric acid from DMS oxidation

in the marine

boundary

layer. It is this SO2 and its oxidation product sulfuric acid that are 

### thought to be the major sources of non-sea-salt

sulfate(nss sulfate) in aerosols

in the remote marine boundarylayer. Oxidation

of DMS is the largest

biogenic source of SO2 in the 

### remote marine boundary

layer. Hydrogen

sulfide (H2S), another biogenic reduced sulfur gas, also oxidizes to form SO2. However,

H2S is present

only at very low concentrations over the remote open ocean and is not considered to be a significant


### source of SO2[Saltzman

and Cooper,

1988;l/yon

et al., 1993]. Recently,simultaneous measurements

of atmospheric

DMS 

### and SO2 have been made in the marine boundary layer [Berresheim

1987; Andreaeet al., 1988; Ferek et al., 1991; l/yon

1Now at National Oceanic and Atmospheric

Administration, Climate Monitoring

and Diagnostics

Laboratory,

Boulder, Colorado.

Copyright1996 by the American Geophysical

Union.

Papernumber

95JD03355. 0148-0227/96/95 JD-03355505.00 

### et al., 1991; Bandyet al., 1992;Huebert et al., 1993; Thornton 

### et al., 1995]. Withtheexception ofthose ofFerek et al. [1991], thesemeasurements

have shown

surprisingly

low SO2 mole fractions given theamounts

of DMS present.

The measurements reported by Fereket al. [1991

] were made

from an aircraft

in the


### same region andduring

the same time period

asthe shipboard measurements

reported by l/yon et al. [1991] and Bandyet al. 

### [1992] aspart of the Pacific

Stratus/Sulfur

Investigation

(PSI3). The above

mentioned

studies focused on the spatial

rather than temporal

distributions

of these species. Of the various

methods 

### employed

in the studies

mentioned

above, only the methods

used 

### by l/yon et al. [1991]

and Bandy et al. [1992]utilizesampling times that are short

enough to allow the results

to be examined for diel cycles

and short

timescale

trends. 

### Simultaneous

measurements

of atmospheric

DMS and SO2

were made during a recent cruise aboard the R/V John V. 

### Vickers in the equatorial

Pacific Ocean. This field campaign
was part of the International Global Atmospheric 

### Chemistry/Marine Aerosols and Gases Experiment (IGAC/MAGE). A station at 12øS, 135øW was occupied

for 6 days (Juliandays

64.0-70.5,GMT) in March 1992 in order to 

### study the diel cycles

of these and other

traceatmospheric

and surface seawater species. Theresults

of these atmospheric

DMS and SO2measurements are investigated

by using a gasphase photochemical

box model [l/yon and $altzman,

1993].



### Experimental Methods

The sample

inlets were mounted

approximately

15 m above
sea level on the forward facingside of a laboratory van which


### wasbolted

to the deck

of the flying

bridge. The van

waslocated forward of the ship's

stacks. The ship

was positioned

with its 

### bow facing into thewindwhilewe wereat the station to prevent contamination

of the air samples

by stack gases.Samples

were6912 YVON AND SALTZMAN: ATMOSPHERIC SULFUR CYCLING, 2, SULFUR DIOXIDE



### taken only when thewind wascoming

from forward of the beam. Atmospheric

DMS was measured

by using an automated

gas 

### chromatograph

with flame photometric detection

[Cooper and Saltzman,

1993;t'von

et al., this issue]. SO2

wasmeasured

with


### an automated technique involving high pressure liquid chromatography

with fluorescence

detection. The method and instrument

are described,

in detail, in Saltzrnan

et. al. [1993]. A 

### brief description

of the analytical

technique

is given here. A sample

air stream

is drawn at 1 standard

liter per minute through a coarse (10 !•m) Teflon particle filter and a Nafion membrane

dryer. The resulting

dry airstream

is then drawn 

### through a gas/liquid

exchange

coil. The gas phaseSO2 is absorbed

into an aqueous

absorber solution containing

10 gM 

### formaldehyde

and 0.84 mM Na2EDTA.The resulting solution is then serially

mixed with (1) ethanolamine in a pH 9 borate buffer,(2) orthopthalaldehyde (OPA), and (3) a 2.5-M sodium acetate solution buffered at pH 5.7. At pH 9, Siv exists 

