dissolved organic carbon and organic acids in coastal new zealand rainwater
TRANSCRIPT
Atmospheric Environment 36 (2002) 3557–3563
Dissolved organic carbon and organic acidsin coastal New Zealand rainwater
Robert J. Kiebera,*, Barrie Peakeb, Joan D. Willeya, G. Brooks Averya
aDepartment of Chemistry and Marine Science Program, University of North Carolina at Wilmington,
Wilmington, NC 28403-3297, USAbDepartment of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
Received 7 December 2001; accepted 29 March 2002
Abstract
Dissolved organic carbon (DOC), formate, acetate, oxalate and a variety of inorganic ions were measured in 54 rain
events collected on the southern portion of the South Island of New Zealand. Concentrations of DOC ranged from a
low of 10mM to a high of 401 mM with a volume weighted average concentration of 58 mM which is in the range
of concentrations measured at coastal locations in the Northern Hemisphere. The concentration of DOC varied by
season with warm season rain (October–March) having somewhat higher concentrations relative to winter rain events
(April–September). DOC levels in rain were also influenced by storm origin with maritime events having significantly
lower levels relative to terrestrially influenced storms. Formic and acetic acids comprised a relatively small fraction of
the DOC pool in New Zealand rain with volume weighted average concentrations of 1.5 mM which was not influenced
by storm origin or season. There was a significant correlation between formic and acetic acid concentrations in these
rain events suggesting they have similar sources or have different sources of approximately the same strength. This
DOC and organic acids data will allow for a better understanding of the global tropospheric transport of carbon
compounds and will aid in quantification of atmospheric carbon removal via wet deposition in the Southern
Hemisphere. r 2002 Elsevier Science Ltd. All rights reserved.
Keywords: DOC; Organic acids; Rainwater composition; New Zealand
1. Introduction
Dissolved organic carbon (DOC) is an ubiquitous,
key component of atmospheric waters (Willey et al.,
2000). It is a major constituent of both marine and
continental rain where it is present in concentrations
greater than that of nitric and sulfuric acids combined.
One of the most significant aspects of DOC geochem-
istry in the troposphere is its role in the removal of
incompletely oxidized carbon from the atmosphere via
wet deposition (Willey et al., 2000). The rainwater
removal of atmospheric organic carbon plays a sig-
nificant role in global carbon cycling and hence must be
considered along with other parameters in global
warming models.
Despite the significance of DOC in the global carbon
cycle, many questions remain regarding its atmospheric
distribution. DOC variability in the global troposphere
is not well constrained primarily because there are
relatively few measurements in rainwater, especially in
the Southern Hemisphere. In an earlier literature review
of DOC concentrations in rainwater, Willey et al. (2000)
reported only two studies in coastal locations in the
Southern Hemisphere. One study contained data for a
single sample analyzed north of Samoa over thirty years
ago (Williams, 1967) while the second involved four
samples analyzed in the western Pacific approximately a
decade ago (Sempere and Kawamura, 1996). The lack of
rainwater DOC data in the Southern Hemisphere is the
major source of uncertainty in current global flux
*Corresponding author. Tel.: +1-910-962-3865; fax: +1-
910-962-3013.
E-mail address: [email protected] (R.J. Kieber).
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 2 7 3 - X
estimates of carbon removal via wet deposition, an
important component in global models of atmospheric
carbon transport.
The current study presents DOC concentrations in
rainwater collected over a 1 yr sampling period at a
coastal location near Dunedin, New Zealand. The data
set is unique because it is one of the first detailed studies
of rainwater DOC levels in the Southern Hemisphere. In
addition to presenting detailed DOC data for New
Zealand rainwater, this study also contains the only
simultaneous determination of formic, acetic and oxalic
acid concentrations in samples also analyzed for DOC in
the Southern Hemisphere. The influence of season and
storm type on DOC and organic acid levels in New
Zealand rainwater, and their correlation with other
commonly monitored rainwater parameters, is also
evaluated.
