Introduction
Anthropogenic reactive nitrogen (Nr) emissions have dramatically
increased in the last few decades owing to rapidly growing populations and
industry (Galloway et al., 2008). China is one of the largest producers and
emitters of Nr in the world (Nr emission of
12.18 Tg yr-1; Reis et al., 2009). Inevitably, large amounts of
Nr emanate into the adjacent seas through various pathways. Through
the atmosphere, annual nitrogen depositions into the eastern China seas
(ECSs: the Yellow Sea and East China Sea) had been reported to be the same
order of magnitude carried by the Yangtze River discharge (Nakamura et al.,
2005; Zhang et al., 2007). Both observational data and global models revealed
that both of the Chinese marginal seas and the northwestern Pacific Ocean
(NWPO) are under the atmospheric influence of the Asian continent, which
supplied significant amounts of anthropogenic Nr (Duce et al.,
2008) and terrigenous materials (Jickells et al., 2005). The cumulative
effect of atmospheric input in the past decades even altered the nutrient
stoichiometry on a regional scale, including the Chinese marginal seas and
the North Pacific Ocean (Kim et al., 2011; Kim et al., 2014).
To better constrain atmospheric deposition of Nr into the ocean
over large spatial and temporal scales, modeling the transport and deposition
of air pollutants is essential. Models of atmospheric nitrogen deposition
include abundant parameters, such as local emission densities, particle size,
deposition velocity, chemical processes and meteorological conditions (Liu et
al., 2005; Guenther et al., 2006; Kanakidou et al., 2012). However, model
accuracy strongly relies on the validation by observational data.
Unfortunately, shipboard observations, particularly for an offshore gradient
from marginal seas to the open sea, are still limited.
In the marginal seas of China, dust and fog storms are two common
intermittent weather events during the transition period from a cold to a
warm season (Sun et al., 2001; Zhang et al., 2009). Dust aerosols may serve
as a carrier bringing significant amounts of terrigenous and anthropogenic
fingerprints including trace elements (Duce et al., 1980) and Nr
(Chen and Chen, 2008) from inland into the open sea via long-range transport.
By contrast, sea fog is relatively stagnant and restricted on a spatial
scale. Fog is the intermediate stage between precipitation and aerosol. Fog
forms by the activation of particulates with subsequent growth and
incorporation of other gases and particles (Cape et al., 2011). Fog droplets
are smaller in size when compared to rain drops; however, concentrations of
water-soluble species in fog water were not necessarily higher or lower than
those of precipitation because of complicated chemical processes (Sasakawa
and Uematsu, 2002; Watanabe et al., 2006; Jung et al., 2013). Inland fog
chemistry has been well studied and its impacts on terrestrial ecosystems
have been highlighted (Chang et al., 2002; Lange et al., 2003). Researchers
have even designed experiments to investigate the differences in aerosol
chemistry for pre- and post-fog formation periods to explore the inland fog
impact on aerosol chemistry (Biswas et al., 2008; Safai et al., 2008). Fog,
in fact, can be sampled only by specialized fog samplers; however, during
aerosol sampling at sea there is no way to avoid fog once sea fog forms.
Nevertheless, the effect of sea fog on aerosol chemistry has not yet been
well studied, even less so in the coastal and marginal seas of China, where
air pollution is serious. Therefore, compared with inland fog and dust
aerosols, we have less knowledge about sea fog chemistry and the aerosol
chemistry under sea fog influence. This is the first investigation of
Nr speciation and deposition of sea-fog-modified aerosols (aerosol
collected under sea fog influence) on the marginal seas off a continent
producing strong emissions.
Different types of aerosols may be composed of different amounts of nitrogen
species based on their formation history (e.g., origin, flow path, reactions
during transport). In this study, we sampled total suspended particulate
(TSP) marine aerosols on a cruise crossing over the ECSs and NWPO during
spring 2014. Water-soluble nitrogen species and ion characteristics among
different aerosol types, including dust, background and sea-fog-modified
aerosols, were investigated. These observational data promoted our
understanding of the type-specific concentration and deposition of various
nitrogen species and the role of sea fog on nitrogen scavenging. The data may
aid in validating model outputs for the Asian region and potentially evaluate
the framework of nitrogen and aerosol interactions in current models.
Materials and methods
Aerosol sample collection and chemical analyses
A total of 44 TSP samples were collected using a high-volume TSP aerosol
sampler (TE–5170D; Tisch Environmental Inc.) during a research cruise on the
R/V Dongfanghong II from 17 March to 22 April 2014. The cruise
tracks (Fig. 1) covered the ECSs and the NWPO. The samples were taken at
∼ 12 h intervals. To avoid self-contamination from the research
vessel, we sampled only when the vessel was cruising; thus, the sampling
interval was not exactly 12 h. Based on simultaneous 1 s particle number
concentration measurements made using an optical particle sizer (TSI, USA),
we found that ship plumes affected the TSP sampling occasionally during
cruising (these data will be presented in a separate paper). We calculated
the plume contribution to the measured volume particle concentration of
PM10 during each TSP sampling (self-contamination) and the short-period
contribution was less than 3 %. Detailed sampling information including
date, time period and locations for each sample are listed in Table S1 in the
Supplement. Meteorological data (Fig. 2) including wind speed and direction,
relative humidity (RH) and temperature were recorded onboard.
Map of the cruise track. Orange, pink and blue indicate sea fog,
dust and background days during the cruise, respectively. Sample number and the collection
range are shown.
The cruise encountered sea fog in the first few days (orange shading in Fig. 2
for 17–19 March) and five samples (nos. 1–5) were collected. Surprisingly,
sea fog occurred again on 21–22 April, while approaching land (samples
nos. 43 and 44). During the fog events, high RH and slow wind speed were
recorded (see orange shading in Fig. 2). The strong temperature gradient
indicated that the sea fog formed owing to a cold air mass from land
confronting warm air from the sea. Since these samples were collected on fog
days (see orange tracks in Fig. 1) when we could collect aerosols, the sea-fog-modified the aerosols as well as sea fog droplets. Since we could not
separate them from each other through our method, we classified these samples
as “sea-fog-modified aerosol”.
Whatman 41 cellulose filters (Whatman Limited, Maidstone, UK) were used for
filtration. The analytical procedures were described by Hsu et al. (2010b,
2014). Briefly, one-eighth of the filter was extracted using 15 mL of
Milli-Q water on a reciprocating shaker for 0.5 h and left to rest for an
additional 0.5 h at room temperature. Then, the extract was filtered through
a polycarbonate membrane filter (0.4 µm pore size and 47 mm in
diameter from Nuclepore). The filter was leached three times with Milli-Q
water, and then 5 mL of Milli-Q water was used to rinse the filter. The
45 mL extract solution was mixed with the 5 mL from rinsing, poured into a
50 mL clean plastic centrifuge tube and used for the determination of the
ion species and water-soluble aluminum (Al).
