VSIMIOU States
Environmental Protection
Agency
Environmental Sciences Research EPA-600/3-78-061
Laboratory July 1978-
Research Triangle Park NC 27711
Research and Development
Biogenic Sulphur
Compounds in Coastal
Atmospheres
of North Carolina
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EPA-600/3-78-061
July 1978
BIOGENIC SULFUR COMPOUNDS IN COASTAL
ATMOSPHERES OF NORTH CAROLINA
by
Dian R. Hitchcock
Hitchcock Associates
Farmington, CT 06032
and
Lester L. Spiller and William E. Wilson
Environmental Sciences Research Laboratory
Environmental Protection Agency
Research Triangle Park, NC 27711
Contract No. FA-8-0764A
Project Officer
Lester L. Spiller
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISCLAIMER
This report has been reviewed by the Environmental Sciences Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. Approval does not signify that the contents necessarily reflect the
views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or
recommendation for use.
11
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ABSTRACT
2- + - +
Atmospheric H»S, SO , and particulate SO, , Na , Cl , NH, , and NO-
were measured in two experiments on the North Carolina coast to determine
the levels of biogenic sulfur species at marsh and estuarine locations where
dissimilatory bacterial sulfate reduction produces H S in local anoxic muds.
The first (summertime) experiment demonstrated the occurrence of variable
3
and high H S levels—4-h means up to 80 yg/m (57 ppb)—associated with low-
tide mud exposure in a Spartina alterniflora marsh. Little or no SO was ob-
served here, and little or no S0? or tLS were observed at a background site
2.4 km distant. Biogenic sulfate in marine air masses ranged from 2 to 13
yg/m , and was strongly associated with the loss of chloride from marine
aerosols. Both H S and SO were observed in the second (autumn) experiment
3 3
at concentrations up to 7 yg/m (5 ppb)(H-S) and 25 yg/m (17 ppb)(SO ) at
an estuarine site where anoxic muds are not exposed at low tide, under condi-
tions which implied a biogenic origin, and the rapid conversion of biogenic
H S to SO,.,. Particulate excess (non-sea salt) sulfate and chloride loss
from marine aerosols were observed at this site in continental air masses
(marine air masses did not occur).
This report was submitted in fulfillment of contract number FA-8-0764A
by Dian R. Hitchcock under the sponsorship of the U.S. Environmental
Protection Agency. This report covers a period from August 1976 to
February 1977 and work was completed as of February 1977-
111
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CONTENTS
Abstract ill
Figures vi
Tables vli
Acknowledgments viii
1. Introduction 1
Biogenic HoS in the atmosphere 2
Biogenic particulate sulfur in the marine and
coastal atmosphere 4
2. Conclusions 8
3. Methodology 10
Hydrogen sulfide and sulfur dioxide 10
Particulate samples 12
4. Site Descriptions and Experimental Protocols ....... 16
Site descriptions 16
Estuarine experiment 19
5. Results 23
Hydrogen sulfide and sulfur dioxide 23
Particulate abundances at Morehead City 31
Influence of air mass trajectories and collection
site on sample composition 33
Particulate abundances at Swansboro 51
6. Discussion 55
Hydrogen sulfide and sulfur dioxide 55
Particulate sulfate and other constituents 57
References 61
v
-------
FIGURES
Number
Page
1 Morehead City sites 17
2 Swansboro and Morehead City 20
3 Swansboro sites 21
4 Comparison of 4-, 6-, and 12-h mean H~S levels at
Calico Creek marsh 24
5 Means of 4-h average fLS and SCL concentrations and
windspeed at Calico Creek marsh 27
6 Nighttime low tide 4-h average H_S concentrationa and mean
surface windspeed at Calico Creek marsh over 15-d period 28
7 Excess sulfate at Camp Glen roof site and Calico Creek
marsh 39
8 Sodium at Camp Glen roof site and Calico Creek marsh on
days with marine trajectories 40
9 Chloride loss and sodium at Camp Glen roof site and Calico
Creek marsh on days with mixed trajectories 41
10 Excess sulfate and chloride loss at Calico Creek marsh . . 43
11 Excess sulfate and chloride loss at Camp Glen roof site . . 44
12 Nitrate and excess sulfate in marine and mixed trajectories
at Camp Glen roof site 45
13 Ammonium and excess sulfate at Calico Creek marsh 46
14 Ammonium and excess sulfate at Camp Glen roof site .... 47
VI
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TABLES
Number Page
1 Detection Limits for SC>2 and H2S 13
2 Hi-Vol Filter Mean Blank Levels and Detection Limits of
Particulate Analysis Techniques 15
3 Morehead City H S and SO 25
4 Swansboro H_ and SCL Results 30
5 Morehead City Mean Particulate Concentrations . 35
6 Regression Equations by Air Mass Origin 37
7 Regression Equations - All Days 50
8 Swansboro Mean Particulate Concentrations 52
vii
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ACKNOWLEDGMENTS
The cooperation of James Brown, Louis Priddy, and Van Maxwell of the
Division of Marine Fishery, North Carolina Natural Reseources Development,
Morehead City, NC; Dr. A. Chestnut and P.R. Carlson of the Institute of
Marine Sciences, University of North Carolina, Morehead City, NC; and Tom
R. Karl, Meteorology and Assessment Division, USEPA, Research Triangle
Park, NC, are gratefully acknowledged.
Vlll
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SECTION I
INTRODUCTION
The existence of "natural" biogenic sulfur contributions to the atmos-
phere has been recognized for nearly a century (1), but few data have been
complied on which to base estimates of the strength of these natural sources
or their distribution. Global budgets of atmospheric sulfur estimate the bio-
genic contributions on the basis of indirect evidence of the sulfur content of
rain, rivers, and the atmosphere in remote locations, but the uncertainty of
these data and the difficulty of interpreting them yield diverse estimates
ranging from about 33 million tons per year (2) to about 100 million tons per
year (3,4) in the most recent budgets. These budget estimates must be regar-
ed as very uncertain and of little use for determing the strength of biogenic
emissions on a local or even a regional basis.
Interest in the magnitude of these emissions has been heightened by
recent reports to the effect that biogenic sulfur sources may make significant
contributions to the atmospheric load of particulate sulfate, both in urban
and nonurban locations (5,6).
Many biological processes produce volatile sulfur compounds which may
be responsible for biogenic sulfur emissions to the atmosphere. These include
the bacterially mediated decomposition of organic sulfur present in plant and
animal tissue, chiefly in protein, and the respiratory metabolism of the
bacterial sulfate reducers. The former process yields a variety of volatile
organic sulfur gases and hydrogen sulfide (7,8). The latter produces hydro-
gen sulfide, which in some circumstances may be transformed either bio-
logically or by means of spontaneous chemical reactions to yield other
volatile compounds. Lovelock et al. (9) have proposed that most of the
biogenic sulfur released to the atmosphere is dimethyl sulfide derived
from the bacterial decomposition process described above. However
1
-------
extrapolation of these data and recent reports of experimental observations
indicate that these processes are unlikely to yield more than a few billion
kilograms per year to the global atmosphere (10,11,12).
The total sources of atmospheric S derived from organic S decomposition
are limited by the availability of this substance in the standing crop and the
proportion that is released as volatile sulfur. Organic sulfur is present
only in low concentrations in plant and vegetable matter ( 0.1 to 1% of
the mass of dry organic matter), and the data of Lovelock et al. (9) imply
annual release totaling at most only 100 yg/g of organic S present in plant
and animal tissue. Even if this release were greatly concentrated in time,
the result could not yield an atmospheric concentration exceeding about 1
3
yg/m (or about 1 ppb) even in locations where the organic sulfur production
of the local biota is extremely high, since the latter rarely exceeds a few
grams per square meter per year even in very productive locations. It there-
fore seems likely that if biogenic sulfur is present in the atmosphere at
3
concentrations exceeding a few ng/m (a few parts per trillion), it must have
been produced by sulfate reducers and released either as H-S, or as DMS or
other gases derived from H.S.
Bacterial sulfate reducers are obligate anaerobes which inhabit aquatic
environments where oxygen is absent and sulfate is present in solution to-
gether with oxidizable organic matter. The environmentally most important
class of sulfate reducers appears to comprise the species Desulfovibrio desul-
furicans, but several other species of sulfate reducers exist. It is possible
that a recently described bacterium that reduces elemental sulfur to hydrogen
sulfide during the respiratory oxidation of acetate may also play an important
role in the sulfur cycle of wet sediments (13).
BIOGENIC H S IN THE ATMOSPHERE
Direct evidence of H.S in the natural atmosphere is very scarce, partly
^ 3
because concentrations below about A yg/m (3 ppb) are difficult to measure.
Several workers have recently developed techniques for measuring H_S
3
and/or dimethyl sulfide at concentrations below 1.5 ug/m (1 ppb) and
available evidence implies that tropospheric air sampled at points not near
significant anthropogenic or biogenic sulfur sources usually contains less
3 3
than 0.1 yg/m (100 ppt) and may have less than 0.01 yg m ,
-------
(lOppt). Slatt et al. (17) report that even air that has passed over an
estuary, a mangrove swamp, forested parks, residential areas, and/or a sewage
3
plant typically contains less than 1 yg/m (1 ppb) H S. (Their sampling
site appeared to be at least 1.5 km from the sewage plant and the nearest
mangrove swamp.) These measurements imply either that the lifetime of H~S
in the atmosphere is extremely short or that it is not released to the
atmosphere in significant quantities.
It is known that H»S production by bacterial sulfate reducers may pro-
ceed at extremely rapid rates under some conditions. Field studies and
-2 -I
laboratory experiments indicate that rates as high as 50 to 400 mg m hr
may be common in suitable habitats (18-23). Relatively little is known
about the fate of this H~S or about the proportion of the sulfide produced
in muds that reaches the atmosphere.
Ostlund and Alexander (24) present evidence of extremely rapid aqueous
oxidation of H~S by dissolved 0« in seawater, and conclude that little tLS
produced in submerged marine muds more than a few meters deep can reach the
atmosphere, since most of it is oxidized in the water column. They infer
that H_S can be released in significant quantities only from exposed muds
of tidal flats or from water only a few meters deep. They estimated the
half-life of tLS in seawater containing CL present to excess to be 17 min,
but other studies report half-lives in seawater and blackish water ranging
from 2 h to more than 100 h, and reports of 12 to 20 h appear to be common
(25-29). It follows that exposed tidal flats are likely to be sources of
atmospheric H~S, and that this gas may under some circumstances be evolved
from water overlying muds in which tLS is being produced.
Frequent reports of the odor of H_S near coastal and inland marshes
and swamps, harbors, lakes, tidal mud flats, and irrigated farm land signify
that emissions do occur, but few quantitative data are available.
tLS emission rates for isolated areas have been measured by determining
the amount of sulfur released from the surface to a closed chamber placed
over the ground or a shallow tidal pool. Using this method, Brannon (30)
-2 -1
observed H»S emission rates ranging from 0.06 to 5.8 mg m hr in Barataria
Bay (Louisiana) Spartina alterniflora marsh soils. He noted that the release
-------
rate varied greatly with the spot sampled, and that H S appeared to be
escaping preferentially in certain areas through "channelized" escape routes.
