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 the tropical South Pacific Ocean is 

investigated by using results from field measurements and numerical models. Simultaneous real 

time measurements of sulfur dioxide and its biogenic precursor dimethylsulfide were made at 

12øS, 135øW for a 6-day period from March 3 through March 9, 1992. The mean SO2 and DMS 

mole fractions were 71 _+ 56 pmol mol 4 (lc0 and 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. Model estimates of the yield of SO2 from DMS oxidation range from 

27% to 54%. Even with low yields, DMS is the dominant source of SO• in this region. 

Estimates of vertical entrainment velocities based on the tropospheric 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/sea exchange 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, 

and references therein]. The relative yields of these products in 

the remote marine boundary layer is not well established owing 

to the difficulties in creating a laboratory environment that is 

suitably similar to that found over the remote oceans. 

Sulfur dioxide (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 boundary layer. 

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; Andreae et al., 1988; Ferek et al., 1991; l/yon 

1Now at National Oceanic and Atmospheric Administration, 

Climate Monitoring and Diagnostics Laboratory, Boulder, Colorado. 

Copyright 1996 by the American Geophysical Union. 

Paper number 95JD03355. 

0148-0227/96/95 JD-03355505.00 

et al., 1991; Bandy et al., 1992; Huebert et al., 1993; Thornton 

et al., 1995]. With the exception of those ofFerek et al. [1991], 

these measurements have shown surprisingly low SO2 mole 

fractions given the amounts of DMS present. The measurements 

reported by Ferek et al. [1991 ] were made from an aircraft in the 

same region and during the same time period as the shipboard 

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

[1992] as part of the Pacific Stratus/Sulfur Investigation (PSI 3). 

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] utilize sampling 

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 (Julian days 64.0-70.5, GMT) in March 1992 in order to 

study the diel cycles of these and other trace atmospheric and 

surface seawater species. The results of these atmospheric DMS 

and SO2 measurements are investigated by using a gas phase 

photochemical box model [l/yon and $altzman, 1993]. 

Experimental Methods 

The sample inlets were mounted approximately 15 m above 

sea level on the forward facing side of a laboratory van which 

was bolted to the deck of the flying bridge. The van was located 

forward of the ship's stacks. The ship was positioned with its 

bow facing into the wind while we were at the station to prevent 

contamination of the air samples by stack gases. Samples were 
6912 YVON AND SALTZMAN: ATMOSPHERIC SULFUR CYCLING, 2, SULFUR DIOXIDE 

taken only when the wind was coming 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 was measured 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 phase SO2 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_• =,which reacts with ethanolamine and OPA 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 through the addition of the strong 

sodium acetate solution. This decrease in pH slows further 

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 point was obtained by drawing 

outside air through a carbonate-impregnated filter. Calibration 

was carried out by adding gas standards downstream of the 

carbonate filter. Gas phase standards were generated by passing 

dry nitrogen (100 standard cubic centimeters per minute) over a 

wafer-type permeation device held at 40øC. A variable fraction 

of the total gas flow over the device was pumped to waste via a 

mass flow 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 in the airstream 

being sampled. The calibration system also provides the 

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 during this cruise, the 

detection limit during the course of this experiment was 

approximately 20 pmol 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 found in the 

remote marine boundary layer are not reliable [Gregory et al., 

1993]. The SO2 instrument used in 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 

time series in Figure la. The mean 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 in the DMS 

mole fractions approaching Julian day 66 and an increasing trend 

toward the end of Julian day 67. This data and simultaneous 

seawater DMS measurements are discussed in detail by t'von et 

al. [this issue]. 

The mean SO2 mole fraction at the site 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 can be seen in the data. SO2 

appears anticorrelated with DMS during the first day at the 

station (JD 64); however, this trend does not continue 

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

correlated on Julian day 67. 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- 

hour intervals (Figure lb). Averaged in this way, the sulfur 

dioxide data do not exhibit a significant diel cycle. 