The water-soluble and total concentrations of Al in the TSPs were analyzed
using inductively coupled plasma mass spectrometers (ICP-MS). For total
Al, briefly, one-eighth of the filter was digested with an acid mixture
(4 mL HNO3 + 2 mL HF) using an ultra-high-throughput microwave
digestion system (MARSXpress, CEM Corporation, Matthews, NC, USA), and the
efficiency of the digestion scheme was checked by subjecting a certain amount
of a standard reference material (SRM1648, urban particulate matter, National
Institute of Standards and Technology, USA) to the same treatment. The
recoveries of Al in the SRM 1648 through digestion with the HNO3–HF
mixture fell within ± 10 % (n = 5) of the certified values.
Details regarding the ICP-MS analysis are described in Hsu et al. (2008).
The meteorological parameters collected during the sampling period
(solid line). Wind speed is in purple, wind direction in green, RH in blue
and temperature in red. The orange shadings indicate the period of sea fog
contact and pink indicate the dust period. Non-sampling period is shown with dashed
curves.
The major ionic species (Na+, NH4+, K+, Mg2+, Ca2+,
Cl-, NO3-, NO2- and SO42-) in the extract were analyzed
using an ion chromatograph (model ICS-1100 for anions and model ICS-900 for
cations) equipped with a conductivity detector (ASRS-ULTRA) and suppressor
(ASRS-300 for the ICS-1100 and CSRS-300 for the ICS-900). Separator columns
(AS11-HC for anions and CS12A for cations) and guard columns (AG11-HC for
anions and CG12A for cations) were used in the analyses. The precision for
all ionic species was better than 5 %. Details of the analytical
processes can be found in Hsu et al. (2014). Only five samples contained
NO2- (1.39 nmol m-3 for no. 2, 2.32 nmol m-3 for no. 4,
3.69 nmol m-3 for no. 5, 5.96 nmol m-3 for no. 43 and
3.76 nmol m-3 for no. 44), which accounted for < 1 % of the
total dissolved nitrogen (TDN).
The TDN was analyzed using the wet oxidation method to convert all nitrogen
species into nitrate with re-crystallized potassium persulfate, and then the
concentration of nitrate was measured using chemiluminescence (Knapp et al.,
2005). Monitoring with laboratory stock
(NO3- + NH4+ + glycine + EDTA) showed that the
recoveries of TDN by the persulfate oxidizing reagent (POR) digestion fell
within 95–105 % (n=6) over the range of detection.
Data analysis
The amounts of non-sea-salt Ca2+ (nss-Ca2+) and non-sea-salt
SO42- (nss-SO42-) in the aerosol, as well as the Ca2+ and
SO42- fractions in excess over that expected from sea salt, were
calculated using the unit of equivalent concentration (neq m-3) in the
following equations:
[nss-Ca2+]=[Ca2+]-[ss-Ca2+],where[ss-Ca2+]=0.044×[Na+],
[nss-SO42-]=[SO42-]-[ss-SO42-],where[ss-SO42-]=0.121×[Na+],
where the factors 0.044 and 0.121 are the typical
calcium-to-sodium and sulfate-to-sodium equivalent molar ratios in seawater
(Chester, 1990).
Relative acidity (RA) was calculated using all the observed ion species in
their equivalent concentrations following Yao and Zhang (2012):
RA=([Na+]+[Mg2+]+[K+]+[Ca2+]+[NH4+])/([Cl-]+[NO3-]+[SO42-]),
where [Na+], [Mg2+], [K+], [Ca2+], [NH4+], [Cl-],
[NO3-] and [SO42-] are the equivalent concentrations of those
water-extracted ions. The relative acidity is based on the imbalance of
cations and anions, which was caused by the non-detected ions such as H+,
HCO3- and CO32- (Kerminen et al., 2001). When the total ions
were distributed over a wide range (by a factor of 20 in our case), the ratio
of total anions to cations in neq m-3 is more effective in presenting
the relative acidity than the absolute value of imbalance (total cations –
total anions).
The concentration of water-soluble organic nitrogen (WSON) was calculated
using the following equation:
[WSON]=[TDN]-[NO3-]-[NH4+]-[NO2-],
where [TDN], [NO3-], [NH4+] and [NO2-] are molar
concentrations (nmol N m-3) of those water-soluble nitrogen species in
TSPs. The standard errors propagated through WSON calculation varied from
sample to sample (17 to 1500 %). The average standard error was 116 %
when all samples were considered, and when the extreme value was excluded, the
average standard error was reduced to 81 %.
Flux calculation
The dry deposition flux (F) was calculated by multiplying the aerosol
concentrations of water-soluble nitrogen speciation (C) by the dry
deposition velocity (V):
F=C×V,
where V is a primarily function of particle size and meteorological
parameters, such as wind speed, RH and sea surface roughness (Duce et al.,
1991). According to previous reports, dry deposition velocity varies by more
than 3 orders of magnitude at a particle size ranging from 0.1 to
100 µm (Hoppel et al., 2002). In general, ammonium appears in
submicron mode from 0.1 to 1 µm, with a small fraction residing in
the coarser mode; however, nitrate is mainly distributed in a supermicron
size ranging from 1 to 10 µm (Nakamura et al., 2005; Baker et al.,
2010; Yao and Zhang, 2012; Hsu et al., 2014). The non-single-mode size
distribution appears in not just nitrogenous elements but also metals,
including aluminum and iron (e.g., Baker et al., 2013). Thus, for any
compound or elements, using a fixed deposition velocity to calculate dry
deposition flux might cause under- or over-estimation, as discussed by Baker
et al. (2013). Unfortunately, we collected TSPs with no information for size
distributions. Meanwhile, the meteorological parameters were highly variable
during sample collection. In our observation wind speed ranging from 0.8 to
18 m s-1 under a RH ranging from 40 to 100 % (Fig. 2). Thus, it is
very difficult to provide variable dry deposition velocities under a wide
range of environmental conditions (Hoppel et al., 2002; Baker et al., 2013);
thus, assumptions were made based on existing knowledge. Based on the model
and experimental results for aerosol deposition to the sea surface (Duce et
al., 1991; Hoppel et al., 2002) and the size distribution of nitrate and
ammonium in particles as reported above, deposition velocity of
2 cm s-1 was applied for nitrate and 0.1 cm s-1 for ammonium.
Both deposition velocities were often used in calculating the specific
nitrogen deposition fluxes, especially for the maritime aerosols, though
uncertainties were involved (de Leeuw et al., 2003; Nakamura et al., 2005;
Chen et al., 2010; Jung et al., 2013). As for WSON, the size distribution of
WSON in previous studies showed that WSON appears in a wide size spectrum
(Chen et al., 2010; Lesworth et al., 2010; Srinivas et al., 2011). In
previous studies, different orders of magnitude of deposition velocity were
employed for WSON deposition (1.2 cm s-1 by He et al., 2011;
0.1 cm s-1 for fine and 1.0 cm s-1 for coarse by Srinivas et
al., 2011; 0.075 cm s-1 for fine and 1.25 cm s-1 for coarse by
Violaki et al., 2010). Our TSP aerosols covered the entire size distribution;
thus, 1.0 cm s-1 was applied for WSON deposition. Since
1.0 cm s-1 is near the upper boundary of velocities previously applied
for WSON deposition, our calculation of WSON deposition may represent the
upper boundary.
