GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 16, NO. 4, 1076, doi:10.1029/2002GB001892, 2002
Atmospheric deposition of inorganic and organic nitrogen and base
cations in Hawaii
Jacqueline H. Carrillo1
Department of Oceanography, University of Hawaii, Honolulu, Hawaii, USA
Meredith Galanter Hastings and Daniel M. Sigman
Department of Geosciences, Guyot Hall, Princeton University, Princeton, New Jersey, USA
Barry J. Huebert
Department of Oceanography, University of Hawaii, Honolulu, Hawaii, USA
Received 15 February 2002; revised 28 May 2002; accepted 6 June 2002; published 8 November 2002.
[1] Atmospheric deposition of nitrogen (N) and base cations was measured for 5–7 years
on the island of Hawaii and for 1.5 years on Kauai. On Hawaii, mean annual fluxes of K+,
Mg2+, and Ca2+ were 15, 17, and 13 kg ha1 yr1, respectively. Fog interception was
the largest deposition pathway. Sea salt contributed the majority of cations, although
biomass burning and Asian dust were significant sources for some years. Total N
deposition (inorganic and organic) averaged 17 kg N ha1 yr1. Fog interception was also
the largest source of N, depositing 16 kg N ha1 yr1. Precipitation deposition was 1.0 and
0.2 kg N ha1 yr1, respectively on Hawaii and Kauai. Dry deposition on Hawaii was
0.1 kg N ha1 yr1. Organic N averaged 16 and 12% of total N in rain and fog,
respectively. The d15N values for NO3-N are consistent with long-range transport of N
from Asia in the spring/summer and from North America in the fall/winter as nonvolcanic
sources. Atmospheric deposition on Hawaii may completely account for a previously
INDEX TERMS: 0315 Atmospheric Composition and Structure:
identified soil N imbalance.
Biosphere/atmosphere interactions; 0370 Atmospheric Composition and Structure: Volcanic effects (8409);
0322 Atmospheric Composition and Structure: Constituent sources and sinks; 1615 Global Change:
Biogeochemical processes (4805); KEYWORDS: nutrient, fog, isotopic N, long-range transport, Asian dust
Citation: Carrillo, J. H., M. G. Hastings, D. M. Sigman, and B. J. Huebert, Atmospheric deposition of inorganic and organic nitrogen
and base cations in Hawaii, Global Biogeochem. Cycles, 16(4), 1076, doi:10.1029/2002GB001892, 2002.
1. Introduction
[2] Atmospheric deposition of plant nutrients is of special
interest in Hawaii due to its important role in ecosystem
growth limitations, and also due to the way those limitations
change through time. As Hawaiian ecosystems age from 0
to 4 Myr, processes shift from limitation by atmospherederived compounds, to limitation by compounds present in
high concentrations in new volcanic substrates [Chadwick et
al., 1999], as atmosphere-derived compounds accumulate in
the soil and the substrate-derived compounds weather away.
Therefore growth on the young island of Hawaii is limited
by atmosphere-derived N [Vitousek et al., 1993], while
growth on 4.1 Myr old Kauai is limited by initially
substrate-derived phosphorus (P) [Chadwick et al., 1999].
1
Now at Department of Atmospheric Science, Colorado State
University, Fort Collins, Colorado, USA.
Copyright 2002 by the American Geophysical Union.
0886-6236/02/2002GB001892$12.00
[3] The atmosphere also becomes an increasingly important source of the base cations, potassium (K+), calcium
(Ca2+), and magnesium (Mg2+), through time [Chadwick et
al., 1999]. While recent research on base cation deposition
has focused on their important role in neutralizing acid
deposition [Hedin et al., 1994; Larssen and Carmichael,
2000; Lee et al., 1998], base cations are also important plant
nutrients. On young islands, there is a high availability of
cations in the soil because of high concentrations present in
the relatively unweathered rock substrate. However, in wet,
tropical climates, weathering can rapidly lead to very low
availability of these materials [Vitousek and Sanford, 1986].
On Kauai, concentrations of cations in the soil are extremely
low and atmospheric deposition is likely to be the only
significant source of new material [Chadwick et al., 1999].
[4] We measured atmospheric nutrient deposition to a
300-year-old Hawaiian ecosystem from 1993 to 2000. Our
research site was located on the island of Hawaii in the
Hawaii Volcanoes National Park (Figure 1). The original
motivation for this study was an apparent imbalance in the
soil N budget. Crews et al. [1995] found that a 33 kg N
ha1 yr1 source was needed to account for the amount of
24 - 1
24 - 2
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
Figure 1. Map showing our Thurston and Kokee research sites. Data copyright 1996, Dr. John R. Smith
and T. Dunnebier, University of Hawaii, SOEST.
N present in the soil. In situ fixation contributed only 1.2 kg
N ha1 yr1 and preliminary precipitation deposition measurements revealed similarly low values [Vitousek, 1994].
We began measuring precipitation deposition in 1993, adding dry deposition measurements in 1994, and fog interception measurements in 1995. While our original
motivation was to determine N fluxes (measured as nitrate
+
(NO
3 ) and ammonium (NH4 )), analyses for chloride (Cl ),
2
+
2+
2+
+
sulfate (SO4 ), sodium (Na ), K , Ca , and Mg were
performed concurrently.
[5] Initial deposition results indicated that large amounts
of inorganic N are deposited by fog influenced by the
nearby, active volcano Kilauea [Heath and Huebert,
1999]. In 1996, over 20 kg of inorganic N ha1 yr1 was
deposited; the majority occurred during a small number of
very concentrated, volcanically influenced fog events.
Investigation of the volcanic N source led to the discovery
of significant thermal fixation of atmospheric dinitrogen
(N2) in air contacting the Kilauean lava flows [Huebert et
al., 1999]. It appears that the volcano actually fertilizes
ecosystems developing on new lava flows through this
mechanism.
[6] Though the initial focus of this study was the atmospheric deposition of N, we found that the atmosphere was
also a significant source of base cations. The majority (68%)
of cation input was of marine origin [Coeppicus, 1999],
which was expected, given the exposure of the site to the
persistent, marine trade winds. However, this left 32% of
the total cation deposition with an unidentified source. The
transport and deposition of dust from Asia is the most likely
source [Parrington et al., 1983; Leinen et al., 1994], but
until now this possibility has not been evaluated. Quantifying this source is important for understanding base cation
deposition and for evaluating the ways in which that
deposition may change through time.
[7] N also has an important local source (the volcano), but
as with cations, we have little information on nonvolcanic N
sources. Long-range transport of material from Asia and
North America is likely to contribute to Hawaiian N
deposition, but there was no strong evidence to support
this. The isotopic value of NO
3 -N in samples can be useful
in distinguishing among different sources of N [Wania et
al., 2002; Yeatman et al., 2001]. We analyzed a subset of
fog and rain samples in order to determine the isotopic
signature of volcanically produced N, and to evaluate longrange transport sources.
[8] Kilauea Volcano exerts enormous influence on the N
deposition to proximate ecosystems, contributing up to 75%
of the total N deposition during some years (Heath et al.,
Volcanically produced nitrogen in Hawaii, submitted to
Global Biogeochemical Cycles, 2002). As a result, N
deposition measurements made near the volcano are
unlikely to represent N deposition on other islands. Any
differences between islands, in the deposition of N, as well
as of base cations, are significant due to the differing
nutrient limitations and availability on each island. In 1999,
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
we began measuring precipitation deposition on the 4.1 Myr
old island of Kauai to identify any differences in deposition
rates.