Previous measurements of SO2 in this region have shown 

mean mole fractions of54 pmol mol 4 using aGC/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 

SO2 mole fractions at the MAGE station (a) plotted as a timeseries (LT = GMT - 9 hours) and (b) as data averaged over time 

of day. In Figure 1 b the local time has been adjusted for the fact 

that solar noon occurred at 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 pmol mo1-1 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 mole fractions 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 sulfur dioxide, (3) SO2 

is the principle product of DMS oxidation, and (4) SO2 and 

DMS are in steady state, then one would expect an DMS/SO2 

ratio of about 0.15, based on 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 SO2 in the sub-Antarctic 

and Antarctic Oceans. His results suggest a DMS/SO2 ratio of 9. 

Andreae et al. [1988] measured DMS and SO2 from an aircrai• 

over the Northeast Pacific Ocean. Their results suggest a 

DMS/SO2 ratio of 6. It has been suggested that these high ratios 

may be due to a low yield of SO2/•om the oxidation of 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 sufficient to 

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 sinks tbr sulfur dioxide [Sievering 

et al., 1991, 1992; Chameides and Stelson, 1992]. Several 

workers have recently suggested that uptake of sulthr dioxide by 

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

boundary layer which has previously been overlooked [Sievering 

et al., 1991, 1992; Chameides and Stelson, 1992]. In order to 

assess the relative importance of various sulthr dioxide sources 

and sinks we have modified a photochemical model of marine 

boundary layer chemistry to include the processes thought to 

control the atmospheric sul/hr dioxide 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, SO2 is 

produced via the OH-initiated oxidation of DMS and/•om the 

vertical entrainment of SO2 i•om the i•ee troposphere into the 

boundary layer. The sinks include gas phase oxidation of SO2 

via OH, uptake and oxidation in cloud droplets and in sea-salt 

aerosols, and dry deposition tothe sea surface. In this paper we 

use a photochemical box model of the marine 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 indetail by Yvon and Saltzman [1993]. A description 

of the gas phase chemistry and the conditions used to describe 

the M_AGE cruise were given by Yvon et al. [this issue] and are 

summarized inTable 1. A model run using test conditions gives 

a midday maximum OH concentration of 3.0 x 106 cm '3 and a 

24-hour mean concentration of 7 x 105 cm '3. 

One method for estimating the photochemical production rate 

of SO2 is to take the average observed DMS diel profile and 

SO 2 

Free Troposphere 

r.t•ca• Manne Bounda

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 data from 

this study this calculation gives a diurnally averaged DMS 

oxidation rate of 4.4 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 wind fields) and the 

calculated OH levels. They demonstrated that by using currently 

accepted cross sections, quantum yields, and rate constants itis 

not possible to reproduce the observed mean DMS levels and the 

amplitude of diel variation. They concluded that the oxidation 

of DMS must occur at roughly twice the predicted rate, 

approximately 8.0 x 10 4 molecules cm -3 s -1. The difference 

between these two estimates introduces an uncertainty of a factor 

of 2 into the SO2 budget. 

The SO2 production rate is also sensitive to assumptions 

about the yield of SO2 tkom DMS oxidation. Estimates of the 

yield based on laboratory studies vary widely, 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]. This line of reasoning 

assumes that other reservoirs of methylated sulihr, such as 

dimethylsul/bxide or dimethylsul/bne, are minor. Bandy et al. 

[1992] argued that the yield of SO2 ikom DMS is extremely low, 

based on field observations 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 oxidation and SO2 

losses. 

The vertical entrainment of SO2 from the 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 mean mole 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 SO2 in the free troposphere is

assumed tobe 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, SULFUR DIOXIDE 

Table 1. 

Calculations. 