Note that a period of our aerosol sampling was influenced by sea fog, which
we could not avoid as mentioned earlier in the Introduction. Apparently, the
deposition velocity for sea-fog-modified aerosol differs from that of common
aerosol, thus, the deposition velocity needs to be revised once we have
sufficient knowledge about the influence of sea fog on aerosol deposition.
Air mass backward trajectory analysis
In order to investigate the likely origins of aerosols in the transporting
air masses, 3 days with three heights of above-sea-level air mass backward
trajectories were calculated using the National Oceanic and Atmospheric
Administration (NOAA) Hybrid Single Particle Lagrangian Integrated Trajectory
(HYSPLIT) model with a 1∘ × 1∘ latitude–longitude
grid and the final meteorological database. Details about the HYSPLIT model
can be found at https://ready.arl.noaa.gov/HYSPLIT.php, as prepared by
the NOAA Air Resources Laboratory. The time period of 3 days was suggested to
be sufficient for dust transport from dust source to the NWPO (Husar et al.,
2001). The three heights (100, 500 and 1000 m) were selected because 1000 m
can be taken as one of the typical atmospheric boundary layers (Hennemuth and
Lammert, 2006).
Results and discussion
Using the Al content, air mass backward trajectories, weather conditions, and
ion stoichiometry, we classified aerosols into three types and then discussed
the speciation and concentrations of Nr for each aerosol type as
well as the potential processes involved. We compared the chemical
characteristics of dust aerosols collected in the ECSs with ours under sea
fog influence. Global aerosol and precipitation WSON data were also compiled
to reveal the significance of WSON. Finally, we estimated the deposition of
individual nitrogen species for the three types of aerosol and highlighted
the importance of atmospheric nitrogen deposition in different regions.
Aerosol type classification
Total Al content in aerosol samples is an often used index to identify dust
events (Hsu et al., 2008). As shown in Fig. 3, the total Al concentrations in
aerosols ranged from 52 to 6293 ng m-3 during our entire cruise. For
the first three samples (from nos. 1 to 3 collected in the Yellow Sea), total
Al increased from 1353 to 6293 ng m-3, and then rapidly decreased
(nos. 4 and 5 in the East China Sea) as the cruise moved eastward to the NWPO
(orange shading in Fig. 3). When the cruise returned to the ECSs, the total
Al concentrations in the aerosols (nos. 43 and 44) increased once again.
Apparently, an abundance of dust is frequently present in the low atmosphere
over the Chinese marginal seas in spring. The air mass backward trajectories
by HYSPLIT (Fig. 4a) revealed that the air masses for these fog samples
mainly hovered over the ECSs at an altitude of < 500 m and the air masses
for nos. 1–5 originated from the east coast of China. The air masses for the
samples of nos. 43–44 were from southern South Korea. The water-soluble Al
followed the same pattern as total Al (Fig. 3), but the leachable
concentrations were significantly higher when compared with dust aerosols
reported for the same area. The relative acidity of aerosols showed that the
values of sea-fog-modified aerosols were all below 0.9 (Fig. 3), indicating
an enhanced acidification relative to those aerosols with sea fog influence.
The low RA values explained the higher concentrations of water-soluble Al.
Total and water-soluble Al concentrations and relative acidity
(RA) for TSP. The orange bars indicate the sea fog period, and the pink bars
indicate the dust period. Sample identifications are shown on the x axis
(see Table S1). The horizontal blue dashed line (590 ng m-3) stands for
the reference to define background aerosols, and black dashed line indicates
the criterion of 0.9 for relative acidity.
As for sample nos. 6, 7, 25–27 and 29 collected in the NWPO (see pink tracks
in Fig. 1), the total Al concentrations ranged from 590 to 1480 ng m-3
with an average of 1025 ± 316 ng m-3 (pink shading in Fig. 3),
which were significantly higher than the remaining samples
(212 ± 120 ng m-3) from the NWPO. Although most of the air mass
backward trajectories of these samples collected in the NWPO originated from
25 to 40∘ N (and beyond) as well as high altitude (Fig. 4b), the
lidar browse images from NASA (Fig. S1) clearly indicated that the air masses
of these aerosol samples pass through dusty regions. The consistency between
high total Al concentration and the occurrence of dust and polluted dust
defined by the lidar browse images from the NASA allowed us to separate dust
aerosols from background aerosols. In this paper, background aerosols stand
for non-dusty and non-foggy aerosol in our classification. Thus, the
background aerosol is more like a baseline aerosol collected within this
study region during the investigating period; thus, the “background” may
vary over space and time and it does not necessarily have to be pristine.
Below we can also see a discernable ion stoichiometry among the three types.
Map and cruise track superimposed on 3-day air mass backward
trajectories corresponding to each sample. Altitudes of 100 m a.s.l.
(triangles), 500 m a.s.l. (asterisks) and 1000 m a.s.l. (squares) are above
sea level during the collection of (a) sea-fog-modified aerosols, (b) dust
aerosols and (c) background aerosols. The color bar represents the altitude
(in km).
Ion stoichiometry in three types of aerosol
Excluding sea-fog-modified aerosols, all the ratios of total anions and total
cations followed close to a 1 : 1 linear relationship (Fig. 5a). Such a
well-defined positive relationship indicated the charge balance and further
emphasized the validity of our measurements. The sea-fog-modified aerosols in
the ECSs contained higher contents of anions than cations, which was
consistent with previous observations for fog water (Chang et al., 2002;
Lange et al., 2003; Yue et al., 2014). The non-measured H+ ion should be
the dominant cation for charge compensation, as indicated previously (Chang et
al., 2002; Lange et al., 2003). The low RA values for sea-fog-modified
aerosols also supported this notion (Fig. 3). Below we set out the
characteristics of the three types of aerosol with ion stoichiometry.
Scatter plots for equivalent concentrations of specific ions.
(a) Total anions vs. total cations, (b) chloride vs. sodium, (c) magnesium vs.
sodium, (d) calcium vs. sodium, (e) potassium vs. sodium, (f) ammonium vs.
nss-sulfate, (g) ∑(nitrate + nss-sulfate) vs. ∑(nss-calcium + ammonium) and (h) nitrate vs. ammonium. Orange, pink and blue are for
sea-fog-modified, dust and background aerosols, respectively.
Since the Cl- / Na+ ratios of all samples including sea-fog-modified aerosols (Fig. 5b) were near 1.17, this indicated that almost all
the Na and Cl for our aerosols originated from sea salt. The relationship
between Mg2+ vs. Na+ (Fig. 5c) indicated that almost all Mg2+
also originated from sea salt sources (Mg / Nass = 0.23),
except sea-fog-modified aerosols, which held a deviated correlation due to Mg
enrichment (y=0.32x+8.7, R2=0.88) because of terrestrial mineral
sources of Mg. Such Mg enrichment was not observed in summer sea fog in the
subarctic North Pacific Ocean (Jung et al., 2013).
