Introduction
Ammonia (NH3(g)) is the dominant alkaline gas in the atmosphere and is
an important component of the global nitrogen cycle. Its transport and
deposition can have harmful effects for N-sensitive ecosystems such as
eutrophication, loss of biodiversity and soil acidification (Krupa, 2003).
The presence of NH3(g) can impact climate by increasing rates of new
particle formation via stabilization of sulfuric acid clusters (Kirkby et
al., 2011). Gas-phase NH3 is also able to partition to acidic fine
particulate matter (PM2.5) to form particulate-phase ammonium
(NH4(p)+), which alters various aerosol properties, such as
scattering efficiency (Martin et al., 2004), hygroscopicity (Petters and
Kreidenweis, 2007), ice nucleating ability (Abbatt et al., 2006), and
heterogeneous chemistry occurring on surfaces (Fickert et al., 1999).
As a result, the accurate quantification of the magnitude and location of
NH3(g) sources is important for chemical transport models (CTMs). The
major anthropogenic source is agriculture (fertilization and animal
husbandry) with biomass burning, transport, and industry being minor
contributors (Reis et al., 2009). Natural sources include soils, vegetation,
oceans and animal excreta (Sutton et al., 2013). Estimates for the annual
global emissions of NH3(g) range from 35 to 54 Tg N yr-1; however,
large uncertainties exist for these values due to the area-wide nature
(emissions spread over a large spatial extent) and poor characterization of
many sources. In remote marine environments, the ocean is thought to be the
dominant source of NH3(g) to the marine boundary layer and delivers an
estimated 6–8 Tg N yr-1 to the atmosphere globally (Sutton et al.,
2013). The dominant sources of oceanic NHx (≡ NH3+ NH4+) include remineralization of organic matter by bacteria and
phytoplankton excretion (Carpenter et al., 2012). However, NHx is an
extremely labile nutrient for microbes such that assimilation by
phytoplankton and bacteria prevents significant accumulation in surface
waters. Nonetheless, there exists a pool of dissolved ammonia (NH3(sw))
available for exchange with the atmosphere.
In order to compute sea–air NH3 fluxes, simultaneous measurements of
both atmospheric NH3(g) and oceanic NHx are required. These
measurements are extremely challenging due to low ambient concentrations and
complications arising from making ship-based measurements (e.g. proximity to
human activity can cause artefacts). As a result, to our knowledge only six
previous studies have simultaneously quantified both [NH3(g)] and
oceanic [NHx], leading to extremely large uncertainties for both the
direction and magnitude of global sea–air NH3 fluxes (Asman et al.,
1994; Geernaert et al., 1998; Gibb et al., 1999; Johnson et al., 2008; Quinn
et al., 1988, 1990). Johnson et al. (2008) provided the most recent data set
and summarized the previous studies to show that the open ocean can be both
a net source and a net sink of NH3(g), with sea surface temperature
(SST) being a key determinant for the direction of flux. Colder SST reduces
the emission potential due to increased solubility of NH3 (because of
both reduced NH3(aq) volatility and increased partitioning of
NH3(aq) to NH4(aq)+); hence, at higher latitudes
the open ocean is more likely to act as a net sink (Johnson et al., 2008).
Of the six previous studies, only Johnson et al. (2008) quantified NH3
fluxes above the Arctic Circle (66∘33′ N) during a summer time
study in the Norwegian Sea. Therefore additional measurements of sea–air
NH3 fluxes in the High Arctic are invaluable for improving constraints
on oceanic NH3 emissions.
During the summertime, freshwater melt ponds are a ubiquitous feature on top
of melting Arctic sea ice and can comprise up to 80 % of the sea ice
surface (Lüthje et al., 2006). These melt ponds form from melting sea
ice and are anywhere from a few cm to over 1 m deep. They are chemically
distinct from the bulk ocean owing to their low salinity and physical
separation from the ocean mixed layer by sea ice or stratification. To our
knowledge, no studies to date have attempted to quantify melt-pond–air
NH3(g) fluxes despite the abundant presence of melt ponds in the
summertime Arctic.
Quantifying sea–air and melt-pond–air NH3 exchange in the Arctic will
help elucidate the role these processes play as either sources or sinks in
the Arctic nitrogen cycle. Many terrestrial Arctic ecosystems are N-limited
and highly sensitive to perturbations in N-input (Shaver and Chapin III,
1980); thus Arctic soils and vegetation are unlikely to represent important
sources of atmospheric ammonia. Major sources at lower latitudes include
agriculture, vegetation, transport, and industry (Reis et al., 2009; Sutton
et al., 2013) but these are expected to contribute minimally north of the
Arctic Circle. Since the lifetime of NH3(g) is typically less than
24 h, long-range transport from lower latitudes is likely not important (Lefer
et al., 1999). Substantial NH3 emissions have been measured from both
seabird guano (Blackall et al., 2007) and seal excreta (Theobald et al.,
2006) so large colonies may be relevant point sources throughout the Arctic
region. Biomass burning can also inject significant quantities of NH3
into the free troposphere and/or boundary layer (Bouwman et al., 1997).
Although vegetation in the High Arctic is sparse, there can be large
wildfires in boreal regions, and emissions may be transported poleward. The
potential for the ocean and melt ponds to act as sources to the atmosphere
will depend on the relative importance of sources and sinks within the
atmosphere and the aqueous systems.
NH3 emission to the atmosphere can affect the extent of non-sea-salt
sulfate (nss-SO42-) neutralization, which has implications for
N-transport (Lefer et al., 1999). Therefore, it is important to also
consider the relative abundances of atmospheric NHx and
nss-SO42-. The dominant source of the latter in the summertime
Arctic is oxidation of dimethylsulfide (DMS) emitted from the Arctic Ocean
(Leaitch et al., 2013; Sharma et al., 1999, 2012). Measurements of
PM2.5 composition in the summertime Arctic marine boundary layer are
rare (e.g. Chang et al., 2011; Leck et al., 2001). Previous chemical
transport model (CTM) studies with GEOS-Chem predict highly acidic aerosol
(i.e. nss-SO42- ≫ NHx) with
negligible amounts of NH3(g) throughout the summertime Arctic boundary
layer (Breider et al., 2014).
The region for this study is the eastern Canadian Arctic Archipelago where
ship-based atmospheric (NH3(g), NH4(p)+,
SO4(p)2-) and oceanic ([NHx], pH, SST) measurements were
taken over a 4-week period in July and August 2014. To our knowledge, this
study presents the first measurements of NH3(g) in the Canadian Arctic.
