Introduction
Particulate air pollution is one of the most important environmental issues
due to its severe impact on visibility and air quality, and has been a great
issue over East Asia, particularly in China (Zhang and Cao, 2015; Cui et al.,
2015). On the other hand, its impacts on not only climate but also public
health may be more severe and intricate (Pöschl, 2005; Menon et al.,
2002). Carbonaceous aerosols are ubiquitous in the Earth's atmosphere and
potentially cause harmful effect on human health (Bond et al., 2013;
Kanakidou et al., 2005; Ramanathan and Carmichael, 2008; Fatima et al., 2012;
Chung and Seinfeld, 2002). They are traditionally divided into two fractions:
organic carbon (OC), which contains less volatile and more reflective
species, and elemental carbon (EC; alternatively referred to as black carbon,
BC), which is the least reflective and most light-absorbing component
(Pöschl, 2005). However, the role of OC on cooling or warming has been a
matter of debate (Chung et al., 2012; Cazorla et al., 2013) because a class
of OC (brown carbon) may absorb sunlight (Feng et al., 2013; Lu et al., 2015;
Laskin et al., 2015; Bahadur et al., 2012). In the ambient atmosphere,
however, these two fractions (EC and OC) are mixed and consequently
complicate the estimation of net radiative forcing (Jacobson, 2001).
Therefore, studying carbonaceous aerosols and their sources is essential to
understand how the different sources of carbonaceous particles may influence
the radiative balance on a regional and global scale.
The major sources of carbonaceous aerosols are fossil fuel and biomass
burning in addition to the atmospheric oxidation of anthropogenic and
biogenic volatile organic compounds (VOCs) (Chung et al., 2012; Szidat et
al., 2006). The global emission of organic aerosols (OAs) from biomass and
fossil fuel sources has been estimated at 45–80 and 10–30 Tg yr-1,
respectively (Scholes and Andreae, 2000). Due to the presence of polar
functional groups, particularly carboxylic acids, many organic compounds in
OA are water-soluble (Boreddy et al., 2016) and hence aid the particles
acting as cloud condensation nuclei (CCN) (Novakov and Penner, 1993;
Matsumoto et al., 1997; Asa-Awuku et al., 2009). According to the recent
report of the intergovernmental panel on climate change (IPCC, 2013), the
radiative forcing of BC and OA associated with fossil fuel and biofuel
combustions is in the range of +0.05 to +0.8 (mean: +0.4) W m-2
and -0.4 to -0.1 (-0.12) W m-2, respectively. It is +0.0
(-0.2 to +0.2) W m-2 as a result of their change offset when BC
and OA are emitted by biomass burning (Boucher et al., 2013). Therefore,
carbonaceous aerosols have a net warming effect on the climate as per the
IPCC 2013 report. However, there still exist large uncertainties in
quantifying radiative forcing of carbonaceous aerosols, particularly with
regard to OA (Reddy and Boucher, 2004).
The atmosphere over East Asia is becoming worse due to not only the dense
population but also rapid urbanization/industrialization (Fu et al., 2012;
Cao et al., 2007). On a global scale, China has the largest carbonaceous
aerosol emissions from combustion with contributions of about 24 and 30 %
for OC and BC, respectively (Bond et al., 2004). Recently, Wang et al. (2016)
suggested that coal combustions and vehicular emissions are the dominating
sources of carbonaceous aerosols in China (Kirillova et al., 2014). Using the
emission estimated by Model of Emissions of Gases and Aerosols from Nature
(MEGAN) and combined with the MOdel of HYdrocarbon Emissions from the CANopy
(MOHYCAN), Stavrakou et al. (2014) reported an increased emission of biogenic
isoprene over Asia (0.16 % yr-1) with more pronounced trend over
China (0.52 % yr-1) during 1979–2012. In contrast, SO2
emissions over China have been declining since 2006 because of the wide usage
of flue-gas desulfurization (FGD) equipment in power plants (Lu et al., 2010,
2011). All these East Asian pollutants along with soil dust are transported
to the North Pacific via long-range atmospheric transport by westerly winds
and perturb the remote marine background conditions and the ocean
biogeochemistry by heterogeneous reactions (Boreddy et al., 2015; Matsumoto
et al., 2004). In addition to East Asian pollutants, the western North
Pacific also receives biomass burning emissions from Southeast Asia (Tsay et
al., 2016; Lin et al., 2013; Huang et al., 2013).