(Martens 31 has reported a similar phenomenon facilitating the escape of
methane from submarine sediments.) Aneja (32) also found that H^S release
rates in the intertidal zone of a _S. alterniflora marsh on the coast of
Long Island (New York) to be highly variable, ranging from below detection
-2 -1
level to 4.7 mg m hr . He reports that, on some runs, the tLS concen-
tration inside the chamber exceeded the measurement capability of the gas
chromatograph he employed. Hansen et al. (33) employed the chamber method
to measure the release of H?S from shallow-water sediments on a protected
shore in Denmark. They found that the observed emission rate from a sandy
sediment with a relatively low organic matter content ranged from below the
-2 -1 -2 -1
detection limit up to 11 mg m hr and averaged 2.1 mg m hr , while
that from a small lagoon whose surface sediments contained 20 to 30%
-2
organic matter in the form of decomposing eelgrass ranged up to 118 mg m
-1 -2 -1
hr and averaged 53 mg m hr The sediments were covered by 1 to 3 cm
of water.
The literature contains no measurements of biogenic sulfides in the air
near sites where they are produced.
BIOGENIC PARTICULATE SULFUR IN THE MARINE AND COASTAL ATMOSPHERE
If natural sources of sulfur contribute to the atmosphere in marine
and coastal locations, then one would expect to find it present in aerosol
form in these locations. Aerosol sulfate is a normal constituent of the
atmosphere in such locations. Sulfate makes up about 8'% of the mass of the
principal ions in bulk seawater, and it is injected into the atmosphere
together with other seawater constituents by the action of wind and waves.
The occurrence of non-sea salt sulfate in the marine aerosol may be demon-
2- +
strated on the basis of the observed SO. /Na ratio in the aerosol, or on
4 '
the basis of the size distribution of sulfate-containing particles and
their relationship to that of some other conservative tracer of sea salt
aerosols. The first method requires the assumption that the process of
forming sea salt aerosols does not result in aerosols enriched in sulfate,
while the latter requires the assumption that the tracer employed is a
conservative one which is neither fractionated in the formation of the
-------
aerosol nor depleted or enriched in the aerosol after it has entered the
atmosphere.
In analyzing the composition of aerosols, it is customary to determine
the enrichment factor EF (X) of the ion or element relative to bulk sea-
——• sw
water (34) . This may be defined as the ratio of the observed ratio of the
element or ion X concentration in the aerosol to a seawater tracer such as
Na, to the ratio of the two in bulk seawater:
EF (X) = (X/Na) , / (X/Na ) (1)
sw ob seawater
If concentration measurements are available, the enrichment may be
expressed in terms of the amount by which the abundance of X in aerosol form
exceeds the amount that would be present if seawater were the only source,
and the ion or element were present together with the seawater tracer in
concentrations that match their ratio in bulk seawater:
EX = X ,-(Na , (X/Na )) (2)
sw ob ob sw
It will be noted that, when the enrichment factor is less than one, depletion
has occurred, and expression (2) will be negative.
Although the marine aerosol has been the subject of intense study for
the last decade, there are few reports of the presence of particulate sulfur
in marine or coastal locations, and fewer still that include measurements
that permit determination of the presence of excess sulfate or an estimate
of its abundance. The most interesting studies are those of Buat-Menard
et al. (35) and Meinert and Winchester (36). The former measured sodium,
chloride, magnesium, potassium, calcium, and the mass of insoluble salts
in aerosols collected on shipboard in the open Atlantic Ocean and off the
coast of Africa. They observed sulfate enrichment relative to sodium in
combination with calcium enrichment and the presence of insoluble salts,
which implied that the sulfate was derived from gypsum (CaSO.) in dust
advected from the Sahara. Sulfate enrichment was also observed in conjunc-
tion with potassium enrichment and chloride depletion in the Gulf of Guinea,
and the authors inferred that it was derived as H~S and/or CH SCH_ from the
productive upwelling waters present in the region. The ratio of excess
-------
sulfate to chloride loss implied complete replacement of the chloride
anions by sulfate anions.
Meinert and Winchester (36) studied the distribution of Cl, K, S, Fe,
and other elements in size-fractionated aerosols collected from marine air
on the coast of Bermuda. They observed that all chloride occurred in the
largest particles (>2 ym aerodynamic diameter) and determined that it was
approximately 20 to 40% depleted relative to K, which was presumed to be of
marine origin. In contrast, Martens et al. (37) and Martens and Harriss (38)
observed that both Cl and Na of apparent marine origin are present on all
stages of an Anderson cascade impactor and also on the after-filters in
samples collected in Puerto Rico and at San Francisco Bay. Chloride deple-
tion relative to sodium was observed in most samples and was greatest in the
small-particle fraction. In the San Francisco Bay area, the loss of chloride
in samples on stages 6 and 7 often exceeded 90%, while losses of 10 to 20%
were observed in stages 1 through 4. Total losses averaged for all particle
sizes ranged from 6 to 90%. These samples were collected in coastal loca-
tions and at sites up to about 30 km from the nearest shore.
Chesselet et al. (39) report an average chloride loss of 4% in aerosols
collected over the open ocean. More recently, Gordon et al. (40) present
convincing evidence that chloride was not depleted relative to sodium when
they sampled aerosols from an aircraft (altitude 75 to 2000 m) on five
consecutive days near the Bahamas. The mean ratios of these two elements
did not differ significantly from that of bulk seawater throughout the
altitude range sampled, though their total concentrations declined from about
4 to 7 ug m at 75 m to about one-tenth that value at 2000 m.
There is evidence that chloride loss, when it occurs, is related to
the chemistry of the aerosol and to local gaseous sources of N09 or biogenic
sulfur. Robbins et al. (41) proposed that HC1 is volatilized off the
aerosol particle by HNO , which results from the uptake of NCL. Eriksson
(42) proposed that chloride loss may be due to uptake of S0? followed by
its conversion to sulfur trioxide. Martens et al. (37) present evidence
supporting the hypothesis of Robbins et al. (41) in the form of correlations
between the NCv abundance and observed chloride loss in the San Francisco
aerosols they studied. They found no correlation between S0? and chloride
-------
loss in these samples, but theorized that much of the chloride loss in the
Puerto Rican samples (collected in locations where pollutant sources of NO-
are small) may be due to uptake of SO,., derived from natural sources in
Puerto Rico or in the offshore waters.
The latter explanation is compatible with the results of Junge (43)
who found a S/C1 ratio that decreased with increasing particle size, and
with those of Meinert and Winchester (36) which show the same kind of
relationship. Green (44) reported that the Cl/Na ratio declined with the
pH of the aerosol in marine and mixed marine and polluted continental air
collected at the coast of southern California. Kadowaki (45) reported the
presence of NaNO., in large particles collected at a seacoast location, and
of NH.NCL in small particles, from which he infers that chloride volatilized
off large particles by reaction with N0?.
Taken together, these results imply that biogenic sulfur emissions may
lead to the formation of aerosol sulfate, and that this may be reflected in
an associated depletion of chloride relative to sodium in sea salt aerosols,
which should be most pronounced in the small particle fraction. Chloride
depletion in both size fractions may result from the uptake of NO.-,, which
may also result in the presence of nitrate in the particulate sample. The
picture is further complicated by the conclusions of Marker et al. (46)
who demonstrate that the nitrate abundance of particulates may be reduced by
volatilization of the nitrate (probably as nitric acid vapor) due to inter-
action with H~SO, produced either locally by photochemical oxidation of
S0~ or contributed as a sulfuric acid aerosol by long distance transport.
We report here the preliminary results of a program of field measure-
ment designed to survey the distribution of hydrogen sulfide, sulfur
dioxide, and particulate sulfur, sodium, and chloride in the atmosphere
near sites where bacterial sulfate reduction is active. The first study
reported here was conducted to determine the sulfide levels present in a
tidal marsh and their relationship to the tidal cycle. The second was con-
ducted in an estuarine environment where HLS-producing muds occurred in
bottom sediments but were never exposed to the atmosphere.
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SECTION 2
CONCLUSIONS
1. Biogenic sulfur gases were observed in the atmosphere at concen-
trations ranging up to 80 yg/m (60 ppb) in a tidal marsh in summer at a
site where muds containing high concentrations of biogenic H?S are regularly
3 z
exposed by tidal movements, and up to 30 yg/m (22 ppb) in the fall at
three estuarine sites where fine-grained bottom sediments containing H«S
occur one or more meters beneath the surface. In the summer, the biogenic
sulfur consisted of H-S and was found to be highly localized in the atmos-
phere near the marsh; little was observed at a background site 2.4 km distant
from the marsh, even though this site was similar in regard to underwater
and exposed sediments to the sites studied in the fall. In the fall, both
SO- and H«S of biogenic origin were observed. These two constituents were
correlated at one site, which implies that the SO was derived from precursor
fLS. H»S observed at two other sites 2.5 km apart was correlated, which
implies that tLS had a longer atmospheric lifetime and/or that the sources
were widespread. The absence of exposed H_S-rich muds or of correlation
between biogenic sulfur gases in the atmosphere and the tidal cycle implies
that the sulfur was evolved from the surface of estuarine water and/or
from coarse sand sediments containing relatively little organic matter.
2. This biogenic gaseous sulfur is derived from the metabolism of
bacterial sulfate reducers, not from the decomposition of organic sulfur
present in plant and animal tissue. This implies that the global and even
local sources of significant quantities of atmospheric biogenic sulfur
are extremely uneven and that source strengths are highly variable.
3. Biogenic particulate sulfate was observed in the summer at con-
-3 -3
centrations ranging from about 2 to 13 yg m and averaging about 4 yg m
in air masses with marine trajectories; non-sea salt sulfate was observed in
-------
air with mixed continental and marine trajectories at concentrations
_3
ranging from 3.2 to 20 yg m , and an unknown proportion of this also
appears to be biogenic.
4. Chloride loss from sea salt particles, estimated on the basis of
the observed sodium concentrations, ranged from 0 to 96% and was strongly
correlated with excess sulfate in air samples collected with both marine
and mixed marine and continental trajectories in the summer. Chloride loss
showed no correlation with excess sulfate, nitrate, ammonium, or excess
potassium in the samples collected from air with continental trajectories
in the fall.
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SECTION 3
METHODOLOGY
HYDROGEN SULFIDE AND SULFUR DIOXIDE
Hydrogen sulfide and sulfur dioxide were collected on treated filters
for subsequent analysis by X-ray fluorescence spectroscopy. Millipore
WSWP04700 filters (47 mm o.d) were mounted in a modified Millipore single
filter holder No. XX-50-04700 which was elongated so that it could hold
three filters in series, separated with stainless steel rings fitted with
Teflon or Viton rings to seal the filter elements in the holders. Air is
drawn through the filter holder by a vacuum pump, the flow rate being con-
trolled by a critical orifice attached to the outlet of the filter holder.
