Introduction
The atmosphere is mostly composed of gases but also contains suspended
liquid and solid particles called aerosols. Knowledge of the physical and
chemical properties of aerosols is important, because of their role in
atmospheric processes and climate change. Marine aerosols perturb Earth's
radiation balance directly by scattering and absorbing the incoming solar
radiation or indirectly by acting as cloud condensation nuclei (CCN) and thus
altering their water uptake properties (Twomey, 1977; Charlson et al., 1991;
Ramanathan et al., 2001). The strength of these direct and indirect effects
depends on the concentration, size distribution, and chemical composition of
the atmospheric aerosols (Coakley et al., 1983). In addition, marine aerosols
play an important role in the atmospheric sulfur cycle of the marine portion
(O'Dowd et al., 1997; Faloona, 2009). Thus, meticulous information on the
chemical and physical properties of marine aerosol is crucial for the aerosol
studies.
Sea salt, a ubiquitous and major component in the marine total suspended
particulate (TSP) mass has been recognized as the dominant contributor to the
clear-sky albedo over the oceans (Haywood et al., 1999). Sea salt aerosols
are produced at the ocean surface through the bubble bursting mechanism
(Woodcock, 1953). They can affect the chemical and microphysical properties
of other aerosol components by taking up and releasing chemically reactive
compounds including sulfur and halogen compounds. The sea salt concentration
primarily depends on wind speed ranging from 2 to 100 µg m-3
(Fitzgerald, 1991). Additionally, sea salt aerosol particles are hygroscopic
by nature (Tang et al., 1997) and hence act as CCN (O'Dowd et al., 1999;
Quinn et al., 2000; Ayash et al., 2008).
Non-sea salt (nss-) SO42- acts effectively as a reflector of solar
radiation and as CCN and, therefore, controls the cloud microphysical
properties and cloud albedo (Charlson et al., 1987). The principal source of
nss-SO42- in the marine atmosphere is the oxidation of gaseous
dimethyl sulfide (DMS) emitted by marine phytoplankton (Charlson et al.,
1987). Graf et al. (1997) reported that the global burden of
nss-SO42- (0.78 Tg sulfur) is composed 37 % from fossil fuel
burning, 36 % from volcanoes, 25 % from marine DMS, and 1.6 %
from biomass burning. On the other hand, continental anthropogenic
nss-SO42- and nitrate (NO3-) are transported over the remote
marine locations and perturb the marine background conditions (Duce and
Tindale, 1991; Uematsu et al., 1992; Matsumoto et al., 1998).
Methanesulfonate (MSA) is also derived by the oxidation of DMS that
originates from the biological activity in the ocean/land (Uematsu et al.,
1992; Pavuluri et al., 2011; Miyazaki et al., 2012; Kunwar and Kawamura,
2014).
Anthropogenic and mineral aerosols have significant impact on global climate
and also influence the atmospheric chemistry as well as marine ecosystems in
remote oceanic regions (Matsumoto et al., 2004). Bridgman (1990) reported
that on average about 185–483 × 106 ton global aerosols per
year are caused by anthropogenic sources including transportation, stationary
combustion, industrial process, solid waste disposal and other miscellaneous
sources. East Asia is one of the most swiftly developing regions in the world
and consumes a significant amount of fossil fuels leading to an apparent
increase in anthropogenic emission of gaseous pollutants and particulate
matter. In addition, high dust loading in springtime is another discernible
feature of air quality over the East Asian region (Sun et al., 2001). The
long-range atmospheric transport of anthropogenic and mineral aerosols from
the Asian continent to the North Pacific (Kawamura et al., 2003; Matsumoto et
al., 2004) and sometimes even North America (Jaffe et al., 2003) by the
westerlies may have significant impacts on global radiation balance,
atmospheric chemistry, and ocean biogeochemistry (Satheesh and Moorthy, 2005;
Rudich et al., 2002; Jickells et al., 2005; Houghton et al., 2001).
Chichijima Island, a remote marine site in the western North Pacific, is
located on the lee side of a large industrial area; therefore, this site
is well suitable for the study of long-range transport of air pollutants in
East Asia and also the perturbation of anthropogenic activity in the remote
marine atmosphere. However, the observational data on aerosol chemistry over
the western North Pacific are very sparse (Kawamura et al., 2003; Mochida et
al., 2003, 2010; Matsumoto et al., 2004; Chen et al., 2013; Boreddy et al.,
2014). There is no study on the long-term observations of ionic chemical
species from the western North Pacific. In order to investigate the annual
and seasonal behavior of water-soluble inorganic ions and to clarify the decadal
trend of the long-range transport of continental aerosols to the remote ocean
area, we carried out measurements of atmospheric aerosols at Chichijima Island in
the western North Pacific.
Experimental
Sampling site and aerosol sampling
Figure 1 shows the sampling location of Chichijima Island in the western
North Pacific and its surrounding East Asian regions. This island is about
1000 km from the main Japanese island, Honshu, and 2000 km away from the
Asian continent. The area within 40 km of this station is covered by oceans
and seas. The population of Chichijima is about 2300 and the island's area
about 24 km2 according to the report of the Tokyo metropolitan
government bureau of general affairs
(http://www.soumu.metro.tokyo.jp/07ogasawara/28.html, accessed in
November 2011). The observatory is not affected by local pollution but by the
long-range transport of polluted air from the Asian continent during winter
and spring. Therefore, the observations at
Chichijima Island are useful in discussing the long-range transport of
polluted air on a regional scale.
The geographical location of Chichijima Island (indicated by red colored star) in
the western North Pacific.
NCEP/NCAR reanalysis of the mean synoptic wind vector (m s-1) at 850 mb pressure level
for each month over the study area during 2001–2012.
TSPs were collected on a weekly basis at the
Satellite Tracking Center of Japan Aerospace Exploration Agency (JAXA,
elevation: 254 m) in Chichijima Island (27∘04′ N,
142∘13′ E) at a height of 5 m above ground level during
2001–2012. Aerosol particles were collected on precombusted
(450 ∘C, 3 h) quartz filters (20 × 25 cm, Pallflex
2500QAT-UP) using a high volume air sampler with a flow rate of
1 m3 min-1 (Kawamura et al., 2003). Filters were placed in a
clean glass jar with a Teflon-lined screw cap during the transport and
storage. After the sampling, the filters were recovered into the glass jar
and stored in a freezer room at -20 ∘C prior to analysis.