As for Ca2+ (Fig. 5d), all types of aerosol were enriched in Ca2+
but at different levels, indicating various degrees of terrestrial mineral
influence on the marine aerosols. For background aerosols, a strong
correlation between Ca2+ and Na+ (y=0.044x+6.6, R2=0.92)
was observed. The slope was identical to that of sea water
(Ca / Nass = 0.044), suggesting that most Ca2+ and
Na+ in background aerosols were sourced from sea salt. An unusually high
regression slope (20 times that of the sea salt) observed between Ca2+
and Na+ in sea-fog-modified aerosols (y=0.90x-1.8, R2=0.71)
was attributable to the reaction between mineral CaCO3 and H+ in
fog droplets during the formation of sea fog (Yue et al., 2012). The more
excessive Ca2+ observed in dust aerosols implied that stronger
heterogeneous reactions between the acid gas and dust minerals had occurred
during long-range transport (Hsu et al., 2014). Similar to Ca2+,
patterns between K+ and Na+ can also be seen in Fig. 5e. However,
besides the contribution from inland dust (Savoie and Prospero, 1980), excess
K+ may also originate from biomass burning in China (Hsu et al., 2009).
Note that statistically significant intercepts could be seen in Ca2+
against Nass and K+ against Nass scatter plots for
background aerosols. Although small, such excesses in Ca2+ and K+
relative to Na+ in widespread background aerosols deserve further
explanation.
As shown in Fig. 5f, a correlation was found between NH4+ and
nss-SO42-. Except for three sea fog samples, all ratios fell close to
the 1 : 1 regression line, suggesting the dominance of
(NH4)2SO4 rather than NH4HSO4. Complete neutralization
of NH4+ by nss-SO42- had likely occurred, and a similar
phenomenon was found elsewhere (Zhang et al., 2013; Hsu et al., 2014).
The ratio of
[NO3- + nss-SO42-] / [NH4+ + nss-Ca2+]
for background aerosols (Fig. 5g) closely followed unity, thus suggesting
that NH4+ + nss-Ca2+ was neutralized by the acidic ions
NO3- and nss-SO42-. However, for the dust and foggy aerosols,
[NO3- + nss-SO42-] / [NH4+ + nss-Ca2+]
ratios located between 1 : 1 and 2 : 1 indicated that the excess
anthropogenic acidic ions that originated from coal fossil fuel combustion
and vehicle exhaust had been transported to the ECSs and NWPO by the Asian
winter monsoon as previously indicated (Hsu et al., 2010a). On the other
hand, Liu et al. (2013) suggested that NHx emission in China is important
and may play a major role in neutralizing the acidic ions. As shown in
Fig. 5h, the scatter plot of NH4+ against NO3-, revealed that
almost all dust and background aerosols sampled in the NWPO had
NH4+ / NO3- ratios larger than 1, which is common in
aerosol observation. However, significantly enriched NO3- in
sea-fog-modified aerosols drew the ratio down to < 1. Such high nitrate to
ammonium ratios had been observed in a previous study of sea fog water
collected from the South China Sea (Yue et al., 2012). In summary, the three
types of aerosol had distinctive features in nitrogen speciation and ion
stoichiometry including relative acidity (Fig. 6a), further supporting our
aerosol type classification.
Box plots for (a) concentrations of
NO3-,
NH4+, WSON and
nss-Ca2+, and RA, and (b) fractions of nitrogen
species in total dissolved nitrogen and proportion of
nss-Ca2+ in Ca2+, in sea-fog-modified, dust and background aerosols. The large boxes represent the
interquartile range from the 25th to 75th percentile. The line inside the
box indicates the median value. The whiskers extend upward to the 90th and
downward to the 10th percentile.
Nitrogen speciation in various aerosols reported from different
regions.
Sample type
Date
Location
NO3-
NH4+
WSON
NO3-
NH4+
WSON
Reference
nmol m-3
nmol m-3
nmol m-3
%a
%a
%a
TSP (sea fog)
Mar–Apr 2014
ECSs
Shelf
536 ± 300
442 ± 194
147 ± 171
48 ± 7
42 ± 9
10 ± 6
This study
TSP (dust)
Mar–Apr 2014
NWPO
Remote ocean
100 ± 23
138 ± 24
11.2 ± 4.0
41 ± 5
56 ± 7
5 ± 2
This study
TSP (bgd.)
Mar–Apr 2014
NWPO
Remote ocean
26 ± 32
54 ± 45
10.9 ± 6.8
27 ± 9
60 ± 11
14 ± 8
This study
TSP (dust)
Aug–Sep 2007, 2008
Barbados, Atlantic
Island
10 ± 4
11 ± 7
1.4 ± 1.3
45c
49c
6c
Zamora et al. (2011)
TSP (dust)
May 2007–July 2009
Miami, FL, Atlantic
Coast city
28 ± 9
26 ± 10
3.0 ± 2.0
50c
45c
5c
PM>2 (dust)
Tropic Atlantic Ocean
14
Violaki et al. (2015)
TSP (dust)
Mar 2005–Apr 2007
Southwest ECS
Shelf
84 ± 98
177 ± 151
–
–
–
–
Hsu et al. (2010b)
TSP (dust)
Feb 1992–May 2004
Island of Jeju
Island
71 ± 44c
72 ± 48c
–
–
–
–
Kang et al. (2009)
TSP
Feb–Mar 2007
Northwest ECS
Shelf
68c
193c
–
–
–
–
Shi et al. (2010b)
TSP
Mar 2005–Apr 2007
Southwest ECS
Shelf
38 ± 45
89 ± 76
–
–
–
–
Hsu et al. (2010b)
TSP
Sep–Oct 2002
ECS
Shelf
34c
136c
54 ± 36
15c
61c
24
Nakamura et al. (2006)
TSP
Mar 2004
ECS
Shelf
39c
91c
16 ± 19
27c
62c
10
TSP
Mar 2005, Apr 2006
Yellow Sea
Shelf
–
–
–
–
–
20
Shi et al. (2010a)
TSP
Apr 2010
Northwest ECS
Island
111c
76c
–
–
–
–
Zhu et al. (2013)
TSP
Mar 2011
Northwest ECS
Island
137c
202c
–
–
–
–
Zhu et al. (2013)
TSP
Spring 2003–2004
Northeast ECS
Island
85 ± 47c
133 ± 78c
–
–
–
–
Kundu et al. (2010)
TSP
Jul–Aug 2008
NWPO
Remote ocean
2.5
5.6
–
–
–
–
Jung et al. (2013)
TSP
Aug 2003–Sep 2005
Gulf of Aqaba
Coast
39 ± 19
25 ± 14
8 ± 5
53c
34c
11c
Chen et al. (2007)
TSP
Nov–Dec 2000
Tasmania
Island
11 ± 7
2.6 ± 3.0
3.6 ± 5.7
63
15
21
Mace et al. (2003a)
TSP
Aug–Sep 2008
NWP
Remote ocean
1.8 ± 1.5
1.2 ± 1.1
1.1 ± 0.93
43c
30c
28c
Miyazaki et al. (2011)
TSP
Apr 2007–Mar 2008
Marina, Singapore
Urban
50 ± 31c
14 ± 8c
56 ± 22c
40 ± 15c
11 ± 6c
49 ± 17c
He et al. (2011)
TSP
Jan–Dec 2006
Keelong, Taiwan
Coast city
76 ± 28
26c
Chen et al. (2010)
TSP (sea spray)
6.7 ± 2.7
4.2 ± 1.7
0.5 ± 0.3
59c
37c
4c
Zamora et al. (2011)
TSP (Bb)b
11 ± 11
18 ± 13
3.3 ± 2.0
34c
56c
10c
TSP (Bb)b
28 ± 16
48 ± 48
6.2 ± 6.4
34c
58c
8c
Zamora et al. (2011)
TSP (pollution)
22 ± 11
23 ± 24
3.7 ± 2.8
45c
48c
8c
PM1.3-10
2005, 2006
Crete, Greece
Island
26 ± 9
8.9 ± 4.0
5.5 ± 3.9
64
23
13
Violaki and Mihalopoulos (2010)
PM1.3
2005, 2006
Crete, Greece
1.5 ± 1.3
70 ± 35
12 ± 14
2
85
13
PM2.5
Jan–Dec 2005
Indian Ocean
Remote ocean
0.3 ± 0.2
1.3 ± 1.0
0.8 ± 1.4
14
53
32
Violaki et al. (2015)
PM2.5-10
0.2 ± 0.1
0.3 ± 0.1
0.2 ± 0.4
26
39
35
PM2.5
Jan 2007
Middle S. Atlantic
Remote ocean
1.3 ± 0.8
51
a Percentage in total dissolved nitrogen.