The first filter in the series (the prefilter) was untreated and was
used to remove particulate matter from the sampled air. The second filter
was treated with a LiOH solution to collect S0?, and the third with AgNO
to collect H?S. After use, the filters were returned to the laboratory
and analyzed for sulfur content by energy dispersive X-ray fluorescence to
determine the abundance of S on the surface of each filter (47). This
_2
method of analyzing for sulfur determines the mean S abundance cm by a
procedure which in effect "weighs" the result in favor of the abundance
present in the center of the filter, hence it tends to give erroneous
results if the collected sample is not evenly deposited over the treated
filters.
Several potential sources of measurement error in this collection
system were evaluated in the laboratory: (a) absorption of sulfur gases
on the prefilter; (b) inefficiencies in the collection of S0~ or H?S by
the treated filters; (c) penetration of the collected gas into the treated
filter, so that it cannot be detected by the X-ray fluorescence technique;
(d) interference through collection of H_S or other sulfur gases by the
10
-------
LiOH filter, or of S0? that has passed through the first filter or other
reactive gases by the AgNCL filter; (e) losses of the collected gas after
exposure in the field, or migration of the sulfur into the filter where it
cannot be detected; and (f) inhomogeneous deposition of the sample on the
treated filters.
These error sources were evaluated in the laboratory by a series of
tests in which an air stream containing gas of known concentration was
passed through the filters, separately or in combination, with and without
a prefilter. Sulfur gas concentrations were measured in the air stream
ahead of and behind the filters, and the concentrations collected on the
filters were measured by X-ray fluorescence. Tests were conducted in a
variety of combinations of flow rate, relative humidity, exposure duration,
temperature, etc. The efficiency of the collection system was determined
by comparing the observed results with what should have been observed
assuming 100% efficiency of collection and even deposition on the surface
of the filter.
These tests demonstrated that the apparent collection efficiency of
the LiOH filters varies with flow rate and relative humidity, and that the
optimum flow rate for S0_ collection by the LiOH filters is 1.0 min . At
this rate the collection efficiency averages 89% for relative humidities
in the range 30 to 90%, while collection efficiency declines to 77% at a
relative humidity of 100%.
The collection efficiency of the AgNO_ filters for H~S was found to be
lower and more variable than that of the LiOH filters for SO-. The average
observed for flow rates in the range 0.5 to 2 1 min and relative humidities
in the range 10 to 90% was 78 ± 18%. At 1.0 1 min the mean observed
collection efficiency was 81 ± 3%, while at 2.0 1 min" it was 78 ± 4%.
Temperature variations characteristic of ambient air had little effect on
these collection efficiencies.
Both filters were found to have good selectivity for their respective
gases when freshly made, and the LiOH retained good selectivity for a
month after preparation. However, the AgNO_ filters began to collect
small quantities of S02 after approximately 2 days. For this reason, the
11
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LiOH filter was placed ahead of the AgNCL filter in the holders when used
in the field.
The collection efficiency of the AgNCL filter for dimethyl sulfide
was tested, and no uptake was detected. Shelf life tests indicated that
the collected SCL is stable on the filter for longer than 2 weeks, but that
the measured S concentration on the AgNO filters exposed to ELS begins to
decline after about 1 week of storage and may reach 50% or less in a month.
The factors influencing this decline have not been examined.
These tests indicate that in field use, the SCL measurements obtained
with the LiOH filters will underestimate the amount present in the sampled
air by about 10%, while the H_S measurements obtained with the AgNCL filters
will underestimate the amount by about 20%.
Detection thresholds for each gas depend on the flow rate, the duration
of sample collection (exposure) and the sensitivity of the X-ray fluo-
_2
rescence measurements, which was 0.05 yg S cm . Detection thresholds for
the various exposure times and flow rates used in the two studies are
shown in Table 1, calculated under the assumption of 100% collection
efficiency and the most likely efficiencies obtained in the field (85% for
S02 and 84% for H2S at 1.0 1 min"1 and 80% for H S at 2.0 1 min"1).
PARTICULATE SAMPLES
Particulates were collected in standard Hi-Vol samplers equipped with
flow rate controllers to insure constant flow rates. Pallflex TX40120WW
filters were employed because of their inherent low blank values for S, and
because they do not significantly collect SO, from ambient air.
SO^ , Cl and NCL were determined by the ionic chromatographic techni-
que using a water extract of half of the filter. This extract was also used
to determine NH. by the ion selective electrode technique, and the Na and K
by atomic absorption. An acid leach of one quarter of each filter was used
to determine Fe, Ca, Pb, and V by atomic absorption. The graphite furnace
technique was used to determine V, and the conventional flame technique
was used for the determination of Fe, Ca, and Pb.
12
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TABLE I. DETECTION LIMITS FOR S02 AND
Sampling Flow rate Theoretical limit
time (hours) (liters min ) yg/m~3 ppb
4 1 S02 3.7
H2S 2.0
6 1 S02 0.9
H2S 0.9
2 H2S 0.5
1.4
1.4
2.4
1.3
0.7
Most Proabably limi
yg/m~3 ppb
4.5
2.4
2.7
1.4
0.9
1.7
1.7
1.0
1.0
0.6
-2
^Assumes 100% collection efficiency, and a detection level of 0.05 yg cm
of filter.
Assumes average collection efficiencies of 89% for SO- and 81% for tLS at
1.0 1 min-1 and 78% at 2.0 1 min-1.
13
-------
Table 2 shows the mean blank levels observed during the Morehead City
studies reported below together with the detection limits of the techniques
-3
employed, expressed in mg per filter, and in yg m , calculated on the basis
of the average sample volume (1800 m ). The blank levels observed in the
second series of studies at Swansboro were very similar. Ca blank levels
were too high to make useful measurements of this element possible.
Total suspended particulate loadings were determined gravimetrically
for the second series of studies at Swansboro, but the filter weighings
prior to the first series were found to be insufficiently accurate to permit
accurate estimates of TSP values, because the mass loading turned out to be
very low.
14
-------
TABLE 2. HI-VOL FILTER MEAN BLANK LEVELS AND DETECTION
LIMITS OF PARTICULATE ANALYSIS TECHNIQUES
Constituent Mean blank
(mg filter"-*-)
SO™ 0.98
Cl~ 0.24
NH")"
-------
SECTION 4
SITE DESCRIPTIONS AND EXPERIMENTAL PROTOCOLS
SITE DESCRIPTIONS
The two sites chosen for this study are located on the coast of North
Carolina, one in Morehead City, and one approximately 30 km west near the
small village of Swansboro (Figure 1). The Atlantic coast from New England
to Florida has a series of narrow offshore barrier islands, which result
in the creation of protected sounds between the islands and the mainland
coast. These sounds and the mouths of rivers and creeks flowing into them
make up a series of shallow estuaries which are flushed to varying degrees
by the tides. On the North Carolina coast, the bottom sediments of sounds
and estuaries are relatively coarse grained compared to those found north
of Delaware Bay or to the south on the shores of South Carolina and Georgia.
On this coast, tidal volume is low compared to the area of the sounds and
estuaries behind the barrier islands; consequently, tidal excursions are
minimal, and the area of tidal mud flats is negligible in most locations.
These sites were chosen because of (a) their convenience to Raleigh-Durham,
North Carolina; (b) the presence of access roads, docks, sources of power,
and other amenities; and (c) the absence of sites of significant sources
of anthropogenic S0~ or other common atmospheric pollutants.
The surrounding countryside is forested, and much of it is swamp land,
part of the Great Dismal Swamp of Virginia and North Carolina. This is a
freshwater, peat-forming system with relatively low sulfate levels (less
than 10 mg/liter and low pH, which we judged to have relatively
little capacity for the production of H?S by sulfate reduction on account
of its similarity to other freshwater peat-forming swamps on this coast (48).
Morehead City is a port town with a population of 5300 whose major
industry is summer tourism and several military bases located in the vicinity.
16
-------
MOREHEADCITY
BOGUESOUND
Figure 1. Morehead City sites.
17
-------
There are few anthropogenic S0? sources here or in adjacent Beaufort, and
fewer still in Swansboro. The nearest city with even modest SCL-producing
industries is Wilmington, which is about 85 km southwest of Swansboro.
The first series of measurements was made in August and September 1976
in Morehead City (Figure 1). Samples were collected at two sites, one
(Calico Creek) an extremely productive tidal marsh where H_S-rich muds are
exposed twice daily at low tide, and the other (Camp Glen) a background
site about 2.4 km ESE of Calico Creek on the shore of Bogue Sound, which
is not near any organic-rich tidal marshes.
At the Calico Creek site, gas and particulate samplers were located
on the end of a dock extending about 100 m from the edge of the marsh out
to a narrow channel in its center. Muds were exposed about 2 m below the
gas collectors at every low tide. These muds are biologically extremely
productive due in part to high nutrient supply from the outflow of a sewage
plant located at the head of the creek about 0.6 km from our sampling site.
As is common under such conditions, the muds are very rich in organic matter,
and bacterial sulfate reduction is very active. The H S level in inter-
stitial water in these muds at this season is often very high and has been
measured at concentrations ranging from 0.5 to 3.6 m M 1 (P. R. Carlson,
unpublished data, 1976). Spartina alterniflora is the dominant plant in the
marsh.
All sulfur gas collection sampling intervals were synchronized to local
tidal movements. Four-hour samples were collected four times per day, the
sampling interval timed to begin 2 h prior to the estimated time of high
or low water, and to continue 2 h after it. Sampling intervals of approx-
imately 6 h duration were timed to begin at the midpoint between each
successive high and low water, and to continue to the midpoint of the
interval between the next two. In a similar manner, sampling intervals of
approximately 12 h duration were synchronized with each successive pair of
6 h sampling intervals. Hi-Vol particulate collections coincided with
each successive pair of 12 h sampling intervals.
Wind speed and direction were recorded by means of a portable
meteorological tower also located on the end of the dock.
18
-------
Gas and Hi-Vol samples were collected at the background site on the
roof of a two-sotry building approximately 12 m above ground level, and 50 m
north of the shore of Bogue Sound. This building is approximately 100 m
south of a moderately well-traveled highway (U.S. 70) and 250 m south of a
hospital, both of which probably constitute modest sources of antoropogenic
SCL. Gas sampling intervals of 6- and 12-h and Hi-Vol sampling intervals
were synchronized with local tide movements in the sound in the manner
described above for Calico Creek. The tidal movements in the sound at this
point are about 1 h ahead of those in the marsh at Calico Creek. There are
no tidal marshes in the near vicinity of the Camp Glen site, although
anoxic sediments occur offshore in the sound (49, 50). The nearest tidal
marshes are across the sound at least 1 km distant, but these are neither
extensive nor biologically very productive compared to those at Calico
Creek.
Wind speed and direction data for this site were supplied by the
University of North Carolina Marine Sciences Institute, which occupies
an adjacent building.