NOAA HYSPLIT 10-day backward air mass trajectories at 500 m a.g.l. for each month
over Chichijima Island during 2001–2012.
Analysis of chemical species
All samples were analyzed at the Institute of Low Temperature Science, Hokkaido
University, Japan. The procedure of chemical analysis is as follows: a punch
of 20 mm in diameter from each filter sample was extracted with 10 mL
organic-free ultrapure water (resistivity of > 18.2 MΩ cm,
Sartorius arium 611 UV) and ultrasonicated for 30 min. These extracts were
the filtrated through a disk filter (Millex-GV, 0.22 µm pore size,
Millipore) to remove filter debris and particles and were analyzed for major
inorganic ions (MSA-, Cl-, SO42-, NO3-, Na+,
NH4+, K+, Ca2+, and Mg2+) using an ion chromatograph
(761 Compact IC, Metrohm, Switzerland).
Major anions were separated on a SI-90 4E Shodex column (Showa Denko, Tokyo,
Japan) using a mixture of
1.8 mM Na2CO3 + 1.7 mM NaHCO3 solution at a flow rate
of 1.2 mL min-1 as an eluent and 40 mM H2SO4 for a
suppressor. For cation measurements, a Metrosep C2-150 (Metrohm) column was
used by using a mixture of 4 mM tartaric acid
(C4H6O6) + 1 mM dipicolinic acid (C7H5NO4)
solution as eluent at a flow rate of 1.0 mL min-1. The injection loop
volume was 200 µL. A calibration curve was evaluated using
authentic standards along with a sequence of filter samples. The analytical
error in duplicate analysis was about 10 %. Contributions from the field
blanks varied between 0 to 8 % and 0 to 2 % of real samples for
anions and cations, respectively, during the sampling period. The
concentrations of all inorganic ions reported here are corrected for field
blanks that were collected during the sampling period (2001–2012). A total
of 545 samples were used in this study.
Synoptic wind pattern and general meteorology
Figure 2 shows monthly mean wind vectors at 850 mb pressure level over
Chichijima Island and its surrounding regions, as obtained from the National
Centers for Environmental Prediction (NCEP)/National Center for Atmospheric
Research (NCAR) reanalysis
(http://www.esrl.noaa.gov/psd/data/gridded/reanalysis/); the data have been used
to ascertain the synoptic conditions during the study period 2001–2012. It
is very clear that from January to April the synoptic winds are stronger,
circulation is westerly (from the Asian continent to the Pacific) and the
observation site experiences long-range continental aerosols (anthropogenic
and dust). The winds are weakening by May/June and the wind direction changes
to southeasterly and continues until August/September. The observation site
gets pristine marine air masses, low wind speed and also much rainfall during
the southeasterly regime. Again the wind starts shifting from southeasterly to
northwesterly/westerly by October and becomes stronger towards December and
January–April again. Therefore, on the basis of major synoptic meteorological
conditions as above, a year is divided into four seasons: winter
(December–February), spring (March–May), summer (June–August) and autumn
(September–November) over Chichijima Island.
Based on the historical records from 1974 to 2011 (see Fig. S1 in the
Supplement)
(http://weatherspark.com/averages/33165/Chichijima-Chichi-Shima-Chubu-Japan),
the temperature typically varies from 16 to 30 ∘C and is rarely below
13 ∘C or above 31 ∘C over the course of a year. In summer the average daily high temperature is above 28 ∘C, whereas in the winter
the average daily high temperature is below 22 ∘C. The relative humidity
typically ranges from 55 (winter) to 94 % (summer) over the year,
rarely dropping below 45 % and reaching as high as 98 %. The highest
average wind speed of 4 m s-1 occurs in spring, when the average daily
maximum wind speed is 6 m s-1. The lowest average wind speed of
2 m s-1 occurs in summer, when the average daily maximum wind speed is
4 m s-1. In this region, westerly winds dominate in winter and spring
and trade winds dominate in summer and autumn.
Backward air mass trajectories
Figure 3 shows daily 10-day backward air mass trajectories arriving over the
observation site, Chichijima at 500 m above the ground level, which were
computed for each month using the HYSPLIT model, developed by NOAA/ARL
(http://ready.arl.noaa.gov/HYSPLIT.php) (Draxler and Rolph, 2003), for
the study period of 2001–2012. The selection of 500 m altitude for air mass
trajectories was due to the potential impact of the air–sea surface
interactions within the boundary layer (Zielinski et al., 2014; Rozwadowska
et al., 2010). The sampling site Chichijima is in the western North Pacific,
located in the outflow region of Asian dusts and polluted air masses from
China. At 500 m altitude, all trajectories come from the East Asian
countries during winter and spring. Therefore, based on the sampling point
(JAXA, 254 m) and source regions, we assumed that 500 m is the minimum
suitable altitude to calculate backward air mass trajectories over Chichijima
Island. As we discussed above, during the winter and spring months, the air
masses originate from Siberia passing over northeastern Asia, whereas in the
summer months they mostly originate from the Pacific, where pristine air
masses exist.
Evaluation of non-sea salt analysis
The contributions from other sources excluding sea salts are calculated using
Na+ as a sea spray marker. However, in this study, for better accuracy,
non-sea salt components were evaluated from the sea salt (ss) Na+
fraction (Bowen, 1979; Becagli et al., 2005).
nss-SO42-=[SO42-]-0.253⋅ss-Na+,nss-Ca2+=[Ca2+]-0.038⋅ss-Na+,nss-K+=[K+]-0.037⋅ss-Na+,
where [SO42-], [Ca2+] and [K+] are the total measured TSP
mass concentrations and ss-Na+ was calculated using the four-equation
system reported below and knowing total Na+, total Ca2+, the mean
Ca2+ / Na+ ratio in the crust
((Na+ / Ca2+)crust = 1.78 w/w; Bowen, 1979) and
the mean Ca2+ / Na+ ratio in sea water
((Ca2+ / Na+)seawater = 0.038 w/w; Bowen, 1979).
ss-Na+=Na+-nss-Na+nss-Na+=nss-Ca2+⋅(Na+/Ca2+)crustnss-Ca2+=Ca2+-ss-Ca2+ss-Ca2+=ss-Na+⋅(Ca2+/Na+)seawater
Crustal contribution to water-soluble sodium ranged from 0.004 to 0.94 with a
mean of 0.078 ± 0.071 during the study period.
