Aerosol chemical composition over the SCS and comparison with global
marine aerosols
Aerosol characteristics over the SCS
Annual average mass concentrations of aerosol chemical species at
Yongxing Island. Box boundary indicates 25th and 75th percentile.
Lines within the box show the mean. Whiskers above and below the box
indicate 90th and 10th percentiles.
Mean contributions of each major ionic component to total ionic
mass concentration of (a) Yongxing Island (YXI) annual, (b) YXI cool season,
(c) YXI warm season and (d) YXI transition season.
Figures 3, 4 and Table 1 provide information on atmospheric concentrations of
TSPs and major inorganic ions during the sampling period at Yongxing Island.
The annual average TSP concentrations at the island were 89.6 ± 68.0 µg m-3,
with a range of 16.4 to 440.1 µg m-3. The major
inorganic ionic concentrations (Na+, Cl-, SO42-,
Ca2+, Mg2+, K+, NH4+ and NO3-) accounted
for 24.8 % of TSPs. Total major inorganic ionic concentrations were 22.21 µg m-3. Cl-
had the highest concentration among these ions,
from 0.39 to 36.47 µg m-3, with an annual average of 7.73 ± 5.99 µg m-3.
It was followed by SO42- (range 0.52–23.34 µg m-3;
average 5.54 ± 3.65 µg m-3), Na+
(0.9–8.86 µg m-3;
average 4.00 ± 1.88 µg m-3),
Ca2+ (0.17–9.65 µg m-3; average 2.15 ± 1.54 µg m-3),
NO3- (0.10–10.05 µg m-3; average 1.95 ± 1.34 µg m-3),
Mg2+ (0.02–1.55 µg m-3;
average 0.44 ± 0.33 µg m-3), K+ (0.06–1.13 µg m-3;
average 0.33 ± 0.22 µg m-3) and NH4+
(0.01–0.32 µg m-3; average 0.07 ± 0.07 µg m-3).
Annual average, minimum and maximum mass concentrations (µg m-3) of TSPs and aerosol chemical species at Yongxing
Island.
TSPs
Na+
Cl-
K+
Ca2+
Mg2+
SO42-
NO3-
NH4+
Annual
89.6 ± 68.0
4.00 ± 1.88
7.73 ± 5.99
0.33 ± 0.22
2.15 ± 1.54
0.44 ± 0.33
5.54 ± 3.65
1.95 ± 1.34
0.07 ± 0.07
Minimum
16.4
0.90
0.39
0.06
0.17
0.02
0.52
0.10
0.01
Maximum
440.1
8.86
36.47
1.13
9.65
1.55
23.34
10.05
0.32
Aerosols over SCS compared with global marine aerosols
The annual average TSP concentrations at Yongxing Island is also lower than
the annual average value over the northern Yellow Sea (123.2 µg m-3),
another Chinese marginal sea (L. Wang et al., 2013). However, the
mean TSP concentrations at Yongxing Island are not lower than those at other
remote islands or other seas, such as the Indian Ocean (21.1 µg m-3),
Pacific Ocean (36.7 µg m-3), Mediterranean
Sea (46.9 µg m-3), South Atlantic (39.1 µg m-3) and three
islands of Okinawa (22.6 µg m-3 in summer and 44.5 µg m-3
in spring) (Arakaki et al., 2014; Balasubramanian et al., 2013; Zhang et
al., 2010).
Comparisons of major ions in aerosol at Yongxing Island with global
ocean. Data of Oki, Ogasawara and Hedo are from EANET (www.eanet.asia);
data of Rishiri Island are from Okuda et al. (2006); data of Hawaii are
from Carrillo et al. (2002); data of Bermuda are from Moody et al. (2014);
data of Amsterdam Island are from Claeys et al. (2010); data of the
Arabian Sea and Indian Ocean are from Kumar et al. (2008); data of
Helgoland are from Ebert et al. (2000); and data of the Mediterranean Sea,
North Atlantic 1 and 2, Pacific and South Atlantic are from Zhang et
al. (2010). Pentagrams represent sampling sites on islands while others represent
cruises. N.A. indicates no data.
The aforementioned annual average TSP and ionic concentrations are comparable
to those reported in many remote oceans (Fig. 5), e.g., Hedo, which is at
the junction of the East China Sea and northwestern Pacific. The marine ions
(Na+ and Cl-) accounted for 53 % of total major ions (Fig. 4).
Na+ and Cl- concentrations at Yongxing Island were higher than
most reported values among all locations and remote sites (Fig. 5), such as
the Indian Ocean (3.0 and 4.4 µg m-3, respectively), Arabian Sea
(1.9 and 2.2 µg m-3, respectively), Oki (3.3 and 4.3 µg m-3, respectively)
and Rishiri islands (1.2 and 2.7 µg m-3,
respectively) in the Sea of Japan, Amsterdam Island (1.4 and
2.3 µg m-3, respectively) in the Southern Ocean, Bermuda (3.3 and 4.8 µg m-3, respectively)
in the Atlantic Ocean and
Hawaii (0.5 and 0.4 µg m-3, respectively) in the Pacific (Kumar et al., 2008; Okuda et
al., 2006; Claeys et al., 2010; Moody et al., 2014; Carrillo et al., 2002).