Motivated by a lack of atmospheric and oceanic measurements in the region,
as well as substantial uncertainties in sea–air and melt-pond–air NH3
fluxes, the specific goals of this study were the following:
to simultaneously quantify NH3(g) and oceanic/melt pond [NHx] to
infer surface–air NH3 fluxes
to assess the relative abundances of NH3(g), NH4(p)+ and
SO4(p)2- to determine the extent of SO4(p)2-
neutralization
to elucidate the major sources and sinks of atmospheric NH3 throughout the
summertime Arctic marine boundary layer
and to evaluate whether atmospheric NHx deposition could be an important
N-input to aquatic and terrestrial Arctic ecosystems.
Materials and methods
2014 CCGS Amundsen cruise
Measurements were taken aboard the Canadian Coast Guard Ship Amundsen between
13 July and 7 August 2014 as part of the Network on Climate and Aerosols:
Addressing Key Uncertainties in Remote Canadian Environments (NETCARE). The
CCGS Amundsen departed from Québec City, Québec on 8 July 2014 and sailed
throughout the eastern Canadian Archipelago heading as far north as 81.47∘ N eventually reaching Kugluktuk, Nunavut on 13 August 2014. A
detailed map of the ship's route for this leg is shown in Fig. 1 along with
the ship's position at the start of selected days. All times are given in
coordinated universal time (UTC).
CCGS Amundsen ship track coloured by gas-phase NH3 concentrations
measured by the AIM-IC. Invalid measurements (e.g. instrument
troubleshooting, influenced by ship) are purple along the ship track. Units
of ng m-3 were chosen as a convenience for flux calculations. At STP,
100 ng m-3≈ 130 pptv. Relevant landmarks are also labelled.
Dates and arrows indicate the position of the ship at 00:00 UTC on that day.
Atmospheric measurements
Ambient levels of water-soluble ions in PM2.5 (NH4+,
SO42-, and NO3-) and their precursor gases (NH3,
SO2, and HNO3) were measured using the Ambient Ion Monitor-Ion
Chromatograph (AIM-IC) system (Model 9000D, URG Corp., Chapel Hill, NC). The
AIM-IC is a continuous online system which provides simultaneous gas-phase
and particle-phase measurements with hourly time resolution. The system has
been adapted to locate the gas and particle separation and collection
hardware as close as possible to the inlet sampling point (Markovic et al.,
2012). Ambient air is pulled through a PM2.5 impactor to remove coarse
(> 2.5 µm in diameter) particles at a flow of 3 L min-1. Air then enters a parallel-plate wet denuder where water-soluble
gases are dissolved in a 2 mM H2O2 solution (to enhance the
solubility of SO2) which is continuously flowing across the denuder
membranes. The remaining PM2.5 particles have sufficient inertia to
pass through the denuder into a supersaturation chamber where they are
collected as an aqueous solution via hygroscopic growth. These components
were contained within an aluminum inlet box that was mounted to the hull
near the bow of the ship (about 4 m back of the bow). The height of the
inlet was 1 m above the deck. Influence from ship-generated sea spray was
likely minimal due to the benign nature of the summertime Arctic Ocean, in
addition to the PM2.5 impactor designed to remove coarse particles. The
aqueous solutions collected in the inlet box were pulled down a 22 m sample
line through a conduit leading to the IC systems which were housed in a
laboratory below deck. Half of each ∼ 10 mL aqueous aliquot
(representing 1 h of sampling) was then separately injected onto both a
cation IC and anion IC for quantification of water-soluble ions.
The IC systems (ICS-2000, Dionex Inc., Sunnyvale, CA) were operated using
CS17/AS11-HC analytical columns, CG17/AG11-HC guard columns and
TCC-ULP1/TAC-ULP1 concentrator columns for improved detection limits.
Reagent-free gradient elution schemes and suppressed conductivity were also
employed. Aqueous standards of known concentration were prepared via serial
dilution of commercially available mixed standards (Dionex Corp., Sunnyvale,
CA) containing six cations (P/N 040187) and seven anions (P/N 056933). Manual
injection of these standards yielded reasonable (R2 > 0.99)
six-point calibration curves.
During the campaign, three zero-air overflow experiments were performed to
quantify the background signal of each analyte measured during AIM-IC
ambient sampling. For each experiment, the inlet was flooded with high-purity zero air (AI 0.0 UZ-T, PraxAir, Toronto, ON) at 4.5 L min-1 for
18 h. The average peak area during the final 8 h of each experiment
was used as a background and subtracted from each ambient measurement.
Detection limits were calculated by taking 3 times the standard deviation of
each analyte peak area during the final 8 h of each zero air overflow.
This value was then converted to either a mixing ratio or mass loading
assuming standard temperature and pressure (STP). Detection limits for
species of interest during the cruise were 29 ng m-3 (NH3), 17 pptv
(SO2), 8 pptv (HNO3), 12 ng m-3 (NH4+), 36 ng m-3 (SO42-), and 64 ng m-3 (NO3-). For the
convenience of flux calculations, NH3 values are reported in ng m-3 (at STP 100 ng m-3 NH3≈ 130 pptv).
Standard meteorological parameters were measured using a Vaisala HMP45C212
sensor for temperature, an RM Young model 61205V transducer for pressure,
and an RM Young Model 05103 wind monitor for wind speed and direction
located at the bow ship of the ship at a height of 8.2–9.4 m above the
deck. Data were averaged to 1 h to match the time resolution of the
AIM-IC. In order to remove any influence from activities aboard the ship,
gas-phase measurements are only reported if the following conditions were
met: (1) average hourly ship speed > 4 knots (∼ 7.4 km h-1), (2) average hourly apparent wind direction ±90∘
of the bow, and (3) standard deviation of apparent wind
direction < 36∘. Similar cut-offs for speed and wind
direction have been used in previous studies of NH3 in the marine
boundary layer (e.g. Johnson et al., 2008; Norman and Leck, 2005).