[9] Our past N deposition results have represented inorganic N deposition only. In California’s San Joaquin Valley,
Zhang and Anastasio [2001] measured organic N that was
16% of the total N in fog water, while at a Chilean site that
is more comparable to ours, Weathers et al. [2000] found
organic N concentrations to be more than four times those
of inorganic N. Scattered measurements by this and other
groups indicate that organic N may be significant in Hawaii
as well, representing 100% of the N present in some
precipitation samples [Coeppicus, 1999; Cornell et al.,
2001; Vitousek and Walker, 1989]. In 1999, we added
analyses for organic N in rain and fog samples.
[10] Because fog concentrations can vary by more than 10
times between events [Heath and Huebert, 1999], sampling
a small percentage of the events in a year may give a
skewed annual average. During 1999, the number of fog
events sampled was increased from an average of 9 to 48
events per year, so that our measured concentrations would
be more representative of the actual annual mean concentration. This more comprehensive data set not only gives us
higher confidence in our annual deposition results, but it
also allows us to evaluate a critically important problem that
is frequently encountered in deposition research: What is
the best way to extrapolate intermittent data to longer
periods of time?
[11] This paper presents a more comprehensive view of
nutrient deposition to Hawaiian montane ecosystems than
was possible in the past. Results from Kauai allow us to
assess the variability of chemical deposition between two
Hawaiian ecosystems. Organic N analyses make our N
deposition results more complete. Our most current results
from the Thurston site are presented along with past results
that have been recalculated using an updated fog interception methodology. The result is chemical deposition data
spanning 7 years: one of the most comprehensive records in
the tropical Pacific and certainly the most complete in
Hawaii.
2. Description of Sites
2.1. Thurston Site
[12] Our research site on the island of Hawaii was near the
Thurston Lava Tube in Hawaii Volcanoes National Park
(Figure 1). It is at 1190 m elevation, on the windward side
of the island, and it was frequently influenced by orographic
fog and rain. The average annual rainfall was between 2 and
3 m [Giambelluca et al., 1986] while fog interception
contributed an additional 1.6 m annually (J. H. Carrillo
and B. J. Huebert, Fog interception in Hawaii calculated
with a water balance approach: Results and uncertianties,
submitted to Journal of Hydrology, 2002). The forest
canopy at this site was approximately 13 m high and the
vegetation was predominantly Meterosideros polymorpha
(Ohia lehua), Cibotium glaucum (tree fern), and Hedychium
gardnerianum (Kahili ginger).
[13] Our research was conducted at two sites at approximately the same elevation. The majority of measurements
24 - 3
were made from a 14 m tower in the middle of a 50 m by 30
m clearing. Measurements made at this location include
rainfall amount (R), temperature (T), relative humidity
(RH), wind speed (WS) and direction (WD), and liquid
water content (LWC) of fog. This was also the location of
our fog collector and aerosol/gas filters. A precipitation
collector that collected samples for chemical analyses was
located on the ground about 30 m away from the tower.
Samplers on the tower were arranged across the predominant wind direction at varying heights to minimize interferences. From 1995 to the fall of 1998, when the large
tower was installed, instruments were mounted on an 8 m
tower in the same clearing. Prior to 1995, instruments were
mounted on a 4 m telephone pole in the clearing. Throughfall (TF) and stemflow (SF) measurements (for our fog
interception calculation described in section 3.4) were made
at a second location approximately 1.5 km from the tower.
2.2. Kokee Site
[14] Our research sites on the island of Kauai were located
in the Kokee State Park and in Na Pali Kona Forest Reserve.
Both sites are at approximately 1130 m elevation and also
received 2 – 3 m rainfall per year [Giambelluca et al., 1986].
We have no measurements of the Kokee fog interception
amount, although because the sites had similar elevation
and orientation to the trade winds, the fog water input may
be similar to the 1.6 m measured at the Thurston site. Like
the Thurston site, the vegetation was M. polymorpha (Ohia
lehua) dominated forest, although it was somewhat lower in
stature at approximately 10 m.
[15] The two sampling locations were separated by less
than 2 km. The westernmost site was located on the western
edge of a ridge, in an approximately 10 by 20 m clearing off
of the road. This was the location of our precipitation
collector (for chemical samples). It was also the location
of the Hawaii Ecosystems Project’s R, T, RH, WS, WD, and
solar radiation measurements.
[16] The other site was to the northeast of the precipitation
site, on the eastern-facing side of the same ridge. Our
instruments were located on a 10 m tower surrounded by
forest. The top section of the tower was approximately 1 m
above the nearby vegetation. We measured R, T, and RH
from this tower. WS and WD were measured from a pole
extending about 1 m above the tower top.
3. Measurement Techniques
[17] Table 1 summarizes our measurements at Thurston
and Kokee. At Kokee, we have used Hawaii Ecosystems
Project data for time periods when our instruments had
either not yet been installed or were not functioning
properly.
3.1. Meteorological Parameters
[18] WS and WD were measured with RM Young propeller-vane anemometers from the top of the Thurston
(since December 1993) and Kokee (since October 1999)
towers. In April 1999, the RM young anemometer at
Thurston was replaced with a Solent, Ltd. sonic anemometer. T and RH were measured from the top of the Thurston
(since December 1993) and Kokee (since December 1999)
24 - 4
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
Table 1. Timetable of Measurements and Instruments Installed at Thurston and Kokee
Parameter
Data logger
WS
WD
R
Rain collection
T, RH
LWC
Aerosol chemical concentration
HNO3 concentration
Fog collection
TF collection
TF rate
SF
Data logger
WS, WD
R
Rain collection
T
RH
TF collection
TF rate
Instrument
Dates
Thurston
Campbell Scientific Inc. 21X
RM Young prop-vane anemometer
Solent, Ltd. sonic anemometer
Aerochemetrics rain collector bucket weight
Texas Instruments TE – 525 tipping bucket rain gauge
Aerochemetrics wet-only rain collector
Vaisala HMP133Y, HMP35C, or HMP45C
Gerber Scientific Inc. PVM-100
Gelman Sciences 47 mm Zeflour Teflon filters, 1 mm pore size
Gelman Sciences 47 mm Nylasorb nylon filters
Active Teflon string collector
1-inch aluminum angle
Texas Instruments TE-525 tipping bucket rain gauge
hand-made sheet metal collars that drain to plastic containers
Dec. 1993 – present
Dec. 1993 – March 1999
April 1999 – present
Aug. 1993 – July 1995
July 1995 – present
Aug. 1993 – present
Dec. 1993 – present
Oct. 1998 – present
Dec. 1993 – present
Dec. 1993 – present
July 1995 – present
July 1995 – present
July 1995 – present
Nov. 1995 – present
Kokee
Campbell Scientific Inc. 21X
RM Young propeller-vane anemometer
Texas Instruments TE-525 tipping bucket rain gauge
Aerochemetrics wet-only rain collector
Vaisala HMP45C
Jan. 1999 – present
Oct. 1999 – present
Oct. 1999 – present
Jan. 1999 – present
Dec. 1999 – present
1-inch aluminum angle
Texas Instruments TE-525 tipping bucket rain gauge
Jan. 1999 – present
Jan. 1999 – present
towers using Vaisala T and RH sensors (either model
HMP133Y, HMP35C, or HMP45C).
[19] Since October 1998, a Gerber Scientific, Inc. Particle
Volume Monitor (PVM, model 100) was installed on the top
section of the tower at Thurston. This measured fog LWC.
The sum or average of all measurements was recorded every
10 min (Thurston) or every hour (Kokee) by a Campbell
Scientific, Inc. 21X data logger. Radiation was measured by
the National Park Service Air Resources Division using a
LI-COR, Inc., LI-200SZ radiometer. This instrument was
mounted on the roof of our field lab, about 4 m from the
tower. Sums were saved on an hourly basis and were
available since October 1999.
ocean surface) and 1.0 cm s1 (calculated by Zhang et al.