Results and discussion
Ion balance
In order to assess the quality of the analysis, we performed an ion balance
calculation using major anions (Cl-, SO42-, NO3-)
and cations (Na+, NH4+, K+, Ca2+, and Mg2+)
assuming that most of the ions are in the solutions. Based on the electro
neutrality principle, the sum of total anions (µeq m-3) should be
equal to the sum of total cations (µeq m-3) in the solutions and
this ratio is a good indicator to study the acidity of aerosols over the
sampling site. The following equations are used here to calculate the charge
balance between cations and anions.
Cation equivalent(Σ+)=Na+23+NH4+18+K+39+Mg2+12+Ca2+20Anion equivalents(Σ-)=SO42-48+NO3-62+Cl-35.5
The relation between anions and cations for
different seasons are shown in Fig. 4. We found that correlation coefficients
of anions vs. cations were higher than 0.92 for all seasons, which indicates
a good quality of data and
that ions share a common origin (Zhang et al., 2011). The
slopes of linear regression lines for the seasonally stratified data are
> 1 with the following order: summer (1.264) > spring
(1.256) > autumn (1.252) > winter (1.231). This result suggests that
in all seasons, the TSP was apparently acidic. As most of the major ions were
measured except for hydrogen ions (H+), the cation deficits are probably
due to the H+ ion.
Charge balance of ions (µeq m-3) on a seasonal scale.
Temporal variations of major inorganic species,
MSA- / nss-SO42- and Σ+ / Σ- ratios
Figure 5 presents temporal variations of major water-soluble ionic species,
MSA- / nss-SO42- and Σ+ / Σ- ratios
for the period 2001–2012 over the sampling site. All the measured ions
showed a clear temporal trend for each year during the study period. The
Σ+ / Σ- ratio (µeq m-3), which is a
good indicator of acidity of aerosols over the environment, ranged from 0.8
to 1.6 with a mean of 1.2 ± 0.1, demonstrating that aerosol particles
are acidic over Chichijima Island (Fig. 5a). The
MSA- / nss-SO42- ratio, which can be used as a tracer to assess
the contribution of biogenic sources to sulfate in the atmosphere (Savoie and
Prospero, 1989), varied between 0.002 and 0.064 with a mean of
0.014 ± 0.01 and summertime maxima (Fig. 5b).
Temporal variations of different measured/derived inorganic ions
(µg m-3) and
mass ratios over the western North Pacific during 2001–2012. Each data point represents
1 month in each year.
Sea salt species (Cl- and Na+) are found as the most abundant
ranging from 0.92 to 16.6 µg m-3 with a mean of
6.31 ± 2.61 µg m-3 and from 0.61 to
7.36 µg m-3 with a mean of
3.39 ± 1.20 µg m-3, respectively (see Fig. 5i and k).
Concentrations of nss-SO42- varied from 0.09 to
7.85 µg m-3 with a mean of
2.17 ± 1.53 µg m-3 (see Fig. 5e), whereas those of
nitrate ranged from 0.09 to 1.17 µg m-3 (mean
0.57 ± 0.37 µg m-3). Although NH4+ was less
abundant throughout the sampling period, we found significant levels under
the influence of continental air masses in the spring. Its concentrations
ranged from 0.01 to 1.10 µg m-3 with a mean of
0.17 ± 0.16 µg m-3 (Fig. 5h). Concentrations of
MSA-, a marker of biogenic source, varied from 0.006 to
0.055 µg m-3 with a mean of 0.021 ± 0.009 (Fig. 5f).
Nss-Ca2+ (nss-K+), a tracer for dust (biomass burning), ranged from
0.002 to 0.84 µg m-3 (0.002 to 0.19 µg m-3)
with a mean of 0.13 ± 0.15 µg m-3
(0.04 ± 0.03 µg m-3) (Fig. 5c, d). Concentrations of
Mg2+ ranged from 0.06 to 0.78 µg m-3 with a mean of
0.40 ± 0.14 µg m-3 (Fig. 5j). It is also noteworthy
that the sum of all the water-soluble inorganic ions (WSIM) ranged from 2.9
to 25.7 µg m-3 with a mean of
13.1 ± 4.8 µg m-3 in Chichijima TSP aerosols for the
study period of 2001–2012 (not shown as a figure).
Monthly variations of major chemical species and
MSA- / nss-SO42-
Figure 6 shows box and whisker plots of monthly variations of different
chemical species at Chichijima Island in the western North Pacific for the
period of 2001–2012. Almost all of the ions showed a clear monthly/seasonal
variation with higher concentrations during the long-range atmospheric
transport of continental air masses and lower concentrations under the
influence of marine air masses. Seasonally averaged concentrations of
major ions (mean ± SD) during 2001–2012 at Chichijima Island in the
western North Pacific are reported in Table 1. The presence of a monthly
averaged trend is demonstrated by Theil–Sen slope test (Sen, 1968; Theil,
1950). The results show that these differences are statistically significant
with Theil–Sen slope values of less than 0.01.
Box and whisker plots of monthly variations of different measured/derived ionic
species (µg m-3) and mass ratio for the period 2001–2012 over the western North Pacific.
As illustrated in Fig. 6a and b, sea salt particles are characterized by a
gradual increase from autumn to winter, with a peak in early spring (March).
Thereafter, Na+ and Cl- decreased to a minimum in early summer (June) and again increased toward winter.
We found a significantly high concentration during August, probably due to
the influence of southeast Asian air masses (see Fig. 3). This trend of sea
salts is similar to that of wind speed over the sampling site; that is,
higher wind speeds during spring/winter and lower in the summer. This result
suggests that the concentrations of sea salts are mainly dependent on wind
speed. It is also worth noting that a similar seasonal pattern can also be
seen in the concentrations of Mg2+ (see Fig. 6c), indicating that
Mg2+ comes from the oceanic rather continental sources. This is further
supported by the existing correlation between Mg2+ and Na+. We
found a strong correlation (R2 = 0.94 and slope = 0.117) between
Mg2+ and Na+ with the ratio being very close to seawater (0.12).
Seasonal mean concentrations of major water-soluble ions
(mean ± SD) and the Theil–Sen slope value for the
seasonal trend during the period 2001–2012 at Chichijima Island in the
western North Pacific.