b Bb indicates biomass burning.
c Calculated value from the original data.
Nitrogen speciation and associated processes in different types of
aerosol
Sea-fog-modified aerosols
Only a few studies concerning water-soluble nitrogen species in sea fog water
have been published (Sasakawa and Uematsu, 2002; Yue et al., 2012; Jung et al.,
2013). To the best of our knowledge, ours includes the first first-hand data from
the Chinese marginal seas (the ECSs) in spring concerning water-soluble
nitrogen species in aerosols collected under the influence of sea fog. As
shown in Table 1 and Fig. 6a, in sea-fog-modified aerosols the concentrations
of nitrate ranged from 160 to 1118 nmol N m-3 with a mean of
536 ± 300 nmol N m-3, and ammonium was slightly lower than
nitrate, ranging from 228 to 777 nmol N m-3 with a mean of
442 ± 194 nmol N m-3. WSON in sea-fog-modified aerosols was the
lowest nitrogen species ranging from 23 to 517 nmol N m-3 with a mean
of 147 ± 171 nmol N m-3 (Table 1 and Fig. 6a). The sea-fog-modified aerosols contained 2–11 times higher concentration of nitrate, 2–6
times higher ammonium and 3–6 times higher WSON when compared with aerosols
in the ECSs and other regions (Table 1). Such high concentrations of
Nr not only highlighted the seriousness of the nitrogen air
pollution in Chinese marginal seas but also underscored that water-soluble
nitrogen species can be scavenged efficiently during sea fog formation.
Mean molar concentrations (nmol m-3) of major ionic species
together with Al (ng m-3) in sea-fog-modified aerosols and dust
aerosols in the ECS.
Sea foga
Dustb
Dustc
mean ± SD
mean ± SD
mean ± SD
Na+
123.2 ± 97.5
294.8 ± 238.3
130.4 ± 85.2
NH4+
441.5 ± 193.9
177.6 ± 150.7
72.2 ± 47.7
Mg2+
24.1 ± 16.5
41.2 ± 32.4
25.0 ± 12.9
K+
17.5 ± 9.9
21.8 ± 19.1
17.9 ± 9.2
Ca2+
54.7 ± 52.2
61.7 ± 39.5
76.9 ± 58.5
Cl-
125.2 ± 111.3
280.9 ± 349.1
121.3 ± 101.6
NO3-
535.9 ± 299.7
83.6 ± 98.4
71.0 ± 43.5
SO42-
172.5 ± 54.1
145.2 ± 103.2
104.0 ± 47.2
nss-SO42-
165.1 ± 50.3
94.9 ± 89.0
96.1 ± 47.3
Total Al
2460 ± 2160
3470 ± 2730
4900 ± 6500
Water-soluble Al
124 ± 36
38 ± 45
nd
Al solubility
5.0 ± 1.7 %
1.1 ± 1.6 %
nd
Relative acidity
0.73 ± 0.13
1.07
1.06
a This study; b Hsu et
al. (2010b); c Derivation from Kang et al. (2009); nd: no data.
Since no chemistry data of sea-fog-modified aerosols had been reported
before, we can only compare with the dust aerosols from the same regions in
spring. The concentrations of leachable ions, water-soluble Al, and total Al
and RA for dust aerosols and sea-fog-modified aerosols sampled in the ECSs
are listed in Table 2. The seven sea-fog-modified aerosols were distinctive
in chemical characteristics. For all except NH4+, NO3- and
SO42-, sea-fog-modified aerosols had lower or similar molar
concentrations relative to dust aerosols. The anthropogenic species,
particularly NO3- and NH4+, were the most abundant ions in the
sea-fog-modified aerosols. However, Na+ and Cl- were the highest among
all the ions in dust aerosols from the island of Jeju and the East China Sea.
Taking Jeju as an example, the concentration levels of Na+ and Cl- were
similar to those of our sea-fog-modified aerosols, yet both NO3- and
NH4+ in sea-fog-modified aerosols were > 6 times higher than those
from the island of Jeju.
The pie charts for ion fractions of aerosols from the ECSs are shown in
Fig. 7. Note that the fraction distribution of ions for the dust aerosols
from a previous cruise in the ECSs (n=8, Fig. 7b; Hsu et al., 2010b)
resembled that collected from the island of Jeju (n=49, Fig. 7c; Kang et
al., 2009) despite the fact that their sampling was performed in different areas and
at different times. Such consistency in the ion pie chart indicates the
representativeness of these dust aerosols. However, the pie chart for sea-fog-modified aerosols revealed that NH4+ and NO3- occupied
approximately 30 and 36 % of the total ionic concentration (Fig. 7a).
Such an overwhelmingly high occupation of nitrogenous ions emphasizes the
role of sea fog in modifying the chemistry of non-foggy dust aerosols.
Pie charts of ion distribution for (a) sea-fog-modified aerosols
(this study), (b) dust aerosols collected over the East China Sea (n=8)
(Hsu et al., 2010b), and (c) dust aerosols collected on the island of Jeju
(n=49) (Kang et al., 2009).
In a previous study in the Po Valley, the average scavenging efficiencies for
aerosol nitrate and ammonium were reported to be at similar levels (70 and
68 %; Gilardoni et al., 2014), while in our case the concentrations of
nitrate in sea-fog-modified aerosols were higher than those of ammonium
(Table 1 and Fig. 6a). Since the gas-phase HNO3 is rapidly dissolved in
liquid water particles during the early stages of fog formation (Fahey et
al., 2005; Moore et al., 2004), it was reasonable to infer that the enriched
nitrate in sea fog was attributed to gaseous HNO3 owing to the
gas–liquid equilibrium between NO3- and HNO3 in fog droplets.