[2001]) for a vegetated surface). Daytime (6:00 A.M. to
6:00 P.M.), nighttime (6:00 P.M. to 6:00 A.M.), and control
(no airflow) filter packs were collected weekly. To reduce
contamination by R or fog (F), the filters faced downward in
an enclosure that was open only on the bottom. The filter
pump was programmed to shut off when the precipitation
collector opened or if the RH was over 95%. In November
1998 when the PVM was installed, the pump was set to shut
off if the precipitation collector was open or if the LWC was
above 0.005 g m3. Filter measurements are not available
for Thurston from the fall of 1996 to the fall of 1998. Dry
deposition was not measured at Kokee.
3.2. Precipitation Deposition
[20] Precipitation was collected for chemical analyses at
Thurston and Kokee using Aerochemetrics, Inc., wet-only
precipitation collectors. Precipitation was collected weekly
since September 1993 at Thurston and since January 1999
at Kokee. The rainfall amount at both sites was measured
with Texas Instruments, Inc. TE-525 tipping bucket rain
gauge (0.254 mm per tip). Prior to 1995, the precipitation
amount at Thurston was determined by weighing the
precipitation bucket.
3.4. Fog Interception
[22] Fog water was collected at Thurston since July 1995,
using an active Teflon string collector [Daube et al., 1987].
Samples were collected intermittently, at random times
throughout the year, from 1995 to 1998 and 2000. To better
characterize the chemistry of Thurston fog, sampling was
intensified for the calendar year 1999.
[23] The fog interception amount was calculated using a
water balance approach shown in the following equation
[Juvik and Nullet, 1993]
3.3. Dry Deposition
[21] Since January 1995 at Thurston, aerosol inorganic
ions and nitric acid vapor (HNO3) concentrations were
measured using 47 mm Teflon per nylon filter packs (1
mm Zeflour Teflon and Nylasorb nylon, both from Gelman
Science). Meteorologically derived deposition velocities
[Hicks et al., 1985] were used to calculate HNO3 deposition. To estimate the importance of aerosol deposition, we
calculated deposition amounts using both a deposition
velocity of 0.1 cm s1 (used by Duce et al. [1991] for the
R þ F ¼ TF þ SF þ CS þ E
ð1Þ
Here the water inputs, rain (R) and F, are equal to the sum of
the measurable or calculable parameters TF, SF, canopy
storage (CS), and evaporation (E). If we also measure R, we
can solve for F as a residual term.
[24] TF was measured using four 6.4 m pieces of aluminum angle, arranged to create collection troughs, which
drained into tipping bucket rain gauges. Aluminum or
galvanized steel collars were used to collect SF from eight
representative trees. CS was determined by examining
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
‘‘rain-only’’ (no fog) periods, using the relationship
described by Juvik and Nullet [1993]. Finally, E was
calculated using the Penman-Monteith equation [Monteith,
1965]. The details of these measurements, the fog interception calculation, and its uncertainty are presented in a
related paper (J. H. Carrillo and B. J. Huebert, Fog interception in Hawaii calculated with a water balance approach:
Results and uncertianties, submitted to Journal of Hydrology, 2002). It should be noted that, because fog interception
is calculated as the difference between two often large
terms, the uncertainty of this value can be substantial, with
a mean annual deposition uncertainty of +39%/30%. For
fog chemical deposition, the fog interception amount dominates the uncertainties.
3.5. Sample Handling
[25] Details of cleaning and sample handling procedures
are reported by Heath [1996] and Coeppicus [1999]. Only
new or altered procedures are described here.
[26] At the Thurston site, all fog samples analyzed for
organic N were collected in Teflon bottles, while R samples
were collected in a polyethylene bucket. After R and F
samples were collected they were separated into aliquots
that were analyzed for inorganic ions, organic N, P, isotopic
N, and pH (in that order if there was not enough sample for
all analyses). P results are presented in a related paper
(Benitez-Nelson et al., Volcanic phosphorus deposition to
the Hawaiian Island Chain, submitted to Biogeochemistry,
2002). Organic N samples were stored in cleanroom bags
and isotopic N samples were stored in Nalgene bottles. Both
were frozen until analyses.
[27] At Kokee, when the precipitation was collected, all
but approximately 2 l of water was discarded. Chloroform
(3 – 4 ml) was added to inhibit biological activity, and the
sample was shipped to either the Thurston laboratory for
processing, then to Honolulu or directly to the Honolulu
laboratory. These samples remained unrefrigerated for up to
2 weeks. At the laboratory, aliquots were taken for inorganic
ion and P analysis. Samples were refrigerated in Nalgene
bottles. Since all Kokee samples had chloroform added,
they were not analyzed for organic N or pH.
3.6. Sample Analyses
[28] Organic N samples were thawed and photolyzed for
3 hours with ultraviolet (UV) light using a Metrohm Model
705 UV Digester at 75– 85C. The UV light oxidized
+
organically bound N to NO
3 , NO2 , or NH4 , which we
analyzed with ion chromatography. Results were compared
with unphotolyzed samples and the difference in total N was
assumed to be organic N. In a comparison study, samples
photolyzed for longer times yielded no additional N. No
oxidant was added because this has been shown to increase
contamination but not oxidation of sample N in precipitation [Cornell and Jickells, 1999; Scudlark et al., 1998].
[29] All the liquid samples and filter extracts were ana
2
+
+
lyzed for inorganic ions (NO
3 , NO2 , SO4 , Cl , Na , NH4 ,
+
and K ) using a Dionex 300 Series ion chromatograph.
Analyses for Mg2+ and Ca2+ began in January 1997. Anions
were analyzed on an OmniPac Pax-500 column with a 25
mM H2SO4 autoregenerant and a 1– 35 mM NaOH per 5%
methanol eluant solution. Cations were analyzed on an
24 - 5
IonPac CS-12 column with a 20 mM HCl per 2mM MSA
eluant and a self-regenerating cation suppressor.
3.7. Blank and Control Samples
[30] For the aerosol and gas filters, a weekly control filter
pack was mounted on the sampling tower alongside the
weekly sample filter packs. It was treated identically to the
sample filters, except that no air was pulled through.
Concentrations measured on the control filters were subtracted from the corresponding sample filter concentration
for that week.
[31] For precipitation at both Thurston and Kokee, bucket
blanks were taken for each weekly sample. Prior to installation in the precipitation collector, each collection bucket
was soaked with deionized water for 1 week. An aliquot of
the soaking water was saved and was processed identically
to a precipitation sample. The concentration of the blank
was subtracted from the measured precipitation concentration from that bucket.
[32] Prior to fog collection, the collector strings were
sprayed with deionized water using a hand-held spray
bottle. An aliquot of the rinsate was saved and processed
identically to a fog sample. After rinsing, the collector was
covered with plastic. If an event did not occur within 48
hours, the strings were rerinsed and a new blank was taken.
Concentrations in the blank were subtracted from the first 4hour sample of a fog event. We assumed that any contamination would have been collected in this first sample.
3.8. Nitrogen Isotopic Analyses of Nitrate
[33] The nitrogen isotopic composition of nitrate was
measured in 26 fog samples and nine precipitation samples
(six from Thurston, three from Kokee) collected during
1999 and 2000. An attempt was made to select samples
collected under a variety of meteorological conditions.
Measurements were made by the quantitative conversion
of nitrate to N2O by bacterial denitrification, followed by
isotopic analysis of the product N2O by continuous flow
isotope ratio mass spectrometry [Sigman et al., 2001].