MSA-
Cl-
NO3-
nss-SO42-
Na+
NH4+
nss-K+
nss-Ca2+
Mg2+
Winter
0.02 ± 0.00
7.10 ± 0.88
0.78 ± 0.14
3.06 ± 0.43
4.12 ± 0.47
0.19 ± 0.06
0.05 ± 0.03
0.12 ± 0.03
0.48 ± 0.05
Spring
0.03 ± 0.01
6.18 ± 1.20
0.84 ± 0.15
2.97 ± 0.89
3.32 ± 0.59
0.23 ± 0.10
0.05 ± 0.02
0.30 ± 0.12
0.42 ± 0.07
Summer
0.02 ± 0.00
4.94 ± 1.54
0.24 ± 0.09
1.06 ± 0.59
2.52 ± 0.71
0.11 ± 0.13
0.02 ± 0.01
0.04 ± 0.04
0.29 ± 0.09
Autumn
0.01 ± 0.00
7.12 ± 2.61
0.43 ± 0.11
1.31 ± 0.42
3.62 ± 1.06
0.11 ± 0.05
0.05 ± 0.04
0.04 ± 0.03
0.40 ± 0.11
Theil–Sen slope (2001–2012)
Slope
0
-0.0067
-0.0004
-0.0045
-0.0048
-0.0002
0.0005
0
-0.0005
The seasonal variations of NH4+ and NO3- are characterized by
spring maxima and summer minima. NH4+ concentrations are low
throughout the sampling period over Chichijima Island (Fig. 6d, e),
probably because the sampling site is far away from the source regions of
ammonia over the Asian continent (Boreddy et al., 2014; Matsumoto et al.,
2007). The residence time of NH3 is of approximately several hours in the marine
boundary layer (Quinn et al., 1990) and the concentration of NH3
transported from continental to remote marine locations should be
considerably low. Interestingly, we found a significantly higher
concentration of NO3- than of NH4+ over the sampling
site, which may result from some additional NO3- sources. The
heterogeneous reaction, HNO3 + NaCl → NaNO3, can provide
an additional source of NO3- in TSP aerosols (Wu et al., 2006) over
the observation site. Furthermore, the low temperature over East Asian regions in
winter and spring would favor the shift from the gas phase of nitric acid to
nitrate in the particle phase, which could lead to a higher concentration of
NO3- that is transported to the western North Pacific in winter and
spring. On the other hand, nss-K+, which is derived mainly from biomass
burning, was also quite low in Chichijima TSP aerosols, although it shows a
higher concentration in winter and spring than in summer and autumn. The
seasonal variation of nss-SO42- showed maxima in the spring/winter
and minima in summer (see Fig. 6h), being similar to that of NO3-.
This result indicates that the higher levels of nss-K+ during the winter
and spring are mainly associated with the long-range atmospheric transport of
anthropogenic/biomass burning particles over the observation site.
The concentrations of nss-Ca2+ drastically increased in spring when the
Asian dusts were transported over the observation site by westerly winds (see
Fig. 6i). This result is consistent with the previous studies (Kawamura et
al., 2003; Suzuki et al., 2008; Guo et al., 2011) where nss-Ca2+
increased to a maximum in spring. A strong seasonal variability was found in MSA-
concentrations with higher values in spring followed by winter and lower
values in autumn and summer. This strong seasonal variability in MSA-
can be ascribed to seasonality of photochemistry, biology, and meteorology.
It is worth noting that the mass concentration of MSA- showed similar
seasonal variation with nss-Ca2+ and NO3-, although its
concentrations are much lower than that those of nss-Ca2+ and NO3-.
This result suggests that there should be a link between dust and biological
emissions and NO3 radicals (see Fig. 6g). This point will be discussed
in more details in the subsequent sections. The mass ratio
MSA- / nss-SO42- showed a clear, distinct variation
characterized by a gradual increase from winter to spring with a peak in
summer; it again gradually decreased toward winter (see Fig. 6f). This result
illustrates that the contribution of marine biogenic sources to
nss-SO42- was higher in summer, because of higher solar radiation
which enhances the biological activity over the sampling site. We also found
co-variation between the MSA- / nss-SO42- ratio and air
temperature, both of which showed a maximum in summer followed by spring and
minimum in winter.
Annual mean concentrations of major inorganic ions
(mean ± SD) and the Mann–Kendall test for annual trends
during 2001–2012 at Chichijima Island in the western North Pacific.
MSA-
Cl-
NO3-
nss-SO42-
Na+
NH4+
nss-K+
nss-Ca2+
Mg2+
2001
0.01 ± 0.00
5.65 ± 2.82
0.47 ± 0.29
1.67 ± 1.11
2.87 ± 1.36
0.09 ± 0.04
0.03 ± 0.01
0.15 ± 0.19
0.33 ± 0.16
2002
0.02 ± 0.01
6.84 ± 2.66
0.61 ± 0.41
2.81 ± 1.66
3.59 ± 1.20
0.27 ± 0.17
0.05 ± 0.03
0.18 ± 0.26
0.44 ± 0.15
2003
0.02 ± 0.01
7.23 ± 2.16
0.60 ± 0.36
2.17 ± 1.21
3.85 ± 1.19
0.13 ± 0.07
0.05 ± 0.03
0.10 ± 0.11
0.45 ± 0.15
2004
0.02 ± 0.01
8.41 ± 4.14
0.54 ± 0.39
2.27 ± 1.65
4.46 ± 1.42
0.16 ± 0.07
0.06 ± 0.04
0.08 ± 0.08
0.50 ± 0.16
2005
0.02 ± 0.00
7.25 ± 2.24
0.69 ± 0.40
2.32 ± 1.24
4.12 ± 1.03
0.10 ± 0.07
0.04 ± 0.01
0.11 ± 0.10
0.46 ± 0.13
2006
0.02 ± 0.01
6.58 ± 2.56
0.64 ± 0.43
2.20 ± 1.56
3.58 ± 1.07
0.14 ± 0.15
0.03 ± 0.03
0.16 ± 0.23
0.41 ± 0.14
2007
0.02 ± 0.01
5.63 ± 1.51
0.67 ± 0.35
2.77 ± 1.39
3.36 ± 1.01
0.36 ± 0.32
0.07 ± 0.04
0.17 ± 0.15
0.39 ± 0.13
2008
0.02 ± 0.01
4.83 ± 2.35
0.49 ± 0.29
2.28 ± 1.42
2.89 ± 1.12
0.16 ± 0.11
0.08 ± 0.06
0.08 ± 0.09
0.33 ± 0.13
2009
0.03 ± 0.01
6.46 ± 2.64
0.57 ± 0.37
1.51 ± 1.18
3.47 ± 1.03
0.13 ± 0.08
0.03 ± 0.02
0.09 ± 0.08
0.40 ± 0.11
2010
0.02 ± 0.01
5.15 ± 2.31
0.55 ± 0.38
1.71 ± 1.19
2.71 ± 1.25
0.15 ± 0.14
0.03 ± 0.02
0.15 ± 0.20
0.32 ± 0.15
2011
0.02 ± 0.01
5.56 ± 1.51
0.51 ± 0.31
1.67 ± 1.34
2.85 ± 0.81
0.14 ± 0.15
0.03 ± 0.04
0.10 ± 0.13
0.34 ± 0.11
2012
0.02 ± 0.01
7.04 ± 1.87
0.64 ± 0.51
2.03 ± 1.43
3.49 ± 0.97
0.18 ± 0.18
0.04 ± 0.02
0.15 ± 0.15
0.43 ± 0.12
Mean
0.02 ± 0.00
6.39 ± 1.04
0.58 ± 0.07
2.12 ± 0.42
3.44 ± 0.54
0.17 ± 0.07
0.05 ± 0.01
0.13 ± 0.03
0.40 ± 0.05
Mann–Kendall non-parametric test (2001–2012)
Z value
2.34*
-1.71
-1.56
-2.49*
-2.34*
-0.31
0.62*
-2.02*
-2.18*
* Correlation is significant at the 0.01
level (two-tailed).