2. Experimental
2.1. Sample collection and storage
Rainwater samples were collected on the southeastern
coast of the South Island of New Zealand (45151S,
170130E) at a site 3 km from the city of Dunedin
in a water reservoir area approximately 10m above
ground level. Rainwater was collected on an event basis
using an Aerochem Metrics (Bushnell, FL) ACM
Model 301 Automatic Sensing Wet/Dry Precipitation
Collector equipped with high-density polyethylene
(HDPE) buckets and a HDPE funnel. The ACM Model
301 automatically removes an airtight cover from the
wet deposition bucket at the onset of precipitation
and replaces the cover approximately 5min after
precipitation ceases minimizing airborne particulate
contamination. Tygon tubing with a fluoropolymer
FEP (inert) liner connected the funnel to an acid-cleaned
2 l Teflon bottle inside a closed HDPE housing.
Procedural blanks performed with MQ water in place
of rainwater had undetectable DOC and organic acid
concentrations.
Rain samples for organic acid analysis were placed in
30ml Teflon vials, preserved with chloroform, and
stored in a refrigerator (41C) immediately after collec-
tion to stop biological activity (Avery et al., 2001, 1991;
Keene and Galloway, 1984). Chloroform blanks made
with deionized pH 4.5 water in place of authentic
rainwater showed no organic acids contamination.
Previous studies demonstrated organic acids stabilized
with chloroform in this manner are stable for several
months (Avery et al., 2001, 1991). Rain samples for
DOC analysis were placed immediately after collection
in 50ml muffled glass vials and frozen for later analysis.
Earlier storage experiments with rainwater samples
preserved in this manner indicated DOC concentrations
were stable (73%) for 1 yr (Peierls, 1997; Willey et al.,
2000).
2.2. Season definition, storm type and data analysis
Seasons were defined as winter (April–September) and
summer (October–March). Storms were classified via
back trajectory analysis supplied by the New Zealand
Institute of Water and Atmospheric Research (NIWA).
Rainwater component concentration averages and
standard deviations were volume weighted which mini-
mizes effects of small rain events on averages and is the
mathematical equivalent to combining all rain samples
into one container prior to analysis (Topol et al., 1985).
Rainfall amount averages are simple averages with
standard deviations based on n � 1: All average pH
values were computed from volume weighted hydrogen
ion concentrations. Non-sea-salt sulfate (NSS) concen-
trations were calculated using seawater chloride/sulfate
ratios assuming all chloride present was from seasalt
which is appropriate for rain in a coastal location
(Kieber et al., 1999).
2.3. Analytical methods
2.3.1. DOC determination
DOC was determined by high-temperature combus-
tion (HTC) using a Shimadzu TOC 5000 total organic
carbon analyzer equipped with an ASI 5000 autosam-
pler (Shimadzu, Kyoto, Japan). Standards were pre-
pared from reagent grade potassium hydrogen phthalate
(KHP) in Milli-Q Plus Ultra Pure Water. Samples and
standards were acidified to pH 2 with 2M HCl and
sparged with carbon dioxide free carrier gas for 5min at
a flow rate of 125mlmin�1 to remove inorganic carbon
prior to injection onto a heated catalyst bed (0.5% Pt on
alumina support, 6801C, regular sensitivity). A non-
dispersive infrared detector measured carbon dioxide
gas from the combusted carbon. Each sample was
injected 4 times. The relative standard deviation was
p3%.
2.3.2. Organic acids determination
Formic and acetic acid concentrations were measured
with a Dionex 4000i/SP ion chromatograph equipped
with a SP4290 integrator, Dionex IonPacR AS11 4mm
analytical column, AG11 4mm Guard column and
anion micromembrane Suppressor Model AMMS-11. A
gradient system was used to separate formic and acetic
acids from other organic acids. The three eluents
consisted of deionized (DI) water, 0.005M NaOH and
0.1M NaOH. At the beginning of each week, eluents
were prepared using degassed DI water to avoid CO2contamination. A He atmosphere was constantly main-
tained over eluents to minimize CO2 adsorption into
solvents. The eluent flow rate was 1.5mlmin�1. The
R.J. Kieber et al. / Atmospheric Environment 36 (2002) 3557–35633558
regenerant was 0.05N H2SO4 at a pressure of 5 PSI
which produced a flow rate of 5mlmin�1. Formate and
acetate standards were prepared daily from concentrated
stock solutions prepared every other month.