Individual analyses are referenced to injections of N2O
from a pure gas cylinder and then standardized using an
internationally accepted nitrate isotopic reference material,
IAEA-N3 [Bohlke and Coplen, 1995]. Despite the high
sensitivity of the denitrifier method, which requires 10–
20 nmol N per analysis, sufficient sample volume was
available for only a few of the precipitation samples,
explaining the smaller number of isotopic analyses on
precipitation relative to fog.
4. Data Analysis
4.1. Computing Fog Deposition
[34] Chemical deposition by fog was computed by taking
the product of chemical concentrations in fog water and the
fog interception amount. Because fog water was collected
for chemical analysis intermittently, there are many events
for which we have a water flux (measured continuously),
but no chemical data. Additionally, due to instrument failure, there are a few periods of time for which we have
chemical concentrations, but no water flux data, and periods
of time with no data at all. In order to compute annual N
24 - 6
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
Table 2. Precipitation Concentrations and Rainfall Amounts at Thurston and Kokee
K+
Mg2+
Ca2+
Cl
Na+
mmol l
Year
1993a
1994a
1995a
1996a,b
1997b
1998b
1999
2000
Mean
1.8
1.3
21
5.6
9.3
1.7
0.5
0.6
5.3
0.8
2.0
5.0
1.9
2.4
1999
2000
Mean
1.1
0.4
0.7
6.5
5.2
5.9
a
b
NO
3
SO2
4
NH4+
Inorg. N
Org. N
1
0.9
1.3
2.2
0.6
1.3
85
97
125
123
65
40
16
24
72
51
72
43
36
26
51
21
33
42
12
12
19
21
8
10
10
13
13
1.1
0.8
1.0
2.0
1.4
1.7
0.5
1.2
1.2
Thurston
0.8
1.5
1.0
0.4
0.7
1.3
1.2
1.0
1.0
2.1
2.0
2.0
54
31
43
62
96
76
6
9
8
0.5
1.1
0.8
Kokee
0.02
0.5
0.00
1.2
0.01
0.8
2.0
2.3
2.0
2.4
2.2
3.0
1.7
2.2
2.2
Org.% of Total N
Rain, cm
Sampling Days
14
19
16
248
331
177
325
214
276
357
138
258
117
347
221
344
245
253
356
194
260
168
171
170
270
162
216
0.3
0.5
0.4
Inorganic nitrogen data from these years appear in the works of Heath [1996] and Heath and Huebert [1999].
Inorganic nitrogen and cation data from these years appear in the study of Coeppicus [1998].
fluxes, we must determine the best way to extrapolate our
data to annual values.
[35] Heath [1996] used the average of three different
approaches, since she had no way to determine which was
the most accurate. For the first approach, an average N flux
per fog interception event was computed for events with
both chemical and water flux data. To extrapolate to an
annual value, this average deposition was multiplied by the
number of fog interception events in a year. For the second
approach, rather than scaling the measured deposition by
the number of events, an average N deposition per cm of fog
interception was calculated (concentration may vary with
size of the event) and this was then normalized to the cm of
fog interception for the year. For the third approach, the
average N concentration of all the collected fog water for a
year was multiplied by the annual fog interception amount.
[36] The relatively large number of chemical data that we
have for 1999 has allowed us to evaluate for the first time,
the success of each of the three techniques at correctly
estimating the deposition. There are 48 events for which we
have both chemical and water flux data for 1999, compared
to an average of six events per year for other years. For this
analysis, we defined the total deposition as the measured
deposition for the 48 events. We then randomly selected six
events of the 48 and used the three approaches previously
described to extrapolate to the total value. This was repeated
with 12 different combinations of events and results were
compared with the total measured deposition.
[37] We found that Methods 1 and 2 consistently overestimated the deposition amount, by as much as 10 times
for N. Method 3 (the product of average concentration and
the total water input) consistently produced results that
were by far the closest to the measured values. For N, the
deposition amount estimated using Method 3 was within
an average of 8% of the actual deposition. For sea salt
compounds, the estimated values were often within 10%
of the actual values. Besides its higher degree of accuracy,
an advantage of this approach is that it allows us to use all
Table 3. Annual Precipitation Deposition With Uncertainties at Thurston and Kokee
K+
Mg2+
Year
Ca2+
kg ha
1
Na+
yr
Cl
SO2
4
1.8 ± 0.7
1.7 ± 0.7
13 ± 5.3
5.1 ± 2.1
14 ± 5.5
1.9 ± 0.1
0.8 ± 0.05
0.8
1.8
3.4
1.6
0.3
0.7
0.2
0.1
1.3 ± 05
2.0 ± 0.8
2.5 ± 0.2
0.9 ± 0.06
48 ± 20
73 ± 30
45 ± 18
67 ± 27
56 ± 23
26 ± 2
13 ± 1
2000
0.3 ± 0.02
1.2 ± 0.07
0.6 ± 0.04
8±1
18 ± 1
Mean
4.8
1.8
1.5
42
39
1999
2000
Mean
0.7 ± 0.05
0.3 ± 0.02
0.5
2.7 ± 0.2
2.1 ± 0.1
2.4
1.4 ± 0.1
1.3 ± 0.1
1.4
21 ± 1.3
12 ± 0.8
16
37 ± 2
52 ± 1
44
a
b
NH4+
kg N ha
1993a
1994a
1995a
1996a,b
1997b
1998b
1999
±
±
±
±
NO
3
1
45
84
24
30
35
52
27
±
±
±
±
±
±
±
3
5
2
2
2
3
2
Thurston
29 ± 2 0.4
39 ± 3 0.4
30 ± 2 0.2
49 ± 3 0.6
27 ± 2 0.8
27 ± 2 0.7
35 ± 2 0.3
±
±
±
±
±
±
±
Inorg. N
1
yr
Rain, cm
0.03
0.03
0.02
0.06
0.06
0.06
0.02
0.3 ± 0.2
0.7 ± 0.4
0.2 ± 0.1
0.1 ± 0.1
0.4 ± 0.2
0.5 ± 0.05
0.6 ± 0.05
0.7
1.1
0.4
0.8
1.1
1.2
0.9
18 ± 1
0.3 ± 0.02
0.2 ± 0.02
0.5 ± 0.03
32
0.5
0.4
0.9
0.01 (+0.06/0.0004)
0.001 (+0.01/0.0001)
0.003
0.1 ± 0.01
0.3 ± 0.02
0.2
Kokee
10 ± 1 0.1 ± 0.01
15 ± 1 0.3 ± 0.02
12
0.2
Inorganic nitrogen data from these years appear in the works of Heath [1996] and Heath and Huebert [1999].
Inorganic nitrogen and cation data from these years appear in the study of Coeppicus [1998].
Org. N
1
±
±
±
±
±
±
±
0.04
0.07
0.03
0.07
0.07
0.08
0.05
0.2 (+0.06/
0.03)
0.1 (+0.04/
0.02)
0.1
248 ± 10
331 ± 13
177 ± 3
325 ± 4
214 ± 4
276 ± 3
357 ± 3
138 ± 4
258
168 ± 3
171 ± 3
170
24 - 7
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
Table 4. Nitric Acid and Aerosol Concentrations at Thurston
Aerosol, nmol m3
Year
1994a
1995a
1996a
1997
1998
1999
2000
Mean
a
K
+
0.9
NA
0.6
0.5
0.7
0.7
2+
Mg
NA
2.3
2.1
3.2
2.6
2+
Ca
NA
0.6
0.7
1.3
0.9
Na+
20
NA
23
16
26
22
Cl
6
NA
12
9
14
10
SO42
NO
3
NH4+
HNO3 Vapor,
nmol m3
Sampling
Days
15
NA
17
15
21
17
1.3
7.9
1.2
NA
0.8
1.1
1.8
2.4
1.3
2.8
8.0
NA
2.5
2.1
4.1
3.5
0.4
1.3
0.1
NA
0.3
0.7
1.3
0.7
159
203
161
NA
85
355
188
192
Inorganic nitrogen data from these years appear in the works of Heath [1996] and Heath and Huebert [1999].
of our data, not just data for events with both chemical
and water flux data.