### primarily

as SO_• =, whichreacts

with ethanolamine

andOPA to form a fluorescent isoindole derivative. After the addition of the OPA, the reaction

mixture flows through a 2-min mixing coil. The pH is then lowered throughthe addition of the strong sodiumacetate solution. This decrease in pH slowsfurther reaction with the OPA and reduces degradation

of the column packing material. The aqueous solution containing the derivative

flows continuously

through a 600-[tL injection

loop. A sample integrated

over 1.5 min is injected onto a reversed phase C-18 HPLC column once every 10 min. The chromatographically

separated

derivative is then detected by using a Hitachi F1000 fluorescence spectrophotometer

with excitation

wavelength

at 330 nm and emission wavelength

at 380 nm. The instrument

blank or zero pointwas obtained by drawing outside air through

a carbonate-impregnated filter. Calibration
was carried out by adding gas standards downstream

of the carbonate

filter. Gasphase

standards

were generated

by passing dry nitrogen

(100 standard

cubic centimeters

per minute)overa wafer-type

permeation

device held at 40øC. A variable fraction of the total gas

flow over

the device

was pumped

to waste via a
massflow controller

and vacuum

pump. The remainder of the gas flow was directed to the inlet of the instrument during calibration. This technique allows the delivery of low-level sulfur dioxide standards

without passage of the analyte

through a mass flow controller,

where large losses

have been found to


### occur. Using a 2-ng/min permeation tube, we were able to 

### generate mole fractions as low as 10 pmol mol 4 inthe airstream being sampled. The calibration systemalso providesthe capability

to spike ambient

air samples

to carry out standard addition tests. The lower limit of detection for this instrument is 

### approximately

7 pmol mol 4 under normal operating conditions. Because of excessive

fluorometer

noise duringthis cruise, the detection limit during the course of this experimentwas 

### approximately

20pmol mol 4, and the accuracy was +(10-15)%. The results of a recent airborne intercomparison of the 

### different techniques for measuring SO2 during the NASA/GTE/CITE 3 project indicated that the methods

most 

### commonly

used for measuring

the low levels SO2

foundin the remotemarine boundary layer are not reliable [Gregoryet al., 

### 1993]. The SO2instrument usedin this study

was not part of this intercomparison.During CITE 3, very little agreement between the various instruments involved could be seen in their results. However, results from this instrument

agreed well with results from measurements

made by using an isotope dilution 

### gas chromatography/mass spectrometry

(GC/MS) technique during the PSI 3 project

[t'von et al., 1991;

Bandy et al., 1992]. Results and Discussion

The measurements

made at the MAGE station are shown as a 

### timeseries

in Figure la. Themean

DMS mole

fraction was 453 

### pmol mol '1 (+93 pmol mo1-1, 1 (•) with a pronounced

diel cycle of 85 pmol mol '1. There is a slightly

decreasing trend inthe DMS mole fractions approaching

Julian day 66 and anincreasing

trend toward the end of Julian

day 67. This dataand simultaneous seawater

DMS measurements

are discussed

in detail by t'von et al. [this issue]. 

### The mean SO2 mole fraction at thesite

was 71 pmol mol '1 (+56 pmol mol '1,1 (•). The lack of SO2 data near the beginning and end of the time series is due to instrument problems. 

### Substantial

short-term

variability

canbe seen

in the data. SO2 

### appears anticorrelated

with DMS during

the first dayat the station (JD 64); however,

this trend does not continue 

### throughout

the time series. In fact, SO2 and DMS are positively correlated

on Julian

day67. In order

to further

examine diel variations in sulfur dioxide the measurements were sorted 

### according tothe time of day of each sample and averaged over 3- 

### hourintervals

(Figure lb). Averaged

in thisway,the sulfur dioxide data donotexhibit

a significant

diel cycle. Previous

measurements

of SO2 in this region have shown 

### mean mole fractions of54 pmol mol 4 using a GC/FPD technique

8OO

600

400

200

0 .

.

.

63 DMS



## i i,

64 65 66 67 68 69 Julian Day (local time) 

### (a)

I i

70 71

6OO

5OO

400

300

200

1. oo 107

_

- DMS

_

so
- 2 106



### :•: 89 102 

## =•= 88 • (b)

97

0 6 18 24 

# •6 60 • 60 • :I: 35 • • 31 61

12 Time of Day (hours) 

### Figure 1. Results

of measurements

of atmospheric

DMS and SO2mole fractions

at the MAGE station

(a) plottedas a time

### series (LT = GMT - 9 hours)

and(b) as data

averaged over time 

### of day. In Figure

1 b the local

time has been adjusted forthe fact that solar noon occurredat 1230 hours local time, and the error 

### bars indicate 1 standard error in the mean ((•/(nm)), where n is given above each averaged point.YVON AND SALTZMAN: ATMOSPI-IERIC SULFUR CYCLING, 2, SULFUR DIOXIDE 6913



### [Maroulis

et al, 1980] and approximately 50 pmol mol

q [IZegorov,

1990] using a modified West-Gaeke method with a 

### detection limit of 4 pmolmo1-1

and an accuracy

of +10% [Huebert et al., 1993]. These

values are -27% lower than the
mean observed

during this study. The mole fractions

of SO2 are quite low relative to the high DMS molefractions

observed.