Surface measurements
A total of 37 surface ocean and 9 melt pond samples were collected
throughout the study. Melt pond samples were collected directly into a
cooler jug using an electrical pump fixed on a telescopic arm. The water was
sampled as far from the side of the melt pond as possible, between 1 and 2 m
depending on the size of the melt pond. Temperature was measured in situ
with a VWR high-precision thermometer, and total aqueous [NHx] was
determined within 10 h of sampling using a fluorometric technique that has
been optimized for low concentrations and complex matrices (Holmes et al.,
1999). The method detection limit was 20 nM. Surface ocean samples were
obtained with a Rosette sampler equipped with GO-FLOW bottles and a CTD
(Seabird Electronics SBE911+) recording temperature. Total aqueous
[NHx] was determined as above within 1 h of sampling. Surface water
temperature along the ship's track was continuously measured by a
thermosalinograph (Seabird Electronics SBE 45) connected to the seawater
inlet. For the purposes of flux calculations, the ocean pH and salinity were
assumed to be 8.1 and 35 g kg-1, respectively, which are representative
for the region of interest (Takahashi et al., 2014). These assumptions have
been made previously and were not found to be a major source of uncertainty
when calculating sea–air NH3 fluxes (Johnson et al., 2008). The melt
pond pHs were measured using a pH-meter within 4 h of sampling. A
three-point calibration of the pH probe (Orion™ Model 91–72,
Thermo Scientific) was performed using commercially available pH 4.01, 7.00
and 10.00 buffers. Salinity of the melt ponds were determined with a WTW Cond 330i handheld conductivity meter.
Flux calculations
The direction of sea–air NH3 fluxes can be assessed by comparing
ambient measurements of NH3(g) to the atmospheric mixing ratio
predicted from Henry's Law equilibrium calculations using seawater
[NHx] and surface temperature measurements (e.g. Asman et al., 1994;
Johnson et al., 2008; Quinn et al., 1988, 1996). This equilibrium NH3
concentration signifies the ambient value at which the net flux changes
direction, and is known as the compensation point (denoted χ). In
other words, one expects a net downwards flux if ambient NH3(g) exceeds
χ and a net upward flux if it is below χ. The magnitude of these
fluxes are commonly computed using the “two-phase” model first developed
by Liss and Slater (1974), which describes the sea–air transfer of gases as
being controlled by molecular diffusion on either side of the interface. The
transfer of NH3 across this interface is predominantly dictated by the
air-side transfer velocity, given the relatively high water solubility of
NH3 (Liss, 1983). Hence, the equation to calculate sea–air NH3
fluxes is as follows:
FNH3=kg⋅(χ-NH3(g))⋅17.03,
where FNH3 is the sea–air flux of NH3 (ng m-2 s-1),
kg is the air-side transfer velocity (m s-1), NH3(g) is the
measured ammonia concentration (nmol m-3), χ is the compensation
point (nmol m-3), and the molecular weight of 17.03 g mol-1 is to
convert nmol to ng. Numerous parameterizations exist for kg with
varying degrees of complexity (Johnson, 2010). Here we adopt the approach
established by Duce et al. (1991):
kg=u770+45⋅MW1/3,
where u is the wind speed (m s-1) and MW is the molecular weight of the
gas of interest (17.03 for NH3). Although simple, this parameterization
has been used previously to estimate sea–air NH3 fluxes (e.g. Johnson
et al., 2008) and has been shown to be in good agreement (within 20 %)
with a more complex scheme, particularly at lower wind speeds (Johnson,
2010). The following equation is used to calculate χ:
χ=KH⋅[NH3(sw)],
where KH is the Henry's law constant (dimensionless) and [NH3(sw)]
is the concentration of dissolved ammonia in the surface pool (nmol m-3). The temperature-dependent equation for KH is (McKee, 2001):
KH=117.93⋅T273.15⋅e(4092/T)-9.70,
where T is the surface temperature (in K). The following equation is used to
relate the NH3(sw) to the concentration of total dissolved NHx
([NHx(sw)]), which is the value actually measured by the procedure
outlined in Sect. 2.3:
[NH3(sw)]=[NHx(sw)]⋅Ka10-pH+Ka,
where Ka is the acid dissociation constant of NH4+. The
pKa (≡-logKa) is calculated according to Bell et al. (2008), which provides an empirical correction for salinity (S,
dimensionless) at a given temperature (T, in ∘C):
pKa=10.0423+0.003071⋅S-0.031556⋅T.
Equations () and () closely follow that of Johnson et al. (2008) but are
sufficiently similar to analogous approaches for calculating KH and
kg used in other sea–air NH3 exchange studies (e.g. Asman et al.,
1994; Gibb et al., 1999; Quinn et al., 1992). Johnson (2004) reported that
fluxes calculated with these various schemes usually agree within 2 %.
Melt-pond–air exchange was also examined using Eqs. () to ().
GEOS-Chem
The GEOS-Chem chemical transport model (www.geos-chem.org) is
used to aid in the interpretation of the atmospheric measurements. We use
GEOS-Chem version 9-02 at 2∘ × 2.5∘ resolution globally,
and with 47 vertical layers between the surface and 0.01 hPa. The
assimilated meteorology is taken from the NASA Global Modelling and
Assimilation Office (GMAO) Goddard Earth Observing System version 5.11.0
(GEOS-FP) assimilated meteorology product. Boundary layer mixing uses the
non-local scheme implemented by Lin and McElroy (2010). Our simulations use
2014 meteorology and allow a 2-month spin-up prior to the simulation.
The GEOS-Chem model includes a detailed oxidant-aerosol tropospheric
chemistry mechanism as originally described by Bey et al. (2001). Simulated
aerosol species include sulfate–nitrate–ammonium (Park et al., 2004,
2006), carbonaceous aerosols (Park et al., 2003; Liao et al., 2007), dust (Fairlie et
al., 2007, 2010) and sea salt (Alexander et al., 2005). The
sulfate–nitrate–ammonium chemistry uses the ISORROPIA II thermodynamic
model (Fountoukis and Nenes, 2007), which partitions ammonia and nitric acid
between the gas and aerosol phases. For our simulations, the natural NH3
emissions are from Bouwman et al. (1997) and biomass burning emissions are
from the Quick Fire Emissions Dataset (QFED2) (Darmenov and da Silva, 2013),
which provides daily open fire emissions at
0.1∘ × 0.1∘resolution. Anthropogenic NH3
emissions are from Bouwman et al. (1997). The model includes natural and
anthropogenic sources of SO2 (van Donkelaar et al., 2008; Fisher et al.,
2011)
and DMS emissions based on the Nightingale (2000) formulation and oceanic DMS
concentrations from Lana et al. (2011). Oxidation of SO2 occurs in clouds by
reaction with H2O2 and O3 and in the gas phase with OH
(Alexander et al., 2009) and DMS oxidation occurs by reaction with OH and
NO3.