Moreover, our sea-fog-modified aerosols were collected from the air masses
moving around eastern China and the ECSs, where the NOx emission is the
highest in China (Gu et al., 2012). The lifetime of NOx in the boundary
layer is generally less than 2 days (Liang et al., 1998). Based on our air
mass backward trajectories analysis, the travel time of air masses from
inland China to the marginal seas is long enough for oxidation of NOx into
HNO3. Thus, nitrate enrichment in the sea-fog-modified aerosol was
likely a synergistic consequence due to the sea fog formation and gas–liquid
equilibrium of gaseous HNO3.
As for SO42-, both the concentration and percentage occupation were
comparable in sea-fog-modified aerosols and dust aerosols (Table 2 and
Fig. 7). However, the concentrations of nss-SO42- in sea-fog-modified
aerosols were 60 % higher than those of dust aerosols (Table 2),
suggesting the addition of anthropogenic SOx emission during sea fog
formation as indicated by Gilardoni et al. (2014). In the marginal seas
adjacent to the anthropogenic emission source, acidified sea fog induced by
additional sulfuric and nitric acid was common (Sasakawa and Uematsu, 2005;
Yue et al., 2014).
In general, Al in marine aerosols originated from terrestrial minerals
(Uematsu et al., 2010). The mean concentrations of total Al in our seven sea
fog samples were the lowest among those in dust aerosols from the ECSs
(Table 2). However, the concentrations as well as the fractions of
water-soluble Al in sea-fog-modified aerosols were significantly higher than
those of dust aerosols. Because of the high acidity (low RA values) for sea-fog-modified aerosols (Fig. 6a), we suspected that during the seasonal
transition period the formation of sea fog at the land–ocean boundary may
acidify the aerosol to effectively promote the solubility of metals in
aerosol minerals.
Finally, it has been shown that dissolved organic matter can be scavenged by
fog, but its scavenging efficiency was lower than those of nitrate and
ammonium due to hydrophobic organic species being more difficult to
scavenge than hydrophilic ones (Maria and Russell, 2005; Gilardoni et al.,
2014). In our case, although concentrations of WSON in sea-fog-modified
aerosols (147 ± 171 nmol N m-3) were significantly higher than
those of background aerosols, the ratio of WSON to TDN in sea-fog-modified
aerosols (10 ± 6 %) was similar to those (ranging from 10 to
24 %) of background aerosols sampled in the ECSs (Table 1). Such a high
WSON concentration but low WSON % in TDN in sea-fog-modified aerosols may
indicate the lower scavenging efficiency of WSON relative to other nitrogen
species or that its source region is different or both.
Note that all these aerosols in our study were sampled by using TSP.
Conventional knowledge indicates that aerosol may act as a precursor for fog
formation, but this does not necessarily mean all the aerosols we sampled
were directly associated with fog. Nevertheless, we observed distinctive
chemistry for this type of aerosol either comparing with aerosols sampled
during the same cruise or comparing with “non-foggy” aerosols collected in
the ECSs in previous study. More studies are needed to explore the effect of
sea fog formation on aerosol chemistry.
Dust aerosols
For dust aerosols collected in the NWPO, nitrate ranged from 79 to
145 nmol N m-3 with an average of 100 ± 23 nmol N m-3,
and ammonium ranged from 94 to 163 nmol N m-3 with an average of
138 ± 24 nmol N m-3 (Table 1 and Fig. 6a). Relative to
background aerosols, both nitrate and ammonium were significantly higher in
dust aerosols revealing the anthropogenic nitrogen fingerprint carried by the
Asian dust outflow along with westerlies (Chen and Chen, 2008).
Interestingly, dust aerosols contained a low concentration of WSON
(11.2 ± 4.0 nmol N m-3) resembling that of background aerosols
(Table 1 and Fig. 6a). Moreover, dust aerosols held the lowest WSON fraction
in total dissolved nitrogen among the three types (Table 1 and Fig. 6b).
Based on the good correlation between nss-Ca2+ and WSON, previous
studies demonstrated that dust can carry anthropogenic “nitrogen” activity
into remote oceans and simultaneously promote the ratio of WSON / TDN in
aerosol (Mace et al., 2003b; Lesworth et al., 2010; Violaki et al., 2010).
However, in our case there was no correlation between WSON and nss-Ca2+
(not shown), likely illustrating that these aerosols had less chance to come
in contact with WSON along their pathway from a high altitude, or that WSON had been
scavenged during transport. However, the latter was less likely.
Scatter plots of concentrations of (a) Na+,
(b) Cl-, (c) Mg2+, (d) WSON,
(e) NO3-, and (f) NH4+ against corresponding wind
speed for background aerosols. Wind speed was derived by averaging wind speed
(5 min average) in corresponding sampling intervals. Crosses in
(d), (e) and (f) were not considered during the
linear regression.
Background aerosols
For the 31 background aerosol samples, the mean concentrations of NO3-
and NH4+ were 26 ± 32 and 54 ± 45 nmol N m-3
(Table 1). Both were 10 times higher than those collected in the same region
during summer (2.5 ± 1.0 nmol N m-3 for nitrate and
5.9 ± 2.9 nmol N m-3 for ammonium; Jung et al., 2011). The 10
times higher Nr for springtime background aerosols indicated that
the “spring background” was not pristine at all. Such distinctive
seasonality was ascribed to the origins of air mass, since in summer the air
masses in our study area were mainly from the open ocean, while in spring the
air masses came from the northeast of China through the Japanese Sea and
Japan (Fig. 4c), where they were strongly influenced by anthropogenic
nitrogen emission (Kang et al., 2010). The concentration of WSON in
background aerosols was 10.9 ± 6.8 nmol N m-3, which fell
within the wide range reported previously (∼ 1 to
76 nmol N m-3; Table 1). In the open ocean, the WSON in aerosols may
come from natural and anthropogenic sources. For example, the highest
percentage of WSON in TDN in the southern Atlantic (84 %) was attributed
to high biological productivity (Violaki et al., 2015). Unfortunately, no
marine biological data (i.e., special amines or amino acids as summarized by
Cape et al., 2011) existed in our case to directly support marine-sourced
aerosol WSON.
Nevertheless, our sampling cruise experienced a wide range of wind speed with
variable sea salt contents during the collection of background aerosols. The
correlations between ion content and wind speed may reveal some useful
information as indirect evidence. Higher sea salt, e.g., Na+, Cl-, and
Mg2+, appeared with higher wind speed conditions (Fig. 8a–c). Positive
correlations can be seen although r-square values were small, possibly due to
time-integrated sampling (∼ 12 h) and averaged wind speed over the
sampling period. The positive correlation illustrated that the emission of
sea salt aerosols was driven by wind intensity as indicated by Shi et
al. (2012). Except for WSON (Fig. 8d), which was consistent with sea-salt-associated ions, no statistically significant relationships can be derived
from scatter plots of nitrate and ammonium against wind speed (Fig. 8e and
f). An analogous tendency between WSON and sea salt ions suggested that WSON
might come from the surface ocean. Since the concentration of dissolved
organic nitrogen (DON) in surface sea water was less variable, ranging from
4.5 to 5.0 µM in the Pacific Ocean (Knapp et al., 2011), DON can be
taken as a relatively constant component in surface sea water similar to
Na+, Cl- and Mg2+. Very likely, breaking waves and sea spray
brought DON into the atmosphere under higher wind speed. In fact, using free
amino acids and urea compositions in the maritime aerosol, Mace et
al. (2003a) indicated that live species in the sea surface microlayer may
serve as a source of atmospheric organic nitrogen.