If we assume

that (1) DMS is the only source

of SO2, (2) gas phase OH chemistry

controls both the production

and destruction

of DMS and sulfurdioxide, (3) SO2 is the principle productof DMS oxidation,and (4) SO2 and DMS are in steady

state, then one would expect an DMS/SO2 ratio of about 0.15, basedon the ratios of their respective OH rate constants.

The ratio observed

in this study

is approximately 6. High DMS/SO2 ratios have been seen in results from measurements

in the remote marine boundary layer be/bre [Berresheim

1987;Andreae et al., 1988; Huebert et al., 1993]. The measurements discussed by Huebert et al. [1993] were made in this region in 1990. The ratio of DMS/SO2 tbr that cruise was 18 with mole fractions of SO2 as discussed above. Berresheim

[1987] measured

DMS and SO2in the sub-Antarctic and Antarctic

Oceans. His results

suggest a DMS/SO2 ratio of 9. Andreae et al. [1988] measured DMS and SO2from an aircrai•
over the Northeast Pacific Ocean. Their results suggesta DMS/SO2 ratio of 6. It has been suggested that thesehigh ratios may be due to a low yield of SO2/•om the oxidationof DMS [Bandy et al., 1992]. Chameides and Stelson [1992] have suggested

that there is a large sink in the marine boundary layer that has been overlooked. Clearly, the assumptions cited above are not sufficientto explain the SO2 budget

in this region. The difference must lie either in assumptions about the yield of SO2 from DMS oxidation (as suggested by Bandy et al. [1992]) or in the importance

of heterogeneous

sinkstbr sulfur dioxide [Sievering et al., 1991, 1992; Chameides and Stelson, 1992]. Several workers haverecently

suggested

that uptake

of sulthrdioxide

by sea-salt aerosols may provide a large sink in the marine boundary

layerwhich has previously beenoverlooked

[Sievering et al., 1991, 1992; Chameides and Stelson, 1992]. In order to
assess the relative importance of varioussulthr dioxide sources and sinks

we have modified a photochemical model of marine boundary

layer chemistry

to include the processes thoughtto control the atmospheric

sul/hrdioxide

budget in this region.

Model Description



### A schematic

diagram of the SO2

budget in the remote

marine 

### boundary

layer is shown

in Figure

2. In this budget,

SO2is produced

via the OH-initiated oxidationof DMS and/•om the 

### vertical entrainment

of SO2 i•omthe i•ee troposphere into the boundary layer. The sinks

include gas phase oxidation

of SO2 

### via OH, uptake

andoxidation

in cloud droplets andin sea-salt 

### aerosols, and dry deposition

tothe sea surface. In this paper we use a photochemical

box model of themarine

boundary layer to simulate these processes and to assess their relative magnitudes under the conditions of the M_AGE cruise. The model used is 

### described in detail byYvon and Saltzman [1993]. A description of the gasphase

chemistry

and the conditions

used to describe 

### theM_AGE

cruise were given by Yvon et al. [this

issue] and are summarized

in Table1. A model

runusing test conditions

gives a midday

maximum

OH concentration

of 3.0 x 106

cm '3 and a 24-hourmean concentration of 7 x 105

cm '3. 

### One method for estimating

thephotochemical

production

rate 

### of SO2 is to takethe average

observed DMS diel profile

and SO

2 Free Troposphere 

# r.t•ca• Manne

Boundary Layer

ve entrainment

loud oxidation

+ OH scavenging._ ß

..... 

## DMS by

sea-salt "- .-;,.. 

## .••+ OH

.... ß 

# I :: t'.'.': :.:.. H2 SO4 dry

deposition

. ..,.:. - ..

Sea DMS 

### Figure 2. Diagram of the processes controlling the 

### concentration

of SO2 in the remote

marine boundary

layer.

calculate the DMS oxidation rate based on model OH 

### concentrations

and laboratory

rate constants.

For the datafrom 

### this study

this calculation

gives a diurnally

averaged DMS 

### oxidation rate of4.4 x 104 molecules cm '3 s 'l. However,

Yvon et al. [this issue] used the model to attempt to examine

the consistency

between the sea-to-air

flux of DMS (based

on 

### measured water column concentrations

and windfields)

and the 

### calculated

OH levels.

They demonstrated

that byusing currently accepted cross sections, quantum yields, andrate constants

it is not possible

to reproduce

the observed

meanDMS levelsandthe 

### amplitude

of diel variation. They concluded that the oxidation 

### of DMS must occurat roughly twice the predicted

rate, 

### approximately

8.0 x 10 4 molecules

cm -3 s -1. The difference between these twoestimates

introduces

anuncertainty

of a factor of 2 into the SO2

budget. 