GEOS-Chem simulates both wet and dry removal of aerosols and gases. Dry
deposition follows a standard resistance in series scheme (Wesley,
1989) with an aerosol dry deposition velocity of 0.03 cm s-1 over snow and
ice (Fisher et al., 2011). Wet removal in GEOS-Chem takes place in large-scale
clouds and convective updrafts (Liu et al., 2001). In-cloud scavenging of
hydrophilic species takes place at temperatures warmer than 258 K, and
hydrophobic black carbon and dust are also removed at temperatures colder
than 258 K (Wang et al., 2011).
Parameterizations and options used for the NETCARE WRF simulations.
Atmospheric process
WRF option
Planetary boundary layer
Mellor–Yamada–Janjic Scheme (MYJ) (Janjic, 1994)
Surface layer
Monin-Obukhov Janjic Eta similarity scheme (Monin and Obukhov, 1954; Janjic, 1994, 1996, 2002)
Land surface
Unified Noah Land Surface Model (Tewari et al., 2004)∗
Microphysics
WRF Single-Moment 5-class scheme (Hong et al., 2004)
SW radiation
Goddard Shortwave Scheme (Chou and Suarez, 1994)
LW radiation
RRTMG (Iacono et al., 2008)
Cumulus parameterization
Kain–Fritsch Scheme (Kain, 2004)
∗ with corrected calculation of skin temperature over sea ice when snow
melting is occurring, see
http://www2.mmm.ucar.edu/wrf/users/wrfv3.7/updates-3.7.1.html.
FLEXPART-WRF
FLEXPART-WRF (Brioude et al., 2013, website:
http://flexpart.eu/wiki/FpLimitedareaWrf) is a Lagrangian particle dispersion model
based on FLEXPART (Stohl et al., 2005) that is driven by meteorology from
the Weather Research and Forecasting (WRF) Model (Skamarock et al., 2005).
Here we use FLEXPART-WRF run in backward mode to study the emission-source
regions and transport pathways influencing ship-based ammonia measurements.
A WRF simulation for the summer 2014 NETCARE campaign was performed using
WRF 3.5.1 with initial and boundary conditions provided by the operational
analysis (0.25∘ × 0.25∘ resolution) from European Centre
for Medium-Range Weather Forecasts (ECMWF). Parameterizations and options
for the WRF simulations are given in Table 1. The WRF model was run from 1 July to 13 August 2014 and nudged to ECMWF winds, temperature, and
humidity every 6 h above the atmospheric boundary layer. The WRF run was
evaluated using meteorological measurements made onboard the Amundsen and
from the Polar-6 aircraft flights during this period. FLEXPART-WRF was run in
backward mode to produce retroplume output that is proportional to the
residence time of the particles in a given volume of air. Runs were
performed using the location of the ship, with one model run performed every
15 min while the ship was in the model domain (13 July–13 August 2014).
For each run, 100 000 particles were released at the ship location (100 m
extent horizontally and vertically) and the FLEXPART-WRF was run backwards
for 7 days prior to release. The output provides retroplume information (the
residence time of air prior to sampling) which is used to calculate the
potential emission sensitivities (PES) integrated over the 7 days prior
to sampling by instruments aboard the Amundsen.
Results and discussion
Surface–atmosphere NH3 fluxes
Figure 1 shows the ambient NH3(g) concentrations measured by the AIM-IC
throughout the cruise. Measured values of NH3(g) range between 30–650 ng m-3 with the highest values occurring in Lancaster Sound as the ship
was steaming eastward into Baffin Bay. Only two measurements of NH3
were below the detection limit (29 ng m-3) throughout the entire
cruise. NH3 consistently exceeded 100 ng m-3 during later parts of
the cruise along the eastern shores of Ellesmere Island and western shores
of Greenland. Lower values (< 100 ng m-3) were observed at the
beginning of the campaign along the eastern shores of Baffin Island.
Measurements of NH3(g) in the marine boundary layer at northern
latitudes (> 50∘ N) are sparse; however, the
concentrations measured in this study are within the few previously reported
ranges for the regions above 50∘ N. Johnson et al. (2008)
reported NH3(g) between 20–300 ng m-3 in the Norwegian Sea during
spring and summer, but a lower range (20–90 ng m-3) in the northern
North Sea in winter. In the southern North Sea, Asman et al. (1994) measured
higher values (30–1500 ng m-3) in a study lasting from February to
October.
The relevant measurements needed to calculate χ for both the open
ocean and melt ponds are listed in Tables S1 and S2 in the Supplement, respectively. Only four
unique co-ordinates are listed for the nine melt pond samples because
multiple melt ponds were sampled at each location. Roughly half of the
surface ocean samples had [NHx] below the detection limit (20 nM) and
in general values were significantly lower than in the melt ponds. Open
ocean samples ranged from < 20 to 380 nM whereas seven of the nine
melt pond samples were between 640 to 1260 nM (with the other two below
detection limit). These concentrations and their spatial variability are
typical for the region during summer (Martin et al., 2010).
Parameters listed in Tables S1 and S2 were input into Eqs. () to () to
calculate χ for both the surface ocean and melt pond samples. For
samples with [NHx] below the detection limit, a value of 10 nM (half of
the detection limit) was assumed. A comparison of the calculated
compensation points for the ocean (χocean) and melt ponds (χMP) are shown in Fig. 2. Also shown is the range for the nearest valid
measurement (see Sect. 2.2) of ambient NH3(g). The NH3(g)
concentration taken during the hour of surface sampling could not be used
since the ship remained stationary for up to 12 h while melt pond or
ocean work was being conducted. Hence, the NH3(g) measurement from
several hours prior (as the ship approached the surface sampling site) had
to be used. This approach should not significantly impact the analysis given
that the ambient levels of NH3(g) were observed to be fairly uniform
from 1 h to the next (i.e. no rapid spikes of NH3(g) were
measured). Shown in lighter yellow are the ranges of NH3(g) observed
over the entire study (∼ 30–650 ng m-3). Figure 2 clearly
shows that the ambient concentrations of NH3(g) exceed both χocean and χMP by several orders of magnitude throughout the
entire region. This conclusively demonstrates that during the summertime,
the ocean and melt ponds are net sinks of atmospheric NH3(g). This
finding is consistent with Johnson et al. (2008) who found a tendency for
downward net fluxes at higher latitudes, primarily as a result of colder sea
surface temperatures. Assuming an upper limit for the ocean pH of 8.2 would
increase χocean by less than 20 %.
Box-and-whisker plot showing the observed ranges of χ (on a
log scale) for both the ocean surface (dark blue) and melt ponds (light
blue). The range of NH3(g) measured by the AIM-IC near the time of
surface sampling is shown in darker yellow whereas NH3(g) over the
entire campaign is shown in lighter yellow. The box represents 25th to
75th percentile while the line within the box denotes the median.