Input Conditions for the Photochemical Model O_•, resulting in a high initial scavenging rate. This fast 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) 

Bates et 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 layer height, km 1.30 

Wind speed, ms 4 7.1 

Vertical entrainment 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 dry deposition 

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 the unitless drag coefficient 

which has a value of 1.22 x 1 if_a, as determined from Hicks and 

Liss [1976] and Garratt [1992] for a wind 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 camera with a wide-angle lens (180 ø) was used to 

photograph the sky during daylight hours throughout the cruise. 

However, the chemistry of the air mass observed at the station 

represents the result of conditions over several days prior to 

arrival. Hegg [1985] estimated an in-cloud oxidation rate 

coefficient for SO2 of 1.275 x 10 -6 s 4, for the 0 ø - 15øN latitude 

band. He recommended an adjustment for the lower water 

content of the Southern Hemisphere resulting in a rate 

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

limit for the MAGE study. 

The loss of SO2 to sea-salt aerosols was parameterized in the 

model as a constant first-order loss. This loss rate was estimated 

by using an approach similar to that of Chameides and Stelson 

[1992], who used a coupledroplet/gas phase chemistry model 

to study the scavenging of SO2 by sea-salt aerosols in the marine 

boundary layer. Their time-dependent calculations demonstrated 

that in 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 per equivalent of alkalinity (HCO_a' + 2COif). 

As the pH decreases, the rate of oxidation by O_a decreases, and 

the oxidation of S•v by 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 that most of the uptake of SO2 occurs by reaction 

with O_at high pH, with only about 30% occurring by other 

oxidants. Because the O3 reaction is so rapid, they suggested 

that the uptake of SO2 could be reasonably approximated as one 

half the flux of alkalinity associated with sea-salt aerosols plus 

30% for the lower pH reactions. 

This approach was applied to the MAGE case as follows. 

The production rate of sea-salt aerosols icalculated according 

to Monahah et. al. [1986] as shown here: 

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

where 

W 0.38- log (r(80)) = 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 10 m above the sea surface, r(80) 

is the radius (gm) at a standard relative humidity of 80% as 

determined by using the equilibrium radius equation 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 this paper 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 SO2 by these aerosols is then calculated as 

shown below: 

scavenging rate- •- 1 (Pdry)(rata•k)(N•

(P(r)BLH I (Vol•)Ar (2) 

where Pdry is the density of the dry particle [H•inel, 1976], 

ratal kis the ratio (eq 84) of equivalents of alkalinityo mass of 

sea-salt [Millero and Sohn, 1992], N• is Avogadro's number, r is 

the radius of 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 dry particle, and Ar is the width of the radius 

interval, 0.05 gm. 

The results from cascade impactor samples taken at the 

station (1 sample spanned 24 hours) and analyzed using ion 

chromatography indicated that the sodium mass was found only 

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

communication, 1992). Therefore, only aerosol particles larger 

than 0.15 [tm are used in the model calculations. Aerosols 

larger than 10.0 [tm have a short residence time which may not 

be long enough to allow for 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 adopthe sea-salt aerosol 
YVON 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 of 4278 cm -3 s -t from equation (2) for the MAGE station. 

Increasing this rate by 30% to account for free 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. The results 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 profiles are shown 

with their error bars for comparison to the results from the 

model. 

Case 1. The first model simulation includes SO2 losses by 

gas phase reaction, in-cloud reactions, and deposition to the sea 

surface. Heterogeneous losses to sea-salt aerosols are not 

included. The model predicts a mean SO2 mole fraction of 307 

pmol mol -• (Figure 3, case 1). The results also show adiel cycle 

in SO2 that is anticorrelated to that of DMS. Both the mean 

mole fraction and the amplitude of the diel cycle predicted by 

the model are considerably larger than the field results. In this 

case the largest source of 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. 