Compared with DON in the surface ocean, it is not possible that nitrate and
ammonium in the surface seawater are a source of atmospheric aerosol nitrate
and ammonium since the concentrations of nitrate and ammonium are very low
(a few tens to hundreds of nM) in the surface ocean. However, under a wide
range of wind speed, we observed relatively narrow concentration ranges of
aerosol ammonium and nitrate. This was strange, given that high wind speed
implied vigorous exchange on the air–sea interface, during which both sea
salt emission and scavenging were supposed to be high. Under efficient
scavenging conditions, to maintain a relatively uniform aerosol nitrate or
ammonium concentration (quasi-static), some supply processes are needed for
compensation. Since the surface ocean is not a possible source for both
aerosol ammonium and nitrate, we suggested alternative supplies which
included deposition from the upper atmosphere and photochemical
production/consumption.
Based on δ15N–NH4+ in aerosol (Jickells et al., 2003) and
rainwater (Altieri et al., 2014) collected in the Atlantic, the ocean was
suggested to be one of the ammonium sources for the atmosphere. Because of
the low concentration of ammonium in the ocean surface, direct ammonium
emission via sea spray was less likely. Based on our observation, we
hypothesized that the emitted marine WSON in the atmosphere may serve as a
precursor for ammonium and/or nitrate via the photodegradation and
photooxidation processes reported previously (Spokes and Liss, 1996; Vione
et al., 2005; Xie et al., 2012). A recent study by Paulot et al. (2015)
supported our hypothesis. By modeling global inventories of ammonia
emissions, they found that the ammonia source from the ocean cannot
neutralize the sulfate aerosol acidity; thus photolysis of marine DON at the
ocean surface or in the atmosphere was suggested to be a source of
atmospheric ammonia. More studies about the exchange processes among nitrogen
species through the ocean–atmosphere boundary layer are needed.
(a) Scatter plot of published aerosol WSON and TDN
concentrations from around the world (red circles for this study, black
crosses from Lesworth et al., 2010; Chen et al., 2007; Mace et al., 2003a;
Miyazaki et al., 2011; Shi et al., 2010a; Srinivas et al., 2011; Zamora et
al., 2011; and Violaki et al., 2015). (b) Frequency histograms for
percentage WSON in aerosol TDN (grey bars, data from a) and in
rainwater (blue bars, data from Cornell, 2011; Zhang et al., 2012; Altieri et
al., 2012; Cui et al., 2014; Chen et al., 2015; and Yan and Kim, 2015).
WSON in aerosol and rainwater: a global comparison
Organic nitrogen, distributed in the gas, particulate and dissolved phases,
is an important component in the atmospheric nitrogen cycle. In our case,
mean fractions of WSON in aerosol TDN were 10 ± 6, 5 ± 2 and
14 ± 8 % for modified sea fog, dust and background aerosols, respectively. All
values fell within the wide range reported previously (also in Table 1). Here
we synthesized a published data set about aerosol WSON from around the world
for comparison (Fig. 9a). The synthesized data revealed that aerosol WSON
concentrations varied over 3 orders of magnitude and the fraction of WSON in
TDN ranged from 1 % to as high as 85 %. Additionally, the fraction of
WSON was the less variable towards high WSON concentrations. The slope of the
linear regression between WSON and TDN indicated that WSON accounted for
18 % of aerosol TDN. Although the positive correlation between WSON and
TDN may imply WSON's anthropogenic origin (Jickells et al., 2013), the
marine-sourced WSON cannot be ignored in the open ocean as discussed in Sect. 3.3.3.
In Fig. 9b, we made a comparison between the distribution of the WSON
fraction in rainwater TDN and that in aerosol. The distribution pattern of
WSON fractions in aerosols (Fig. 9b, grey bar) was relatively concentrated,
revealing a tendency towards lower fractions. Its peak frequency appeared at
the category of 10–20 % and at least 80 % of the observed WSON
fractions fell within < 25 %. However, for WSON/TDN in rainwater
(Fig. 9b, blue bar), the distribution pattern was relatively diffusive,
shifting towards a higher percentage and peaking at around categories of
25–40 % with a mean value of 33 % (n=332), which is slightly
higher than that (24 %, n=115) obtained by Jickells et al. (2013).
Although values of the coefficient of variation for both aerosol and
rainwater were high, the results were still statistically meaningful. The
mean WSON fraction for rainwater was around 2 times that for aerosol
(18 %), but the sampling bias inherent in such a comparison should be
noted. In a previous study, Mace et al. (2003a) reported that the fractional
contribution of dissolved free amino acids to organic nitrogen in rainwater
was 4 times higher than that in aerosol. The higher fractional contribution
of WSON to TDN for rainwater may imply that precipitation washed out
hydrophilic organic matter or WSON from the atmosphere more effectively
(Maria and Russell, 2005).
The depositional fluxes reported or calculated for the Asian region
and Pacific Ocean based on assumed deposition velocity.
Locations
Collection type
Date
NO3-a
NH4+a
WSONa
Totala
Reference
ECSs (sea fog)
Cruise
Mar–Apr 2014
926 ± 518
38 ± 17
127 ± 148
1090 ± 671
This study
NWPO (dust)
Cruise
Mar–Apr 2014
172 ± 40
11.9 ± 2.1
6.5 ± 5.7
190 ± 41.6
This study
NWPO (bgd.)
Cruise
Mar–Apr 2014
44.6 ± 55.3
4.66 ± 3.90
7.6 ± 6.5
56.8 ± 59.1
This study
Subarctic western North Pacific
Cruise
Jul–Aug 2008
3.3 ± 2.3
1.9 ± 0.63
–
5.3 ± 2.6
Jung et al. (2011)
Subtropical western North Pacific
Cruise
Aug–Sep 2008
3.0 ± 1.5
2.7 ± 2.1
–
5.7 ± 3.5
Jung et al. (2011)
Central North Pacific
Cruise
Jan 2009
1.6 ± 0.44
1.4 ± 0.96
–
3.1 ± 1.4
Jung et al. (2011)
Northwest ECSb
Cruise
Feb–Mar 2007
117
17
–
134
Shi et al.(2010b)
Southwest ECSb
Cruise
Spring 2005–2007
66
8
–
74
Hsu et al. (2010b)
Northwest ECSb
Coastal island
Apr 2010
192
6.6
–
198.6
Zhu et al. (2013)
Northwest ECSb
Coastal island
Mar 2011
237
17.5
–
254.5
Zhu et al. (2013)
a In µmol N m-2 d-1.
b Recalculated fluxes based on assumed deposition
velocity.