Whiskers extend to the 10th and 90th percentile.
Figure 3 shows the magnitude of the sea–air and melt-pond–air flux of
NH3. Average net downward fluxes of 1.4 and 1.1 ng m-2 s-1
were calculated for the open ocean and melt ponds, respectively, using
Eqs. () and (). Net fluxes were exclusively downwards (net
deposition into the ocean and melt ponds) due to the relative abundances of
NH3(g) and NH4(aq)+ in these surface pools as
well as cold surface temperatures as suggested by Johnson et al. (2008). It
is unlikely that this represents a significant input of NH4+ into the
open ocean except in cases of extremely low [NHx]. A simple calculation
assuming a mixed layer depth of 25 m results in an increase of only
∼ 0.3 nM d-1 to the ocean (assuming complete mixing and no loss
pathways). However, for the much shallower melt ponds (assumed depth of
0.25 m) the same calculation yields an input of ∼ 22 nM d-1.
Furthermore, this does not account for atmospheric inputs from either wet
deposition or dry deposition of particulate NH4+, and these melt
ponds are cut-off from the upwelling currents in the ocean, which deliver
reactive N to the surface. Rates of nitrification, mineralization and
N2-fixation in the open ocean and melt ponds would help put this
atmospheric input into perspective and give insight as to whether or not it
is an important process in the nitrogen cycle in these environments.
Box-and-whisker plot of the estimated fluxes into the open ocean
and melt ponds. The percentiles are represented in the same fashion as Fig. 2.
Time series of neq m-3 for NH3(g) (black dots),
NH4+ in PM2.5 (orange trace), and SO42- in
PM2.5 (red trace). Interruptions in the data are a result of zero air
experiments, calibrations, values below detection limit, instrument
downtime, and (for gas-phase species) periods when the wind direction/speed
were not conducive for ambient sampling (as explained in detail in Sect. 2.2).
Sulfate neutralization
The extent of neutralization of PM2.5 influences aerosol properties as
discussed previously. Figure 4 depicts the relative abundances (in neq m-3) of gas-phase ammonia and particulate-phase ammonium and sulfate.
It is important to note that the value for sulfate is total PM2.5
sulfate as opposed to non-sea-salt sulfate (nss-SO42-), which is
commonly reported for marine boundary layer studies. High and variable
backgrounds of Na+ from the AIM-IC prevented the calculation of
nss-SO42-; hence this data set provides an upper limit for
nss-SO42-. Given the low wind speeds (< 5 m s-1) that
dominated the campaign, it is likely the nss-SO42-≈ SO42- since the contribution from sea salt to PM2.5 was
likely small. It should also be noted that measurements of SO2,
HNO3 and NO3- were almost always below their respective
detection limits.
Particle loadings of NH4+ and SO42- were extremely low
(typically < 5 neq m-3) throughout the duration of the cruise.
During the first third of the cruise (before 18 July), gas-phase NH3
was also low and neutralization (i.e. the ratio
NH4+ : SO42- in units of equivalents) was ambiguous due to
numerous values near or below detection limit. On the other hand, after 25
July the nanoequivalents of NH3(g) were substantially higher than
either NH4+ or SO42- (i.e. NHx≈ NH3), which implies a nearly neutralized sulfate aerosol. It is
important to note that a nearly neutralized aerosol does not equate to an
aerosol with a pH of 7 since aerosol pH is highly sensitive to liquid water
content as well as the precise NH4+ : SO42- ratio. An
aerosol with NH4+ : SO42- approaching 1 can still have an
acidic pH. For example, a deliquesced ammonium sulfate particle containing
20 neq m-3 of SO42- and 19.98 neq m-3 NH4+ at
85 % RH will have a pH of ∼ 3.1 under equilibrium conditions
despite having an NH4+ : SO42- equivalents ratio of 0.999.
Figure 5 shows the distribution of the NH4+ : nss-SO42-
ratio (on a per equivalent basis) measured in Alert, Nunavut (82.50∘ N, 62.33∘ W) as a function of month from 1996–2011.
Weekly averaged PM2.5 speciation measurements in Alert are made by
Environment Canada and are available online (Environment Canada, 2014). The
contribution from NO3- is minor and has not been included in this
analysis. In warm environments, volatilization of NH4NO3 off of
filters can cause an underestimation of NH4+, but this is not
expected to be an issue in Alert due to cold weather and low loadings of
NH4NO3. During July and August the nss-SO42- is, on
average, completely neutralized by the NH4+ in PM2.5 as shown
by a median neutralization ratio approaching 1 during these months. This
implies there is sufficient NH3(g) throughout the region to neutralize
nss-SO42- produced from DMS oxidation which is consistent with the
measurements shown in Fig. 4. However, there is no denuder upstream of the
Hi-Vol filters to remove NH3, so the observed
NH4+ : SO42- ratio (Fig. 5) may be higher than for ambient
PM2.5. This effect is difficult to characterize, but if it is important
then it is still evidence for the abundance of NH3 in the summertime
Arctic boundary layer. Lastly, Johnson and Bell (2008) show that a
sufficiently neutralized sulfate aerosol will tend to “push” gas-phase
NH3 into the ocean in the aerosol-gas-ocean system, also consistent
with Fig. 3. The AIM-IC and Alert measurements are both inconsistent with a
previous study that used GEOS-Chem to predict a highly acidic aerosol and
insignificant gas-phase ammonia (NHx≈ NH4+)
throughout the summertime Arctic marine boundary layer (Breider et al.,
2014). This inconsistency implies a missing process in a widely used CTM
that we investigate further below.
Box-and-whisker plot of neutralization (defined as
NH4+/2∗nss-SO42-) for 15 years (1996–2011) of
weekly PM2.5 speciation measurements taken in Alert, Nunavut. The
percentiles are represented in the same fashion as Fig. 2.
Evidence for the importance of seabird guano
Observations collected onboard the Amundsen and in Alert strongly suggest a
significant source of NH3 in the Baffin Bay region. Decomposition of
uric acid in seabird guano (excreta) has been recognized as a significant
source of NH3 where large colonies exist (Blackall et al., 2007; Wilson
et al., 2004). However, studies measuring NH3 from seabird colonies are
limited due to the remoteness of most colonies and technical challenges in
quantifying NH3 in isolated locations (Blackall et al., 2007). The few
studies that have been done have focused on colonies located in the United
Kingdom (Blackall et al., 2004; Wilson et al., 2004), Antarctica (e.g.