However, Na+ and Cl- concentrations were lower than samples from
cruises, such as over the North Atlantic 2 (7.0 and 9.6 µg m-3,
respectively) and Pacific (5.8 and 9.0 µg m-3, respectively) (Zhang
et al., 2010). The Ca2+ concentration was the highest among all
locations and contributes 10 % of major ions (Fig. 4), followed by the
Mediterranean Sea, South Atlantic and North Atlantic 2 (Fig. 5). The
relatively high Ca2+ concentration may be because of Asian terrestrial
dust transported to Yongxing Island. NSS Ca2+
accounted for 93 % of
total Ca2+, ranging from 0.14 to 9.31 µg m-3 with an annual
average of 1.99 µg m-3. Large contributions of NSS Ca2+ were
also found in the Mediterranean Sea, South Atlantic and North
Atlantic 2, being at 88.4, 90.3 and 90.0 %, respectively (Zhang et
al., 2010). The relatively high NSS-Ca2+ concentrations in those oceans
were potentially from the crust or dust from some deserts (Zhang et al.,
2010). Comparing Yongxing Island with all locations, average
Mg2+ concentrations were higher at Yongxing Island than most reported
values among all locations (Fig. 5). K+ was also the highest among all
locations (Fig. 5). NSS K+ ranged from 0 to 0.87 µg m-3, with
an annual average of 0.18 µg m-3 and a contribution of 55 % to
total water-soluble K+ at Yongxing. In general, SO42-,
NO3- and NH4+ were major in the form of secondary
inorganic aerosols. They accounted for only 34.0 % of total inorganic
ionic concentrations, giving them an intermediate position among all
locations (Fig. 5). The average SO42- concentration at Yongxing
was the highest among all locations. As shown in Fig. 4, the mean
contribution of SO42- to major inorganic ionic components was
∼ 25 % at Yongxing. The NSS-SO42- concentration
was 3.66 µg m-3, with a contribution of 66.1 % to total
SO42-. Similar to SO42-, the average concentration of
NO3- in this study was the highest among all locations. It
accounted for 9 % of major ions at Yongxing Island. This indicates that a
large number of anthropogenic sources affected the concentrations of
SO42- and NO3-. It was surprising that NH4+
had relatively low concentrations over most oceans, except for the South
Atlantic and Mediterranean Sea (Fig. 5). The average NH4+
concentration was 0.07 ± 0.07 µg m-3 in aerosol at Yongxing
Island (Fig. 3), representing < 1 % of total major ions (Fig. 4).
Further, low concentrations of NH4+ were also observed in
rainwater on the island (Xiao et al., 2016).
Global marine aerosol chemical patterns
Mole equivalent ratios for major ionic species in aerosols at
Yongxing Island (annual, cool, transition and warm seasons), together with
seawater ratios for comparison.
Yongxing Island
Seawater*
annual
cool
transition
warm
Cl- / Na+
1.25
1.31
1.06
1.12
1.17
Mg2+ / Na+
0.21
0.21
0.19
0.23
0.22
K+ / Na+
0.048
0.051
0.040
0.042
0.021
Ca2+ / Na+
0.62
0.64
0.83
0.47
0.044
SO42- / Na+
0.66
0.71
0.73
0.51
0.12
NSS SO42- / Na+
0.54
0.58
0.61
0.39
–
NO3- / Na+
0.18
0.18
0.22
0.16
–
NH4+ / Na+
0.022
0.021
0.044
0.016
–
NO3- / NSS SO42-
0.34
0.32
0.36
0.41
–
NH4+ / NSS Ca2+
0.038
0.035
0.056
0.038
–
* Seawater ratios from Keene et al. (1986).
Globally, sea salt ions (Na+ and Cl-) were the most important
components in marine atmospheric aerosol, with higher concentration of
Cl- than Na+, except over the Mediterranean and northern seas (Fig. 5; Zhang et al., 2010; Ebert et al., 2000).
In the marine atmosphere, sea
salt aerosol (NaCl) can react with sulfuric acid and nitric acid to release
HCl, which results in a deficit of Cl- relative to Na+ (Zhang et
al., 2010). It is also found that a deficit of Cl- in transition season
at Yongxing Island (Fig. 4 and Table 2) was most likely because air masses
were primarily from remote ocean regions far from the continent (Fig. 1),
where wind is weak (Fig. 2). The mole equivalent ratios of Cl- / Na+
(neq L-1) in aerosols were slightly larger than seawater in annual, cool and
warm seasons at Yongxing Island (Table 2). This suggests that Cl-
enrichment had an anthropogenic or other natural origin (Duan et al., 2006;
Jung et al., 2012; Xiao et al., 2013). For SO42- with
SS SO42-
and NSS SO42-, NSS SO42- was greatly
influenced by anthropogenic sources from developed industrial areas, leading
to higher concentrations of SO42- than Na+ and Cl-.