ESTUARINE EXPERIMENT
The estuarine experiment was conducted at three sites approximately
35 km west of Morehead City near the small village of Swansboro at the
mouth of the White Oak River (Figures 2 and 3). This site was selected
because there are few local sources of anthropogenic sulfur. The surround-
ing land is forested, with no farms and few occupied residences within 10 km.
One of the collection sites was located at the Swansboro Coast Guard
Station on the western tip of Bogue Banks on the shore of the sound about
300 m east of the inlet, where large volumes of water are exchanged four
times per day through Bogue Inlet between the Atlantic Ocean and Bogue
Sound and other estuaries (Figure 2). These exchanges result in a highly
variable current flow, which is quite swift at times. There are a number
of sand bars in the near vicinity, which are exposed by tidal movements, and
some marshes with relatively coarse-grained sediments within 1 to 3 km of
this sampling site (Figure 3). Gas collectors operated at 1 1 min were
positioned within 2 m of the water's edge (high tide), and 6-h H S and S09
19
-------
ho
O
Figure 2. Swansboro and Morehead City. Locations marked 1 and 2 are Morehead City sites; 3,4, and 5 are Swansboro sites. The Great
Dismal Swamp is a freshwater swamp.
-------
TOWN
0,23456 yi^SALT MARSH
Figure 3. Swansboro sites.
21
-------
collection intervals were synchronized with the local tide movements in the
manner described above. Hi-Vol sample collection intervals of four complete
tidal cycles (about 25-h duration) were made daily.
The two other collection sites were located on the White Oak River
at Little Kinston and Breezy Point, which are 9.2 and 7.0 km north of the
Coast Guard site, and 2.5 km apart (Figure 3). Samples were collected from
docks extending out about 100 m from shore, in water which ranged from
about 1.0 to 1.5 m deep (low tide). At both sites, the underwater sediments
were very fine-grained, black muds which smelled of H_S, although the
sediments at the shore consisted of coarse to fine sand. At Breezy Point,
S0~ was collected for 6-h intervals with a system operated at 1 1 min ,
but the H«S collection system was operated at twice this flow rate, to
reduce the minimum detectable concentration as far as practical. Only H_S
_1 2-
was collected at Little Kinston, also at a flow rate of 2 1 min . Hi-Vol
samples were collected at both sites. Tidal movements are synchronous at
the two White Oak River sites, but high tide at these sites is delayed about
1 h relative to the Coast Guard Station, and low tide is delayed about 1
h and 45 min. Consequently, while there is 100% overlap between S09 and
H«S samples at these locations, overlap between them and at the Coast
Guard site is only about 75 to 80%.
22
-------
SECTION 5
RESULTS
HYDROGEN SULFIDE AND SULFUR DIOXIDE
Morehead City
Hydrogen sulfide was observed in the air about the exposed muds at
Calico Creek at every low tide, and comparison of 4-, 6-, and 12-h measure-
ments showed that the average H~S concentrations appeared to vary system-
atically with the amount of mud exposed (Figure 4). We infer, therefore,
that the peak concentrations corresponding to the lowest water levels were
much higher than the 4-h means reported here. Since sulfide was detected
at low tide on every 4-h filter, we will confine our attention to the
measurements obtained with these filters. The measurements reported assume
100% collection efficiency and hence probably underestimate the H_S concen-
trations in the sampled air.
In contrast with the high sulfide levels observed in the marsh, the
measurements made at the Camp Glen background site indicated that H S
3
almost never exceeded the detection threshold 1.6 yg/m (1.1 ppb) there.
Sulfur dioxide was rarely detected at either site.
The results of this study are summarized in Table 3, where mean H«S
and SO. concentrations are calculated from the measurements that exceeded
the detection limit, and the number of observations where the detection
limit was not exceeded is also shown. At Calico Creek, hydrogen sulfide
exceeded the detection threshold at high tide only three times, when the
3
measured concentrations ranged froir 2.0 to 2.7 yg/m (1.4 to 1.9 ppb).
In contrast, the 4-h mean H?S observations at low tide ranged from 3 to 80
3 ^
yg/m , (1.9 to 56.6 ppb). Sulfur dioxide exceeded the detection limit
3
on only 8 of the 52 observation periods and ranged from 3.7 to 5.6 ym/m ,
(1.4 to 211 ppb). The average low tide H~S concentration was 15.3 +
23
-------
60
50
40
30
20
4-h
AVERAGE
n
HJU
40
* 30
10
n n.
i i i
6-h
AVERAGE
n
30
20
12-h
AVERAGE
rrnl
2400 1200 2400 1200 2400 1200 2400 1200 2400 1200 2400 1200 2400 1200 2400 1200 2400 1200
TIME OF OAY.h
Figure 4. Comparison of 4-, 6-, and 12-h mean H2S levels at Calico Creek marsh.
24
-------
TABLE 3. MOREHEAD CITY H AND SO,
A. Calico Creek (4-h
Low Tide
Night
Day
All points
High Tide
Night
Day
All points
B. Calico Creek (4-h
High Tide
Calm
280° to 20°
Other
directions
C. Camp Glen (roof)
Low Tide
Night
Day
All points
High Tide
Night
Day
All points
filter)
Mean
14.4+7.0 (16)
5.011.2 (10)
10.8+7.6 (26)
1.7+0.5 ( 2)
1.4 ( 1)
1.6+0.3 ( 3)
filter)
1.9 ( 1)
1.6+0.3 ( 3)
(6-h filter)
Mean
1.0 ( 3)
1.0 ( 3)
1.0 ( 1)
1.0 ( 1)
-------
6.5 yg/m (10.8 ± 4.6 ppb). Hydrogen sulfide concentrations were found
to be highest at night, when air movement was limited. At night, the
3
measured 4-h mean concentrations ranged from 4 to 8.0 yg/m ( 2.8 to
56.6 ppb) and averaged 20.5 ± 9.9 yg/m ( 14.4 ± 7 ppb ). Wind-
speeds averaged 1.8 m s at night. In the daytime, the low tide H?S
3
4-h concentrations ranged from 2.7 to 10.7 yg/m ( 1.9 to 7.5 ppb ) and
averaged 7.1 ± 1.7 yg/m ( 5 ± 1.2 ppb ) while the windspeed averaged 5.5
m s~ (Figure 5). Analysis of the dependence of low tide hydrogen sulfide
level on windspeed showed that the concentrations declined exponentially
with increases in windspeed, the coefficient of correlation of hydrogen
sulfide mean with log windspeed being -0.73, which is significant beyond
the 0.01 level (Figure 6). In contrast, no significant relationship
existed between daytime low tide hydrogen sulfide levels and windspeed.
We found no relationship between S0~ and the tidal cycle, windspeed, or
local H S level.
Sewage plants can be significant sources of atmospheric H~S and other
sulfur gases; consequently, it is possible that the nearby sewage plant
contributed some contaminants to our observations. We evaluated this
possibility by comparing samples collected under air movement conditions
that would promote such contamination with those collected under other
conditions. We assumed that if such contamination occurs, it would be
most evident either when there was little air movement (which would pro-
mote the accumulation of sewage emissions in local surface air), or when
the wind blew directly from the sewage plant to the collection site. Since
the depth of the water in the estuary does not influence the atmospheric
release of sewage effluents (the outflow pipe being below the low tide
level), it is clear that sewage plant contamination can be determined by
comparing the hydrogen sulfide levels observed at high tide during condi-
tions promoting contamination with those observed at high tide under other
conditions.
The 27 high tide observations were found to be distributed as follows:
4 occurred when the wind direction favored contamination (mean wind direction
within ± 50° of the direction of the sewage plant for one or more hours),
and 7 occurred under calm conditions (mean hourly wind speed less than 0.45
26
-------
0
16
12
LU
CJ
o
u
S02
H2S
NiGHT
WIND
SO?
H2S
DAY
WIND
E
a"
CO
Q
Figure 5. Means of 4-h average H2S and S02 concentrations and windspeed at Calico
Creek marsh.
27
-------
0.01
10
50
20 30 40
4-h AVERAGE H2S, ppd
Figure 6. Nighttime low tide 4-h average H2S concentrations and mean surface windspeed
at Calico Creek marsh, over 15-d period.
60
28
-------
m s for one or more hours). The concentrations observed at these times
are summarized in Table 3. H?S exceeded the detection limit during only
3
one of these 11 intervals, at which time it averaged 2.7 yg/m (1.9 ppb).
In contrast, during the remaining 15 high tide intervals, H^S exceeded the
3
detection limit only twice, averaging 2.0 yg/m (1.4 ppb) both times.
These observations support the conclusion that there was no sulfide con-
tamination from the sewage plant.
The results observed at the Camp Glen roof are shown in Table 3. H«S
3 2~
was detected only twice, and never exceeded 2.8 yg/m (1 ppb). SO was
3
detected in the range 2.4 to 6.1 yg/m (0.9 to 2.3 ppb) only 9 times out
of the total of 53 observation intervals and was observed equally often at
low tide and at high tide, night and day. At these times the SCL levels
exhibited no significant relationships to windspeed or wind direction, or
to the levels observed nearly simultaneously in Calico Creek, only 2.4 km
3
distant. On the night on which the FLS reached 80 yg/m (56.6 ppb)
during the 4-h period of lowest water in Calico Creek, it averaged only
3
2.0 yg/m (1.4 ppb) at the Camp Glen roof site during the 6-h interval
beginning about 2 h before the corresponding Calico Creek sample and ending
almost simultaneously with it.
Swansboro
Adverse weather conditions and power outages greatly restricted the
number of useful observations from this experiment, so that only about 14
6-h samples were ultimately available for each site. However, the results
were extremely interesting in that they contrasted surprisingly with those
obtained in the marsh earlier in the year (Table 4). We found the H?S was
detectable at all sites most of the time, while sulfur dioxide was present
in measurable concentrations during all but two sampling intervals. tLS
o
ranged up to nearly 7.0 yg/m (5 ppb) at the estuarine sites, and from
3
about 1.4 to 3.7 yg/m (1 to 2.6 ppb) at the Coast Guard Station. SO
3
ranged up to 42.6 to 45.2 yg/m (16 to 17 ppb) at both locations where
it was measured.
We observed no systematic differences in mean H S or SO,-, concentrations
with tide at any site, and no significant differences between mean FLS and
29
-------
TABLE 4. SWANSBORO H^S AND S02 RESULTS
Mean
Mean SO,
A. Breezy Point
Low Tide
High tide
All points
1.811.0 ( 6)
2.0±0.8 ( 8)
1.9±0.4 (14)
3.812.6 ( 5)
4.515.6 ( 5)
4.113.0 (10)
Little Kinston
Low tide
High tide
All points
1.3+0.4 ( 5)
2.111.4 ( 5)
1.7+0.8 (10)
C.