Legrand et al., 1998; Zhu et al., 2011) and remote tropical islands (Riddick
et al., 2014; Schmidt et al., 2010). Recently, Riddick et al. (2012a)
developed a global inventory to estimate the magnitude and spatial
distribution of NH3(g) from seabird guano. The authors employed a
bioenergetics model, first developed by Wilson et al. (2004), to calculate
the NH3(g) emissions (in g bird-1 yr-1) for 323 different
seabird species. After compiling a list detailing the populations and
locations of 33 225 colonies, they were able to estimate global annual
emissions between 97–442 Gg NH3 per year. Although this is less than
2 % of total global NH3(g) emissions, it can be the dominant source
in remote regions where seabird populations are large and other sources are
negligible.
In order to assess the impact of seabird guano on NH3 across the Baffin
Bay region, seabird colony NH3 emissions were implemented in the
GEOS-Chem model, and the impact on monthly mean surface layer NH3 was
examined. The NH3 emissions inventory used in the standard GEOS-Chem
v9-02 (and in many other CTMs) is from Bouwman et al. (1997) and does not
include seabird emissions. Paulot et al. (2015) recently showed the oceanic
emissions from this original inventory are roughly a factor of 3 too high
since the initial inventory assumes atmospheric NH3 is equal to zero.
The Riddick et al. (2012a, b) seabird colony NH3 emissions inventory
(scenario 3) was added to the original inventory in GEOS-Chem following
Paulot et al. (2015). Scenario 3 was chosen since this represented the
midpoint between the minimum and maximum emissions of scenario 1 and 2,
respectively. Close inspection of this seabird inventory revealed that some
large seabird colonies in our study region were not accounted for. To
investigate this, the spatial co-ordinates of northern colonies
(> 50∘ N) in the Riddick et al. (2012a) inventory were
cross-referenced against colonies in the online Circumpolar Seabird Data
Portal (Seabird Information Network, 2015). Annual emissions for large
colonies in the seabird data portal were calculated in the same manner as in
Riddick et al. (2012a). In total, there were 42 colonies present in the
seabird data portal but absent in the Riddick inventory in the region north
of 50∘ N. These additional emissions were added to the inventory
we implemented in GEOS-Chem. These colonies totaled 7.5 Gg
NH3 yr-1 (approximately one quarter of the existing emissions north of 50∘ N) and were primarily in Siberia and western Alaska. The Riddick
et al. (2012a) bioenergetics model only counts emissions that occur during
breeding season and while the seabirds are at the colony. Hence, the annual
emission estimates (Gg NH3 yr-1) per colony were temporally
allocated evenly between the 15 May to 15 September. This period is when the
majority of seabirds in the Baffin Bay region are nesting (e.g. Gaston et
al., 2005; Mallory and Forbes, 2007; McLaren, 1982). One limitation to this
approach is that it does not account for additional temporal variations in
NH3 emissions. For instance, moisture increases the rate of uric acid
degradation, and fluxes of NH3 from guano have been observed to
increase 10-fold for up to a day after rain events (Riddick et al., 2014).
GEOS-Chem simulation of NH3 mixing ratio (ppb) of the July
monthly mean surface layer for (a) no seabird emissions and (b) with seabird
emissions. Circles in (a) represent the ship track coloured by NH3
measurements. Panels (c) and (d) show GEOS-Chem simulations for the ammonium
to non-sea-salt sulfate ratio during the same period for (c) no seabird
emissions and (d) with seabird emissions. The star indicates the average
ratio observed in Alert during July.
Figure 6 shows the July mean output for surface layer NH3 mixing ratio
both without (Fig. 6a) and with (Fig. 6b) seabird emissions, along with the
NH3(g) measured by the AIM-IC denoted by circles in Fig. 6a. Comparing
the top two panels reveals that seabird emissions make a substantial impact
on modelled NH3 levels in the boundary layer. Much better
model-measurement agreement is achieved with the inclusion of the seabird
colonies. Without the seabird emissions, NH3 mixing ratios are
underpredicted by several orders of magnitude. Surface NH3 is still
underpredicted in Fig. 6b (with guano NH3 emissions) which could be the
result of modelled emissions being independent of rainfall, which can
substantially increase NH3 emissions. Episodic rainfall was persistent
throughout the latter half of the campaign. Other contributing factors may
include the following: challenges in representing boundary layer mixing, uncertainties in
deposition rates, comparing monthly averages (GEOS-Chem) to ambient hourly
measurements, missing/underestimated bird colonies, and/or excreta from
other fauna (e.g. seals, caribou, musk-ox) absent in the updated inventory.
The bottom two panels (Fig. 6c and d) show the influence of seabirds on
the ammonium to non-sea-salt sulfate ratio. Without seabirds (Fig. 6c) the
ratio is less than 0.3 throughout most of the study region, which is
inconsistent with the abundance of NH3 relative to SO42-
measured by the AIM-IC. Adding the seabird emissions (Fig. 6d) increases the
ratio to above 0.7 in most grid cells along the ship track. Although the
high ratio (July average is ∼ 1) observed in Alert (denoted by
the star in Fig. 6c and d) is underestimated in the GEOS-Chem simulation,
the bias is reduced by nearly a factor of 2 (from 0.32 to 0.57) when seabird
emissions are included.
Wildfires are also a source of NH3 to the free troposphere and/or
boundary layer. Particularly strong wildfire events were persistent in the
Northwest Territories (NWT) during the study period. Blue circles in Fig. 7
show the location and average fire radiative power (representative of fire
strength) of wildfires across the Arctic from 20–26 July. It was constructed
using data from NASA's Fire Information Resource Management System (FIRMS)
database (NASA, 2015). We used FLEXPART-WRF retro plumes to assess the
importance of wildfire NH3 emissions, as well as to further corroborate
the influence of seabird guano.
PES plots of FLEXPART-WRF 7-day retroplumes from the ship's
location on (a) 14 July 00:00, (b) 26 July 00:00, (c) 2 August 00:00, (d) 3 August 00:00 and (e) 4 August 00:00. The ship track is shown in black, and
the ship location at the release time is indicated in red. Colours show the
air-mass residence time prior to arrival at the ship (PES) in seconds. The
plume centroid locations at 1 and 2 days (the approximate lifetime of
NH3) before release are shown (numbers 1 and 2). Purple circles
represent the location of bird colonies with the size of each circle
indicating the magnitude of estimated NH3 emissions (in Mg NH3 yr-1). Blue circles show the location of wildfires from the NASA FIRMS
measurements of fire radiative power from 20–26 July (in MW). The bottom
panel is a time series of NH3(g) and particle-phase NH4+ and
SO42- measured by the AIM-IC with arrows indicating times of
retroplume initiation in the upper panels. The NASA FIRMS data set was
provided by LANCE FIRMS operated by NASA/GSFC/ESDIS with funding from
NASA/HQ.