Examples were Bermuda, Ogasawara and the Arabian Sea (Fig. 5; Moody et al.,
2014; Kumar et al., 2008), where NSS SO42- was the preferred
species for acid displacement (Zhang et al., 2010). As another important ion
of anthropogenic sources, NO3- concentrations were often good
relationships with those of NSS SO42- (Zhang et al., 2010), with
relatively high concentrations among major ions (Fig. 5). Relatively high
concentrations of SO42- and NO3- were also found over
the SCS (Figs. 3 and 4). NH4+ had the lowest concentrations among
the major ions in most marine atmospheric aerosols, suggesting little
ammonia transport to the open ocean, such as Yongxing Island. However, there
were some exceptions. For example, the South Atlantic and Mediterranean
Sea had the highest NH4+ concentrations among major ions (Fig. 5).
Over most seas, the order was Ca2+ > Mg2+ > K+. However, we found that Mg2+ had higher
concentrations than Ca2+ in some remote ocean areas, such as in the
Pacific, Atlantic and Southern oceans (Zhang et al., 2010). This indicates
that Ca2+ of crustal origin was difficult to transport to the remote
oceans, and Mg2+ may mainly be from sea salt over the open ocean (Moody
et al., 2014).
Seasonal variations of TSP mass concentrations and associated
species, including Na+, Cl-, SO42-, Mg2+, K+,
Ca2+, NH4+ and NO3- at Yongxing Island (cool
season: C; warm season: W; annual: A). Shown are the mean and standard
deviation for each bar.
Seasonal patterns of aerosol chemical species over SCS and adjacent
areas
Seasonal characteristics at Yongxing Island
As illustrated in Figs. 4 and 6, seasonal and monthly TSP concentrations and
major inorganic water-soluble ion concentrations had distinct features at
Yongxing Island. Generally, concentrations of TSPs and major inorganic ions
were higher in the cool season than in the warm season (Fig. 6). Seasonal
variations were the same as those in other studies, such as Okinawa, and 18
urban, rural and remote sites in various regions of China (Arakaki et al.,
2014; Wang et al., 2006; Xiao and Liu, 2004; Zhang et al., 2012).
Average TSP concentrations were 114.7 ± 82.1, 60.4 ± 27.0 and
59.5 ± 25.6 µg m-3 in the cool, warm and transition seasons,
respectively, with the highest monthly average in November 2014 and the
lowest in April (39.4 µg m-3) and September (39.9 µg m-3)
of the same year (Fig. 6). There were lower concentrations in the warm season
than in the cool season because 70 % of rainfall at Yongxing Island
happens during the warm season (Fig. 2), being the same as other studies,
such as Shanghai and over the China Sea (Wang et al., 2006; Zhao et al.,
2015). The positive correlation between TSP concentrations and wind speed
(p<0.01) shown in Table 3 suggests that relatively high
speeds can produce many particles from both sea spray and terrigenous
matter. We discovered negative correlations between TSP concentrations and
temperature (p<0.01) and relatively humidity (p<0.01) (Table 2), indicating that warm temperatures and high
relatively humidity enhance particle activation and scavenging is happening
(Liu et al., 2011).
Correlation coefficients among major ions in aerosol and
meteorological parameters.
TSPs
Na+
Cl-
SO42-
Ca2+
Mg2+
K+
NH4+
NO3-
WS
T
RH
R
TSPs
1
0.77**
0.92**
0.77**
0.92**
0.32**
0.75**
-0.05
0.52**
0.36**
-0.47**
-0.44**
-0.20
Na+
1
0.91**
0.69**
0.72**
0.57**
0.78**
-0.03
0.48**
0.44**
-0.51**
-0.46**
-0.27*
Cl-
1
0.71**
0.83**
0.49**
0.77**
-0.04
0.45**
0.43**
-0.37**
-0.36**
-0.19
SO42-
1
0.86**
0.56**
0.85**
0.26*
0.87**
0.04
-0.56**
-0.58**
-0.29*
Ca2+
1
0.36**
0.81**
-0.03
0.69**
0.24
-0.51**
-0.53**
-0.27*
Mg2+
1
0.63**
0.45**
0.59**
0.04
-0.12
-0.08
-0.18
K+
1
-0.18
0.72**
0.15
-0.51**
-0.45**
-0.27*
NH4+
1
0.36**
-0.13
-0.18
0.11
-0.18
NO3-
1
-0.05
-0.50**
-0.50**
-0.31**
** Correlation significant at 0.01 level (two-tailed), * significant at 0.05
level (two-tailed). WS: wind speed (m s-1); T: temperature (∘C); RH:
relative humidity (%); R: rainfall (mm h-1).