Coast Guard Station
Low tide
High tide
All points
1.5+0.8 ( 8)
1.610.5 ( 6)
1.510.3 (14)
5.414.0 ( 7)
5.8+6.2 ( 5)
5.613.4 (12)
"Arithmetic mean in ppb l 2 standard errors of the mean. N shown in
parenthesis. Means calculated from values exceeding detection threshold,
30
-------
SO values at any site. However, comparison of the tLS levels observed
during simultaneous collection intervals revealed a systematic tendency for
slightly higher levels to be recorded at Breezy Point than at Little Kinston,
and this tendency was found to be significant beyond the 0.01 level as
determined by the Sign Test (51). S0? values observed at the Coast Guard
site and at Breezy Point were examined for any tendency to correlate, but
none was found. In contrast, tLS at Breezy Point was found to correlate
significantly with H2S at Little Kinston (r = 0.61, which is significant
beyond the 0.05 level). S0? and H?S values measured at the Coast Guard
Station were positively correlated (r = 0.60, which is significant beyond
the 0.005 level), but no evidence of such a correlation was observed at
Breezy Point. Analyses of the relationship between windspeed and SO,., or
H S levels showed no correlation at any location, but the total number of
sampling periods was far too small to enable us to distinguish any inter-
action among tide, diurnal cycle, and windspeed that may have been present.
PARTICULATE ABUNDANCES AT MOREHEAD CITY
Data Analysis Methods
The observed sodium concentrations in each sample were used to estimate
the total sea salt contributions of sulfate, chloride, and potassium on the
basis of their abundance relative to sodium in bulk seawater:
Cl/Na = 1.79
sw
S0,/Na =0.25
4 sw
K/Na = 0.037.
sw
Excess sulfate, chloride, and potassium were calculated for these con-
stituents, together with the corresponding enrichment factors. Air mass
trajectory analyses were employed to classify samples according to the
apparent origin and history of the air mass sampled.
Differences in the sample characteristics as a function of site and
air mass origin were investigated by several techniques. Since particulate
abundances and enrichment factors tend in general to be lognormally
31
-------
distributed, estimates of central tendency and judgments of the statistical
validity of differences between mean abundances, correlation coefficients,
and so forth require that the original data points be converted to their
logarithms. Therefore, comparisons of the differences in mean abundances
were based on the means of the logs of the individual observations. In
addition, some nonparametric tests of the significance of the difference
between the abundances of paired roof/marsh samples were used. These were
the Sign Test and Wilcoxson's Signed Rank Test (51).
Physically realistic models of the relationships between particulate
components suggest that they should be linearly related. This is especially
true regarding the possible causes of element or ion enrichment in the forma-
tion of sea salt particles, anion replacement, and the relationship between
the abundances of constituents of interest collected at the two different
sites. Consequently, we explored these phenomena by linear regression
analyses, recognizing that it is, strictly speaking, meaningless to speak
of the statistical significance of the resulting correlations or of the
values of the slope and intercept of the resulting regression lines. In
discussing these, no reference is made to their possible statistical
validity.
Air Mass Origins
Sampling days were classified as "marine" or "mixed marine and conti-
nental" on the basis of 96-h backwards trajectories calculated by the method
of Heffter and Taylor (52) at 6-h intervals. If all trajectories showed at
least 72 hours over the ocean and no time over land, the day was classified
as "marine", but if the time over the ocean were shorter, or the trajectory
began on the continent, passed over the ocean, and then returned to the coast
at our site, it was classified as "mixed marine or continental". Of the
total 14 sampling intervals, 7 were classified as "marine" and the remainder
"mixed marine and continental". One of the latter was purely continental,
three corresponded to days when the air mass left the coast and spent 12 to
60 hours at sea before returning to land, and the remaining three had wholly
marine trajectories, but were only 36 to 72 hours long (lack of reporting
weather ships making longer trajectories impossible to calculate).
32
-------
We observed no consistent relationships between trace metal abundances
and air mass origin, but this is not surprising in view of the possibility
that local sources of both fugitive dust and automobile lead pollution
could lead to variable concentrations of iron and lead independently of the
trajectory of the air mass. Vanadium concentrations on "marine" days were
_3
below the detection level (2 ng m ) in 3 of the 14 samples, and averaged
3.4 ng m in the remaining. This is high compared to marine air (0.2 to
0.4 ng m ), but compares well to concentrations in very remote continental
_3
regions at some isolated shore locations (1 to 3 ng m ) (34). Vanadium
concentrations were somewhat high on "mixed" days, when they averaged nearly
6 ng m in the 12 samples with values above the detection threshold.
INFLUENCE OF AIR MASS TRAJECTORIES AND COLLECTION SITE ON SAMPLE COMPOSITION
The analyses of these data may be approached in two ways. One is to
group together both air mass trajectory classes, on the grounds that differ-
ences between them will probably be slight and difficult to interpret, in
view of the fact that all samples appear to contain a significant marine
component, and the effects of continental contributions collected before
the air mass arrived at our site cannot be readily distinguished from
"continental" contributions of local origin. The other way is to analyse
each of the two types of samples separately and look for characteristics
that are consistent within air mass trajectory classes, and not present
when the classes are combined. We have conducted both kinds of analyses.
The marine influence inferred from the air mass trajectories was
apparent in the presence of sodium in all samples. It ranged from about
_3
1 to 4 yg m . Excess sulfate was present in all, in concentrations ranging
_3
from about 1.5 to over 20 yg m , and chloride enrichment factors ranged
_3
1.0 down to 0.04. Sea salt concentrations ranged from about 2 to 8 yg m
and appeared to be slightly higher in samples from "mixed" air mass tra-
jectories, although the differences were not statistically significant.
Sulfate, chloride, sodium, and potassium concentrations at the two
sites were very similar on all sampling days except the last, when a high
sulfate concentration at the roof site combined with other anomalies to
suggest contamination of this sample. Consequently, we have not included
it in the results reported here.
33
-------
Geometric mean concentrations of all constituents of interest and of
enrichment factors are shown in Table 5. Statistical comparisons of the
means revealed no significant differences between sites or between types of
days at either site. Analyses of the differences between concentrations
in the roof site sample and the marsh sample on corresponding days by means
of the Sign Test showed that when all 13 pairs of measurements are con-
sidered, chloride, nitrate, sulfate, and excess sulfate are significantly
more abundant at the roof site than at the marsh. This is not true of
sodium or excess chloride.
The presence of excess sulfate in all samples implies that it is
derived at least in part from natural marine sources of biogenic sulfur, but
it is not possible to determine whether these sources are local or are
contributed from the open ocean. Previous workers have shown that chloride
loss in marine aerosols may be due to absorption of gaseous biogenic sulfur,
and that the presence of biogenic sulfate in the aerosol may be associated
with excess potassium. Buat-Menard et al. (35) found both in aerosols
collected near upwelling regions in the Atlantic Ocean and speculated that
the potassium was derived from the film of surface organic matter, while
the sulfate was derived from biogenic sulfides that evolved from the water.
Therefore, it is instructive to examine the relationships among excess
sulfate, excess chloride, and excess potassium in our samples to see
whether it is possible to observe an effect local to one of the two sites,
which may implicate local sources of biogenic sulfur or other important
constituents.
Systematic differences at the sites were explored by determining
whether the concentrations of particulate constituents at one site vary
linearly with those at the other. If the correlation between sites is
high, and the intercept value is near 0.0 and the slope near 1.0, then we
may tentatively conclude that the concentrations at the two sites are
effectively identical. If the correlation is high and the slope is 1.0,
but the intercept differs from 0.0, we may conclude that there is an
excess of the constituent at one site which is not dependent on the abun-
dance at the other. This could be due to the presence of sources local to
the site with the excess, or to differences in the particle-size distribu-
34
-------
TABLE 5. MOREHEAD CITY MEAN PARTICULATE CONCENTRATIONS*
Constituent
Na+
so4
cr
Sea salt
Excess SO,
Excess Cl
Excess K
EF S0?~
sw- 4
EF Cl~
sw
EF K
sw
N0~
+**
Trajectory
marine
mixed
both
marine
mixed
both
marine
mixed
both
marine
mixed
both
marine
mixed
both
marine
mixed
both
marine
mixed
both
marine
mixed
both
marine
mixed
both
marine
mixed
both
marine
mixed
both
marine
mixed
both
Marsh
1.611.5
2.111.5
1.711.4
3.1+2.2
5.711.9
4.212.1
1.6+1.4
2.111.5
1.8+1.5
3.4H.5
3.711.6
3.511.5
2.6±2.3
5.111.9
3.712.2
- 1.412.5
- 2.711.5
- 2.012.1
0.18+1.52
0.23+1.6
0.19±1.4
7.7 ±1.7
10.8 ±1.6
9.1 ±1.7
0.38±3.0
0.21±2.5
0.28+2.9 -
4.1 ±1.4
3.9 ±1.4
4.0 +1.4
0.5 11.7
1.9 +1.9
1.0 ±2.4
0.32±4.5
0.14±5.6
0.23+4.6
Roof
1.711.5
2.511.5
2.011.3
3.712.1
6.6+1.9
4.812.1
1.7±1.5
2.5H.5
2.011.5
3.711.5
5.011.6
4.311.6
3.1+2.3
5.911.9
3.612.6
1.2+2.4
- 1.9+2.5
1.5+2.4
0.16H.4
0.2012.1
0.17±1.7
8.7 ±1.9
10.7 ±1.6
9.6 ±1.7
0.47±2.2
0.37+2.4
0.42±2.2
3.5 +1.3
3.2 il.6
3.4 H.4
0.8 H.3
1.6 +2.0
1.1 il. 9
0.1411.2
0.5314.4
0.28±4.2
*Geometric mean (ug m ) t 1 geometric standard deviation. Values below
the detection limit not used to calculate means.
**NH, was detectable at the roof only twice on each kind of day.
35
-------
tions collected at each site, and to the particle-size dependence of the
constituents in question. If the correlation is high, and the slope is not
equal to 1.0, then we may conclude that the constituent has a source
common to both sites, but that there is more at one than at the other, due
perhaps to particle-size concentration dependence and distribution effects,
or to the fact that one site is closer to a local source than the other.
Table 6 lists the results of the intersite regression analyses per-
formed separately for each class of days. In this table, the roof site is
viewed as the dependent variable and the marsh site as the independent
variable, so that the equations are for the roof concentration as a function
of the marsh concentration.
The roof site is the more exposed of the two, and somewhat closer to
the surf zone on the south side of Bogue Banks. In contrast, there are
trees and other obstacles around the collector at the marsh, which is at
ground level. Therefore, it seems likely that if systematic differences
do exist between the correlation of sodium at the two sites, they will be
due to a larger proportion of large-particle sodium at the roof.
On "marine" days, the intersite correlation for sodium is extremely
high, the intercept of the regression line is near 0.0, and the slope near
1.0. On "mixed" days, the intersite correlation is equally high, but the
intercept value indicates that some of the sodium at the marsh is inde-
pendent of that at the roof. The value of the slope indicates that some of
the sodium at the two sites is apparantly of common origin, but the amount
at the roof is about 1.36 times that at the marsh. These results on
"marine" days therefore seem to imply little difference in particle-size
distributions between the two sites, while those on "mixed" days suggest
a larger proportion of large-particle sodium at the roof site, and—perhaps—
an excess of small-particle sodium at the marsh site.