2.3.3. Supporting analyses
Anions (chloride, nitrate and sulfate) were measured
using suppressed ion chromatography using synthetic
rain as a standard (Fitchett, 1983). Rainwater pH was
determined using a Ross electrode calibrated with low
ionic strength 4.10 and 6.97 buffers (Boyle et al., 1986).
Ionic strength adjuster (pHix from Orion Research
Incorporated, Boston, MA) was added to each sample
to match the ionic strength of samples to that of the
buffers.
3. Results and discussion
3.1. DOC in New Zealand rainwater
The volume weighted average concentration of DOC
collected during 54 rain events between April 1999
and March 2000 in Dunedin, New Zealand was
58mM79mM (Table 1). The range of DOC values
varied from a low of 10 mM to a high of 401mM with a
simple average concentration of 68 mM. During this
sampling period, 560mm of rainfall was collected which
is somewhat o30 yr average annual rainfall in Dunedin
of 1102mm rain (NIWA). The number of events
sampled for DOC analysis represents approximately
95% of the total number of storms during this time
interval.
The concentration of DOC measured at Dunedin was
remarkably similar to DOC concentrations determined
at other island locations (Table 2). Eklund et al. (1997)
reported a volume weighted mean of 58 mM organic
carbon in rainfall falling at La Selva, Costa Rica in 110
events sampled during a three year study between
February 1992 and 1995. In a similar study, McDowell
et al. (1990) reported a volume-weighted mean of 52 mMDOC in precipitation from 34 storms at El Verde,
Puerto Rico. The most detailed study of DOC concen-
trations was by Willey et al. (2000), who measured
DOC concentrations on the southeastern coast of the
United States near the city of Wilmington, NC.
The authors reported a somewhat higher volume
weighted DOC concentration relative to the island
locations of 114mM (n ¼ 205). The higher concentration
determined at Wilmington most likely reflects
greater continental influences at this site relative to the
island locations because Wilmington is located near a
much larger land mass, which increases DOC concen-
trations via several mechanisms including increased
biogenic and anthropogenic emissions (Willey et al.,
2000).
3.2. Seasonal variations in DOC
Cold and warm seasons DOC levels were compared
in order to assess seasonal variations in DOC concen-
trations in New Zealand rainwater. Warm season
rain falling between October and March had slightly
higher DOC concentrations compared with winter
rainwater falling between April and September
(Table 1, t test, po0:03). This seasonality in rain DOCconcentrations probably reflects increased inputs
from terrestrial vegetation in the southern part of the
South Island during the growing season relative to the
winter season. It may also represent differences in
anthropogenic emissions from varied sources during
the course of a year. The only other study of seasonal
patterns in DOC concentrations was in the Northern
Hemisphere in Wilmington, NC (Willey et al., 2000). A
much greater seasonal difference was observed with
higher DOC concentrations in continental rain in
summer compared with winter. Marine rain events at
Wilmington, however, did not display any seasonal
variation. The relatively weak seasonality in New
Zealand DOC falls between the strong seasonality
observed in continental storms and lack of seasonality
in marine storms at Wilmington, most likely because NZ
precipitation is a mixture of these two event type end
members.
Table 1
Volume-weighted average concentration (vw ave) and standard deviation (vw s) of dissolved organic carbon (DOC), formate andacetate in rainwater collected in Dunedin New Zealand with all storms considered and broken down by season
n Amt (mm) DOC vw
ave (mM)DOC vw
s (mM)Formate
(mM)Acetate
(mM)% DOC as
formate and
acetate
pH
All 54 560 58 9 1.5 1.5 8 4.90
Summer 13 152 69 22 1.2 1.0 5 4.75
Winter 41 408 51 9 1.6 1.7 10 5.01
Rain amount is expressed as number of samples (n) and mm collected. % DOC as formate and acetate was calculated as total carbons
contributed by the vw ave concentrations of formic and acetic acids divided by vw ave DOC concentration.