5. Results and Discussion
5.1. Cation Deposition
[38] Cation deposition at Thurston shows the same trend
as the N deposition [Heath and Huebert, 1999]: dry
deposition is very small (less than 0.2 kg ha1 yr1 for
cations); precipitation deposition is slightly larger (averaging 2 – 5 kg ha1 yr1 for K+, Ca2+, and Mg2+); and fog
interception is by far the largest chemical input (averaging
10 –15 kg ha1 yr1 for K+, Ca2+, and Mg2+; Tables 2 –7).
For each deposition mechanism, Tables 2 – 4 list species’
concentrations while Tables 5 – 7 tabulate deposition fluxes.
The deposition amounts and chemical concentrations in
precipitation were within the range of values measured at
other tropical locations [Cavelier et al., 1997; Vitousek and
Sanford, 1986]. Few data exist on tropical cation deposition
by fog. Our results are higher, but within the same range as
those of Asbury et al. [1994] in Puerto Rico. Our deposition
results are higher than reported by Clark et al. [1998] for a
Costa Rican forest, in part because our fog interception
amount is nearly twice as great.
[39] Weathering of the rock substrate at Thurston contributes almost 150 kg Ca2+ ha1 yr1 to the ecosystem, but
this weathering input decreases rapidly with substrate age
and is more than 8 orders of magnitude lower at Kokee
[Chadwick et al., 1999]. While atmospheric deposition is
not a negligible input at Thurston (an average total deposition of 15 kg Ca2+ ha1 yr1) it is an order of magnitude
smaller than the weathering source. If we assume that fog
interception contributes a similar Ca2+ input at Kokee, then
the atmospheric source must sustain the Ca2+ needs of this
ecosystem, as suggested by Chadwick et al. [1999].
5.2. Biomass Burning
[40] A striking feature of our cation measurement results
is the large interannual variability in the K+ concentrations
and deposition amounts (Tables 2 – 7). The K+ values at
Thurston for 1995 – 1997 were higher than other years for
precipitation, while for fog, 1996 had higher values. The
majority of this K+ deposition is contributed by a handful of
weeks (for precipitation) or events (for fog) with very high
K+ concentrations.
[41] It is possible that the elevated K+ we occasionally
measure may result either directly or indirectly from
Kilauea Volcano. Concentrations of K+ in fumarolic vapor
condensate can be extremely high (28,000 mmol l1 in
condensate from Galeras Volcano; Alfaro and Zapata
[1997]). Though these high K+ samples contained other
and Cl, in elevated
volcanic indicators, such as SO2
4
concentrations, this is common for samples from Thurston.
However, the majority of other samples containing volcanic
indicators do not have elevated K+ concentrations. Nonetheless, because of the variety of volcanic activity on
Hawaii, it is possible that some of this elevated K+ may
be a direct emission from the Pu’u ’O’o Vent.
Table 5. Annual Dry Deposition Estimates for Thurstona
Aerosol Deposition
Year
1994b
1995b
1996b
1997
1998
1999
2000
Mean
a
b
K+,
kg (x)
ha1 yr1
Mg2+,
kg (x)
ha1 yr1
Ca2+,
kg (x)
ha1 yr1
Na+,
Cl,
SO2
4 ,
kg (x)
kg (x)
kg (x)
ha1 yr1 ha1 yr1 ha1 yr1
0.016 – 0.16
0.30 – 3.0
NA
NA
NA
NA
0.002 – 0.02 0.05 – 0.05 0.002 – 0.02 0.05 – 0.5
0.004 – 0.04 0.01 – 0.1 0.006 – 0.06 0.07 – 0.7
0.006 – 0.06 0.02 – 0.2
0.01 – 0.1
0.1 – 1
0.007 – 0.07 0.01 – 0.1 0.006 – 0.06
0.1 – 1
0.05 – 0.5
NA
0.03 – 0.3
0.06 – 0.6
0.09 – 0.9
0.06 – 0.6
1 – 10
NA
0.2 – 2
0.3 – 3
0.5 – 5
0.5 – 5
NO
3,
kg N
ha1 yr1
NH4+,
kg N
ha1 yr1
0.002 – 0.02
0.003 – 0.03
0.001 – 0.01
NA
0.001 – 0.01
0.003 – 0.03
0.005 – 0.05
0.002 – 0.02
0.002 – 0.02
0.001 – 0.01
0.03 – 0.3
NA
0.003 – 0.03
0.006 – 0.06
0.01 – 0.1
0.008 – 0.08
HNO3
Deposition, Aerosol HNO3, Fraction
kg (N)
Vd,
Vd ,
‘‘Dry’’
ha1 yr1 cm s1 cm s1
Time
0.1 ± 0.1
0.1 ± 0.1
0.1 ± 0.1
NA
0.1 ± 0.1
0.4 ± 0.4
0.7 ± 0.8
0.2
Uncertainties are shown for HNO3 deposition.
Inorganic nitrogen data from these years appear in the works of Heath [1996] and Heath and Huebert [1999].
0.1 – 1
0.1 – 1
0.1 – 1
NA
0.1 – 1
0.1 – 1
0.1 – 1
0.1 – 1
14
12
20
NA
17
17
17
16
0.18
0.26
0.24
NA
0.32
0.63
0.69
0.39
24 - 8
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
Table 6. Annual Average Fog Concentrations and Water Fluxes at Thurston
K+
Mg2+
Ca2+
Cl
Na+
mmol l
Year
1995a
1996a,b
1997a,b
1998b
1999
2000
Mean
a
b
SO2
4
11
46
13
11
9
15
18
13
21
30
63
32
11
11
11
29
15
174
1918
106
189
275
538
537
178
1125
112
302
353
624
449
NO
3
NH4+
Inorg. N
28
270
5
5
23
30
60
7
24
11
11
23
29
18
35
290
16
16
46
59
77
Org. N
1
29
251
43
239
137
131
138
9
5
7
Org.%
of Total N
Fog,
cm
Sampling Days – Water
Input, days
Events Sampled –
Chemistry
17%
7%
12%
122
109
170
200
204
182
164
106
155
152
300
338
167
203
12
10
13
10
48
4
16
Inorganic nitrogen data for these years appear in the works of Heath [1996] and Heath and Huebert [1999].
Inorganic nitrogen and cation data for these years appear in the study of Coeppicus [1998].
[42] Another explanation is that these samples were
indirectly influenced by Kilauea Volcano through biomass
burning. Occasionally, when new lava outbreaks occur the
flows intercept vegetation causing it to ignite. Elevated K+
concentrations in biomass burning plumes are ubiquitous
[Andreae, 1983; Maenhaut et al., 1996; Pereira et al.,
1996], so if our site was impacted by such a plume it is
likely that it would result in elevated K+ concentrations in
precipitation and fog [Lacaux et al., 1992]. In order to
evaluate this possibility, we searched archived weekly
reports of the Hawaiian Volcano Observatory (HVO). We
found that on several occasions when we observed an
elevated K+ concentration in precipitation, there was
reported biomass burning due to new flow outbreaks onto
forested land during our sampling period (Figure 2) [HVO,
1995, 1996, 1997]. While there are samples with elevated
K+ concentrations for which we could not find reports of
biomass burning, this does not preclude the possibility that
there were unreported fires.