### The SO2production

rate is alsosensitive

to assumptions aboutthe yield of SO2 tkom DMS oxidation.Estimates of the 

### yield based

on laboratory

studies varywidely,

from <50% to
more than 90%, with the more recent, low-NOx measurements 

### supporting

the highest

yields [Thornton

et al., 1995]. Most modeling studies have also assumed high yields of SO2, based on the high ratios of non-sea-salt sulfate to methanesulfonate in 

### marine aerosols [Saltzman et al., 1986]. Thisline of reasoning
assumes that other reservoirs

of methylated

sulihr, such as 

### dimethylsul/bxide

or dimethylsul/bne,

areminor. Bandy

et al. [1992] argued that the yield of SO2 ikom DMS is extremely

low, 

### based on fieldobservations

in the northeast Pacific. In this study
we attempt

to constrain

the yield of SO2 ikom our field data, based on independent estimates

of DMS oxidationand SO2 losses. 

### The vertical

entrainment

of SO2 fromthe ikee

troposphere

is estimated by examining the budget of 03. Given a free 

### tropospheric

03 mole ikaction of 30 nmol

mol 4 [Routhier

et al., 1980] and an observed boundary

layer meanmole fraction of 9.3 

### nmol tool ' (J. Johnson,

personal communication,

1992), the vertical entrainment

rate was calculated

at approximately

0.168 

### cm s -1 for a boundary

layer height of 1.30 km[Yvon et al., this issue]. The mole ikaction of SO2in the free troposphere is 

### assumed to be 90 pmol

tool 'l based on measurements

from this region as discussed

by Maroulis et al. [1980]. The source flux 

### of SO2 ikom the free troposphere

into the marine

boundary layer 

### is calculated

by using the 0.168 cm s

-1 entrainment

rate determined/?om the 03 difference.

### 6914 YVON AND SALTZMAN: ATMOSPHERIC

SULFUR CYCLING, 2, SULFURDIOXIDE

Table 1. Calculations. 

### Input Conditions for the Photochemical

Model O_•,

resulting in a high initial scavenging rate. Thisfast rate

Parameter Value Source

03, nmol mol 4 9.3
CO,nmol mol 4 60.0
CH4, ytmol mol 4 1.97 H202, pmol mol 4 580 NON, pmol mol 4 3
C2H4, pmol mol 4 200
C2H6, pmol mol 4 800

Temperature,

K 301.5

Total ozone

column density, mmol m'2 118 Relative humidity,% 72

0.87 1.1 x 10 '6 J. Johnson

(personal. Communication,

1992) J. Johnson

(personal. Communication,

1992) Bateset al. [ 1993] Thompson

et al. [1993] D. Jaffe (personal. Communication,

1992) Donahue and Prinn [1993] and Atlas et a/.[1993] Donahue and Prinn [1993] and Atlas et al. [ 1993

] J. Johnson

(personal. Communication,

1992) P. Newman

(personal. Communication,

1992) J. Johnson

(personal. Communication,

1992) Yvon et al. [this

issue] Yvon et al. [this

issue] Yvon et al. [this

issue] Yvon et al. [this

issue]

Lenschow

et al. [1982] calculated

Hegg [1985] Saltzman

et al. [1986] 

### DMS flux, yanol m '2 d 4 16 Boundary

layerheight,

km 1.30 

### Wind speed, m s 4 7.1 Verticalentrainment

velocity,
cm s 4 0.168 O3 deposition velocity, cm s 4 0.053 SO2 deposition

velocity,
cm s '1 In-cloud oxidation,

s 4 SO2 percent yield from DMS, % 94



### The sinks

for SO2 include (1) homogeneous

oxidation

by OH 

### radicals, (2) dry deposition,

(3) in-cloud

oxidation,

and (4) 

### uptake by sea-salt

aerosols.

Homogeneous

oxidation rates are determined from the model calculated OH concentrations as 

### discussed by Yvon and Saltzman [1993]. An SO2

drydeposition 

## velocity of 0.87 cm s -1 is assumed.

This value was obtained using the equation

for deposition

velocity (va)as given

by Liss 

### [1983] (va = Cso2U) where Cso2 is theunitless

drag coefficient which has a value of 1.22 x 1 if_a, asdetermined

from Hicks and 

## Liss [1976] and Garratt [1992] for awind speed of 7 m s 4, and U is the mean

wind speed

for MAGE station. The rate of in-cloud

oxidation

of SO2 was estimated

by using 

### the climatological

approach of Hegg [1985]. During

the MAGE cruise a camerawith a wide-angle lens (180 ø) was used to 

### photograph

the sky during daylight hours throughout

thecruise. However,

the chemistry

of the air mass

observed at the station 

### represents

the result

of conditions

overseveral

days priorto arrival. Hegg [1985] estimated an in-cloud

oxidation

rate 

### coefficient

for SO2 of 1.275 x 10 -6 s 4, forthe0

ø - 15øN latitude band. He recommended an adjustment

for the lower water 

### content of the Southern Hemisphereresulting in a rate 

### coefficient

of 1.1 x 10 '6s 4. This loss rate is considered

anupper limit for the MAGE study. 