Dry deposition of TDN and the implications
As shown in Fig. 10, the atmospheric nitrogen dry deposition over the cruise
revealed a large spatial variance under different weather conditions. In the
ECSs, the mean DIN (NH4+ + NO3-) deposition on fog days
was estimated to be ∼ 960 µmol N m-2 d-1
(926 ± 518 and 38 ± 17 µmol N m-2 d-1 for
nitrate and ammonium), which was around 6 times higher than the average
values for ordinary aerosols derived from literature reports
(153 µmol N m-2 d-1 for aerosol nitrate and
12.3 µmol N m-2 d-1 for aerosol ammonium; see
Table 3). The WSON deposition ranged from 20 to
446 µmol N m-2 d-1 with an average of
127 ± 148 µmol N m-2 d-1. Since the
bioavailability of aerosol WSON to phytoplankton was reported to be high
(12–80 %; Bronk et al., 2007; Wedyan et al., 2007), by taking WSON into
consideration, the deposition of TDN will be
∼ 1100 µmol N m-2 d-1.
Dry deposition of aerosol nitrogen against sample identification.
Nitrate is in blue, ammonium in red and WSON in green. Sample
identifications, which match with Table S1, are shown on the x axis.
Taking 1150 × 103 km2 for the total area cover by the
ECSs, we calculated the daily nitrogen supply from atmospheric deposition
associated with sea fog to be 18 ± 11 Gg TDN d-1, which is
around 6 times the nitrogen input from the Yangtze River in spring (total
amount of 3.1 Gg DIN d-1; Li et al., 2011) and 2 times the supply
from the subsurface intrusion of the Kuroshio (7.9 Gg NO3--N d-1;
Chen, 1996). In the ECSs, the sea fog occurrence was around 3–5 days in
March and 8–10 days in April (Zhang et al., 2009). Given such high TDN
deposition per day, the contribution of foggy weather should really be taken
into account in a monthly estimate even though the occurrence of sea fog is
limited in time and space. Moreover, with a focus on the plume area, the
atmospheric influence is more widespread than the river.
Assuming that nitrogen was the limiting nutrient and that all the total
dissolved nitrogen deposited from atmosphere into the sea was bioavailable
and would be utilized for carbon fixation, we obtained a C-fixation rate of
∼ 87 mg C m-2 d-1 in spring for the ECSs based on the
Redfield C / N ratio of 6.6. Since atmospheric nitrogen deposition is an
external source, such a conversion represents new production. When compared
with the primary productivity in the East China Sea
(292–549 mg C m-2 d-1; Gong et al., 2000), the new production
associated with sea fog nitrogen deposition may account for 16–30 % of
the primary production in the ECSs on foggy days in spring.
Similar to sea fog on the ECSs, sporadic dust events are frequently observed
from March to May in the NWPO (Shao and Dong, 2006). In our spring case, the
average deposition of dust aerosol nitrate and ammonium
(172 ± 40 µmol N m-2 d-1 for nitrate and
11.9 ± 2.1 µmol N m-2 d-1 for ammonium) were
significantly higher than that of background aerosols
(44.6 ± 55.3 µmol N m-2 d-1 for nitrate and
4.7 ± 4.0 µmol N m-2 d-1 for ammonium; see
Table 3). However, both dust and background aerosols depositions were
significantly higher in spring when compared to summer dry deposition in the
subtropical western North Pacific (3.0 ± 1.5 for nitrate and
2.7 ± 2.1 µmol N m-2 d-1 for ammonium) and the
subarctic western North Pacific (3.3 ± 2.3 for nitrate and
1.9 ± 0.63 µmol N m-2 d-1 for ammonium) (Jung et
al., 2011). Likewise, the C-fixation rate in the NWPO during spring was
estimated to be 4.5–15 mg C m-2 d-1 based on the above
assumptions and observations. The minimal level of C fixation induced by dry
deposition, in fact, equals to the maximum carbon uptake
(3.6 mg C m-2 d-1; Jung et al., 2013) in summer by the total
atmospheric DIN deposition (wet + dry + sea fog) in the western North
Pacific Ocean. Thus, the contribution of atmospheric nitrogen deposition to
primary production in the NWPO could be significantly different between
seasons.
Conclusions
We presented the total dissolved nitrogen species including water-soluble
organic nitrogen in TSP sampled over the ECSs and NWPO during spring and the
samples of the ECSs were collected under sea fog influence. Three types of
aerosol – the sea-fog-modified, dust and background aerosols – were
classified. We found that sea fog formation significantly altered the aerosol
chemistry, resulting in the highest concentrations of all nitrogen species
among the three types of aerosol, accompanied by higher acidity and higher
cation deficiency. On a daily basis, the nitrogen supply from sea-fog-associated atmospheric deposition into the ECSs was around 6 times the
nitrogen supply from the Yangtze River in spring (total amount of
3.1 Gg DIN d-1) and 2 times the supply from the subsurface intrusion
of Kuroshio (7.9 Gg NO3--N d-1). Sea-fog-associated deposition
and chemical processes require more attention and need to be considered in
future aerosol monitoring and modeling works, especially in marginal seas
during seasonal transition.
In the open sea, the spring background aerosol ammonium and nitrate were 10
times higher than previous report for summer, indicating an anthropogenic
influence and the importance of the seasonality of the air mass source. The
ammonium and nitrate varied in narrow ranges showing no correlation with wind
speed, which may represent the degree of sea salt emission and scavenging. It
is likely that nitrate and ammonium in the atmosphere above sea surface had
reached a budget balance. Since the supply of nitrate and ammonium from
surface ocean (bottom) is not possible, their sources might come from upper
atmospheric boundary layer (top) or photochemical production of nitrogenous
compounds. However, WSON revealed a similar pattern to the sea salt
ions (Na+, Mg2+ and Cl-), in which concentrations increased as
the wind speed increased. Such a similarity indicated that at least a portion
of the WSON should come from the surface ocean, where DON is emitted with sea
salt. Future studies of nitrogen isotopic compositions of aerosol WSON and
marine DON may shed light on the role of marine DON in nitrogen cycling of
the air–sea interface.
The dust aerosols were significantly enriched in nitrate and ammonium, but
not in WSON. Unless WSON-depletion processes had occurred, such a
disproportionate enrichment suggests that dust aerosols from high latitude
and altitude may have less chance to come in contact with WSON during long-range
transport.
The WSON to TDN ratios of aerosols collected in the ECSs and NWPO fell within
that of the global pattern of aerosols. Since nitrate and ammonium are mainly
anthropogenic, the significantly positive correlation between WSON and TDN
may imply WSON's anthropogenic origin. When TDN concentrations were low
(< 100 nmol m-3), the proportions of WSON in TDN were more
diffusive, indicating that factors other than anthropogenic ones were
involved. The mean ratio of WSON to TDN in aerosols was only 1/2 of that for
precipitation over the world. Such a low proportion of WSON in aerosol TDN
suggests that the aerosol was less capable of scavenging hydrophilic organic
nitrogen when compared with precipitation. Nevertheless, WSON occupies a
significant portion of the TDN for both aerosol and precipitation and thus
cannot be overlooked in the atmospheric nitrogen cycle.