[43] The high K+ deposition that seems to result from
biomass burning caused by volcanic activity represents
another way in which the volcano actively contributes
nutrients to proximate ecosystems. In this case, the K+
deposited to nearby ecosystems is not truly new material,
the way the volcanically fixed N is, because it was originally
part of the local plant biomass. However, in the absence of
this atmospheric transport and deposition, the K+ in plant
material covered by lava flows would have become unavailable as it was buried underneath layers of fresh lava. In this
sense, this mechanism represents a pathway for material in
condemned ecosystems to be recycled rapidly.
5.3. Asian Dust Deposition
[44] If we assume all of the Na+ found in our samples is of
marine origin, we may compute the amount of non –sea salt
(NSS) derived K+, Mg2+, and Ca2+ by comparing their
ratios to Na+ in the sample to those in sea salt (SS). Figure 3
shows SS and NSS deposition amounts of the base cations
in precipitation at Thurston and Kokee for 1999 through
June of 2000, the period of time for which the sampling was
done at both sites. The left side of Figure 3 shows the SS
components of cation deposition. For these marine-derived
elements, there is both a wintertime and springtime peak,
resulting from increased storm activity during these times of
year. Higher wind speeds result in the generation of more
sea-spray. It seems likely that with additional years of data,
these two peaks might merge into one, lasting from November until May.
[45] The NSS-derived base cation deposition is shown on
the right side of Figure 3. For K+ and Mg2+, there is very
little material that is not associated with SS. However, Ca2+
shows very clear springtime peaks at both Thurston and
Kokee, most likely due to the transport and deposition of
dust from Asia.
Table 7. Annual Fog Deposition With Uncertainties for Thurstona
K+
Mg2+
Ca2+
kg ha
Year
1995b
b
1996
1997b
b
1998
1999
2000
Mean
a
b
5
+2/1
20
+11/6
9
+3/2
9
+3/2
7
+3/2
11
+3/2
10
5
+2/1
10
+4/3
15
+6/4
28
+8/6
15
7
+3/2
9
+4/3
9
+4/3
21
+8/7
11
Cl
Na+
1
yr
SO42
NO
3
1
49
+19/11
479
+253/137
41
+15/9
87
+32/19
129
+52/33
233
+65/47
170
NH4+
Inorg. N
1
kg N ha
77
+30/17
433
+228/122
68
+24/14
214
+78/45
255
+101/63
403
+107/76
241
34
+14/8
262
+138/74
70
+25/15
458
+169/98
268
+106/68
229
+62/45
220
5
+2/1
40
+21/11
1
±1
1
±1
7
+3/2
8
+2/1
10
1
±1
4
+2/1
3
±1
3
±1
6
+3/2
8
+2/1
4
Org. N
1
Fog, cm
6
+2/1
44
+23/12
4
±1
4
+2/1
13
+5/3
15
+4/3
14
122
+43/32
109
+45/34
170
+66/51
200
+65/50
204
+79/61
182
+53/40
164
yr
3
±2
1
±1
2
Values shown here have been recalculated using an updated procedure for computing fog deposition.
Inorganic N data and cation data for these years appear in the works of Heath [1996], Coeppicus [1998], and Heath and Huebert [1999].
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
Figure 2. K+ concentrations in weekly precipitation
samples at Thurston from 1995 to 1998. Several of the
peaks in K+ correspond to episodes of biomass burning
caused by lava flows.
[46] Springtime dust storms over central Asia suspend
tremendous amounts of material in the air. This material
may be transported great distances in the free troposphere,
frequently reaching Hawaii and occasionally even the con-
24 - 9
tinental United States [Braaten and Cahill, 1986; Betzer et
al., 1988; Parrington et al., 1983; Uematsu et al., 1983]. In
the measurements of springtime Asian dust taken by our
group off the coast of China during the ACE-Asia field
project, the dust we measured was characterized by
extremely high concentrations of Ca2+ (as high as 730 nmol
m3), with only slight enhancements of K+ and Mg2+
(14:1:1, molar ratio of Ca2+:K+:Mg2+).
[47] These enhanced aerosol Ca2+ concentrations, with
smaller peaks in aerosol K+ and Mg2+, are consistent with
what we observe in Hawaii, both at our Thurston and Kokee
sites as well as at the Mauna Loa Observatory (MLO),
where our group has been making aerosol measurements
since 1987 (methodologies described by Lee et al. [1993]).
The nightly Teflon/nylon filter packs sample predominantly
free tropospheric air [Lee et al., 1993]. Monthly concentrations of aerosol base cations at Thurston and MLO are
plotted in Figure 4. The Ca2+ peak that we see at Thurston
appears as a more pronounced feature at MLO. The smaller
Mg2+ peak appears in the Thurston aerosol as well as in
Thurston and Kokee precipitation. While the K+ data are
less clear, we should note that our K+ measurements are
often very near our detection limit. Note that the apparent
Figure 3. Monthly averaged precipitation deposition of base cations at Thurston and Kokee from
January 1999 through June 2000.
24 - 10
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
rather consistent from year to year. Lower measured rainfall
and inorganic N deposition during 1995 and 2000 are likely
a result of sampling primarily during the first, typically
+
drier, half of the year. NO
3 and NH4 are roughly equivalent
on average, although in a given year (or week) one or the
other may dominate significantly. This variability may be a
result of changes in volcanic activity. While oxidized N is
formed in air near the lava flows, reduced N should be
primarily emitted at Pu’u O’o Vent. Which of these two
related but different sources influences our site will depend
on the nature of the volcanic activity and the wind direction
at the time (J. H. Carrillo et al., Volcanically produced
nitrogen in Hawaii, submitted to Global Biogeochemical
Cycles, 2002).
[50] There is seasonality in rainfall, with relatively more
rainfall during the winter months from November to January; the amount of N deposition follows a similar trend as
the amount of rainfall from month to month (Figure 6). This
is not generally true for the interannual deposition amount.
The frequency of rain events is consistent from year to year,
with a rain event occurring on average less than every 2
days (J. H. Carrillo and B. J. Huebert, Fog interception in
Hawaii calculated with a water balance approach: Results
and uncertianties, submitted to Journal of Hydrology, 2002).
Higher rainfall years result primarily from heavier, not more
frequent, rainfall. Intensive rain sampling has shown that
most N is scavenged during the beginning of a rain event, so
Figure 4. Monthly averaged aerosol concentrations of
nonsea salt cations at Thurston and at the MLO from 1999
to 2000.
negative NSS concentrations for K+ and Mg2+ are possible
since NSS is calculated by difference and SS concentrations
are typically many times NSS concentrations in Thurston
aerosol.
[48] This same annual pattern of cation deposition is
apparent in the fog deposition at Thurston as well.
Figure 5 is similar to Figure 3, but plotted for Thurston
fog since 1995. As we have noted previously, we collect fog
intermittently so that we do not have as evenly distributed
data as for precipitation and dry deposition (although in the
figure, values have been extrapolated to monthly values).
This fact makes the very similar pattern that we see in fog at
Thurston even more remarkable. Like precipitation, there is
very little deposition of NSS K+ or Mg2+, while NSS Ca2+
deposition is substantial in the spring and early summer.
Additionally, the spring peak in SS is apparent. This
suggests that the spring peak in precipitation SS deposition
is not solely due to increased rainfall, but to increased
atmospheric concentrations as well.
5.4. Precipitation N Deposition
[49] Our longest and most reliable N deposition results are
for precipitation deposition at Thurston. The mean rainfall
total N deposition is 1 kg N ha1 yr1 (Table 3), which is
within the range of values found for nearby sites [Harding
and Miller, 1982; Vitousek et al., 1993]. The deposition is
Figure 5. Monthly average base cation deposition in
Thurston fog from 1999 to 2000.