To better understand the long-range transport of Asian pollutants and their
atmospheric processing over the western North Pacific, we continuously
collect total suspended particulate (TSP) samples since 1990 at Chichijima
(Mochida et al., 2003; Kawamura et al., 2003; Boreddy and Kawamura,
2016). Chichijima is a remote marine island in the western North Pacific,
which is located in the outflow region of East Asian pollutants and dust
during the westerly wind season and in the pristine air masses under the wind
regime of easterlies. This island is about 1000 km south of Tokyo, Japan, and
2000 km from the East Asian countries (China) as shown in Fig. 1. Therefore,
the observation at Chichijima is useful for studying the long-range
transport of East Asian pollutants and their heterogeneous chemistry over the
western North Pacific (Boreddy et al., 2014; Verma et al., 2015). In this study, we discuss the long-term trends in the concentrations
of carbonaceous aerosols (EC, OC, and water-soluble organic carbon (WSOC))
and their ratios during 2001–2012 in addition to seasonal variations. The
role of photochemical oxidation of anthropogenic and biogenic VOCs on OC and
WSOC and their relations to CCN are also discussed.
Location of sampling site (indicated by red star) in the
western North Pacific and its adjacent Asian countries.
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Instrumentation and data analyses
Sampling site and aerosol collection
Figure 1 shows the location of the sampling site and its adjacent Asian
countries in the western North Pacific. TSP samples were collected at the
Satellite Tracking Centre of the Japan Aerospace Exploration Agency (JAXA,
elevation: 254 m) in Chichijima (27∘04′ N,
142∘13′ E) on a weekly basis (Boreddy and Kawamura, 2015). Aerosol
samples are collected on pre-combusted (450∘ C for 3–5 h) quartz
filter (20 cm × 25 cm, Pallflex 2500QAT-UP) using a high-volume air
sampler (HVS) with a flow rate of 1 m3 min-1. The HVS was
installed at a height of 5 m above the ground level. The filters were placed
in a pre-baked (450 ∘C for 6 h) glass jar (150 mL) with a
Teflon-lined screw cap before sample collection. After aerosol collection,
the filters were recovered into the glass jar, transported to the laboratory
in Hokkaido University, Sapporo, and stored in a freezer room at
-20 ∘C prior to analysis. A total of 545 aerosol samples and about
56 field blank samples were used for the analysis of carbonaceous components
during 2001–2012.
Analyses of carbonaceous aerosols
Concentrations of OC and EC were determined using a Sunset Laboratory carbon
analyzer following the IMPROVE (Interagency Monitoring of Protected Visual
Environments) thermal–optical evolution protocol (Wang et al., 2005),
assuming carbonate carbon (CC) in the aerosol samples to be insignificant
(Chow and Watson, 2002). Previous studies have also shown that carbonate,
particularly calcium carbonate, levels were low or negligible in most ambient
samples, which were analyzed using the IMPROVE protocol (Wang et al., 2005; Clarke
and Karani, 1992; Chow et al., 2001). A filter cut of 1.54 cm2 of each
filter was placed in a quartz tube inside the thermal desorption chamber of
the analyzer and then stepwise heating was applied. Helium (He) gas was
applied in the first ramp and was switched to mixture of He / O2 in
the second ramp. The evolved CO2 during the oxidation at each
temperature step was measured by non-dispersive infrared (NDIR) detector
system. The calculated detection limits of OC and EC were 0.05 and
0.02 µgC m-3, respectively. The sum of OC and EC was
considered to as total carbon (TC) in this study.
To determine WSOC, a punch of 20 mm in diameter of each filter was extracted
with 20 mL organic-free ultrapure water (> 18.2 MΩ cm,
Sartorius arium 611 UV) and ultrasonicated for 30 min. These extracts were
passed through a disk filter (Millex-GV, 0.22 µm pore size,
Millipore) to remove the filter debris and insoluble particles and analyzed
using a total organic carbon (TOC) analyzer (Shimadzu, TOC-Vcsh) equipped
with a catalytic oxidation column and non-dispersive infrared detector
(Miyazaki et al., 2011).
Concentrations of water-soluble methanesulfonate (MSA-), non-sea-salt
sulfate (nss-SO42-), non-sea-salt potassium (nss-K+), and sodium
(Na+) were taken from the study of Boreddy and Kawamura (2015), in order
to support the inferences related to carbonaceous species over the western
North Pacific, which were determined using ion chromatography (761 Compact
IC, Metrohm, Switzerland).