Case 2. Sievering et al. [1991 and 1992] and Chameides and 

Stelson [1992] suggested that heterogeneous loss to sea-salt 

aerosols is a large sink for 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 5566 molecules cm -3 s 'l for SO2 as 

calculated from the alkalinity flux in the previous section. The 

loss of 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 results of this model run show a 

mean SO2 mole fraction of 283 pmol mo1-1 with an afternoon 

maximum of 352 pmol 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 curve is 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 that the scavenging of SO2 by sea-salt aerosols 

is globally important as a sink for SO2 in the marine boundary 

layer, it does not appear to be fast enough at this site to account 

for the low mole fractions observed during this study. 

Case 3. Another explanation for the presence of such low 

mole fractions of SO2 in the marine boundary layer was proposed 

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 coast of Washington. 

They suggesthat the reaction of 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 

case 3, we set the yield of SO2 tkom DMS equal to zero. The 

predicted mean SO2 mole fraction is <1 pmol mol -• (Figure 3, 

case 3). This is well 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 

support the hypothesis presented by Bandy et al. [1992]. 

Thornton et al. [1995] observed anticorrelated SO2 and DMS 

diel cycles in the remote marine boundary layer and suggests 

that SO2 was produced from DMS with an efficiency of 75%. 

Case 4. In Case 4 we adjusted the yield of SO2 from the 

oxidation of DMS so that the model would produce SO2 mole 

fractions with a mean similar to the observed mean mole fraction 

of 71 pmol mol '• (Figure 3, case 4). This process requires a 27% 

yield of SO2 from the total oxidation of 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 [Yvon et 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 

have to be doubled (-54%) to maintain the observed SO2 mole 

tkactions. This higher SO2 production efficiency is much closer 

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 94 pmol 

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 should e •xhibit a maximum at night and a minimum 

during the day. The SO2 mole fractions measured at the station 

show a slight increase before dawn. 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 SO2 of just 27%. 

The loss of SO2 appears to be controlled by dry deposition (58%) 

and scavenging by sea-salt aerosols (28%). In-cloud oxidation 

and homogeneous oxidation appear to be minor sinks. 

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 of Various 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 5.57xl 03 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 rate constant 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 loss of SO 2 

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

(2%) source of SO2, even large uncertainties in this parameter 

would have little or no effect on the 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 

drag coefficient has an error of <20% [Gatwatt, 1977; Gatwatt, 

1992]. A 20% decrease in the deposition velocity would result 

in the following: (1) a 24% yield of SO 2 from daytime DMS 

oxidation, (2) dry deposition equal to 52% of the total loss of 

SO2, and (3) scavenging by sea salt constituting 31% of the total 

loss of SO2. The value for in-cloud oxidation is given here as an 

upper limit owing to the 1oxv numbers of clouds present. 

Changes in this parameter by even a factor of 2 would result in 

the following: (1) there would be a 29% yield of SO2 from DMS 

oxidation, (2) in-cloud oxidation would become 17% of the total 

loss of SO2, and (3) dry deposition and scavenging by sea-salt 

aerosols would 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 calculated by his 

model with flux estimates from other sources [Eriksson, 1959; 

Blanchard, 1963; Blanchard, 1985]. If the alkalinity flux used 

in the model were doubled, the predicted yield of SO2 from the 

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 SO2 from 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 

major conclusions of this modeling study. Dry deposition and 

sea-salt scavenging remain the dominant sinks for SO2, and a 

substantial fraction of 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) of oxidation ofSO2, 

Zaero is the residence time (seconds) of the aerosol, NA is 

Avogadro's number, Mis the molecular weight (96 g mo1-1) of 

sulfate, and the last two numbers are for converting the units to 

[tg m -3. The sulfuric acid produced from homogeneous and incloud oxidation of SO2 is likely to be incorporated into 

submicron aerosols which have a long lifetime. Assuming the 

homogeneous production rate of H2SO4 shown in Table 2 and a 

5.0 to 10.0-day residence time for submicron aerosols [Wameck, 

1988], approximately 0.06-0.14 [tg m '3 of nss sulfate should be 

produced. In-cloud oxidation of SO2 should produce 0.13-0.26 

[tg m -3 of nss 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 [tg m -3, approximately half the mean measured total 

nss sulfate concentration of 1.36 + 0.35 [tg m '3 (P. Quinn, 

personal communication, 1992) from the station. However, the 

measured nss sulfate from the station is approximately twice the 

mean nss sulfate concentration measured in this region by 

Huebert et al., [1993] and at Fanning Island by Prospero et al. 