### The loss of SO2 to sea-salt

aerosols was parameterized

in the model as a constantfirst-order loss. This loss rate was estimated 

### by using an approach

similar to thatof Chameides

and Stelson 

### [1992], who used a coupled

roplet/gas

phase chemistry model to study thescavenging

of SO2 by sea-salt

aerosols in the marine boundary layer. Their

time-dependent

calculations

demonstrated thatin a fresh

sea-salt aerosol droplet, S•v is rapidly oxidized by continues

until sufficient

sulfuric acid is generated

to lower the 

### pH below

approximately

6. This effect

requires sufficient uptake to break the carbonate buffer in seawater,

approximately 1/2 mole of SO2 perequivalent

of alkalinity

(HCO_a' + 2COif). As the pH decreases,

therate of oxidation

by O_a decreases,

and the oxidation

of S•vby tYee

radicals (OH and HO2) and H202 

### become more important.

At pH 4 and

below, H202 controls the 

### rate of uptake of SO2. Chameides and Stelson

[1992] demonstrated

thatmost

of the uptake

of SO2 occurs by reaction 

### with O_a t highpH, with only

about 30% occurring

by other oxidants. Because

the O3 reaction

is so rapid,they suggested 

### that the uptake of SO2 could bereasonably

approximated

as one half theflux of alkalinity

associated

with sea-salt

aerosols plus 30% for the lowerpH reactions. 

### This approach wasapplied

to the MAGE case

as follows. 

### The production

rate of sea-salt

aerosols i calculated

according to Monahah

et. al. [1986]asshown

here: 

# ]9(1') 3.585 X ]05 •r 1'(80) -3 ] 4' 0.0331'(80) 179

where 

# 0.38- log (r(80)) W = 3.84 x 10 -6 U 3'41 B = 0.65 

# 480)- 4•)0 - P•/•00) ø'• 0_0.80) ø.•

u is the wind speed (ms 4) at 10m above

the sea surface, r(80) is the radius(gm) at a standard relative humidityof 80% as determined

by using the equilibrium

radiusequation

given by Kasten [1969], RI-I is the ambient relative humidity

(percent), 

### and P(r)has units of m -2 s 4 grn 4. For the model calculations discussed in thispaper

we use the mean U 3'4• (Ux) for the MAGE station

to determine

the production

rate of aerosols.

The 

### value of •'4•/Ux is 7.3 m s 4 for the MAGE station. The scavenging

rate of SO2by theseaerosols is then calculated

as shown below: 

# 1 (Pdry)(rata•k)(N•4)r• (P(r) I scavenging rate- •- BLH

(Vol•)Ar (2)

where Pdry is the density of the dry particle [H•inel, 1976], ratal k isthe ratio (eq 84) ofequivalents

ofalkalinity

omass of sea-salt [Millero andSohn,

1992], N• is Avogadro's

number, r is the radiusof the aerosols at ambient

relative humidity,

P(1') is 

### the production

rate of aerosols

from equation

(1), BLH is the 

## boundary layer height (in meters), Voldry is the volume (in cubic centimeters)

of the dryparticle,

and Ar is thewidthof theradius interval, 0.05 gm. 

### The results from cascade impactorsamples

taken at the 

### station (1 sample

spanned 24 hours)

and analyzed

usingion chromatography

indicated thatthe sodium

mass wasfound

only
on aerosols

larger than 0.15 grn in radius (P. Quinn,personal communication,

1992). Therefore,

onlyaerosol

particles larger 

### than 0.15 [tm are used in the model calculations. Aerosols larger than 10.0 [tm havea short residence

time whichmay not 

### be longenough

to allowfor the uptake

of enough

SO2 to break the alkalinity

buffer, and extending

the distribution

to include 

### larger particles

does not significantly

increase the calculated scavenging

rate of SO2. Therefore

we adopt he sea-salt

aerosolYVON AND SALTZMAN: ATMOSPHE•C SULFUR CYCLING, 2, SULFUR DIOXIDE 6915



### size range 0.15-10.0

pm in radius

and obtain a scavenging

rate 

### of SO2 of4278 cm -3 s -t from equation (2) for the MAGE station. Increasing

this rate by 30% to account

forfree radical and H202 

## reactions yields a total rate of 5566 molecules cm -3 s 4.

Model Results

In this section

we discuss

four simulations

carried out with 

### the time-dependent

photochemical

model.Theresults

from all of the runs

are shown

in Figure

3, in which,

the predicted

SO2 

### diel profiles

are shown

with their respective

case numbers. There is only one DMS diel profile, and it remained

the same for all runs. The measured DMS and SO2 diel profilesare shown 

### with their error barsfor comparison to the results

from the
model. 