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
Figure 6. Monthly averaged precipitation N deposition at
Thurston from 1994 to 2000.
longer or heavier events should not necessarily result in
more N deposition. The higher N deposition during months
with more rainfall may result from the typically higher event
frequency during those months, rather than from larger
rainfall amounts.
5.5. Dry N Deposition
+
[51] Dry deposition of HNO3 and aerosol NO
3 and NH4
is the smallest atmospheric N source at Thurston, depositing
on average less than 0.2 kg N ha1 yr1 (Table 5). In recent
years, our estimated dry deposition rose significantly due to
changes in our instrumentation and sampling procedures
that have affected our Vd calculations (note that concentrations have not changed significantly; Table 4). Figure 7
24 - 11
shows monthly dry deposition from September 1998 to July
2000, all measured using the new procedures. During this
time, the filter packs and anemometer were mounted well
above the forest canopy.
[52] In November 1998, we began using the PVM data
rather than RH to determine when ‘‘dry’’ (no rain or fog)
conditions were met, resulting in more measured dry time
(Table 5). Indeed, a comparison has shown that our RH
sensor significantly overestimates the amount of fog (J. H.
Carrillo and B. J. Huebert, Fog interception in Hawaii
calculated with a water balance approach: Results and
uncertianties, submitted to Journal of Hydrology, 2002).
As a result, data since November 1998 are more likely to
represent the actual dry deposition amounts. HNO3 deposition, the largest dry input, is highest during the summer
months, which typically receive less rainfall. This was
exaggerated during the spring of 2000, when rainfall
amounts were significantly below climatological values.
[53] Our calculated HNO3 deposition velocities are higher
than most reported values [Duce et al., 1991; Huebert and
Robert, 1985], but are probably realistic. The forest canopy
at Thurston is directly exposed to the trade winds and 10
min averaged wind speeds are near 5 m s1, with gusts that
are much faster. Additionally, the rough topography of the
forest canopy results in more air turbulence than over
grassland or ocean. The relatively high calculated HNO3Vd
suggests that particulate deposition may also be relatively
high. Although we used a range of published values in our
calculations, a 1 cm s1Vd for particles may be more
realistic for our location than 0.1 cm s1. Since we have
not measured or calculated a Vd for particles, our estimates
Figure 7. Dry deposition at Thurston from September 1998 to July 2000.
24 - 12
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
Figure 8. Monthly averaged fog N deposition and fog interception amounts at Thurston from 1995 to
2000.
contain large uncertainty. However, even using this large Vd,
the very low estimated aerosol dry deposition indicates that
we have not overlooked a major inorganic N deposition
source.
5.6. Fog N Deposition
[54] N deposition by intercepted fog is consistently the
largest N source to the Thurston site, depositing an average
of 16 kg N ha1 yr1 (Table 7). Past reports of intercepted
fog at Thurston have suffered from relatively sparse chemical data and from instrumentation failures that resulted in
missed water input sampling days. During 1999, we
sampled many more events. This, in addition to an
improved procedure for estimating fog interception
amounts, gives us greater confidence in the accuracy of
our results.
[55] The fog interception amount (water deposition only)
does not exhibit the same interseasonal variability that
precipitation does (Figure 8), but is fairly consistent
throughout the year. However, fog N deposition has a
peak in the spring-summer and is lowest during the fall
months, because of interseasonal differences in the N
concentration in fog. Because the trade winds are weakened during the late spring and summer, there is a higher
probability of volcanically influenced (and often N-rich)
air reaching our site.
[56] The original motivation for this study was to explain
an apparent imbalance in the soil N budget at the Thurston
site. Our estimated annual fog deposition of 16 kg N ha1
yr1 accounts for much of Crews’ 33 kg N ha1 yr1 of
missing N source [Crews et al., 1995]. Given the large
interannual variability for fog deposition and the uncertainty
associated with our estimate of this term, it seems likely that
N deposition by fog interception is the source of virtually all
of the previously unaccounted for N in the soil.
[57] Fog deposition on other Hawaiian islands may be
significantly lower than at Thurston. In an analysis of the
volcanic source of inorganic N, it was determined that
between 22 (NH4+) and 38% (for NO
3 ) of the deposited N
at Thurston was volcanic in origin during 1999 (J. H.
Carrillo et al., Volcanically produced nitrogen in Hawaii,
submitted to Global Biogeochemical Cycles, 2002). Though
we have not yet determined the amount of volcanic N that
reaches the other islands in fog, it is likely to be less than
Thurston. On all islands, the amount of volcanic N deposition also should be highly variable, depending on the
nature of volcanic activity and on meteorological conditions
that transport volcanic N.
5.7. Organic N Deposition
[58] The average annual deposition of organic N by
precipitation was 0.1 kg N ha1 yr1 and by fog was 2
kg N ha1 yr1. Organic N was 16% of the total N in
precipitation and 12% of the total N in fog. The greater
number of sampled fog events in 1999 suggests that the
17% measured this year is more representative of long-term
averages (Table 7), and indeed it is consistent with the
precipitation value. The percentage of organic N we saw in
fog and precipitation was less than the 70% measured by
Vitousek and Walker at a nearby site over a 19-week period
of time [Vitousek and Walker, 1989]. These results may not
be completely inconsistent, however. Within our yearlong
24 - 13
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
itation inorganic N deposition at Kokee should be more
representative of inorganic N deposition on islands without
active volcanism.
5.9. N Isotopic Composition of Nitrate in Rain and Fog
[61] Stable isotopic composition can sometimes aid in the
identification of different sources or reaction pathways for
nitrogenous species. The 15N to 14N ratio of a sample is
expressed relative to atmospheric N2, as follows
d15 N ¼
Figure 9. Monthly averaged precipitation N deposition at
Thurston and Kokee from 1999 to 2000.5.
average are the periods of time when organic N dominates,
although the largest fraction of organic N over any 19-week
period during our measurements was 45%.
[59] It is possible that we underestimated organic N in
precipitation due to evaporation and losses on the polyethylene buckets [Scudlark and Church, 1988]. Additionally, we may overestimate it at times due to conversion of
inorganic N to organic N by biological activity over our
weeklong collection period, particularly during the summer
months. Because fog is sampled on an event basis and
Teflon bottles are used for collection, many of the potential
artifacts of our precipitation sampling are avoided. The fact
that the fraction of organic N in rain and fog is similar
suggests that either the positive and negative artifacts in
precipitation roughly balance on average, or that they are
small.
5.8. Precipitation N Deposition at Thurston and Kokee
[60] Since drought conditions persisted at Kokee over
much of the measurement period, it is unclear how representative the chemical fluxes are. Nonetheless, precipitation
inorganic N deposition at Kokee was disproportionately
lower than at Thurston (Figure 9) most likely due to the
proximity of Kilauea Volcano to the Thurston site (J. H.
Carrillo et al., Volcanically produced nitrogen in Hawaii,
submitted to Global Biogeochemical Cycles, 2002). Precip-
15
N=14 NðsampleÞ 15 N=14 NðairÞ
15 N=14 Nðair Þ
1000 ‰15 N
ð2Þ
[62] A broad range in d15N was measured for NO
3 in both
our fog and rain samples. For fog, values ranged from 3.0
to +5.9%, while for rain the range was from 3.8 to +2.4%
(Table 8). The majority of the samples that we believe
contain significant volcanogenic N (J. H. Carillo et al.,
Volcanically produced nitrogen in Hawaii, submitted to
Global Biogeochemical Cycles, 2002) have d15N values
between 0 and +2%. Since these values fall within the
range of our background variability, it is difficult to use the
isotopic results to distinguish volcanically produced N.