If this interpretation is valid, then other results listed in Table
6 for "marine" and "mixed" days would seem to imply the following:
1. Excess sulfate occurs chiefly in the small-particle size fraction,
and the extremely high correlation between the two sites and the fact
that the slope is nearly equal to 1.0 on both classes of days imply
36
-------
TABLE 6. REGRESSION EQUATIONS BY AIR MASS ORIGIN*
Constituent
Marine tralectories
Site
r
intercept
slope
X
m
xr
r
Mixed trajectories
intercept
slope
m
X
r
A. Intersite Correlations**
Excess SO,
Na
Cl loss
Excess K
N03
NH. '•
4
0.995
0.98
0.92
0.94
0.73
0.50
0.41
0.06
0.15
0.04
0.43
—
1.06
1.01
0.75
0.63
0.68
—
3.7
1.7
1.7
0.20
0.6
—
4.3
1.8
1.4
0.17
0.8
—
0
0
0
0
0
0
.998
.98
.94
.98
.96
.997
0.
-0.
-1.
-0.
0.
0.
42
67
34
05
17
07
1.00
1.36
1.30
1.13
1.14
0.90
6.9
2.4
2.8
0.27
1.5
0.5
7.3
2.7
2.3
0.25
1.9
0.4
B. Constituent correlations
Excess SO^ Cl loss
Excess SO, Excess K
Cl loss Excess K
Cl loss N03
Excess SO, NO
NH, Excess SO,
Na N03
NH. NO
4 3
marsh
roof
marsh
roof
marsh
roof
marsh
roof
marsh
roof
marsh
roof
marsh
roof
marsh
roof
0.95
0.90
0.95
0.98
0.93
0.84
0.73
0.56
0.83
0.80
0.96
0.71
0.92
0.70
0.81
0.38
0.15
0.11
0.12
0.10
0.11
0.02
0.37
0.37
0.60
2.11
-0.06
—
0.48
0.41
0.30
0.02
0.02
0.05
0.09
0.12
0.06
0.05
10.04
0.38
—
0.62
3.7
4.3
3.7
4.3
1.7
1.4
1.7
3.7
4.3
0.16
1.71
—
0.16
1.7
1.4
0.20
0.17
0.20
0.17
0.6
0.6
0.8
3.7
0.60
—
0.60
0
0
0
0
0
0
-0
-0
-0
-0
0
0
.97
.92
.98
.96
.93
.92
.45
.69
.61
.78
.995
.99
1.
0.
0.
0.
-0.
0.
-
_
2.
3.
4.
36
31
09
03
04
02
-
_
80
61
6
0.24
0.32
0.03
0.03
0.10
0.09
—
-0.12
6.08
6.69
6.4
7.3
6.4
7.3
2.9
2.7
—
7.3
0.46
0.40
2.9
2.6
0.26
0.25
0.26
0.20
—
1.9
6.4
7.3
-0.03
-0
-0
-0
.33
.58
.76
-
1.
-
76
--
-0.70
—
1.92
—
0.41
* Coefficients are listed only when the correlation ie high enough to be significant for normally distributed data.
"x and x" are arithmetic means of the marsh and roof values.
m r
**Roof measurements correspond to the dependent variable.
-------
that the concentrations at the two sites are nearly identical, except
-3
for a small and consistent excess of about 0.4 yg m excess sulfate at
the roof. These data are plotted in Figure 7.
2. Chloride loss is greater at the marsh than at the roof on both kinds
of days. On "marine" days, this loss does not seem to be due to parti-
cle-size differences at the two sites; consequently, it may reflect
some active process local to the marsh site. On "mixed" days, there is
chloride loss at the marsh which is independent of that at the roof,
while the slope of the regression line—which is almost identical to
that for sodium on "mixed" days—suggests that the apparently higher
chloride loss in constitutents common to the two sites may occur in the
large-particle fraction (Figures 8 and 9).
3. There is good correlation between excess potassium at the two sites
on both kinds of days, but the regression lines indicate that of the
potassium common to the two, there is less at the roof on "marine" days
and slightly more on "mixed" days. The results on "mixed" days may re-
flect a particle-size-related phenomenon, but this does not seem to be
the case for "marine" days.
4. The nitrate intersite correlation is modest on "marine" days, when
about one-half of the nitrate at the roof is independent of nitrate at
the marsh. On "mixed" days the correlation between the sites is high.
On these days, of the nitrate common to the two sites, there is slightly
more at the roof than at the marsh. On both kinds of days, the slopes
of the regression lines for nitrate and excess potassium are similar.
5. Ammonium results show virtually no correlation on "marine" days, and
a very high correlation on "mixed" days, when the ammonium at the roof
averages about 90% of that at the marsh.
The effects of types of days at the two sites may be further studied by
analyzing the interconstituent correlations. Some of these are listed in
Table 6.
The most interesting of these results are the high correlations between
chloride loss and excess sulfate, which indicate that the loss of the chlo-
ride is associated with the presence of gaseous sulfur. In the
38
-------
-O MARINE TRAJECTORIES
MIXED TRAJECTORIES
8 12 16
MARSH EXCESS SULFATE, M9/m3
20
24
Figure 7. Excess sulfate at Camp Glen roof site and Calico Creek marsh.
39
-------
MARSH SODIUM, pg/nf
Figure 8. Sodium at Camp Glen roof site and Calico Creek marsh on days
with marine trajectories.
40
-------
n
E
a
o
so
CO
CO
o
cc
o
o
o
cc
O SODIUM
CHLORIDE LOSS
2.0 3.0 4.0 5.0
MARSH CHLORIDE LOSS AND SODIUM, jug/m3
Figure 9. Chloride loss and sodium at Camp Glen roof site and Calico Creek marsh on days with
mixed trajectories.
41
-------
marsh, on "marine" days, most of the chloride loss is associated with
excess sulfate, while on "mixed" days, almost one-half of the average
chloride loss is independent of excess sulfate (Figure 10). This indicates
that other reactions may be involved in chloride loss on "mixed" days in
the marsh. In contrast, on "mixed" days at the roof, only about 10% of the
chloride loss is independent of excess sulfate (Figure 11). Anion replace-
ment of Cl with SO, implies a mass ratio of chloride lost to sulfate
gained of 0.73, while the slopes listed in Table 6 imply a ratio of about
one-half this value, since they range from 0.24 to 0.41. That is, there
appears to be about 1 mol of sulfate ions added to the particulates per
mole of chloride ion lost.
It is possible that some of the chloride loss is due to anion replace-
ment by nitric acid, which is subsequently converted to nitrate. Kadowaki
(45) reported NaNO^ in large particles collected at an urban coastal loca-
tion. There is little correlation between chloride loss and nitrate in our
data, which may be due in part to loss of nitrate by reaction with sulfuric
acid aerosol, as suggested by Barker et al. (46). Table 6 shows that
nitrate and excess sulfate are positively correlated on "marine" days, but
either poorly correlated or negatively correlated on "mixed" days. The
roof site data are plotted in Figure 12.
Excess potassium and excess sulfate are strongly correlated on both
kinds of days at both sites, but about one-half of the excess potassium is
independent of excess sulfate at the marsh on "marine" days. We found no
correlation between sulfate and potassium enrichment factors.
Submicrometer sulfate has often been observed to be associated with
ammonium in aerosols, and ammonium and excess sulfate are strongly corre-
lated at the marsh on both kinds of days (Figure 13) and at the roof site
on "mixed" days (Figure 14). At these times, a significant fraction of the
excess sulfate is independent of ammonium. Ammonium was almost always
_3
below the detection limit unless excess sulfate exceeded about 3 yg m
Ammonium may also be associated with nitrate, and Kadowaki (45) has
demonstrated that small-particle nitrate collected in a coastal urban
location consisted of NH.NO.,. We observed positive correlation between
42
-------
-O MARINE TRAJECTORIES
MIXED TRAJECTORIES
8 12 16
EXCESS SULFATE,;Ug/m3
Figure 10. Excess sulfate and chloride loss at Calico Creek marsh.
43
-------
7.0,
6.0
5.0
E 4.0
a.
to"
o
cc
o
3.0
2.0
1.0
-0.2
---O MARINE TRAJECTORIES
D MIXED TRAJECTORIES
8 12 16
EXCESS SULFATE, M9/m3
20
24
Figure 11. Excess sulfate and chloride loss at Camp Glen roof site
44
-------
3.0
2.0
E
>
a.
1.0
.'tl
O MARINE TRAJECTORIES
D MIXED TRAJECTORIES
12 16
EXCESS SULFATE,M9/m3
20
24
28
Figure 12. Nitrate and excess sulfate in marine and mixed trajectories
at Camp Glen roof site.
45
-------
O MARINE TRAJECTORIES
MIXED TRAJECTORIES
0 1.0 2.0
AMMONIUM,,
Figure 13. Ammonium and excess sulfate at Calico Creek marsh.
46
-------
O MARINE TRAJECTORIES
MIXED TRAJECTORIES
1.0 2.0
AMMONIUM,/jg/m3
Figure 14. Ammonium and excess sulfate at Camp Glen roof site.
47
-------
ammonium and nitrate in the marsh on "marine" days, but the intercept value
indicates that most of the nitrate is independent of ammonium. Nitrate was
also positively correlated with sodium in the marsh on "marine" days, which
suggests that both NaNO and NH.NO were present. These results are con-
sistent with those of Kadowaki.
There was only a weak correlation between ammonium and sulfate (Figure
14) and ammonium and nitrate at the roof site on "marine" days (Table 6).
On these days, ammonium was almost always below the detection limit at the
roof, and it seems likely that most of it originated in sources located
near the marsh. On "mixed" days, there is no correlation at either site
between sodium and nitrate, or at the marsh between ammonium and nitrate
(Table 6). This is consistent with the hypothesis that on these days
some nitrate was removed by reaction with excess sulfate. The negative
correlation between ammonium and nitrate at the roof on "mixed" days may
be due to the fact that most of the ammonium was associated with excess
sulfate.
These results may be summarized by saying that excess sulfate is
strongly correlated with chloride loss at both sites on both kinds of days,
but that there appear to be real differences in the chemistry of the aerosols
collected at the two sites and on the two kinds of days, as reflected in
the presence of nitrate and ammonium and their relationships to other
constituents. On "marine" days, these differences appear to consist of
the following: (a) chloride loss and nitrate are positively correlated at
the marsh, which appears to reflect a chloride replacement by nitric acid,
since sodium and nitrate are also positively correlated at this site,
while neither of these correlations is observed at the roof; and (b)
ammonium is positively correlated with excess sulfate and with nitrate in
the marsh, but only with excess sulfate at the roof, which seems to imply
the presence of ammonium nitrate at the marsh, but not at the roof. On
"mixed" days, the differences appear to be the following: Excess sulfate
is negatively correlated with nitrate at the roof and so is ammonium
(correlations at the marsh have the same sign, but are low), which may
reflect loss of nitrate by interaction with sulfate that is present together
with ammonium.