R.J. Kieber et al. / Atmospheric Environment 36 (2002) 3557–3563 3559
3.3. Impact of storm origin
Rain events collected at Dunedin were further
subdivided based on storm origin as either marine
dominated or mixed which includes storms with both
marine and terrestrial influences. Back trajectory analy-
sis of events originating in the Southern Ocean with no
air mass travel over land prior to arrival in New Zealand
were classified as marine in nature (NIWA). These often-
intense storms receive little terrestrial or pollutant input
in their precipitation. The ten maritime storms classified
in this manner had significantly lower DOC concentra-
tions (24 mM) relative to the other storms analyzed
(Table 3, t test, po0:03). In addition, marine dominatedevents did not display any seasonal variation in DOC
concentration (p > 0:5) in agreement with marine domi-nated events analyzed in Wilmington (Willey et al.,
2000).
The concentration of DOC measured in marine
storms in New Zealand is almost identical to the value
(23mM) reported in marine rain sampled in coastal
North Carolina by Willey et al. (2000). This latter DOC
value is thought to be representative of all oceanic rain
(not just at Wilmington) and was determined using a
variety of methods including published reports of DOC
concentrations and estimates of DOC made from
aerosol scavenging and formic to acetic acids ratios.
Willey et al. (2000) also found 40% of DOC in coastal
rainwater from Wilmington is resistant to microbial or
chemical degradation. This suggests there is a non-
reactive pool of DOC capable of being transported long
distances before ultimately being removed via wet
deposition. The presence of a recalcitrant DOC fraction
would explain why there appears to be a background
DOC concentration in oceanic rain (23 mM) amonggeographically diverse sites in the Northern Hemisphere
and the present site in the Southern Hemisphere.
3.4. Formic and acetic acids
The volume weighted average concentration of both
formic and acetic acids in New Zealand rainwater was
1.5mM with simple average concentrations of 1.8 and
2.0 mM, respectively (Table 1). The concentration of
formic and acetic acids falls within the range of
concentrations recently measured in marine rains falling
over the open ocean aboard the R.V. Endeavor at the
Bermuda Atlantic Time Series Station (31.401N,
64.101W) (Kieber et al., 2001). The concentration of
formic and acetic acids measured during this cruise were
2.4 and 1.2, respectively, which are very low and similar
to the concentrations measured in the New Zealand
samples.
In general, concentrations reported in the present
study fall within the range of concentrations found in
uncontaminated precipitation in remote regions of the
globe as summarized by Khare et al. (1997). Keene and
Galloway (1986) also measured organic acid concentra-
tions in a limited study of New Zealand rainwater. These
Table 2
Volume-weighted average DOC concentration (mM) in rainwater collected from island sampling sites and at a coastal location in
Wilmington, NC
Location Latitude, longitude Number of samples DOC References
Puerto Rico 181N, 661W n ¼ 34 52 McDowell et al. (1990)
Costa Rica 10.51N, 841W n ¼ 110 58 Eklund et al. (1997)
Enewetak Atoll 121N, 1631E n ¼ 10 22 Zafiriou et al. (1985)
North of Samoa 61S, 1741W n ¼ 1 56 Williams (1967)
New Zealand 461S, 1691E n ¼ 54 58 This study
Wilmington, NC 341N, 781W n ¼ 205 114 Willey et al. (2000)
Table 3
Volume-weighted average concentration (mM) of dissolved
organic carbon (DOC), formate, acetate, oxalate and a variety
of inorganic ions in rainwater collected in Dunedin New
Zealand when all rain events are combined and in marine
dominated storms
All (n ¼ 54) Marine
(n ¼ 10)
DOC 58 24
Formate 1.5 1.6
Acetate 1.5 1.6
Oxalate 1.0 0.2
% DOC as formate and
acetate
8 20
H+ 12.6 6.8
% H+ contributed as
organic acids
26 46
Cl� 137 180
NO3� 2.4 2.1
SO42� 16.7 15.0
NSS 9.6 5.6
% DOC as formic and acetic acid was calculated as total
carbons contributed by the vw ave concentration of formic and
acetic acids divided by vw ave DOC concentration.