Annual variations of different chemical species on a seasonal
scale
Annual mean concentrations of major ions (mean ± SD) for different
seasons during 2001–2012 are reported in Table 2. The presence of an annual
average trend is demonstrated by the Mann–Kendall test, results were also
reported in Table 2. The Mann–Kendall trend test (Mann, 1945; Kendall, 1975)
is one of the widely used non-parametric tests to detect significant trends
in time series. In this test, the absolute value of Z is compared to the
standard normal cumulative distribution to define if there is a trend or not
at the selected level α (= 0.01, in this study) of significance. A
positive (negative) value of Z indicates an upward (downward) trend.
Figure 7 presents the annual variations of selected chemical species for
different seasons in the period of 2001–2012. Although there exist some
seasonal trends of ions, we could not find any clear annual trends for the
species Cl-, Mg2+ and nss-Ca2+ in all seasons. However,
nss-SO42- and NO3- showed a clear annual trend for all
seasons, with an increase from 2001 to 2004 and decrease from 2007 to 2012. Lu et
al. (2010) reported that total SO2 emissions in China increased by
53 % (21.7–33.2 Tg, at an annual growth rate of 7.3 %) from 2000 to
2006, during which period the emissions from power plants were the main sources of
SO2 in China with an increase from 10.6 to 18.6 Tg per year.
Geographically, emissions from northern China increased by 85 %, whereas
those from the south increased by only 28 %. The growth rate of SO2
emissions slowed down around 2005 and began to decrease after 2006 mainly due
to the wide operation of flue-gas desulfurization (FGD) devices in power
plants in response to a new policy of the Chinese government. This change in the
SO2 emissions was exactly recorded in our observations at Chichijima in
the western North Pacific; that is, the decreasing trend of SO42-
concentrations over the observation site can be explained by the decrease in
SO2 emissions in China after 2006. Furthermore, these results are supported
by the annual variation of nss-SO42-/Na+ and
nss-NO3- / Na+ mass ratios (see Fig. 7j and k). The
nss-SO42- / Na+ ratio showed a clear annual trend in winter
and spring with an increase from 2001 to 2004 and a decreasing trend from 2007
to 2012. Therefore, nss-SO42- concentrations in the western North
Pacific are gradually decreasing, because of the suppressed emission of
SO2 over East Asia, especially in China.
Annual variations of different chemical species
(µg m-3) on a seasonal scale over the sampling period of
2001–2012.
In contrast, the annual variation of nss-K+ showed an increasing trend
from 2001 to 2004 and 2006 to 2012, suggesting that biomass burning emissions
in East Asia are continuously increasing and transported to the western North
Pacific by long-range atmospheric transport. This result is further supported
by the study of Verma et al. (2015), who reported long-term measurements of
biomass burning organic tracers (levoglucosan, mannosan and galactosan) for
the period of 2001–2013 over the same observation site, Chichijima Island.
They found a continuous increase in the concentrations of biomass burning
tracers from 2006 to 2013, which is mainly caused by enhanced biomass burning
in East Asia. It is of interest to note that the annual variations of
MSA- concentrations have shown a gradual increase from 2001 to 2012
during the winter and spring seasons, indicating that direct transport of
MSA- from the continental surface to the remote marine locations is
continuously increasing. On the other hand, NH4+ concentrations
showed a gradual decrease from 2006 to 2012 during the winter and spring seasons,
whereas in summer and autumn we could not find any clear annual trends in the
abundance of NH4+.
Correlation coefficient matrix among the chemical species for
(a) winter, (b) spring, (c) summer, and (d) autumn.