As shown in Figs. 4 and 6, sea salt ions Na+ and Cl- were
characterized by a gradual increase from the transition to cool season.
Their concentrations (in µg m-3) in the cool, warm and transition
seasons were 4.91 ± 1.82 and 3.04 ± 1.08, 2.28 ± 1.35 and
9.93 ± 6.78, and 5.25 ± 2.63 and 3.73 ± 3.63,
respectively, with corresponding contributions of 52, 57 and 57 %
to total major ions in those seasons. The highest Na+ and Cl-
concentrations in a single sample were found in November, with the lowest
concentrations in May and April, respectively. The highest average monthly
concentrations were in November. Positive relationships between Na+ or
Cl- and wind speed in Table 3 (p<0.01, correlation
coefficient R=0.44 and p<0.01, R=0.43, respectively) at Yongxing Island suggest that sea salt
concentrations were dependent on wind speed. This is consistent with results
at Chichijima Island (Boreddy and Kawamura, 2015). There was a low negative
relationship between Na+ and rainfall (p<0.05,
R=0.27) but no relationship between Cl- and rainfall
(p>0.05) in Table 3, suggesting that Na+ mainly
existed in coarse particles and was readily removed by rainfall. As shown in
Table 3, concentrations of Na+ and Cl- were also negatively
influenced by temperature and relatively humidity. Although Mg2+ is
often treated as crustal-derived ions and elements in continental studies
(Zhang et al., 2015), its highest monthly average concentrations were in
November at Yongxing Island, the same as Na+ and Cl- (Fig. 5). As
shown in Fig. 5, Tables 1 and 2, similar trends and strong correlation were
observed among Na+, Cl- and Mg2+, and the ratios of Mg2+
to Na+ in aerosols were close to that in seawater, suggesting that
Mg2+ may mainly derive from sea salt rather than continental sources.
However, there were no relationships between Mg2+ and wind speed,
temperature, relatively humidity or rainfall (Table 2), in contrast to
other ions, such as Na+ and Cl-. These results reveal that
Mg2+ has different behaviors in the marine atmosphere at Yongxing
Island. The different behaviors of Mg2+ were also found in rainwater at
Yongxing Island (Xiao et al., 2016).
As shown in Fig. 6, the highest monthly average concentrations of Ca2+
were in February. Its monthly trends were different from those of TSPs,
Na+ and Cl-, and the ratios of Ca2+ to Na+ in aerosols
were much higher than those in seawater (Table 2), suggesting that Ca2+
from terrestrial dust sources may be influenced by different factors.
Ca2+ accounted for 10, 13 and 8 % of total major ions in the
cool, transition and warm seasons, respectively (Fig. 4). There was no
correlation between Ca2+ and wind speed, in contrast to TSPs, Na+
and Cl- (Table 3). However, there was a negative relationship between
Ca2+ and rainfall (p<0.05; Table 3). These results
suggest that Ca2+ existed in coarse particles that can be readily
removed by rainfall. Thus, a low mass concentration was observed for
Ca2+ in the rainy (warm) season (Fig. 6), with a low percentage being
in the warm season in Fig. 4.
K+ concentrations were 0.42 ± 0.23 µg m-3 in the cool
season, 0.22 ± 0.18 µg m-3 in warm season and 0.15 ± 0.07 µg m-3
in the transition season at Yongxing, with the maximum
monthly average concentrations in February and the minimum in July (Fig. 6).
However, the lowest NSS-K+ monthly average concentration was in August.
The results suggest that NSS K+ is derived from Chinese biomass/biofuel
burning in the cool season (Lawrence and Lelieveld, 2010). Streets et al. (2003) computed
that China contributes 25 % of total biomass burning in
Asia. Sites in Chinese coastal regions had higher K+ and NSS-K+
concentrations than those at Yongxing Island (Wang et al., 2006) were
observed, further indicating that Chinese and other northeastern Asian regions'
biomass/biofuel burning have a strong influence on atmospheric composition
over the SCS.
Similar to K+, the highest monthly concentrations of SO42-
and NO3- were observed in February, being at 13.08 ± 9.04 and
4.99 ± 4.33 µg m-3, respectively (Fig. 6). As shown in
the figure, SO42- concentrations in the cool and warm seasons were
7.22 ± 3.92 and 3.26 ± 1.26 µg m-3, respectively,
accounting for 26 and 22 % of total major ions, and NO3-
concentrations were 2.43 ± 1.54 and 1.30 ± 0.64 µg m-3,
accounting for 9 and 9 %. The NH4+ showed maxima in the cool
season and minima in the warm season, being 0.08 ± 0.08 and 0.04 ± 0.03 µg m-3, respectively.