R.J. Kieber et al. / Atmospheric Environment 36 (2002) 3557–35633560
authors reported a volume weighted concentration of
formic and acetic acids of 1.2 and 1.0 mM, respectively(n ¼ 8) in rain collected at 90 Mile Beach in the North
Island of New Zealand. Avery et al. (1991) also
measured volume weighted average concentrations of
formic and acetic acids of 1.5 and 1.3 mM, respectively,in non-growing season rainwater in maritime storms
sampled in the vicinity of Wilmington, NC.
The remarkable similarity of formic and acetic acid
concentrations, in coastal or marine rains at these
geographically diverse locations, suggests there is a
background concentration of these acids in remote
marine storms. These organic acids did not display any
seasonal variation in New Zealand rainwater (t test,
p > 0:5). This is consistent with two studies by Avery
et al. (1991, 2001), who found formate and acetate
concentrations were very similar in winter relative to
summer rain in coastal and marine events in coastal
North Carolina (Avery et al., 2001, 1991; Durana et al.,
1992; Keene and Galloway, 1986; Talbot et al., 1988).
The lack of any seasonal pattern in organic acids in New
Zealand suggests short-term transport of terrestrial
organic matter is not the dominant source of organic
acids in coastal rain at this location.
3.5. Formic acetic acid correlations and F:A ratio
The concentration of formic and acetic acids were
significantly correlated (t test, po0:001) in New Zealandrainwater (Fig. 1). Such a significant correlation between
levels of these acids has also been observed for rain
collected from other vastly different geographic regions.
Khare et al. (1997) found good agreement between
formic and acetic acid concentrations at a rural tropical
site in north central India. Keene and Galloway (1986)
found a high degree of correlation between formic and
acetic acids at a variety of both continental and marine
sites. The significant correlation between formic and
acetic acids in New Zealand precipitation suggests these
acids have similar sources to those for other sites or they
have different sources of approximately the same
strength in rainwater.
Reduced major axis (RMA) analysis has been
employed in several previous studies examining the
ratio of formic to acetic acids in atmospheric samples
(Avery et al., 2001, 1991; Keene and Galloway, 1986;
Khare et al., 1997). This technique is appropriate for
analysis of ratios in environmental samples because it
assumes both variables are subject to measurement
error. The F:A ratio determined in this manner is highly
variable among different geographic locations ranging
from approximately 4:1 to o1:1 (Avery et al., 2001,
1991; Keene and Galloway, 1986; Khare et al., 1997).
The formic to acetic acids ratio determined in the
present study by RMA was 0.8:1 with a correlation
coefficient of 0.7. This is at the low end of the range
observed by others and is similar to the ratio determined
by Avery et al. (1991) in marine dominated events.
3.6. F and A contribution to DOC
The contribution of formic and acetic acids to the
DOC pool was 8% when all rain events were combined
(Table 1), and was approximately the same regardless of
season. There appears to be a significant impact of storm
origin on the fraction of DOC contributed by organic
acids. Marine storms had a significantly higher percen-
tage of organic acids to DOC relative to the other storms
analyzed (Table 3). The higher percentage was caused by
increases in DOC concentrations in more terrestrially
influenced events, rather than changes in organic acid
concentrations, which were approximately the same
in marine rains as in other storms. In the only de-
tailed study where both organic acids and DOC
0
2
4
6
8
0 1 2 3 4 5
Acetate (µM)
Formate( µM)
Fig. 1. Formate concentration (mM) as a function of acetate concentration (mM) in New Zealand rain. n ¼ 22; r ¼ 0:7:
R.J. Kieber et al. / Atmospheric Environment 36 (2002) 3557–3563 3561
concentrations were measured, Tang (1998) also found
formic and acetic acids contributed 20% of the DOC in
marine rains. These results suggest that the continental
influences, causing increasing DOC concentrations, are
not the result of increasing inputs of organic acids but
rather must be the result of some additional terrestrial
source(s) of carbon.