(a) Winter
MSA-
Cl-
NO3-
SO42-
Na+
NH4+
K+
Ca2+
Mg2+
nss-SO42-
nss-K+
nss-Ca2+
MSA-
1
Cl-
-.199a
1
NO3-
.365b
-0.166
1
SO42-
.481b
-0.125
.689b
1
Na+
-0.011
.876b
.209a
.261b
1
NH4+
.562b
-.261b
.622b
.821b
0.081
1
K+
.446b
.281b
.568b
.759b
.561b
.744b
1
Ca2+
.303b
.347b
.513b
.478b
.524b
.372b
.533b
1
Mg2+
0.060
.848b
.240b
.291b
.966b
0.124
.589b
.545b
1
nss-SO42-
.496b
-.262b
.677b
.989b
0.116
.835b
.696b
.412b
0.153
1
nss-K+
.520b
-0.155
.518b
.748b
0.107
.829b
.879b
.338b
0.149
.755b
1
nss-Ca2+
.343b
0.052
.480b
.420b
.195a
.380b
.373b
.936b
.229a
.404b
.338b
1
(b) Spring
MSA-
Cl-
NO3-
SO42-
Na+
NH4+
K+
Ca2+
Mg2+
nss-SO42-
nss-K+
nss-Ca2+
MSA-
1
Cl-
.199a
1
NO3-
.388b
.240b
1
SO42-
.349b
0.089
.619b
1
Na+
.258b
.888b
.418b
.467b
1
NH4+
.368b
-0.150
.504b
.710b
0.026
1
K+
.305b
.416b
.703b
.710b
.639b
.474b
1
Ca2+
.236b
.353b
.665b
.485b
.355b
.258b
.516b
1
Mg2+
.382b
.872b
.519b
.416b
.912b
0.105
.660b
.545b
1
nss-SO42-
.418b
-0.008
.609b
.988b
.456b
.770b
.646b
.473b
.362b
1
nss-K+
.294b
-0.082
.539b
.578b
0.034
.631b
.655b
.360b
0.137
.611b
1
nss-Ca2+
.200a
.220b
.621b
.440b
.200a
.256b
.421b
.988b
.404b
.444b
.369b
1
(c) Summer
MSA-
Cl-
NO3-
SO42-
Na+
NH4+
K+
Ca2+
Mg2+
nss-SO42-
nss-K+
nss-Ca2+
MSA-
1
Cl-
-0.163
1
NO3-
.422b
-0.161
1
SO42-
.425b
0.029
.376b
1
Na+
-0.065
.949b
-0.049
.192a
1
NH4+
.359b
-0.243
.485b
.866b
-0.096
1
K+
0.123
.811b
0.062
.429b
.862b
.513b
1
Ca2+
0.127
.765b
0.148
.258b
.797b
0.195
.776b
1
Mg2+
-0.027
.939b
-0.009
.202a
.980b
-0.046
.885b
.817b
1
nss-SO42-
.535b
-.200a
.455b
.968b
-0.061
.911b
.242b
0.082
-0.039
1
nss-K+
.376b
-0.067
.471b
.666b
-0.007
.876b
.456b
0.212
0.053
.738b
1
nss-Ca2+
.277b
0.006
.259b
0.147
-0.016
.384b
0.151
.601b
0.045
.213a
.266a
1
(d) Autumn
MSA-
Cl-
NO3-
SO42-
Na+
NH4+
K+
Ca2+
Mg2+
nss-SO42-
nss-K+
nss-Ca2+
MSA-
1
Cl-
0.007
1
NO3-
.517b
0.037
1
SO42-
.554b
0.104
.753b
1
Na+
.249b
.925b
.338b
.217b
1
NH4+
.342b
-0.131
.360b
.463b
0.088
1
K+
.410b
.567b
.582b
.734b
.754b
.529b
1
Ca2+
.292b
.505b
.492b
.584b
.629b
.336b
.653b
1
Mg2+
.274b
.895b
.428b
.485b
.970b
0.122
.807b
.637b
1
nss-SO42-
.583b
-0.111
.760b
.970b
.224b
.610b
.626b
.483b
.310b
1
nss-K+
.359b
-0.137
.432b
.667b
0.189
.828b
.738b
.531b
0.230
.699b
1
nss-Ca2+
0.163
0.075
.364b
.442b
0.170
.477b
.343b
.879b
0.180
.434b
.623b
1
a Correlation is significant at
the 0.05 level (two-tailed). b Correlation is significant at the
0.01 level (two-tailed).
Correlation analyses among the inorganic ions
In order to find the crucial information about sources of ions, we performed
a correlation analysis among the ions for different seasons (see Table 3)
because the ion concentrations emitted from the same source or similar
reaction pathway should show a good correlation between them. Tables 3a, b,
c, and d show the results of correlation analyses of major ions for winter,
spring, summer, and autumn, respectively, during the study period. In all
seasons, we found strong correlation (excellent correlation during summer and
autumn) among Na+, Mg2+, and Cl-, indicating that these ions
have a similar source and mainly come from sea spray. Although NH4+
concentrations are low throughout the sampling period, it shows good
correlation with SO42- during all seasons.
During winter, nss-K+, a tracer of biomass burning source, strongly
correlates with nss-SO42-, whereas NO3-, a tracer of
anthropogenic source, correlates with NH4+, Na+, and nss-K+
with a relatively strong correlation coefficient (r > 0.55), suggesting
that they are derived from biomass burning and anthropogenic sources in the
Asian continent, respectively. In spring, Ca2+ shows relatively strong
correlation with NO3- (r = 0.62) and moderate correlation with
Mg2+, nss-K+, and nss-SO42-, indicating that they are
derived from similar sources or reaction pathways. It is important to note
that Na+ moderately correlated with acetic ions (NO3- and
SO42-) during spring, whereas no correlation in summer reveals that
chloride loss is prominent in spring than in summer and that
NH3 and HNO3 probably react with sea salt particles in the marine
atmosphere.
Percent contribution of major ions to total WSIM
The percent contributions of individual inorganic species to the total WSIM
are shown as a pie chart in Fig. 8 for the different seasons. Among all the
inorganic species, sea salt (NaCl) is a major contributor to the WSIM,
followed by nss-SO42- and NO3- during all seasons. Na+
and Cl- together contributed ∼ 70, 66, 80 and 82 % to the
total WSIM for winter, spring, summer and autumn, respectively, whereas
nss-SO42- contributed ∼ 26, 24, 11 and 10 %, respectively.
The nss-Ca2+ shows a significant contribution (about 2 %) to WSIM in
spring, indicating a long-range atmospheric transport of Asian dusts over the
observation site. Similarly, Mg2+ contributed to the total WSIM by about
3 % in all seasons.
Percentage contribution of major ions to total water-soluble ions for different seasons.
We found a significant depletion of chloride during winter and spring,
probably due to the atmospheric mixing of anthropogenic pollutants such as
SO2, NO3, etc. (Boreddy et al., 2014). Figure 9a and b show the
monthly and seasonal variations of the Cl- / Na+ mass ratio
during the study period. The monthly averaged Cl- / Na+ ratio
varied from 1.58 to 2.05 with a mean value of 1.79 ± 0.15. Although the
mean mass ratio is almost equal to that of seawater (1.8), we found
significant chlorine loss in the winter and spring samples. Atmospheric
processing of anthropogenic pollutants/minerals and their mixing with sea
salt particles during the long-range atmospheric transport are probably
responsible for the chlorine loss. On the other hand, acid displacement also
plays an important role in chloride depletion over the marine environment
through the following reactions:
2NaCl+H2SO4→Na2SO4+2HCl,
NaCl+HNO3→NaNO3+HCl.