Seasonal patterns over SCS and adjacent areas
Comparison of aerosol chemical species between Yongxing Island and
around the South China Sea (data from EANET).
The spatial variability in seasonal patterns of the major inorganic ionic
components at Yongxing Island and adjacent sites of the Acid Deposition
Monitoring Network in East Asia (EANET) in 2011 is portrayed in Fig. 7. In
general, total major inorganic ionic concentrations tended to be higher in
cool seasons and lower in warm seasons to the north of Phnom Penh, including
Phnom Penh, Hoa Binh, Hanoi, Hongwen, Hedo, Ogasawara and Yongxing Island,
being consistent with previous studies (Boreddy and Kawamura, 2015; Wang et al.,
2006; Xiao and Liu, 2004). There was no substantial seasonal variation at
other sites of EANET, and there was no strong seasonal variation of rainfall
there either. These results suggest that rainfall, wind patterns and
anthropogenic activities influence the ionic seasonal variations (Lawrence
and Lelieveld, 2010; Wang et al., 2006; Xiao et al., 2013; Xiao and Liu,
2004). Additionally, total major ionic concentrations were higher in the
north than in the south, indicating more anthropogenic pollutants in the
north, such as SO42- and NO3- (Lawrence and Lelieveld,
2010). As it is well known, the most densely populated regions in the north,
including Hanoi, northeastern China and the Pearl River Delta of China, Korea and
Japan release large amounts of pollutants (Lawrence and Lelieveld, 2010),
which then transport to the SCS in the cool seasons (Fig. 1).
Fire spot data from MODIS global fire mapping from March 2014 to
February 2015 around South China Sea (https://firms.modaps.eosdis.nasa.gov/firemap/).
The total ionic concentrations were higher at the three islands than sites
to the south of Phnom Penh. As shown in Fig. 7, relatively high
concentrations of Na+ and Cl- were found at those islands,
suggesting that ions from sea salt had large contributions to total major
ions, i.e., 52.8, 62.5 and 55.6 % at Yongxing, Ogasawara and Hedo,
respectively. This represents high mass concentrations of sea salt in the
marine atmospheric aerosol. The highest concentrations of both Na+ and
Cl- appeared in November at Yongxing and Hedo islands, which were
influenced by a strong northeast monsoon. The highest concentrations of both
Na+ and Cl- were in September at Ogasawara Island, which were
influenced by a strong southeast monsoon from the Pacific. The relationship
between Na+, Cl- and wind speed at Yongxing (p<0.01) is shown in Table 3. Other sites in Fig. 7 were also influenced by
wind speed and winds directly from the ocean. However, the highest Na+
and Cl- concentrations at some sites did not appear in the same month,
e.g., at Hongwen, the highest concentrations of Na+ were in April and
the highest of Cl- were in January. Excess Cl- in January has been
observed by anthropogenic sources in China (Duan et al., 2006).
The highest concentrations of Mg2+ were in the same months as Na+
at most sites, indicating that Mg2+ may be from sea salt with Na+
for almost all stations. The exceptions were at Hoa Binh and Tanah Rata with
the maximum Mg2+ concentrations being in December and July,
respectively. This suggests that Mg2+ originates from the crust rather
than oceans (Xiao and Liu, 2004), or from both crust and ocean at these
sites. Hoa Binh was influenced by the northeast monsoon, which carries
strongly weathering crustal matters from the Chinese Yunnan–Guizhou Plateau karst
(Hien et al., 2004; Xiao et al., 2013), and there was a strong relationship
between Mg2+ and Ca2+ (R=0.7, p<0.05). Ca2+ had its highest concentrations in July at Phnom Penh, Tanah
Rata, Petaling Jaya, Serpong and Danum Valley, all of which are located in
the south of the SCS. In these regions, relatively little rainfall (rainfall
data from EANET) and strong sunlight were observed in that month, leading to
strong weathering that generated Ca2+. However, the highest Ca2+
concentrations were found at other sites in the cool season, during which
there was much dust from northeastern Asia (Fig. S1; Boreddy and Kawamura,
2015; Y. Liu et al., 2014; Wang et al., 2011). This result is consistent with
earlier studies (Boreddy and Kawamura, 2015; Cheng et al., 2000; Y. Liu et al.,
2014; Zhao et al., 2015). The Ca2+ data also proved that Asian dust can
affect northern SCS, but it is difficult for Asian dust to be
transported to southern SCS (Fig. S1).
Figure 8 shows fire spot data from MODIS global fire mapping around the SCS
during March 2014 to February 2015. Additionally, smoke surface
concentrations every day in that period and region are shown in Fig. S1 in the Supplement. The
fire spot and smoke data give information on seasonal variations of biomass
burning around the SCS. This activity was strong from January to April in
the west of the SCS, including Vietnam, Thailand and Laos, and between July
and October in the south of the SCS, including Malaysia and Indonesia (Figs. 8 and S1).