[•9851. 

If we assume that the 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 sulfate concentration from the station, it 

would appear that 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 does not 

produce nss sulfate. One of the oxidation products is MSA, but 

this is a minor product in tropical air masses [Saltzman et al., 

1986]. The other products formed are not known, but it has 

been suggested that the OH-initiated oxidation of DMS may 

produce DMSO and DMSO2 with significant yields [Fin et al., 

1990a and 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 _+ 0.35 
YVON AND SALTZMAN: ATMOSPHERIC SULFUR CYCLING, 2, SULFUR DIOXIDE 6917 

Conclusions 

The results of this study indicate that SO2 levels at the field 

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

predicted on the basis of gas phase chemistry alone, if it is 

assumed that DMS oxidation yields primarily SO2. Including 

heterogeneous sinks such as in-cloud oxidation, dry deposition 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 grant from the National Science 

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

Fellowship (NGT-30114), and the Marine Sulfur, Aerosol and Climate 

component of the NOAA Climate and Global Change Program. 

REFERENCES 

Andreae, M. O., H. Berresheim, T. W. Andreae, M. A. Kritz, T. S. 

Bates, and J. T. Merrill, Vertical distribution of dimethylsulfide, 

sulfur dioxide, aerosol ions, and radon over the northeast Pacific 

Ocean, J. Atmos. Chem., 6, 149-173, 1988. 

Atlas, E., S. M. Schauffier, J. T. Merrill, C. J. Hahn, B. Ridley, J. 

Walega, J. Greenberg, L. Heidt, and P. Zimmerman, Alkyl nitrates, 

nonmethane hydrocarbons, and halocarbon gases over the 

equatorial Pacific Ocean during SAGA 3, J. Geophys. Res., 98, 

16,933-16,947, 1993. 

Bandy, A. R., D. L. Scott, B. W. Blomquist, S. M. Chen, and D.C. 

Thornton, Low yields of SO2 from dimethyl sulfide oxidation in the 

marine boundary layer, Geophys. Res. Lett., 19, 1125-1127, 1992. 

Bates, T. S., K. C. Kelly, and J. E. Johnson, Concentrations and fluxes 

of dissolved biogenic gases (DMS, CH 4, CO, CO2) in the 

equatorial Pacific during the SAGA 3 experiment, JGeophys. Res., 

98, 16,969-16,977, 1993. 

Berresheim, H., Biogenic sulfur emissions from the Subantarctic and 

Antarctic Oceans, J. Geophys. Res., 92, 13,245-13,262, 1987. 

Blanchard, D.C., The electrification of the atmosphere by particles 

from bubbles in the sea, Prog. Oceanogr., 1, 71-202, 1963. 

Blanchard, D.C., The oceanic production of atmospheric sea salt, J. 

Geophys. Res., 90, 961-963, 1985. 

Chameides, W. L., and A. W. Stelson, Aqueous phase chemical 

processes in deliquescent sea-salt aerosols: A mechanism that 

couples the atmospheric cycles of S and sea salt, J. Geophys. Res., 

97, 20,565-20,580, 1992. 

Cooper, D. J., and E. S. Saltzman, Measurements of atmospheric 

dimethylsulfide, hydrogen sulfide and carbon disulfide GTE/CITE3, J. Geophys. Res., 98, 23,397-23,410, 1993. 