3.7. Correlation analysis
No correlation was observed between rainfall amount,
DOC, formate and acetate concentrations. This lack of
correlation in the Southern Hemisphere is similar to
observations made in the Northern Hemisphere where
no correlation was observed between precipitation vol-
ume and concentration (Khare et al., 1997; Sakugawa
et al., 1993; Willey et al., 2000). These results suggest
these components are not simply washed out of the
atmosphere but rather there is continuous supply during
rain events. Chameides and Davis (1983) suggested that
formic acid could be produced from aqueous phase
oxidation of formaldehyde by hydroxyl radicals. If this
or a similar mechanism occurred in the New Zealand
rain it would mask any correlation between rain amount
and formic acid concentrations in these samples.
Concentrations of hydrogen ion, nitrate, and NSS
were all intercorelated in the New Zealand rain (t test,
po0:001) which is similar to results obtained in the
Northern Hemisphere (Gorham et al., 1984; Hooper and
Peters, 1989; Kieber et al., 1999; Willey and Kiefer,
1993). The correlation between hydrogen ion, nitrate
and NSS in the Northern Hemisphere is thought to
be the result of common anthropogenic sources of these
analytes and hence they are often used as pollution
indicators in rain. However, invoking common anthro-
pogenic sources is an unlikely explanation for the
correlation observed in the New Zealand rainwater
because concentrations of NO3� and NSS were approxi-
mately equal to background levels found in uncontami-
nated precipitation in remote regions of the globe and in
tropical rain events (Galloway et al., 1982; Kieber et al.,
1999; Likens et al., 1987; Williams et al., 1997). In
addition, NO3�, NSS and H+ were all correlated with
chloride in New Zealand rainwater, which is not
typically observed in anthropogenically impacted pre-
cipitation.
DOC concentrations were highly correlated with the
seasalt component chloride, as well as nitrate and NSS
(po0:0001). DOC concentrations were not, however,
correlated with formate or acetate concentrations. This
lack of correlation may be because these organic acids
did not comprise a large percentage of the organic
carbon pool (8%) in the New Zealand rainwater and
suggests there are multiple sources of DOC in these rain
events in addition to formic and acetic acids. No
correlation was observed between DOC and hydrogen
ion in any of the storm types which is in contrast to
Willey et al. (2000), who found a strong correlation
between hydrogen ion and DOC in marine dominated
events. The authors suggested this correlation reflected
shifting H+ contributions from inorganic to organic
acids during times when organic acids contributed a
relatively large percentage of free acidity in rainwater.
Organic acids in the present study, however, contributed
a relatively small amount to the free acidity in
the Dunedin rainwater (20%) compared to 47% in
Wilmington possibly explaining the lack of correlation
between DOC and H+ in the New Zealand precipita-
tion.
In summary, DOC concentrations measured in rain
sampled at a site on the coast of the South Island of New
Zealand are similar to those measured at island
locations in the Northern Hemisphere. The concentra-
tion of DOC varied by season with the lowest
concentrations in winter. Marine dominated events
contained the lowest DOC concentrations and did not
display any seasonal variability. Formic and acetic acids
comprised a relatively small fraction of the DOC pool
and, in contrast to DOC, neither of these acids were
influenced by storm origin or season. This DOC and
organic acids data set is unique because it is one of the
first detailed studies of DOC levels reported for the
Southern Hemisphere. The DOC data will allow for a
better understanding of the tropospheric transport of
carbon around the globe which will aid in quantification
of atmospheric carbon removal via wet deposition on a
global scale.
Acknowledgements
This work was supported by NSF Grants ATM-
9729425 and ATM-9530069 and the Chemistry Depart-
ment at the University of Otago, Dunedin, New
Zealand. The Marine and Atmospheric Chemistry
Research Laboratory group at UNC-Wilmington as-
sisted with sampling and analyses. James Renwick, New
Zealand National Institute of Water and Atmospheric
Research, provided air trajectory analysis.
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