The analytical errors in the replicate analyses were less than 10 % for
OC, EC, and WSOC in this study. The concentrations of carbonaceous aerosols
reported in this study were corrected for field blanks. The levels of blanks
were less than 5 % for all the parameters in real samples.
Seven-day daily air mass back trajectories at 500 m a.g.l. computed
using HYSPLIT model for each month during 2001–2012 at Chichijima in
the western North Pacific. The star symbol indicates the sampling site and red
dots represent the MODIS inferred fire spots. Fire spots were downloaded for
the
region (80–150∘ E, -10–70∘ N) during the year 2001.
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Statistical analyses
Two statistical approaches were used to better conduct the trend analyses in
time series of WSOC, EC, and OC and their ratios during 2001–2012. First,
the tendency (linear trend) equation was used for each time series (Draper
and Smith, 1966). Second, all trends were assessed by using the Mann–Kendall
non-parametric test (Mann, 1945; Kendall, 1975), which is completely
independent of the first approach. More detailed information about these
statistical analyses are described in the Supplement.
Results and discussion
Air mass back trajectories and general meteorology
To better understand the influence of heterogeneity in air masses to
carbonaceous aerosols, we computed daily 7-day isentropic air mass back
trajectories at an altitude of 500 m for each month using a Hybrid Single
Particle Lagrangian-Integrated Trajectory (HYSPLIT) model (Draxler and Rolph,
2013) during 2001–2012 as shown in Fig. 2. We also investigated the MODerate
resolution Imaging Spectroradiometer (MODIS)-derived fire count data along
with the back trajectories to understand the intensity of biomass burning
over East Asia and South/Southeast Asia. Fire spot data were downloaded from
the MODIS website over the region (80 to 150∘ E, 10∘ S to
70∘ N) during the year 2001 as an example for all the years
(2001–2012) because of overlapping (there is no much difference in the
intensity and area of fire spots). More detailed information about the
monthly air mass back trajectories and fire data for each year during
2001–2012 are described elsewhere (Verma et al., 2015). From winter
(December–February) to spring (March–May), the air masses carry continental
air pollutants and dusts from East Asia to the sampling site in the Pacific
by a long-range atmospheric transport (Fig. 2). The continental air masses
are absent in summer (June to August) with the pristine air masses coming
from the central Pacific to the observation site. In autumn
(September–November), the air mass pattern shifts from southeasterly to
northwesterly with stronger winds towards winter.
Monthly mean (± standard deviation) values of EC, OC, WSOC
concentrations, and their ratios during 2001–2012 over the western North
Pacific.
Month
EC
OC
WSOC
OC / EC
WSOC / OC
nss-K+ / EC
(µg m-3)
(µg m-3)
(µg m-3)
January
0.18 ± 0.07
0.80 ± 0.41
0.54 ± 0.28
4.85 ± 2.01
0.69 ± 0.14
0.29 ± 0.16
February
0.25 ± 0.07
0.95 ± 0.36
0.55 ± 0.17
3.95 ± 1.31
0.63 ± 0.22
0.35 ± 0.39
March
0.28 ± 0.05
1.13 ± 0.37
0.59 ± 0.22
4.11 ± 1.19
0.56 ± 0.19
0.22 ± 0.09
April
0.22 ± 0.10
0.77 ± 0.32
0.48 ± 0.28
3.89 ± 1.37
0.62 ± 0.20
0.26 ± 0.12
May
0.14 ± 0.08
0.80 ± 0.31
0.35 ± 0.19
7.68 ± 4.11
0.44 ± 0.19
0.40 ± 0.27
June
0.08 ± 0.07
0.74 ± 0.35
0.30 ± 0.18
21.1 ± 30.4
0.44 ± 0.17
0.54 ± 0.36
July
0.06 ± 0.06
0.58 ± 0.35
0.22 ± 0.07
19.0 ± 16.7
0.44 ± 0.17
0.97 ± 0.94
August
0.04 ± 0.03
0.63 ± 0.27
0.27 ± 0.16
33.2 ± 52.5
0.46 ± 0.23
0.70 ± 0.69
September
0.05 ± 0.04
0.60 ± 0.26
0.20 ± 0.10
22.3 ± 17.3
0.38 ± 0.19
1.02 ± 0.82
October
0.08 ± 0.04
0.62 ± 0.18
0.27 ± 0.12
12.2 ± 9.07
0.45 ± 0.19
0.50 ± 0.43
November
0.15 ± 0.10
0.75 ± 0.39
0.42 ± 0.20
6.68 ± 4.89
0.61 ± 0.20
0.44 ± 0.26
December
0.18 ± 0.09
0.73 ± 0.29
0.39 ± 0.08
4.63 ± 1.65
0.59 ± 0.18
0.21 ± 0.12
Box-and-whisker plots of monthly variations of carbonaceous aerosol
components (µg m-3) and some specific mass ratios at
Chichijima in the western North Pacific during 2001–2012. The
horizontal line and small dot inside the box indicate median and mean,
respectively. The vertical hinges represent data points from the lower to the
upper quartile (i.e., 25th and 75th percentiles). The whiskers represent data
points from the 1st to 99th percentiles.