These data are consistent with other studies showing substantial
monthly CO emissions from biomass burning during February–April and
August–October in Southeast Asia, and February–May in southern China and
Taiwan (Streets et al., 2003). K+ is commonly used as a tracer of
biomass and biofuel burning (Deng et al., 2011). As shown in Fig. 7, we
found that the maximum K+ was in the aforementioned months at most
sites, suggesting that Asian biomass burning influenced the SCS region.
Fossil fuel combustion, industrial processes, biofuel burning, and agricultural
and waste handling often generate large quantities of SO2, NOx and
NH3 in Asia (Lawrence and Lelieveld, 2010; Liu et al., 2013; Xiao et
al., 2012a, 2014b, 2015), although natural
emissions of SO2 and NH3 include biomass burning, marine and
soil biological processes (Streets et al., 2003; Altieri et al., 2014;
Boreddy and Kawamura, 2015; Xiao et al., 2012a) and NOx from those
processes and lightning (Price et al., 1997; Xiao et al., 2015). In general,
the three marine sites (Yongxing, Hedo, and Ogasawara islands) had smaller
proportions of SO42-, NO3- and NH4+ than
inland sites, with the three ions accounting for ∼ 35 % at
the three marine sites and up to 65 % at the other sites. This indicates
that anthropogenic contributions are smaller over remote open oceans than at
continental sites. Figure 6 shows that the highest SO42-,
NO3- and NH4+ concentrations were found during the cool
season in the north of Phnom Penh, including Phnom Penh, Hoa Binh, Hanoi,
Hongwen, Hedo, Ogasawara and Yongxing, being consistent with total inorganic
major ions. This indicates that the pollutants from northeastern Asia have a
great impact on the northwestern Pacific. Figure S1 confirms these findings
that pollutants from nature and anthropogenic activities effect the
northwestern Pacific. We also found that most sites in the south of Phnom Penh
had maximum SO42- and NO3- concentrations in the same
months as the highest K+ concentrations occurred, suggesting that
biomass and biofuel burning are important sources for SO42- and
NO3- in those regions. Lawrence and Lelieveld (2010) found that
such burning was important in the emissions of SO42- and
NO3- in southern Asia, whereas fossil fuel combustion and
industrial processes tended to be dominant in northern Asia (Xiao et al.,
2015). However, maximum NH4+ concentrations at some sites (e.g.,
Petaling Jaya, Serpong, Danum Valley) were inconsistent with SO42-
and NO3-. Moreover, there was no relationship between
SO42- or NO3- and NH4+ at these sites in the
south of Phnom Penh, including Phnom Penh and Yongxing Island (both
p>0.05). The results are inconsistent with previous
studies (Boreddy and Kawamura, 2015; Hsu et al., 2007; Wang et al., 2006;
Xiao et al., 2013; Xiao and Liu, 2004). In the marine atmosphere,
SO42- and NO3- are predominant in coarse particles
(Boreddy and Kawamura, 2015; Xiao et al., 2015), whereas NH4+ is
often predominant in fine particles and may exist in the form of
(NH4)2SO4 in their accumulation mode (Ooki et al., 2007;
Ottley and Harrison, 1992).
Source identification, apportionment and region
Relative contributions (%) for different major ions from
potential five sources of TSPs at Yongxing Island over the year, based on PMF
5.0 model.
Source
Na+
Cl-
K+
Ca2+
Mg2+
SO42-
NO3-
NH4+
Sea salt (two species)
77.4
93.9
53.2
33.9
70.4
24.0
13.1
6.7
Crust
6.1
5.5
10.8
9.8
11.9
11.5
15.9
23.6
SIA
8.8
0.0
27.7
56.3
15.8
57.5
69.5
9.1
Oceanic emission
7.7
0.6
8.3
0.0
2.0
6.9
1.6
60.5
Profiles of five sources identified from the PMF 5.0 model,
including sea salt (two species), crust, secondary inorganic aerosol and
oceanic emission.
Based on the PMF 5.0 model, five potential sources of atmospheric chemical
components at Yongxing Island were identified: sea salt (two species),
crust, secondary inorganic aerosol (SIA) and oceanic emission. Table 4
summarizes source apportionment of the relative contributions of each
identified source to major ions. Figures 9 and 10 show the modeled source
profiles and the time series of modeled concentrations for each identified
main source.
Further, CWTs were plotted for TSPs and major ions (Fig. 11) to explore
likely regional sources and transport pathways for the island. Air masses at
Yongxing Island had obvious unique and seasonal variations, from northeast
of the island in the cool season, southwest in the warm season and
southeast in the transition season (Fig. 11). This reveals that aerosol or
chemical compositions at the island originated from different regions in
different seasons (Figs. 1, 8, 11 and S1). The air masses with high TSP
concentrations were from China coastal regions bordering the Yellow and East
China seas and northern South China (Fig. 11). This is consistent with the
seasonal variations of TSP concentrations in Fig. 6. The average AOT over the northwestern Pacific (Fig. 1) confirmed this
result. A relatively large average AOT was found over the northern SCS and
East China Sea in the cool season, and Karimata Strait in the warm season
(Fig. 1). But there was a relatively low average AOT over the entire SCS in
the transition season (Fig. 1).