48
-------
While there are real differences on "mixed" days between the inter-
constituent correlations at the two sites, it is not possible to distinguish
differences in chemical reactions leading to the formation of aerosols in
the two locations from differences in the particle-size distributions in
the samples collected at each site.
These conclusions are dependent on the assumption that the particle-
size distributions at the two sites are the same on "marine" days, but
different on "mixed" days. It is instructive to examine the intersite
and interconstituent correlations calculated from all the data, without
regard to air mass trajectory classifications. These are shown in Table 7.
Table 7 shows that the intersite correlations continue to be extremely
high, but that only K appears to be present in nearly identical concentra-
tions at both sites, as judged from the slope value of 1.02 and the inter-
cept value of -0.03. There appears to be more excess sulfate at the roof
_3
site than at the marsh, as revealed by the intercept value of 0.47 yg m ,
but the value of the slope suggests that there is no other systematic
difference between the two sites in excess sulfate abundance. The Na
relationship shows systematically more sodium at the roof site than at the
marsh, but the negative intercept indicates some sodium present at the
marsh that is independent of that at the roof. These sodium data may be
compared with those for chloride loss, which indicate that chloride loss
is greater at the marsh than at the roof site.
The slopes and intercepts of the regression lines for the intercon-
stituent regressions shown in Table 7 are very nearly the same at both
sites, except for the correlation between excess sulfate and chloride loss,
which indicates that about one-third of the chloride loss observed at the
marsh is independent of excess sulfate, while this is the case for only
about 10% of the total at the roof site. It will be noted that the slopes
of these regression lines imply a loss of about 1.2 mol of chloride ions
per mole of sulfate ion gained.
There are no entries in Table 7 for correlations between nitrate and
the other constituents, because all of these are very low (<0.30) when the
data are grouped. This is to be expected, given that the analyses reported
49
-------
TABLE 7. REGRESSION EQUATIONS - ALL DAYS*
Constituent
r
intercept slope
x
m
x
r
A. Intersite Correlations
Excess 30^
Na
Excess Cl
Excess K
N03
NH,
0.
0.
0.
0.
0.
0.
997
98
91
96
96
95
0.
-0.
-0.
-0.
0.
-0.
47
24
30
03
20
06
1.
1.
0.
1.
1.
0.
01
19
97
02
11
80
5.
2.
2.
0.
1.
0.
2
1
2
24
02
36
5
2
1
0
1
0
.7
.2
.8
.21
.33
.23
B. Constituent Correlations
Excess SO, Cl loss marsh
roof
Excess SO. Excess K marsh
roof
Cl loss Excess K marsh
roof
NH. Excess SO. marsh
roof
0.
0.
0.
0.
0.
0.
0.
0.
93
92
97
93
88
88
96
85
0.
0.
0.
0.
0.
0.
3.
4.
75
16
10
06
07
07
0
2
0.
0.
0.
0.
0.
0.
6.
7.
31
32
03
03
07
07
69
04
5.
5.
5.
5.
2.
2.
0.
0.
1
7
1
7
3
0
31
21
2
2
0
0
0
0
5
5
.3
.0
.23
.21
.23
.21
.1
.7
Coefficients are listed only when the correlation is high enough to be
significant for normally distributed data.
x and x are arithmetic means of the marsh and roof values.
m r
i I
Roof measurements correspond to the dependent variable.
50
-------
above showed that these are strongly dependent on air mass trajectory
classification. These results would appear to validate the air mass
trajectory classifications.
PARTICULATE ABUNDANCES AT SWANSBORO
Only five samples were collected at each site, due to adverse weather
conditions and other problems. The resulting concentration data were
analyzed in a manner similar to that employed for the Morehead City data,
except that the small number of data points precluded use of the Sign
Test. Trajectory analyses indicated that air masses with wholly continental
trajectories were sampled on all days, although local surface winds often
blew from the Atlantic Ocean, and windspeeds up to 10 to 15 m s occurred
some of the time.
As might be expected under these conditions, sea salt was present in
all samples, the more exposed site at the Coast Guard Station showing
-3
systematically higher concentrations and a larger range—2.5 to 40 yg m —
as opposed to the other two sites, where sea salt ranged from 2.2 to 7.0
-3
yg m .
Geometric means of constituents of interest are shown in Table 8.
Statistical comparisons showed that both total chloride and total sodium
at the Coast Guard Station significantly exceeded that at Little Kinston
(p < 0.05 as determined by Student's t-test of the significance of the
difference between the mean logs). The mean levels of these constituents
at Breezy Point were slightly higher than those at Little Kinston, and
differences between this site and those at the Coast Guard Station were
only marginally significant (p^ < 0.1).
All samples contained excess sulfate and exhibited evidence of
chloride loss. At the Coast Guard Station chloride loss ranged from 1.9
_3
to 7.0 yg m , and chloride enrichment from 0.05 to 0.92. Excess sulfate
_3
at this site ranged from 2.3 to 6.6 yg m , and sulfate enrichment factors
from 5 to 16.3. At the other two sites in more protected locations, chloride
_3
loss ranged from 2.8 to 5.8 yg m , and sulfate enrichment factors ranged
from 12.6 to 33.3.
51
-------
TABLE 8. SWANSBORO MEAN PARTICULATE CONCENTRATIONS*
Constituent
Coast Guard Station
Breezy Point
Little Kinston
Na+ +
2-
S°4
Cl~ +
Sea salt
2-
Excess SO,
Excess Cl
Excess K
2-
EF SO,
sw 4
EF Cl~
sw
EF K
sw
N0~ **
NH+
6.
5.
4.
10.
3.
-3.
0.
4.
0.
3.
1.
0.
2 ±
3 i
6 ±
5 ±
4 ±
7 ±
66 ±
4 ±
4 ±
8 ±
00 ±
57 ±
2
1
2
3
1
1
1
1
3
1
3
1
.2
.5
.0
.7
.6
.6
.48
.4
.3
.4
.02
.31
2.
4.
0.
4.
3.
-4.
0.
6.
0.
6.
0.
0.
7 ± 1
5 ± 1
7 ± 2
3 ± 1
8 ± 1
Oil
62 + 1
6+1
1+2
4 i 1
06 ± 3
61 ± 1
.3
.4
.9
.5
.5
.3
.36
.5
.3
.5
.43
.86
2
4
0
3
3
-3
0
6
0
6
0
0
.3
.1
.4
.3
.2
.6
.55
.6
.1
.2
.13
.57
± 1
i 1
± 2
± 1
i 1
i 1
+ 1
i 1
i 1
+ 1
i 3
+ 1
.3
.4
.5
.4
.4
.2
.52
.3
.9
.3
.55
.87
* Geometric mean and geometric standard deviation. Values below detection
limit were not used in calculating the means. Units are yg m 3 except
for enrichment factors, which are dimensionless.
Differences between Coast Guard Station and Little Kinston means are
significant beyond the 0.05 level. Differences between Coast Guard
Station and Breezy Point means are marginally significant.
I i
Differences between Coast Guard Station and Breezy Point means are
significant beyond the 0.05 level. Differences between Coast Guard
Station and Little Kinston means are marginally significant.
52
-------
_3
Ammonium was observed in all samples and ranged from 0.3 to 1.1 yg m ,
while nitrate was observed in all but four samples, and ranged up to 3.1
yg m . Potassium was present in all samples, and ranged from 0.4 to 1.5
-3
yg m .
The most striking feature of these data was the chloride depletion
exhibited in all samples. It ranged from 7 to 95% at the Coast Guard
Station, 59 to 96% at Breezy Point, and from 77 to 96% at Little Kinston.
The mean losses were 40% at the Coast Guard Station and 90% at the other
two sites. The air masses sampled were always continental and had spent
effectively no time over the ocean, and there are no significant nonmarine
sodium sources in this region. We therefore infer that the reactions leading
to the chloride depletion were rapid ones.
Analyses of the dependence of the concentrations of constituents at
each site on their concentrations at the others showed high intersite
correlations for excess sulfate, sodium, and excess potassium, but moderate
or low correlations for ammonium, nitrate, and chloride loss. In general,
the two protected sites tended to exhibit stronger intersite correlations
than either did with the Coast Guard Station.. None of the sodium regression
lines implied similar particle-size distributions at any pair of sites.
Chloride loss shows no correlation with excess sulfate, potassium,
nitrate, or ammonium at any site. Ammonium was correlated with excess
sulfate at all sites, but the correlation was higher at the two protected
sites (r = 0.92 and 0.98) than at the Coast Guard Station (r = 0.78).
The wide range of sodium and sea salt concentrations at the Coast
Guard Station implies that the samples collected here on different days
differed in the proportion of large-particle sea salt aerosols present. If
this is the case, then there should be a negative correlation between
sodium and enrichment factors of constituents which are present chiefly
in the small particle fraction, and a positive correlation between sodium
and chloride enrichment. These were observed. The correlation between
sodium and sulfate enrichment factor was -0.999, that between sodium and
potassium was -0.95, and that between sodium and chloride enrichment was
0.86. These results imply that correlations between enrichment factors in
53
-------
non-size-segregated particulate samples do not necessarily imply common
sources for the corresponding elements or ions, since this may be an artifact
of particle-size dependence, combined with variations in particle-size dis-
tributions in the samples in question. Excess sulfate was observed to corre-
late well with excess potassium at all sites (r = 0.80 to 0.98). Since
their enrichment factors were not correlated with sodium at Breezy Point or
Little Kinston, we infer that the association between the two constituents
was probably real.
We may summarize these results by saying that excess sulfate is present
in all samples, but there is no evidence other than its correlation with ex-
cess potassium to suggest that it is of biogenic origin. Chloride loss is
also observed in all samples but showed no systematic relationship to any
other constituent consequently, no simple anion-replacement process is im-
plied. The coastal location of the sites and the fact that all air masses
had traversed the continent before reaching the sampling location implies
that chloride loss is a rapid process in coastal locations.
54
-------
SECTION 6
DISCUSSION
HYDROGEN SULFIDE AND SULFUR DIOXIDE
The observation of HLS in the atmosphere at levels above 1,
(1 ppb) in both Morehead City and Swansboro, and of S00 at Swansboro at
3
levels up to 45 yg/m (17 ppb) during high winds implies that these are
biogenic and that they must be derived from the metabolism of bacterial
sulfate reducers, and not from the decomposition processes described by
Challenger (53), and Kadota and Ishida (8), and proposed by Lovelock et al.
(9) as the source of 100 million tons of atmospheric sulfur per year. This is
implied by the high levels observed, by the association between tidal mud
exposure and H~S concentration in the atmosphere at the marsh site, and by
the absence of anthropogenic sources of S0« or fLS at Swansboro.