Donahue, N.M. and R. G. Prinn, In situ nonmethane hydrocarbon 

measurements on SAGA 3, J. Geophys. Res., 98, 16,915-16,933, 

1993. 

Eriksson, E., The yearly circulation of chloride and sulfur in nature: 

Meteorological, geochemical and pedological implications, 1, 

Tellus, 11, 375-403, 1959. 

Ferek, R. J., D. A. Hegg, J. A. Herring, and P. V. Hobbs, An improved 

filter pack technique for airborne measurement of low 

concentrations of SO2, J. Geophys. Res., 96, 22,373-22,378, 1991. 

Garratt, J. R., Review of drag coefficients over oceans and continents, 

Mon. Weather Rev., 105, 915-929, 1977. 

Garratt, J. R., The Atmospheric Boundary Layer, 316 pp., Cambridge 

University Press, New York, 1992. 

Gregory, G. L., D. D. Davis, N. Beltz, A. R. Bandy, R. J. Ferek, and 

D.C. Thornton, An intercomparison of aircraft instrumentation for 

tropospheric measurements of sulfur dioxide, J. Geophys. Res., 98, 

23,325-23,352, 1993. 

H/inel, G., The properties of atmospheric aerosol particles as functions 

of the relative humidity at thermodynamic equilibrium with the 

surrounding moist air, Adv. in Geophys. vol 19, 74-188, 1976. 

Hegg, D. A., The importance of liquid-phase oxidation of SO2 in the 

troposphere, J. Geophys. Res., 90, 3773-3780, 1985. 

Hicks, B. B. and P.S. Liss, Transfer of SO2 and other reactive gases 

across the air-sea interface, Tellus, 28, 348-354, 1976. 

Huebert, B. J., S. Howell, P. Laj, J. E. Johnson, P. K. Quinn, V. 

Yegorov, A.D. Clarke, and J. N. Porter. Observations of the 

atmospheric sulfur cycle on SAGA 3, J. Geophys. Res., 98, 

16,985-16,995, 1993. 

Kasten, F., Visibility forecast in the phase of precondensation, Tellus, 

21, 631-635, 1969. 

Lenschow, D. H., R. Pearson, and B. B. Stankov, Measurements of 

ozone vertical flux to ocean and forest, J. Geophys. Res., 87, 8833- 

8837, 1982. 

Liss, P.S., Gas transfer: Experiments and geochemical implications, 

in Air-Sea Exchange of Gases and Particle& edited by P.S. Liss 

and W. G. N. Slinn, pp. 241-298, D. Reidel, Norwell, Mass., 1983. 

Maroulis, P. J., A. L. Torres, A. B. Goldberg, and A. R. Bandy, 

Atmospheric SO2 measurements on project Gametag, J. Geophys. 

Res., 85, 7345-7349, 1980. 

Millero, F. J., and M. L. Sohn, Chemical Oceanography, 531 pp., 

CRC Press, Boca Raton, Fla., 1992. 

Monahah, E. C., The ocean as a source for atmospheric particles, in 

The Role of Air-Sea Exchange in Geochemical Cycling, edited by 

P. Buat-M6nard, pp. 129-163. D. Reidel, Norwell, Mass., 1986. 

Prospero, J. M., D. Savoie, R. T. Ness, R. Duce, J. T. Merrill, 

Particulate sulfate and nitrate in the boundary layer over the North 

Pacific Ocean, J. Geophys. Res., 90, 10,586-10,596, 1985. 

Routbier, F., R. Dennett, D. D. Davis, A. Wartburg, P. Haagenson, 

and A. C. Delany, Free tropospheric and boundary-layer airborne 

measurements of ozone over the latitude range of 58øS to 70øN, J. 

Geophys. Res., 85, 7307-7321, 1980. 

Saltzman, E. S., and D. J. Cooper, Shipboard measurements of 

atmospheric dimethylsulfide and hydrogen sulfide in the Caribbean 

and Gulf of Mexico, J. Atmos. Chem., 7, 191-209, 1988. 