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Figure S1 in the Supplement shows the temporal variations of meteorological
parameters such as air temperature (∘C), relative humidity (%),
wind speed (m s-1), and precipitation (mm) at Chichijima during
the study period of 2001–2012. All the meteorological parameters were
downloaded from the Japan Meteorological Agency (JMA). There was a clear
seasonal variation in ambient temperature, relative humidity, and
precipitation with summer maxima and winter minima. Wind speeds were higher
in winter to spring and lower in summer.
Monthly/seasonal variations
Figure 3a–f present the monthly/seasonal variations in the concentrations
of EC, OC, WSOC, and some specific mass ratios at Chichijima in the
western North Pacific during 2001–2012. The corresponding statistical data
were reported in Table 1. All measured species (EC, OC, and WSOC) clearly
showed winter-to-spring maxima (highest concentration was in March) and
summer minima (lowest in July) and then increase towards autumn. The seasonal
variation in carbonaceous aerosols observed in this study was found
consistent with the typical seasonal pattern in ambient carbonaceous aerosols
over China (X. Y. Zhang et al., 2008; Cao et al., 2006), indicating a common
source for these components, which are long-range-transported to the western
North Pacific. This, of course, can also be influenced by seasonal
meteorology and air mass back trajectories over the western North Pacific as
discussed in Sect. 3.1.
Literature values of OC / EC ratios for various sources of
aerosol. Different font styles indicate different OC / EC values from different studies.
Source of aerosol
OC / EC ratio
References
Fossil fuel combustion
4.0, 4.1, 1.1
Koch (2001), Cao et al. (2005), Watson et al. (2001)
Coal combustion
2.7, 12.0
Watson et al. (2001), Cao et al. (2005)
Biomass burning
9.0, 60.3, 5–8
Cachier et al. (1989), Cao et al. (2005), Andreae and Merlet (2001)
Forest fire
∼ 16.0
Watson et al. (2001)
Diesel truck plume
0.06, 0.8, 0.3
Dallmann et al. (2014), Na et al. (2004), Turpin and Huntzicker (1995)
Gasoline vehicle
0.02, 2.2
Dallmann et al. (2014), Na et al. (2004)
Secondary organic carbon
3.3
Saarikoski et al. (2008)
Long-range transported/aged
12.0
Saarikoski et al. (2008)
Traffic
0.7
Saarikoski et al. (2008)
Cooking emissions
4.3–7.7
See and Balasubramanian (2008)
Relatively higher monthly average concentrations up to 0.28, 1.13, and
0.59 µg m-3 were observed for EC, OC, and WSOC in March. In
contrast, their monthly averages were lower in summer or early autumn (July
or September) with the concentrations of 0.04, 0.58, and
0.20 µg m-3, respectively (Table 1). It is well documented
that in summer, a maritime high-pressure wind dominates over the western
North Pacific in which the air masses are pristine and less influenced by the
continental outflow from East Asia (Fig. 2). This observation is consistent
with the fact that concentrations of anthropogenic nss-SO42-,
NO3-, NH4+, and nss-K+ showed similar seasonal
variations with winter and/or spring maxima and summer minima (Boreddy and
Kawamura, 2015). On the other hand, continental air masses blow from the
Asian continent in winter and spring; therefore, the maritime background
condition of the western North Pacific is often influenced by the continental
outflow via long-range atmospheric transport (Duce et al., 1980). Very low
concentrations of EC in summer, whose abundances were up to 7 times lower
than those in the continental outflow, suggest negligible contribution of
local anthropogenic emissions as well as long-range influences over the
sampling site. These results are consistent with previous studies, which
reported that several times lower concentrations of organic compounds in
summer compared to winter/spring over the same observation site (Kawamura et
al., 2003; Mochida et al., 2003). Therefore, it is reasonable to believe that
the sources of carbonaceous aerosols were transported from the adjacent Asian
countries to the western North Pacific via long-range atmospheric transport.