### Case 1. The first modelsimulation includes SO2losses

by gas phase reaction, in-cloud reactions,

and deposition

to the sea surface. Heterogeneous lossesto sea-salt aerosols are not 

### included.The modelpredicts a meanSO2

mole fraction

of 307 

### pmol mol -•(Figure 3,case 1). The results also show a diel cycle in SO2 that is anticorrelated to that of DMS. Both the mean 

### mole fraction

andthe amplitude

of the diel cycle

predicted

by the model are considerably largerthan the field results. In this

case the largest sourceof SO2 is from the oxidation of DMS 

### (>99%)with entrainment of free tropospheric

air providing

<1% of the SO2

in the boundary

layer. Dry deposition

is by far the largest sink (80%), while in-cloud oxidation

and reaction

with 

### OH contribute

only 13% and

7%, respectively,

toward the loss of SO2. Case2. Sievering

et al. [1991 and 1992] and Chameides and Stelson [1992] suggested that heterogeneous

loss to sea-salt 

### aerosols is a largesinkfor SO2,

which may help to explain the low mole fractions of SO2 that have been observed in the marine boundary

layer. Case 2 uses

the conditions

given in Table 1 

### with a scavenging

rate of 5566molecules

cm -3s 'l for SO2

as calculated

from the alkalinity

flux in the previous

section. The 

### lossof SO2

to sea-salt

aerosols is a minor sink for gas phase SO2, reducing the predicted

SO2 mole fractions by approximately 8% compared

to case 1. The resultsof this model run show a 

### mean SO2 mole fraction of 283 pmol

mo1-1 withan afternoon 

### maximum of 352pmol

mo1-1 and a predawn

minimum of 214

6OO

- DMS



# 400

_ 

# 300 200


# 100

0 0 6 12 18 24

Time of Day (hours)
Figure 3. Results

from model calculations for cases

], 2, 3, and 

### 4. The upper curveis the model-predicted DMS diel profile, and the bottom 4 curves are the SO2 diel profiles for the different cases. The solid

circles represent

the average

field data shown in Figure 1

b. 

### pmol mor t (Figure 3, case 2). Although

Chameides

and Stelson [1992] concluded

thatthe scavenging

of SO2 by sea-salt

aerosols 

### is globally

important

as a sinkfor SO2

in the marine

boundary layer, it does

not appear

to be fast enough

at this siteto account 

### for the low molefractions observed during this study. Case 3. Another explanation for the presence

of such low 

### mole fractions of SO2 in the marine

boundary

layer wasproposed by Bandy et al. [1992]. They observed low mole fractions

of 

### SO2 relative to the DMS mole fractions and variability

in DMS which was not reflected in SO2 off the coastof Washington. They suggest

hat the reactionof DMS with the OH radical does not produce SO2 in significant quantities under low-NOx conditions. In this scenario the SO2 found in the marine boundary

layer would originate

solely from vertical

mixing. For
case3, we set the yield of SO2tkom DMS equalto zero. The 

### predicted mean SO2 mole fraction is <1 pmol

mol -• (Figure 3,
case 3). This iswell below

the observed mean of 71 pmol

mol '• for the MAGE station. Therefore it appears unlikely that entrainment

is the only source

of SO2 to the marine boundary layer. Also, the recent results of Thornton

et aI. [1995] do not supportthe hypothesis presentedby Bandy et al. [1992]. Thornton

et al. [1995] observed anticorrelated

SO2 and DMS 

### diel cycles

in the remote

marineboundary

layer and suggests that SO2 wasproduced

fromDMS with an efficiency of 75%. Case 4. In Case4 we adjusted the yield of SO2from the oxidation

of DMS so that the modelwould produce SO2mole fractions with a mean similar to the observed mean mole fraction 

### of71pmol

mol '• (Figure 3, case 4). This process requires a 27% yield of SO2 from the total oxidationof DMS. This is substantially

lower than the 75% suggested by Thornton

et al. 

### [1995]. However,

the oxidation

of DMS in the model

described here is twice that due to oxidation by OH alone in order to model the observed

DMS diel cycle [Yvonet al., this issue]. If this rate were adjusted to that produced

from the oxidation

of 

### DMS by just OH while maintaining the observed

DMS mole fractions,

the resulting

yield of SO2 from this oxidation would haveto be doubled

(-54%) to maintainthe observed SO2mole tkactions.

This higher SO2

production

efficiency

is muchcloser to that suggested

by Thornton

et al. [1995]. Assuming

a 27% yield and

the DMS oxidation

rate suggested by Yvon et al. [this issue],

a diel cycle

in SO2 is predicted

by the model which is not observed in the data with an early morning 

### minimum of 54 pmol

mol '• and an afternoon maximum 94pmol mol -•. The diftbrence

between the model-predicted

diel SO2 profile and the observed SO2 profile may be due to a diel variation in vertical entrainment that is not included in the model. Given the observed SO2 mole fractions, this diel variation shoulde

•xhibita maximum at night and a minimum 

### during the day. The SO2

molefractions

measured

at the station show a slightincrease

beforedawn. The only source of SO 2 at 

### this time of day is vertical entrainment. There is no meteorological

evidence tkom the radiosondes

taken during

this cruise for a strong

diel cycle

in vertical

entrainment

[Yvon et al., this issue]. Given the conditions

of case 4, the SO2 budget

would be as shown in Table 2. The OH-initiated oxidation of DMS is the 

### largest source of SO2 with 98% of the total production of SO2 coming from this source

even with a yield of SO2of just 27%. 