Time series contributions from each identified sources, including
sea salt (two species), crust, secondary inorganic aerosol, and oceanic
emission.
10-day back trajectories of warm (black, June through September 2014), cool (blue, March and April 2014 and October 2014 through February
2015) seasons, and transition seasons (red, May 2014) at Yongxing Island.
Additionally, CWT (concentration-weighted trajectory) plots for daily
weighted-average concentrations of TSPs, Ca2+, Mg2+, K+,
SO42-, NO3- and NH4+ at Yongxing Island.
The first source, sea salt, generally has strong marine elements, such as
Na+ (Cl-) and Mg2+, in which Na+ exists in super-micron
size aerosols, whereas Mg2+ exists in sub-micron size aerosols (Moody et al.,
2014). They contributed 77.4 % (Na+), 93.9 % (Cl-) and
70.4 % (Mg2+) from sea salt at Yongxing Island (Table 4). Although
the CWT for Mg2+ was larger in the cool season and lower in the warm
season, air masses with relatively high concentrations of Mg2+
originated offshore of China (Fig. 10). This further indicates that
Mg2+ was mainly from sea salt. Other significant sources were crust,
SIA and oceanic emission, with contributions < 10 % for Na+
and Cl-, and < 16 % for Mg2+. According to data of
rainwater at Yongxing Island, a part of Na+ and Cl- could be from
crust and produced by burning (Xiao et al., 2016). Coal combustion and
biomass burning also produce Na+ and Cl- (Liu et al., 2000; Tiwari
et al., 2013; Zhang et al., 2015). Zhang et al. (2015) found that coal
combustion was the most likely dominant source of Cl- in Beijing. The
mole equivalent Cl- / Na+ ratios were larger in the cool season than
in the transition and warm seasons at Yongxing, indicating that the crust,
fossil combustion, and biofuel and biomass burning affected Na+ and
Cl- concentrations over the northwestern Pacific (Figs. 8 and S1). As
shown in Table 3, there were strong relationships between Na+
(Cl-) and SO42- (Ca2+, K+), further proving that
crust, fossil combustion and biomass burning can generate Na+ and
Cl-. Moreover, NaCl can react with acids such as H2SO4,
HNO3 and H2C2O4, altering Na+ and Cl-
concentrations in the marine atmosphere and producing secondary
chlorine-containing salt (Boreddy and Kawamura, 2015). Sea salt provided
K+, Ca2+ and SO42-, constituting 53.2, 33.9 and
24.0 %, respectively. The ratios of K+ / Na+, Ca2+ / Na+
and SO42- / Na+ in Table 2 also indicate that part of them are
from sea salt. The results are consistent with other studies (Boreddy and
Kawamura, 2015).
The second source, crust, has substantial crustal elements Ca2+ and
Mn (Fig. 9), which are tracers of crust (Suzuki and Tsunogai, 1988; Xiao et
al., 2013; Xiao and Liu, 2004; Norris et al., 2014). However, Ca2+ from
the crust, only had a contribution of 9.8 % (Table 4). The result
indicates that it is difficult for Ca2+ directly derived from crust to
transport to open ocean. But CWTs for Ca2+ were larger in the cool
season and lower in the warm season, indicating that dust from northeastern
Asia influenced aerosol chemistry in the remote marine areas (Figs. 1, 11
and S1). The result suggests that when it reacts with H2SO4 and
HNO3 to generate secondary inorganic aerosol, e.g., CaSO4 and
Ca(NO3)2, it may transport to a longer distance.
The third source is relevant to secondary inorganic aerosol, which are
typically characterized by remarkable SO42- and NO3-.