The significant impact of seabird colonies on [NH3(g)] is supported by
the analysis of FLEXPART-WRF retro plumes shown in Fig. 7. Periods of low
[NH3(g)] (bottom panel in Fig. 7) correspond to air masses that spent
at least the last 48 h over the ocean and/or aloft above the MBL
(∼ 500 m) where NH3 sources are negligible. This is
clearly shown in Fig. 7a where the air mass sampled on 14 July 00:00 UTC
spent the previous 96 h in the MBL over Baffin Bay, consistent with low
[NH3(g)]. In contrast, on 26 July 00:00 UTC (Fig. 7b) air had recently
passed over seabird colonies (purple circles) surrounding Lancaster Sound as
well as wildfires in the Northwest Territories (NWT) on mainland Canada
(blue circles), coincident with the large increase in [NH3(g)]. A
similar NH3(g) peak occurs on 3 August that can also be examined by
using a retro plume analysis. Low NH3(g) values observed on the morning
of 2 August agree with Fig. 7c showing the air originating from the MBL over
Baffin Bay. At 3 August 00:00 UTC (Fig. 7d) the air had spent the last 12 h
in the boundary layer of western Greenland where large seabird colonies
exist. However, by 4 August 00:00 UTC (Fig. 7e) the retro plume shifted such
that air is now originating from primarily above the boundary layer
(altitude plots not shown) leading to a decrease in NH3(g). In
addition, from 2 to 4 August the ship was north of 79∘ N and in the eastern
Canadian Arctic; hence it is unlikely that this increase in NH3 can be
attributed to wildfires given how far removed this region is from wildfires
in the NWT. While Fig. 7 only highlights five examples from the study
period, retro plumes throughout the entire campaign also support the
hypothesis that NH3(g) in the MBL originates primarily from seabird
colonies (for the eastern Canadian Arctic) with contributions from wildfires
in some regions (central Canadian Arctic). All NH3(g) spikes in the
time series can be attributed to air that had recently passed over seabird
colonies and/or wildfires, whereas low values coincide with air masses from
either the open ocean or free troposphere not influenced by wildfires.
To further investigate the potential influence of wildfires on NH3 in
the Arctic MBL, GEOS-Chem simulations were performed using a wildfire
emissions inventory for 2014 (QFED2). Simulations with/without wildfires and
with/without seabirds revealed that in Lancaster Sound (along 74∘ N) roughly 40 and 55 % of the boundary layer NH3 can be
attributed to seabirds and wildfires, respectively. In other words, air
sampled in Lancaster Sound (20 to 27 July) was likely influenced by
wildfires in NWT in addition to seabird guano. On the other hand, north of
Lancaster Sound, contributions from seabirds and wildfires to surface layer
NH3 were approximately 95 and 5 %, respectively. Wildfires in the
NWT are an important but episodic source of summertime NH3 in the
Canadian Arctic. This is due to periodic transport events associated with
this source that is located remote to our study region. Whereas, seabird
colonies are a local, and persistent source of NH3 from May to
September. Given the observation of consistently neutralized sulfate in Alert each summer, and the large interannual variability and episodic
wildfire influence, emissions from migratory seabirds are likely to be a
significant contributor to NH3 abundance in the Arctic marine boundary
layer.
Implications for N-deposition to ecosystems
Previous studies have highlighted the important role that seabird-derived N
can play in the nitrogen cycle of ecosystems adjacent to bird colonies due
to large deposition rates of NH3 and NH4+ (e.g. Anderson and
Polis, 1999; Lindeboom, 1984). However, little attention has been paid to
the effects of seabird-derived N on deposition at the regional scale. In
this section, we consider the importance of seabird-derived nitrogen as an
input of reactive N to Arctic ecosystems. These ecosystems tend to be
N-limited during the summer and hence have a large sensitivity to N input
(Shaver and Chapin III, 1980). In terrestrial ecosystems, soil N
availability is a key factor in determining both plant community structure
(McKane et al., 2002) and greenhouse gas emissions from soil (Stewart et
al., 2012).
Nitrogen (N2) fixation via microbes is thought to be the primary N
input to remote Arctic terrestrial ecosystems (e.g. Cleveland et al., 1999;
Hobara et al., 2006; Stewart et al., 2014). Numerous field studies have been
conducted to estimate N2-fixation rates via the acetylene reduction
technique (Hardy et al., 1968). The N2-fixation rates for most
terrestrial Arctic sites fall within the range of 10 to
120 mg N m-2 yr-1 (Hobara et al., 2006). However, highly variable rates (due to
spatial heterogeneity of microbial populations) and assumptions in the
acetylene reduction technique yield high degrees of uncertainty for
N2-fixation rates (Stewart et al., 2014).
Total atmospheric N-deposition (wet and dry) in the Arctic is thought to be
smaller than fixation, with typical ranges from 8 to 56 mg N m-2 yr-1 (Van Cleve and Alexander, 1981). Only a few N2-fixation
studies also quantify wet deposition, with dry deposition being ignored
altogether (e.g. Hobara et al., 2006). Nonetheless, in certain Arctic
regions atmospheric deposition may exceed N2-fixation in soils (DeLuca
et al., 2008). These processes are coupled since large inputs of
NH4+ have been shown to inhibit N2-fixation in certain
microbial species and lichens (Chapin and Bledsoe, 1992).
GEOS-Chem simulation of total NHx deposition (in mg N m-2 season-1) for the months May to September (inclusive). Panel
(a) does not include seabird emissions, whereas the panel (b) does. The
difference in total NHx deposition between the two emissions scenarios
(with birds minus without birds) is shown in panels (c) and (d) as an
absolute amount and percentage increase, respectively.