### The loss of SO2 appears to be controlled

by drydeposition

(58%) and scavenging

by sea-salt

aerosols (28%). In-cloud

oxidation andhomogeneous

oxidation

appear to be minorsinks. 

### The SO2 budget shown in Table 2 is fairly insensitive to the known uncertainties involved with our model calculations. For 

### example, the calculations

of the rate of oxidation

of SO2 by OH

### 6916 YVON AND SALTZMAN:

ATMOSPHERIC

SULFUR CYCLING,

2, SULFUR

DIOXIDE

Table 2. Magnitudes

ofVarious

Sources and Sinks for SO2 for the MAGE Station and Their Percent Contributions.

Production relative contribution 

### Sources (molecules

cm '3 s 4) (%)

DMS oxidation 2.17x104 98 (27% yield) Vertical entrainment 4.84 xl 02 2

Destruction relative contribution 

### Sinks (molecules

cm '3 s 'l) (%)

OH oxidation 1.02xl 03 5 

### Dry deposition 1.1

lx104 58 In-cloud oxidation 1.90xl 03 9 Sea-salt aerosols 5.57xl03 28



### radicals has the following

sources of error (1) the calculated

OH concentrations

and (2) the experimental error associated

with the 

### determination

of the rate constants. Thompson and Stewart [1991] have estimated that the error in predicting OH concentrations with a model similar to the one used here is 

### -•25%(1 (•). The rateconstant

for the OH + SO2

reaction has an uncertainty of-50%. A 50% increase in the rate of 

### homogeneous

oxidation

of SO2 would result in a predicted

yield 

### of SO 2 of 28% from

the daytime

oxidation of DMS, and

the contribution

of homogeneous

oxidation

to the total lossof SO

2 would become 7%. Because vertical entrainment is such a small 

### (2%) source

of SO2, even largeuncertainties

in this parameter would have little or no effectonthe budget. 

### The uncertainty

in the dry deposition

of SO2 to the sea

surface is a function

of the uncertainty

in the drag

coefficient

used. This 

### dragcoefficient

has an error of <20% [Gatwatt, 1977; Gatwatt, 1992]. A 20% decrease in the deposition

velocity wouldresult 

### in the following:

(1) a 24% yield of SO

2 from daytime

DMS oxidation,

(2) dry deposition

equal to 52% of the total lossof SO2, and(3) scavenging

by sea saltconstituting

31% of thetotal loss of SO2. The valuefor in-cloudoxidation is givenhere as an 

### upper limit owing to the 1oxv numbersof cloudspresent. Changes in this parameter

by evena factor

of 2 wouldresultin 

### the following:

(1) there would be a 29% yieldof SO2

fromDMS 

### oxidation,

(2) in-cloud

oxidation would become 17% of thetotal lossof SO2,and (3) dry deposition and scavenging

by sea-salt aerosolswould become 53% and 25% of the total loss of SO2. The largest

source of uncertainty

in the determination

of the loss of SO2 to sea-salt aerosols is the error associated with the 

### determination

of the flux of alkalinity [Monahah, 1986]. Monahan [1986] does

not give any estimates

of error associated with his sea-salt

aerosol production

model. However, he does compare globally averaged sea-salt fluxes calculatedby his model with flux estimates from other sources

[Eriksson,

1959; Blanchard, 1963; Blanchard, 1985]. If the alkalinity flux used in the modelwere doubled, the predicted

yield of SO2

fromthe oxidation of DMS would become 34%, the relative contribution of dry deposition

would decrease

to 45% of the total loss, and there would be a corresponding increase to 43% in the contribution

of scavenging

by sea-salt

aerosols.

If the alkalinity flux were halved,the yield of SO2from the oxidation of DMS would become

24%, the relative contribution of dry deposition would increase to 67% of the total loss, and there would be a decrease

to 16% in the contribution of scavenging

by sea-salt aerosols. The uncertainties mentioned above do not affect the 

### majorconclusions of this modeling

study. Dry deposition

and sea-salt scavenging

remain the dominant

sinks for SO2,and a substantial

fractionof the DMS being oxidized does not appear to produce

SO2. Non-sea-salt sulfate concentrations. If we accept the 

### budget shown in Table 2, we can

calculate the expected

steady state aerosol

nss sulfate concentrations (Table 3). The nss sulfate concentration

is determined

by using the following equation: 