However, the d15N results may be helpful in indicating
likely sources for the background (i.e., nonvolcanic) N. In
this context, there is an apparent shift in d15N measured in
both fog and precipitation, from positive d15N values from
about April through September to negative d15N values for
the rest of the year (Figure 10), which may be due to a
change in the N source.
[63] It is likely that much of the N we measure during
trade wind weather arrives by long-range transport from
continents. In a comparison of model predictions and
at MLO, Huebert et al. [2001]
measurements of SO2
4
report that North American sources are most important
from July through September, whereas Asian sources dominate from October through June. Lee et al. [1994] found the
similar pattern for total nitrate at MLO using the back
trajectory analyses summarized in Figure 11 (redrawn from
Lee et al. [1994]). On this map, each number represents a
monthly average starting point for 10-day air mass trajectories that arrive at MLO. From November to May, these
starting points lie west of Hawaii. This is consistent with the
peaks in Ca2+, both at MLO and in our boundary layer sites,
presumably from Asian dust, between March and June
(Figures 3 –5).
[64] What d15N would we expect for nitrate from Asia
versus North America? According to Galloway [2000], N
exported from Asia is due to the roughly equal sources of
fertilizer use and fossil fuel burning. Though we know of no
isotopic measurements specifically for Asian fertilizer or
Table 8. Isotopic N Values of NO
3 -N in Fog and Precipitation
Samples From 1999
Thurston Fog
Thurston Rain
Kokee Rain
d15N of NO
3
Range
Mean
n
3.0 to +5.9%
+0.8%
26
2.9 to +2.4%
0.03%
6
0.4 to 3.8%
2.9%
3
24 - 14
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
Figure 10. Values of d15N for fog and precipitation
samples versus the day of the year.
fossil fuel, in other locations, both fertilizer NO
3 (2 to
+5%; Heaton [1986]) and emissions from coal-fired power
stations (+6 to +13%; Heaton [1990]) tend to be relatively
high in d15N. Alternately, a major source of oxidized N from
North America is vehicle exhaust [Logan, 1983], which
tends to be relatively low in d15N (2 to 13%; Heaton
[1990]). Therefore the high d15N we observe from April to
August at Thurston is consistent with an Asian source and
the low d15N we observe from August to April is consistent
with a North American source. These N isotope results for
our marine boundary layer sites at Thurston and Kokee
seem to be consistent with the long-range transport sources
determined for the free troposphere at MLO.
[65] In an analysis of the vegetation at the Thurston
site, Vitousek et al. [1989] found that leaves, soils, and N
fixers were all depleted in 15N, with d15N values ranging
from 7.7 to 1.2%. The authors proposed that atmospheric deposition may contribute 15N depleted N, resulting in the low d15N values they observed. If atmospheric
N is responsible for the low d15N values, it must be in
the form of either NH4+-N or organic N, since the volume
weighted mean d15N values of NO
3 -N were close to zero,
ranging from +0.8% for Thurston fog to 0.03% and
2.9% for Thurston and Kokee precipitation (Table 8). It
is in fact quite likely that negative d15N values in
Thurston vegetation are because of NH4+-N deposition.
NH4+-N can represent as much as 75% of the total N
deposition during some years (Tables 3, 5, and 7), and
measurements of the d15N of atmospheric NH4+ at other
locations show that it is typically lower than that of
oxidized N at the same location [Freyer, 1978; Garten,
1992; Heaton, 1987]. If the NH4+ ions deposited at
Thurston were depleted in 15N, the result would be a
net low-d15N source, since the d15N of NO
3 is near 0%.
Values for d15N of atmospheric organic N span a broad
range (7.3 ± 7.3%, Cornell et al. [1995]), but tended to
be lower in more remote regions. This implies that
organic N may also be a low-d15N source at our site.
6. Conclusions
[66] Atmospheric deposition is an important source of N
and base cations to Hawaiian ecosystems and the major
pathway for deposition is via fog interception. Though
many researchers have observed concentrations in fog that
are higher than precipitation for a site [e.g., Jordan et al.,
2000; Vong et al., 1997], results from this study indicate that
in Hawaii, high fog water fluxes combine with several
strong, sporadic chemical sources to result in higher deposition rates of several species than might be expected for a
remote, marine environment. High deposition rates for
chemical species in fog have been observed in many
polluted environments and are a concern with regard to
acid and metal deposition [e.g., Dollard et al., 1983; Harvey
and McArthur, 1989; Herckes et al., 2002; Igawa et al.,
1998]. Our study area is unusual in that the chemical
sources determined for our sites are largely natural.
Figure 11. Monthly averaged back trajectories for air masses reaching MLO. The number refers to the
trajectory month and the number location is the trajectory starting point. Figure taken from the study of
Lee et al. [1994].
CARRILLO ET AL.: ATMOSPHERIC DEPOSITION OF N
[67] Measured annual fluxes of K+, Mg2+, and Ca2+
averaged 15, 17, and 13 kg ha1 yr1, respectively and
fog interception was by far the largest deposition pathway.
While sea salt contributed the majority of cations, local
biomass burning and Asian dust were also significant
sources for some years. Though sporadic in nature, the
influence of Asian dust, and owing to the volcanic nature of
the Hawaiian Islands, biomass burning, both are sources
that are likely to have been important historically.
[68] Fog interception is also the largest source of N to
Hawaiian ecosystems, with a measured deposition rate of 16
kg N ha1 yr1 at the Thurston Lava Tube site. Precipitation
and dry deposition contribute an average of 1.0 and 0.3 kg
N ha1 yr1 ,respectively. Precipitation deposition on Kauai
contributed 0.2 kg N ha1 yr1, although drought conditions persisted at Kokee during much of our measurement
period. Organic N was on average 16 and 12% of the N in
rain and fog, indicating that it is an important N source. For
both rain and fog, there were samples for which 100% of the
measured N was organically bound. In fact, during much of
the summer of 1999, all of the precipitation N was organic.
15
[69] Values for NO
3 -d N in fog ranged from 3.0 to
5.9% and in rain from 3.8 to 2.4%. While d15N values for
volcanically produced NO
3 -N fell within the range of background values, the apparent seasonal isotopic variations in
15
NO
3 -d N may be because of different nonvolcanic N
sources. d15N tended to be positive during the spring and
summer, consistent with long-range transport of material
from Asia, and negative during the rest of the year, indicating
long-range transport of N from North America. This result is
consistent with our measured Ca2+ deposition seasonality,
and with free tropospheric sources determined for MLO.
[70] Volume weighted mean d15N values were near zero,
indicating that the strongly negative d15N values measured
by Vitousek et al. [1989] in soils and leaves at the Thurston
site are not a direct result of atmospheric NO
3 deposition.
NH4+ or organic N deposition may provide the light source,
however, since other studies indicate that the d15N of NH4+
tends to be lower than that of NO
3 in atmospheric samples
and that organic N tends to be low in d15N in remote
regions.
[71] Our measurements at Thurston indicate that atmospheric deposition may completely account for the apparent
soil N imbalance [Crews et al., 1995]. The 17 kg total N
ha1 yr1 we measure has large interannual variability.
Since much of the deposited N is volcanic in origin, during
the years of enhanced volcanic activity, deposition rates are
likely to be higher.
[72] Acknowledgments. We are grateful to Liangzhang Zhuang for
performing most of the chemical analyses, to Claudia Benitez-Nelson for
her help in editing the text, and to Peter Vitousek for his encouragement and
insightful suggestions. Many samples were collected by Sirit Coeppicus,
Karin Schlappa, and David Alexander. This work was supported by the
Andrew W. Mellon Foundation, NSF grants ATM-9816637 and ATM9807631 to B.J.H., and NSF grant OCE-9981479 to D.M.S.
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