Furthermore, Mochida et al. (2003) reported high abundance of oxalic acid in the
Chichijima TSP aerosols in spring. Oxalic acid may be internally mixed with
dust-derived minerals. Previous studies of Asian dust showed that oxalate was
largely mixed with dust particles (Sullivan and Prather, 2007). Therefore, it
is reasonable to assume that the springtime chlorine loss over the western
North Pacific was most likely due to the displacement of Cl- with
oxalate through the following reaction:
2NaCl+H2C2O4→Na2C2O4+2HCl.
Variation of the Cl- / Na+ mass ratio on (a) monthly and (b) seasonal scales.
In contrast, during the summer and autumn, we found an excess of chloride
over the observation site, because of some additional source of chloride
added to the TSP aerosols.
In order to investigate which acids are responsible for the depletion of
chloride, we performed regression analysis between the
Cl- / Na+ mass ratio and acidic species, nss-SO42-,
NO3-, MSA- and oxalic acid for different seasons during
2001–2012 as shown in Fig. 10. The regression analysis was verified by
t test. The results show that the differences between the
Cl- / Na+ mass ratio and acidic species are statistically
significant with a two-tailed P value that is ≤ 0.001 for each season during
the study period. For all seasons, nss-SO42- moderately correlated
with the Cl- / Na+ mass ratios with negative correlation
coefficients (R2) of 0.38, 0.29, 0.35 and 0.45 for winter, spring,
summer, and autumn, respectively, whereas NO3- moderately correlated
during winter (R2 = -0.30), weakly correlated in autumn
(R2 = -0.22) and has no correlation in spring and summer. These
results suggest that sulfate has more responsibility for the chloride
depletion than nitrate.
On the other hand, the biogenic tracer, MSA-, moderately correlated
during summer (R2 = -0.29) and has weak correlation in winter and
spring. Freshly emitted MSA and H2SO4 (from oceanic biological
productivity associated with the upwelling of nutrient rich water) also
contribute little to the chloride depletion by coating with sea salt,
especially in summer. Interestingly, during spring, the
Cl- / Na+ mass ratio did not correlate with NO3- or
MSA- but weakly correlated with nss-SO42-. These results suggest
that some other organic acids, such as oxalic acid (because of its high
abundance during spring), are responsible for the chloride depletion during
spring. In fact, we found that oxalic acid significantly correlate with the
chlorine loss in winter (-0.30), spring (-0.28) and autumn (-0.36) (see
Fig. 10d). These results confirm that oxalic acid plays an important role in
a chlorine loss.
Relations between chloride depletion (Cl- / Na+ mass
ratio) and acidic species (a) nss-SO42-,
(b) NO3-, (c) MSA-, and (d) oxalic acid
(C2 di) for different seasons over
the western North Pacific.
Which biological source is more important as a contributor to
MSA-?
To better identify which biological source is a more significant contribution
to MSA-, we compared the monthly mean variation of MSA- with
chlorophyll a (Chl a (mg m-3), a satellite-derived biogenic tracer)
during the study period as shown in Fig. 11. Chl a concentrations were
downloaded from the MODIS (Moderate Resolution Imaging Spectroradiometer)
Aqua satellite over the region of 140–145∘ E, 25–30∘ N
for the period July 2002–December 2012. We found a clear monthly/seasonal
variation in Chl a concentration, which gradually increased from autumn to
early spring and then decreased from mid spring to summer. Surprisingly, a
similar seasonal pattern can also be seen in the concentrations of
nss-Ca2+ (see Fig. 6i), indicating that there should exist a possible
link between the long-range transport of Asian dusts (or a springtime bloom)
and the ocean productivity in the western North Pacific. The production of
algal blooms may quickly respond to dust deposition (nutrients) over the
surface ocean (Gabric et al., 2004). By changing the phytoplankton
productivity, dusts can act as important source of DMS production (Jickells
et al., 2005). However, the mechanisms of marine phytoplankton response to a
dust input from the atmosphere still face numerous
uncertainties, a subject of scientific discussion.
Monthly mean variation of (a) MSA-
(µg m-3) and (b) chlorophyll a
concentrations for the study period. Chlorophyll a concentrations were downloaded
from the MODIS Aqua satellite over the region 140–145∘ E, 25–30∘ N for the study
period.
Ramos et al. (2005) observed the massive Saharan dust storms along with algal
bloom in the North Atlantic in August 2004. Bishop et al. (2002)
observed an increase in chlorophyll a over a couple of weeks in the North
Pacific after passage of a Gobi desert dust cloud. Springtime bloom in the
northern East China Sea and Japan Sea was observed by TOMS (Total Ozone Mapping Spectrometer) and
SeaWiFS (Sea-viewing Wide Field-of-view Sensor)
satellites to be initiated 1 month earlier than usual, being correlated
with an Asian dust event in association with precipitation. Such events lead
to a supply of bioavailable iron and induce a deepening of the critical
depth, which results in an early initiation of the bloom (Jo et al., 2007).
On the other hand, Gabric et al. (2004) revealed that the dust storms in
Australia (2002–2003) lead to advection of large dust plumes over the
Southern Ocean, and observed a coherence between optical characteristics of
the Southern Ocean atmosphere and dust loading by satellite and field data on
surface ocean chlorophyll a. Therefore, it is noteworthy that the
transported atmospheric dust particles can act as a fertilizer to stimulate
the production of microscopic marine plants (plankton/algae blooms).
As discussed in Sect. 3.3, the monthly variation of MSA- gradually
increased from winter to spring, with a peak in April, and gradually decreased
towards summer and autumn months. Interestingly, MSA- reached a maximum in
April, whereas chlorophyll a reached a maximum in March, although both are tracers
for marine biological activity. It is also important to mention that the highest
concentration of MSA- was observed 1 month after the Asian dust
deposition over the ocean surface, suggesting that there may be a time lag
between the dust deposition and DMS emissions. Therefore, we assume that
there are two possible sources for higher MSA- concentrations in
winter/spring over Chichijima Island: (1) direct transport of MSA-
from the continental sources, such as industrial emissions (Lu et al., 2010),
terrestrial higher plants (Pavuluri et al., 2013), and forest floors
(Miyazaki et al., 2012); and (2) springtime bloom of phytoplankton over the
western North Pacific.