They contributed 57.5 and 69.5 % from SIA at Yongxing Island (Table 4)
and good relationship between them was observed (Table 2). Fossil fuel
(especially coal) combustion releases large amounts of SO2 and NOx
(Xiao et al., 2012b, 2014, 2015). Lawrence and Lelieveld (2010)
attributed 61 % of total NOx and 77 % of total SO2 emissions
from fossil fuel combustion in southern Asia, and 76 % of total NOx and
75 % of total SO2 in northern Asia, resulting in the transport of
substantial secondary inorganic aerosols containing SO42- and
NO3- to the SCS (Figs. 1, 11 and S1). As with TSPs and Ca2+,
air masses from China, coastal regions had high SO42- and
NO3- concentrations in the cool season (Fig. 11), owing to rapid
economic development and great coal demand in the country, especially in its
coastal regions (Lawrence and Lelieveld, 2010). Figure S1 also shows that
SO42- from central and eastern China reached coastal regions in
the cool season. In Chinese coastal provinces, emission intensities of
SO2 and NOx were about 10 and 15 tons km-2, respectively, much
higher than other Chinese provinces (China Environment Statistical Yearbook,
2014). Rapidly growing economies and high population densities in these
regions (Kim et al., 2014) release pollutants that are transported to the
northwestern Pacific. In the atmosphere, the products (H2SO4 and
HNO3) of SO2 and NOx can easily combine alkaline ions, such
as Ca2+ and K+ (Xiao et al., 2013), in which K+ is characterized
as an effective tracer of biomass and biofuel burning aerosols (Zhang et
al., 2015). As shown in Table 4, SIA had much bigger contributions of
56.3 % to Ca2+ and 27.7 % to K+. We found that there were
strong relationships between SO42- (NO3-) and Ca2+
(K+) (Table 3). These suggest that SO42- and NO3-
changed source attribution of some alkaline ions to transport to open ocean.
In this case, it is difficult to distinguish the Ca2+ and K+
sources using PMF model.
The fourth source is oceanic emissions, which released NOx, NH3 and
oxidation of dimethyl sulfide (Altieri et al., 2014; Jickells et al., 2003; Boreddy and Kawamura,
2015; Phinney et al., 2006), with respective contributions of 1.6 % to
NO3-, 60.5 % to NH4+ and 6.9 % to SO42-
at Yongxing Island (Table 4). As shown in Fig. 11, air masses with high
NH4+ concentrations were from remote open oceans such as the
southeastern and northeastern SCS. There were relatively higher
NH4+ concentrations in the cool season and lower values in the
warm season at Yongxing Island (Fig. 6). These suggest that it is feasible
for the ocean to be a NH4+ source. Substantial NHx may be
released from degraded organic nitrogen-containing compounds and excretion
from zooplankton in the ocean (Norman and Leck, 2005). Altieri et al. (2014)
suggested that the efficient kinetics of ammonia evasion from surface
seawater causes NH3 to accumulate in the marine atmosphere. The
contribution of oceanic emission to NH4+ was much larger at
Yongxing than at global marine atmospheric NHx sources in the review of
Duce et al. (2008), which showed 87.5 % from anthropogenic sources.
However, Altieri et al. (2014) found that the anthropogenic contribution was
< 87.5 % at Bermuda, an island in the North Atlantic Ocean (Fig. 5).
Atmospheric NHx is usually rapidly deposited near source regions
and has a short residence time, about several hours in the marine boundary
layer (Boreddy and Kawamura, 2015; Xiao et al., 2012a; Xiao and Liu, 2002).
Thus, NHx transportation from continental to remote sea sites is
limited. Therefore, NH4+ in aerosol at Yongxing Island was
possibly from oceanic emission, as being reported at other marine sites
(Altieri et al., 2014; Jickells et al., 2003). DMS is the most abundant
marine biogenic volatile sulfur emitted from the ocean surface to the
atmosphere, and can be oxidized to SO42- in the marine atmosphere
(Phinney et al., 2006). Yang et al. (2015) reported that biogenic
SO42- from the Bohai and northern Yellow seas near China was
0.114–0.551 µg m-3, with an average of 0.247 µg m-3,
accounting for 1.4 % of NSS SO42-. Biogenic SO42- in
the northern SCS was ∼ 1.2 and 0.6 µg m-3 in summer
and winter, respectively (Zhang et al., 2007), constituting ∼ 8 and 12 % of NSS SO42-.
CWTs for SO42- in Fig. 11, also show some SO42- were from marine biogenic source. Thus,
natural sources had large contributions to marine atmospheric aerosols over
the northwestern Pacific.
Figure 10 illustrates the time series of daily concentrations contributed by
each identified source. In order to examine if the results are reasonable,
we compared the modeled results of each source with the observed seasonal
variations of the specific chemical species (Figs. 6 and 11). As shown in
Fig. 9, the highest and lowest contributions of sea salt were in
November–December and April–May, respectively, which are consistent with the
seasonal variations of aerosol Na+, Cl- and Mg2+. For crust,
the resulting time series show that it has a higher concentration in March 2014
and February 2015, relatively close to the observed values of
Ca2+ and K+ (Figs. 6 and 11). SIA has higher contributions from
November 2014 to February 2015 and lower other months, which is also
consistent with the observed and CWTs data of SO42- and
NO3- shown in Figs. 6 and 11. This result is definitely related to
the photochemistry that accounts for SIA formation (Zhang et al., 2013). The
formed SIA species may not appear in their original emission sources (Zhang
et al., 2013), such as coal combustion, biomass burning and crust. It is
obvious that oceanic emissions have the highest contribution in March, and
secondary highest in October, consistent with the seasonal change of
NH4+. The PMF and CWT modeled results seem to be promising
because the corresponding time series of each source's contribution are very
consistent with the observations.