As described earlier, EC particles are primary and predominately come from
biomass and fossil fuel combustion sources. Conversely, OC is of either
primary origin or secondary formation via gas-to-particle conversion and
heterogeneous phase processing in the atmosphere. The precursors of secondary
OC may also come from biogenic sources in addition to fossil fuel combustion
and biomass burning emissions. The OC / EC ratios often used to
distinguish the relative contribution of primary vs. secondary sources as
well as biomass vs. fossil fuel burning sources (Turpin and Huntzicker, 1995;
Castro et al., 1999; Rastogi et al., 2016). Atmospheric aerosols emitted from
fossil fuel combustion are characterized by lower OC / EC ratios
(< 2.0), whereas higher OC / EC ratios (> 2.0) have been used to
point out the presence of secondary OA (SOA) (Cao et al., 2003; Chow et al.,
1996; Kunwar and Kawamura, 2014; Pani et al., 2017) in the atmosphere with a
limited impact of biomass burning. Table 2 summarizes OC / EC ratios
reported for various sources of aerosol particles. The split of OC / EC
ratios at higher or lower than 2 (from Table 2) may not be the best indicator
of SOA because fossil fuel combustion has OC / EC of 1.1 (Watson et al.,
2001), 4.0 (Koch et al., 2001), and 4.1 (Cao et al., 2005). Monthly mean
OC / EC ratios in this study are much larger in the summer and still
greater than the cutoff (∼ 4.0) in winter to spring as shown in
Table 1. This result suggests a dominance of SOA over the western North
Pacific. The seasonal variation of OC / EC mass ratios showed maxima in
summer (∼ 21 to 33) and minima in winter to spring (3.9 to 7.7). The
extremely high OC / EC ratios in summer indicate the secondary formation
of OC via oxidation processes, while low OC / EC ratios in winter to
spring suggests that both biomass burning and fossil fuel combustion
contribute to carbonaceous aerosols over the western North Pacific in
winter to spring.
It is well documented that nss-K+ and EC can be used as tracers for
biomass burning and fossil fuel combustion emissions, respectively.
Therefore, nss-K+ / EC ratios were widely used to better identify
major sources of carbonaceous aerosols (Wang et al., 2005; Rastogi et al.,
2016; Ram and Sarin, 2011). The higher nss-K+ / EC ratios
(> 0.20) indicate the dominance of biomass burning emissions, whereas
lower ratios (< 0.10) suggest the prevalence of fossil fuel combustion
emissions. In this study, higher nss-K+ / EC mass ratios were
observed in midsummer (July) to early autumn (September) (Fig. 3e),
suggesting an influence of biomass burning emissions from southeast Asian
countries via long-range atmospheric transport over the western North
Pacific. This point is consistent with the air mass back trajectory and
MODIS fire count data during summer months (Fig. 2), which clearly showed
that air masses were occasionally coming from Southeast Asia (Indonesia,
Malaysia and New Guinea, etc.) where biomass burning is a common phenomena
during summer to early autumn. Biomass burning products were transported to
the western North Pacific (Fig. 2). Verma et al. (2015) reported significant
concentrations of levoglucosan during summer in Chichijima (in the absence of
East Asian outflows), which were attributed to the occasional transport of
biomass burning influenced air masses from southeast Asia, as inferred from
the air mass trajectories and fire spot data during 2001–2013. Therefore,
carbonaceous aerosols over Chichijima strictly follow the seasonal wind
patterns in the western North Pacific.
Previous studies have shown that SOA is largely composed of oxygenated
compounds that are highly water-soluble (Kanakidou et al., 2005; Kondo et
al., 2007, and references therein). Thus, measurements of WSOC have been used
to estimate the SOA in ambient aerosols (Weber et al., 2007; Snyder et al.,
2009; Sudheer et al., 2015; Decesari et al., 2001; Docherty et al., 2008).
Because a major fraction of biomass burning products is highly water-soluble
(Sannigrahi et al., 2006; Saarikoski et al., 2008), higher WSOC / OC
ratios under ambient conditions with limited biomass burning impact have been
used to better understand the photochemical activity and/or aging of aerosols
and to discuss SOA formation mechanism in the atmosphere during long-range
transport (Miyazaki et al., 2007; Ram et al., 2010b; Ram and Sarin, 2011;
Kondo et al., 2007; Weber et al., 2007). The WSOC / OC ratios exceeding
0.4 have been used to indicate the significant contribution of SOA (Ram et
al., 2010a) and aged aerosols. The WSOC / OC ratios ranged from 0.06 to
0.19 in diesel particles (Cheung et al., 2009) and 0.27 for vehicular
emissions (Saarikoski et al., 2008).