(a) The SeaWiFS images that captured the large Asian dust
storm visible over the Sea of Japan and North Pacific during
17 March–2 April 2002. (b) Temporal variations of MSA- and
nss-Ca2+ concentrations during 2002 over the western North Pacific. The
black regions in (a) are the gaps between consecutive SeaWiFS
viewing swaths and represent areas where no data were collected.
Mean concentrations of major water-soluble species at Chichijima
Island from 2001 to 2012 and those at several other remote marine locations
in the Pacific.
Location (data set)
NO3-
nss-SO42-
MSA-
References
Present study
Chichijima (2001–2012)
0.58 ± 0.07
2.12 ± 0.42
0.02 ± 0.00
Other remote marine locations
Fanning Island (1981–1986)
0.16 ± 0.08
0.67 ± 0.27
0.04 ± 0.01
Savoie et al. (1989)
Nauru
0.16 ± 0.09
Savoie et al. (1989)
Funafuti
0.10 ± 0.07
Savoie et al. (1989)
American Samoa (1983–1987)
0.11 ± 0.05
0.34 ± 0.14
0.02 ± 0.01
Savoie et al. (1989)
Rarotonga
0.12 ± 0.08
Savoie et al. (1989)
Midway (1981–2000)
0.29 ± 0.16
0.56 ± 0.45
0.02 ± 0.01
Prospero and Savoie (2003)
N. Caledonia (1983–1985)
0.42
0.02
Savoie and Prospero (1989)
Another factor that could affect MSA- concentrations
is the concentration of NO3 radicals, which are among the key oxidants
for MSA production. A polluted air mass with higher NOx concentrations
gives higher MSA yields relative to SO2 from DMS oxidation (Yin et al.,
1990). Under prevailing westerly polluted winds, significant amount of
anthropogenic NOx can be transported from East Asia over the western
North Pacific, which could enhance the MSA concentrations relative to the
less polluted pristine air masses. Similar results are reported elsewhere
(Yin et al., 1990; Jensen et al., 1991; Mihalopoulos et al., 1992; Gao et
al., 1996). Furthermore, temperature is also an important factor to control
the MSA- concentrations through the mechanism of DMS oxidation by
hydroxyl radicals (Arimoto et al., 1996). In the present study, we found
lower concentrations of MSA- during summer and autumn months, when
ambient temperature is higher, demonstrating that lower temperature may lead
to higher MSA concentration in this region. However, the MSA concentrations
in the marine atmosphere could be affected by multiple processes related to
primary productivity, such as spatial variability of phytoplankton species,
air–sea exchange rates of DMS, and different oxidation pathways of DMS. In
addition, variations in environmental conditions such as temperatures,
precipitation patterns, sea ice conditions, winds and ocean currents could
also control the concentrations of MSA (Gao et al., 1996).
To further clarify the relations between MSA- and nss-Ca2+, we
examined the intense Ca episodes during the study period (March 2002), which
can be related to variations in MSA- as shown in Fig. 12 as a typical
example. Figure 12a shows the SeaWiFS (flying aboard Orbview-2) images, which captured the large Asian dust storms
over the North Pacific during 17 March–2 April 2002. Dust storms originate
in the deserts of northern China and Mongolia. The East Asian dust storm appears
to have diminished somewhat on 20 March 2002, as compared to previous days.
However, there seems to be a new batch of dust rising toward the left side of this
image. This scene spans from eastern Asia across Japan and over the western
North Pacific, where the dust was partly entrained by a low-pressure system.
On the other hand, possible variations of MSA- concentrations related to
the East Asian dusts are shown in Fig. 11b. Interestingly, we found higher
levels of MSA- after the Asian dust deposition over the ocean surface.
This evidence strongly reveals that Ca episodes supply the nutrients to
significantly stimulate plankton blooms accompanied by statistically
significant variations in MSA concentrations in the atmosphere few days after
the episodes. This result also demonstrates that Asian dusts can act as an
important source of macro and micro nutrients including iron for
phytoplankton and thus sea–air emission of DMS over the western North
Pacific.
Comparison of major inorganic ions over the Pacific
The mean concentrations of NO3-, nss-SO42-, and MSA- at
Chichijima during the period 2001–2012 are compared to those from several other
remote marine sites in the Pacific, as summarized in Table 4. Results from the
Chichijima data show that mean NO3- and nss-SO42- are higher
than those from other remote marine locations. The mean concentration of
nitrate (0.58 µg m-3) at Chichijima is more than 4 times
higher than from other remote marine sites (Fanning, Nauru, Funafuti,
American Samoa, Rarotonga, and N. Caledonia) and more than twice higher than
that from Midway. Moreover, concentrations of nss-sulfate at Chichijima
(2.12 µg m-3) are 4 times higher than at Fanning, Midway, and
N. Caledonia and more than 7 times higher than those from American Samoa and
Norfolk. The mean concentration of MSA- (0.02 µg m-3) at
Chichijima is comparable to those from other remote marine locations (see
Table 4). These results suggest a similarity to those of the oceanic
biological productivity in the North Pacific.
In contrast, the mean MSA- concentration at Fanning in the equatorial
Pacific is about twice higher (0.044 µg m-3) than that of
Chichijima. Savoie and Prospero (1989) have found high biological
productivity is associated with the upwelling of nutrient-rich water near the
equatorial divergence, with mean DMS levels of 3.8 nmol L-1 in the
surface ocean. They also documented that in the oligotrophic regions, the
mean concentrations of MSA in the air and DMS in the sea water vary over the
narrow range from 0.02 to 0.03 µg m-3 and 1.4 to
1.7 nmol L-1, respectively.
The mean concentration ratio (MSA- / nss-SO42-) at
Chichijima is 0.02, which is lower than those of other remote marine
locations by a factor of 5–7, indicating a substantial impact from
continentally derived sulfate. At the tropical stations, American Samoa and
Fanning Island, MSA and nss-SO42- ratios exhibit similar values with
mean ratios of 0.07 and 0.06, respectively, indicating the cleanest locations
in regard to the continental inputs (Arimoto et al., 1987). This result
further supports our assumption that Asian dusts can act as an important
source of nutrients that stimulate DMS production in the ocean surface
following their
emission to the marine atmosphere over the western North
Pacific. However, it is rather less important that yields of MSA from DMS
oxidation are enhanced as a function of temperature (Hynes et al., 1986).