Figure 8 shows results from the GEOS-Chem simulation of total NHx
(≡ NH3+ NH4+) deposition (both wet and dry) for
the months May to September (inclusive) both without (Fig. 8a) and with
(Fig. 8b) seabird NH3 emissions. The difference in total NHx
deposition for birds and no birds is shown in Fig. 8c (absolute difference)
and Fig. 8d (percent difference). Areas near large colonies are heavily
influenced by seabird guano with NHx deposition from seabirds exceeding
10 mg N m-2 yr-1, particularly in western Greenland and near the
mouth of Lancaster Sound. The majority of NHx deposition is caused by
NH3 as opposed to NH4+. Most regions in Fig. 8b are on the
lower end of the annual N-deposition rate of 8 to 56 mg N m-2 yr-1
suggested by Van Cleve and Alexander (1981). However, there are two
important distinctions: the latter is an estimate of total N-deposition and
annual input. Estimates in Fig. 8 might be more useful for comparing
N-deposition to N2-fixation since it captures deposition only during
the growing season, and NHx is likely the dominant form of atmospheric
reactive N in the summertime Arctic boundary layer. Furthermore, Fig. 8b
provides information on regions where N-deposition rates could be comparable
to input from terrestrial N2-fixation (> 10 mg N m-2 yr-1) which can help inform subsequent studies exploring N-cycling in
the region. According to Hobara et al. (2006), Arctic terrestrial
N2-fixation only occurs from May to September (inclusive) and peaks in
July, similar to migration patterns of Arctic seabirds.
Estimates of N2-fixation rates in the Arctic Ocean mixed layer are even
sparser than estimates for terrestrial ecosystems. To our knowledge, only
Blais et al. (2012) have measured oceanic N2-fixation in the summertime
Arctic Ocean mixed layer. The authors found that open ocean N2-fixation
rates averaged 0.12 nM d-1 in the upper 50 m of the water column
throughout the Beaufort Sea to Baffin Bay. For the period of May to
September (inclusive) this represents an input of approximately 13 mg N m-2 which is comparable to inputs we calculate from guano-derived
NH3 in regions close to seabird colonies as shown in Fig. 8b.
Conclusions
Simultaneous measurements of atmospheric and oceanic composition in the
eastern Canadian Arctic revealed that the summertime Arctic Ocean and melt
ponds were net sinks of NH3(g). Concentrations of NH3(g) ranging
from 30 to 650 ng m-3 were observed and represent the first reported
measurements of NH3(g) in the Canadian Arctic. An average downward flux
of 1.4 ng m-2 s-1 into the Arctic Ocean was calculated, consistent
with previous studies showing that higher latitude waters are a net NH3
sink (Johnson et al., 2008). Melt ponds had a smaller net downward flux (1.1 ng m-2 s-1) as well as a slightly higher χ as compared to
the open ocean (median 2 ng m-3 vs. 0.8 ng m-3). To our
knowledge, this is the first study to estimate melt-pond–air NH3
exchange despite the ubiquitous presence of melt ponds throughout the
summertime Arctic.
On a nanoequivalent basis, NH3(g) values were significantly greater (up
to an order of magnitude more) than both NH4+ and SO42-.
This finding was consistent with a 15-year historical data set of weekly
PM2.5 composition from Alert, NU which showed that nss-SO42-
is, on average, completely neutralized by NH4+ during July and
August. These measurements imply strong regional source(s) of NH3(g) in
the eastern Canadian Arctic Archipelago that are sufficient to neutralize
nss-SO42- produced from DMS oxidation. Our surface–air flux
estimates show that the Arctic Ocean and melt ponds are not responsible for
NH3(g) in the marine boundary layer.
It is also noteworthy that even though these melt ponds have significantly
higher [NHx] than the open ocean (average of 670 nM vs. 55 nM),
χMP is only marginally higher. More acidic pHs and slightly lower
temperatures mitigate the effect of higher [NHx] on χ. Chemical
transport models (CTMs) that explicitly account for bi-directional NH3
exchange typically require χ as a predefined model input (e.g. Bash et
al., 2013; Wichink Kruit et al., 2012). Therefore, from a modelling
standpoint, similar values of χocean and χMP are
convenient since they can be parameterized in a similar fashion which would
remove the need for CTMs to resolve the spatial extent and temporal
evolution of melt ponds to properly model surface–atmosphere NH3
exchange in the summertime Arctic.
To investigate the impact of NH3 emissions from seabird guano, we
examined GEOS-Chem simulations both with and without seabird colony NH3
emissions. The seabird NH3 emission inventory developed by Riddick et al. (2012a) was updated for this study to include northern colonies
(> 50∘ N) that had been overlooked in the original
inventory. Without the seabirds, GEOS-Chem underestimated NH3(g) by
several orders of magnitude and predicted highly acidic aerosol at the
surface in July, which is in direct contrast to our measurements. The
inclusion of seabird emissions provided much better agreement with
NH3(g) observations and yielded more neutralized aerosol throughout
most of the Baffin Bay region. The importance of seabird NH3 emissions
is also supported by analysis of FLEXPART-WRF retro plumes throughout the
study period. Air masses enriched in NH3(g) had recently passed through
regions with seabird colonies whereas periods of low NH3(g) involved
air masses originating from the open ocean or above the boundary layer.
Together, these models provide strong evidence that seabird colonies are the
dominant and persistent local source of NH3(g) in the summertime
Arctic. FLEXPART-WRF and GEOS-Chem were also used to assess the influence of
wildfires on NH3. Wildfires are an important but episodic source of
NH3 to the Arctic due to ongoing changes in transport patterns
and fire intensity. Further work should be done to examine the inter-annual
influence of NH3 emissions from wildfires in the NWT on other regions
in the Arctic.
Deposition estimates of NHx from GEOS-Chem during the seabird nesting
season (May to September) exceed 10 mg N m-2 season-1 in grid
cells close to large seabird colonies, which is on the lower end of
microbial N2-fixation in Arctic tundra (Hobara et al., 2006). Hence, in
some regions seabird-derived NHx could be a significant N-input to
terrestrial Arctic ecosystems, which are typically very N-sensitive.
Estimates of NH3 fluxes into the open ocean are unlikely to be an
important input of reactive-N except for waters close to large seabird
colonies; however, these fluxes may be important for the N-cycle in the much
shallower melt ponds.
There is strong evidence that seabird colonies are likely the dominant and
persistent source of NH3(g) to the summertime Arctic boundary layer.
Emissions appear to be significant enough to at least partially neutralize
nss-SO42- throughout most of the study region, in contrast to
previous model simulations that did not consider seabird colony emissions.
Further research is required to better constrain the location, population,
and NH3 emissions of Arctic seabird colonies. It is also important to
quantify meteorological effects (e.g. rainfall, wind speed) on seabird
emissions. The NH3 emissions inventory in CTMs should be updated to
include seabird emissions with correct representation of the breeding season
so that emissions only occur when seabirds are nesting. Summertime
measurements of atmospheric NHx elsewhere in the Arctic are needed to
assess whether the impacts of seabirds observed in this study (substantial
NH3(g), nss-SO42- neutralization, and N-deposition) are
relevant to the entire Arctic.