The observation of variable and sometimes very high concentrations of
H^S associated with the exposure of mud in Calico Creek is consistent with
the observations of Hansen et al. (33) and Brannon (30), who observed
large ELS emissions from tidal pools and exposed tidal muds. They also
confirm the commonplace observation that ELS can be detected by its odor
near tidal marshes and other sites where it is produced by bacterial
sulfate reducers.
The higher low-tide ELS concentrations observed at night in the marsh
may be due to the lower windspeeds experienced then. However, the failure
of low-tide daytime H?S concentrations to exhibit any systematic relation-
ships to surface windspeed suggests that other factors may be at work. There
are several candidates.
The daytime H?S concentrations may reflect atmospheric dispersion
effects that depend on vertical turbulence and mixing heights as well as on
horizontal windspeeds, effects that are probably more important in the
55
-------
daytime that at night. It is also possible that the H2S emissions differed
systematically due to differences in the area and duration of mud exposure
during low tides during the day as compared to low tides at night.
Furthermore, several purely biological processes are known to reduce
the daytime concentrations of H S in the interstitial water of organic-rich
muds like those at the Calico Creek site. Chief among these are the activity
of the green and purple photosynthetic sulfur bacteria which oxidize H2S
in the daytime, and the daytime release of photosynthetically produced C^
from the roots of Spartina plants. JjSrgensen and Fenchel (21), J?5rgensen
(22), Hansen et al. (33), and Brannon (30) have all documented the influence
of one or both of these factors on tLS abundance in interstitial muds. Green
and purple sulfur bacteria could be observed in the surface muds of Calico
Creek, and it is likely that their presence and that of the ^. alterniflora
acted to lower the ELS emissions during the daytime.
The nearly total absence of ELS at the roof site on occasions when it
was abundant in the marsh is interesting. It suggests that either the total
ELS emissions from the marsh were quite low, or that ELS was rapidly removed
from the atmosphere after it was evolved from the surface muds. Since we
have no basis for estimating the flux of ELS from the marsh, we cannot
choose between these alternative explanations. The area of the marsh is
2
only about 0.5 km ; consequently, it represents only about 3% of the land
area of a circle with the marsh sampling site at its center and the roof
site on its perimeter. It should also be noted that if the failure to
observe tLS at the roof site is due in part to its rapid removal from the
atmosphere, then this removal occurs at night as well as during the day-
time, and the H2S does not appear to be converted to SO
The results of the estuarine experiment contrast sharply with those
of the marsh experiment. At this time we observed widespread occurrence
of ELS independent of the local tidal cycle and also observed considerable
SO,.,. The correlation of ELS and S0? detected at the Coast Guard Station
indicates that both have a common source and this fact, together with the
absence of significant local anthropogenic S0_ and ELS sources, implies
that both are derived from biological processes and that the S0? was pro-
duced from the ELS. The correlation between ELS at the other two sites
56
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suggests either that the ELS is derived from a single common source in these
locations, or that it is released in synchrony from sources local to both
Breezy Point and Little Kinston.
The origin of this ELS remains obscure. We have pointed out above that
while many authors have claimed that ELS cannot be evolved from surface
water because it is too rapidly oxidized by 0? in water, the available
evidence indicates that under some conditions ELS may be quite stable in
natural waters. This suggests that the H S we observed may have been
derived from anoxic underwater sediments and may have evolved into the
atmosphere from the surface of the water. The high windspeeds and swift
currents could have promoted this release.
Alternatively, it is possible that biogenic H2S may be released to
the atmosphere from anoxic levels in coarse sand that have been observed
10 to 50 cm below the surface (54). If so, then the exposed sand bars near
the Coast Guard Station and the shore sediments near all three sites could
have contributed to the observed ELS levels. In any case, it seems likely
that the SCL was derived by atmospheric oxidation from ELS, which implies
that the half-life of ELS in this environment may be on the order of
minutes, rather than hours, as proposed by Sprung (55).
The contrast in the results obtained at Morehead City in the summer
and at Swansboro later in the year is puzzling. If both ELS and S0? can be
derived from coarse sand sediments and/or surface water in the fall at
Swansboro, why were these two not observed at the roof site the previous
summer, since these kinds of sources exist near that site? It seems likely
that the differences are due to the difference in the season, either
seasonal variations in the biological or physical processes responsible
for the release of these gases from such sites (which seems unlikely), or
in the processes that govern their atmospheric fate.
PARTICULATE SULFATE AND OTHER CONSTITUENTS
As we have pointed out above, the particulate results obtained at
Morehead City are somewhat ambiguous, in that the interconstituent regres-
sions for excess sulfate, chloride loss, and excess potassium do not differ
greatly between sites if all samples are analyzed together. These results
57
-------
show that chloride loss is greater at the marsh than at the roof site, and
that some of the chloride loss at the marsh is independent of that at the
roof.
If we separate the observations by air mass trajectory classification,
the differences between the sites become more pronounced. High inter-
constituent correlations between some other constituents are observed on
some classes of days. These results involve the abundance and intersite
correlations of ammonium and nitrate, and their relationships to excess
sulfate and sodium. Since these results are compatible with the observations
of other workers, we are inclined to think that they validate the air mass
trajectory classifications.
The presence of excess sulfate on "marine" days indicates that it is
probably biogenic in origin. The fact that excess sulfate is strongly
associated with excess potassium but not with iron or vanadium on both
kinds of days, and that the relationship between chloride loss and excess
sulfate is similar on both "marine" and "mixed" days suggests that much of
the particulate sulfate present on "mixed" days is also biogenic.
The high correlation between chloride loss and excess sulfate observed
at both Morehead City sites indicates that most of the chloride loss was
due to replacement by sulfate anions, at a ratio of approximately one
chloride ion lost per excess sulfate ion gained. The evidence indicates
that ammonium sulfate and ammonium nitrate are present in the fine particle
fraction at the marsh on "marine" days, and that some of the chloride loss
may be due to anion replacement by nitric acid, as proposed by Robbins et
al. (41) and observed by Martens et al. (37), and Kadowaki (45). However,
the failure to observe strong positive correlation between nitrate and
chloride loss on "mixed" days (which would be expected if NaCl were replaced
by NaNO ) may be due to loss of some of the nitrate by reaction with sul-
furic acid aerosols (or even S0?) as proposed by Harker et al. (46). This
appears likely in view of the slight negative correlation between excess
sulfate and nitrate on "mixed" days at the roof site, which contrasts
sharply with the positive correlation between these constituents at both
sites on "marine" days.
58
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The partlculate results observed later in the year at Swansboro are not
comparable to those observed at Morehead City, due to the difference in
season and the fact that all air masses sampled had passed over the conti-
nent and none had spent time over the ocean. The fact that chloride loss
was observed in all these samples is interesting, since it suggests that
chloride loss due to interaction with atmospheric S (as SO,,, H_SO, , or some
other species) occurs rapidly.
The evidence available from Swansboro provides very little insight into
the question of the possible biogenic origin of the excess sulfate, other
than its association with excess K.
Hoffman and Duce (56) showed that potassium is not enriched relative
to sodium in aerosols collected in Hawaii from the trade winds under condi-
tions that carefully eliminate contaminated air from land sources. Hoffman
et al. (57) showed that potassium enrichment in marine aerosols collected
from shipboard in the Atlantic Ocean is very low (e.g., 1.05 or less) if
little iron or dust of continental origin is present, but may otherwise
be large. Their work implies that potassium enrichment is due largely to
contamination from land sources. However, Chesselet et al. (39) demonstrate
that small-particle potassium may be derived from the organic film at the
surface of the ocean, and they report its presence in the Mediterranean Sea
and in the Atlantic Ocean. Buat-Menard et al. (35) observed potassium
enrichment near the Gulf of Guinea in aerosols that did not contain insoluble
dusts, calcium, or other evidence of dusts derived from continental sources,
and inferred that both excess sulfate and potassium were derived from the
ocean surface in this biologically productive region. The simultaneous
observation of chloride loss balanced by excess sulfate indicated to these
authors that the chloride loss was due to anion replacement by sulfuric
acid derived from biogenic gaseous sulfur originating in the productive
surface waters.
The fact that excess sulfate, chloride loss, and excess potassium
are highly correlated at Morehead City, while none of these are correlated
with iron, lead,'or vanadium, suggests that the sulfate is biogenic. However,
it seems that a negative correlation between sulfate and chloride enrichment
59
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factors does not always imply a causal relationship but may reflect day-to-
day variations in the large-particle sodium abundance, as occurred at the
Swansboro Coast Guard Station.
60
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65
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/3-78-061
2.
3. RECIPIENT'S ACCESSI Of* NO.
TITLE AND SUBTITLE
BIOGENIC SULFUR COMPOUNDS IN COASTAL ATMOSPHERES
OF NORTH CAROLINA
. FIE
6. PERFORMING ORGANIZATION CODE
AUTHOR(S)
Dian R. Hitchcock, Lester L. Spiller* and
William E. Wilson*
8. PERFORMING ORGANIZATION REPORT NO.
. PERFORMING ORGANIZATION NAME AND ADDRESS
Hitchcock Associates
Norton Lane
Farmington, CT 06023
10. PROGRAM ELEMENT NO.
1AD712 BB-16 (FY-77)
11. CONTRACT/GRANT NO.
FA-8-0764A
2. SPONSORING AGENCY NAME AND ADDRESS
^Environmental Sciences Research Laboratory - RTF, NC
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final 8/76 - 2/77
14. SPONSORING AGENCY CODE
EPA/600/09
5. SUPPLEMENTARY NOTES
16. ABSTRACT
2-
Atmospheric H2S, S02, and particulate SOi/ , Na', Cl , NH^ ' , and N03 were
measured in two experiments on the North Carolina coast to determine the levels of
biogenic sulfur species at marsh and estuarine locations where dissimilatory bacterial
sulfate reduction produces H2S in local anoxic muds. The first (summertime) experi-
ment demonstrated the occurrence of variable and high H2S levels—4-h means up to
80 yg/m3 (57 ppb)—associated with low-tide mud exposure in a Spartine alterniflora
marsh. Little or no S02 was observed here, and little or no S02 or H2S were observed
at a background site 2.4 km distant. Biogenic sulfate in marine air masses ranged
from 2 to 13 yg/m3, and was strongly associated with the loss of chloride from
marine aerosols. Both H2S and S02 were observed in the second (autumn) experiment
at concentrations up to 7 yg/m3 (5 ppb)(H2S) and 25 yg/m3 (17 ppb)(S02) at an estuarine
site where anoxic muds are not exposed to low tide, under conditions which implied a
biogenic origin, and the rapid conversion of biogenic H2S to S02. Particulate excess
(non-sea salt) sulfate and chloride loss from marine aerosols'were observed at this
site in continental air masses (marine air masses did not occur).
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
COS AT I Field/Group
*Air pollution
^Biological productivity
*Sulfur inorganic compounds
^Aerosols
*Measurement
*Coasts
North Carolina
13B
08A
07B
07D
08F
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
UNCLASSIFIED
21. NO. OF PAGES
74
20. SECURITY CLASS (This page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
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