Saltzman, E. S., D. L. Savoie, J. M. Prospero, and R. G. Zika, 

Methanesulfonic acid and non-sea-salt sulfate in Pacific air: 

regional and seasonal variations, J. Atmos. Chem., 4, 227-240, 

1986. 

Saltzman, E. S., S. A. Yvon, and P. A. Matrai, Low-level atmospheric 

sulfur dioxide measurement using HPLC/fluorescence, ,1. Atmos. 

Chem., 17, 73-90, 1993. 

Sievering, H., J. Boatman, J. Galloway, W. Keene, Y. Kim, M. Luria, 

and J. Ray, Heterogeneousulfur conversion in sea-salt aerosol 

particles: The role of aerosol water content and size distribution, 

Atmos. Environ., 25A, 1479-1487, 1991. 

Sieveting, H., J. Boatman, E. Gorman, Y. Kim, L. Anderson, G. 

Ennis, M. Luria, and S. Pandis, Removal of sulphur from the 

marine boundary layer by ozone oxidation in sea-salt aerosols, 

Nature, 360, 571-573, 1992. 

Thompson, A.M. and R. W. Stewart, Effect of kinetics uncertainties 

on calculated constituents ina tropospheric photochemical model, 

,1. Geophys. Res., 96, 13089-13108, 1991. 

Thompson, A.M., et al., Ozone observations and a model of marine 

boundary layer photochemistry during SAGA 3, J. Geophys. Res., 

98, 16,955-16,968, 1993. 

Thornton, D.C., A. R. Bandy, and B. W. Blomquist, Formation of 

sulfur dioxide from dimethyl sulfide at a remote Pacific Ocean site 

(abstract), Eos Trans. AGU, 76(! 7), S80, 1995. 

Warneck, P., Chemistry of the Natural Atmosphere, 753 pp., 

Academic, San Diego Calif., 1988. 

Yegorov, V., Spatial distribution of ozone, sulfur dioxide and 

sulphates in the marine boundary layer of the atmosphere 

(abstract), Eos Trans. AGU, 71, 1230, 1990. 
6918 YVON AND SALTZMAN: ATMOSPHERIC SULFUR CYCLING, 2, SULFUR DIOXIDE 

Yin, F., D. Grosjean, and J. H. Seinfeld, Photooxidation of dimethyl 

sulfide and dimethyl disulfide, I, mechanism development, J. 

Atmos. Chem., 11, 309-364, 1990a. 

Yin, F., D. Grosjean, R. C. Flagan, and J. H. Seinfeld, Photooxidation 

of dimethyl sulfide and dimethyl disulfide, II, mechanism 

evaluation, J. Atmos. Chem., 11, 365-399, 1990b. 

Yvon, S. A., D. J. Cooper, E. S. Saltzman and P. K. Quinn, 

Measurements of atmospheric DMS and SO2 over the Northeast 

Pacific Ocean during PSI-3 (abstract), Eos Trans. AGU, 72, (44), 

104, 1991. 

Yvon, S. A., D. J. Cooper, V. Korapolov, and E. S. Saltzman, 

Atmospheric hydrogen sulfide over the equatorial Pacific Ocean 

(SAGA 3), J. Geophys. Res., 98, 16,979-16,983, 1993. 

Yvon, S. A., and E. S. Saltzman, A time-dependent photochemical box 

model for atmospheric chemistry (PBMAC), Tech. Rep. 93-008, 78 

pp., Rosenstiel Sch. Of Mar. And Amos. Sci., Univ. of Miami, 

Miami, Fla., 1993. 

Yvon, S. A., E. S. Saltzman, D. J. Cooper, T. S. Bates, and A.M. 

Thompson, Atmospheric sulfur cycling in the tropical Pacific 

marine boundary layer (12øS, 135øW): A comparison 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)