# [nss sulfate] = (Rox)(•racro1⁄2 1-•)(M)/106 ?)/106 cm3) (

where Rox is the rate (molecules

cm '3 s -1) ofoxidation

ofSO2, Zaero is the residence

time (seconds)

of the aerosol,

NA is 

### Avogadro's

number, M isthe molecular weight (96g mo1-1)

of sulfate, andthe last two numbers are for converting

the unitsto 

### [tg m -3. Thesulfuric

acid produced from homogeneous

andincloud oxidation of SO2 is likely to be incorporatedinto submicron

aerosols which have a long lifetime. Assuming the 

### homogeneous

production

rate of H2SO4

shown in Table2 anda 5.0 to 10.0-day

residence

time for submicron

aerosols [Wameck, 

### 1988], approximately

0.06-0.14 [tg m '3of nss sulfate should be produced.

In-cloud oxidation

of SO2should

produce 0.13-0.26 

### [tg m -3 ofnss sulfate. The supermicron

sea-salt aerosol particles

are assumed

to have a shorter

residence

time (-•2.5-5.0 days) 

### [Wameck,

1988]. The amount

of nss sulfate associated

with the loss of SO2 to these aerosols should be approximately

0.27-0.59 

### [tg m '3. The total nss sulfate produced from these pathways i 

### 0.46-0.99 [tgm -3, approximately

half themean measured total nss sulfate concentration

of 1.36 + 0.35 [tg m

'3 (P. Quinn, personal communication,

1992) fromthe station. However, the 

### measured

nss sulfate from the station

is approximately

twicethe
mean nss sulfate concentration measuredin this region by Huebertet al., [1993] and at Fanning Islandby Prospero

et al. [•9851. If we assume

thatthe products

from the remaining

73% of the DMS oxidized

eventually

produce nss sulfate

and use the 5.0 to 10.0-day residence

time assigned

to the submicron

particles,

the 

### nss sulfate t•om this source

should equal 4.04-11.07

[tg m '3. Given the measured

nss sulfateconcentration from the station,

it would appearthat 7-16% of the DMS oxidized produces nss sulfate without SO2 acting as an intermediate. This suggests that most of the remaining 47-57% of DMS oxidized doesnot produce nsssulfate. One of the oxidation products is MSA, but this is a minor product in tropicalair masses

[Saltzman

et al., 1986]. The other products formed are not known, but it has been suggested

that the OH-initiated oxidationof DMS may produce DMSO and DMSO2 with significant yields [Fin et al., 1990aand 1990b].

Table 3. Production of Aerosol Non-Sea-Salt Sulfate

nss Sulfate Produced 

### Pathway (•g rn '3)

Homogeneous

oxidation

of SO2
In-cloud oxidation of SO2 Sea-salt scavenging

of SO2

Total
Measured 0.06-0.14

O. 13-0.26

0.27-0.59

0.46-0.99
1.36 _+0.35YVON AND SALTZMAN: ATMOSPHERIC SULFUR CYCLING, 2, SULFUR DIOXIDE 6917

Conclusions

The results

of this study

indicate that SO2levelsat the field 

### site (!2øS, 135øW) in March 1992 were lower than those predicted

on the basis of gas phasechemistry alone, if it is assumed that DMS oxidation

yieldsprimarily SO2. Including 

### heterogeneous

sinks such asin-cloud

oxidation,

drydeposition

to the sea surface,

and losses

to sea-salt

aerosols significantly decreases

the predicted

SO2 mole fractions,

but these

sinks are insufficient to achieve the observed levels of SO2. These results suggest that the yield of SO2

from DMS oxidation

may be of the order of 27-54%. Under these conditions, DMS is still the dominant

source of SO2 in the marine

boundary

layer.

Aclmowledgments. We would like to thank Trish Quinn and Jim Johnson (both at NOAA/PMEL), Dave Covert (University

of Washington), Dan Jaffe (University

of Alaska),and Anne Thompson (NASA/GSFC). This work was supported in part by a grantfrom the National Science Foundation

(ATM-9120498), a NASA Climate and Global Change Fellowship

(NGT-30114), and the Marine Sulfur, Aerosoland Climate component

of the NOAA Climate

andGlobal

Change Program.

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### Thompson,

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of field data and model results, 1, Dimethylsulfide,

J. Geophys.

Res. this issue.



### E. S. Saltzman,

Rosenstiel

School of Marine

and Atmospheric Sciences, University of Miami, 4600 Rickenbacker

Causeway,

Miami, FL 33149. (email:

saltzman•rsmas.miami.edu) S. A. Yvon,National

Oceanic and Atmospheric

Administration, 

### Climate Monitoring

and Diagnostics

Laboratory,

R/E/CG1,

325 

### Broadway,

Boulder, CO 80303.

(email: syvon•cmdl.noaa.gov) (Received

September

12, 1994; revised March 26, 1995; accepted September

14, 1995)