In this study, we found that monthly mean WSOC / OC ratios were > 4.0
for all months except for September, indicating a significant contribution
from SOA over the western North Pacific. The seasonal variation of
WSOC / OC showed higher values (monthly mean: 0.44 to 0.62) during winter
to spring months (Fig. 3f), implying that SOA formation was enhanced due to
an increased photochemical activity and/or aging of East Asian polluted
aerosols during long-range atmospheric transport. The high WSOC / OC
ratios are traditionally attributed to the atmospheric oxidation of various
VOCs in the presence of oxidants such as ozone and hydroxyl radicals via gas-
and/or aqueous-phase reactions in the atmosphere (Miyazaki et al., 2007; Ram
and Sarin, 2012). However, the atmosphere over the western North Pacific is
always characterized by high relative humidity (> 80 %) and air
temperature (∼ 24 ∘C) during the whole year (Fig. S1).
Therefore, higher WSOC concentrations in winter to spring over the western
North Pacific were largely attributed to the aqueous-phase oxidation of
anthropogenic and/or biogenic VOCs (Youn et al.,
2013), which are emitted over continental East Asia and long-range-transported to the western North Pacific.
On the other hand, we found lower ratios of WSOC / OC in summer. This
result may suggest a minor contribution of water-soluble organic matter in
summer due to a negligible contribution of aged continental air masses and/or
significant contribution from marine biota. Based on the gradient flux
measurements, Ceburnis et al. (2008) found that water-insoluble organic
matter (WIOM) exhibited an upward flux, whereas water-soluble organic matter
(WSOM) exhibited a downward flux, suggesting a primary production for WIOM
and a secondary formation for WSOM. In this study, WIOM/WSOM ratios were
higher in summer (mean: 1.45 ± 0.17) and autumn (0.35 ± 0.57)
than in winter (0.19 ± 0.67) as shown in Fig. 4a. Higher ratios of
WIOM/WSOM in summer over the western North Pacific are consistent with an
idea that the ocean-derived organic matter is emitted from the ocean surface
via sea-to-air flux as a fresh (less aged) organic matter. This result is
further supported by the study of Miyazaki et al. (2010), who reported a
significant amount of WIOM in the western North Pacific during summer, which
may be produced by bubble-bursting processes at the ocean surface. Similarly,
Ovadnevaite et al. (2011) reported higher contributions of primary organic
matter to marine aerosols over the northeast Atlantic.
Monthly variations (a) WSIM / WSOM mass ratios and sea salt
concentrations and (b) regression analysis between water-insoluble organic carbon (WIOC) and sea salt
concentrations. The color scale in (a) indicates the wind speed
over the western North Pacific.
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Statistical report on the annual trends in carbonaceous aerosols
and their ratios during 2001–2012 at Chichijima in the western North
Pacific.
Species
Concentrations (µg m-3)
Mann–Kendall non-parametric test
Min
Max
Mean
SD
Kendall's tau (τ)
p value
Sen's slope
EC
0.00
0.36
0.14
0.10
-0.06
> 0.05
-0.0002
OC
0.26
1.70
0.76
0.36
0.07
> 0.05
0.0008
TC
0.28
2.01
0.90
0.43
0.05
> 0.05
0.0007
WSOC
0.08
1.30
0.38
0.22
0.09*
> 0.05
0.0006
OC / EC
1.91
67
9.74
21.9
0.21*
< 0.05
0.0240
WSOC / OC
0.06
0.94
0.53
0.21
0.09*
< 0.05
0.0007
OC / TC
0.66
1.00
0.85
0.08
0.21*
< 0.05
0.0007
EC / TC
0.00
0.34
0.15
0.08
-0.21
> 0.05
-0.0007
WSOC / TC
0.06
0.86
0.44
0.17
0.14*
< 0.05
0.0009
MSA-
0.00
0.05
0.02
0.01
0.08*
< 0.05
0.00002
nss-K+ / EC
0.02
2.97
0.51
0.40
0.09*
< 0.05
0.0009
“*” indicates that the trends are significant at p < 0.05
level.
Further, laboratory studies have revealed a high abundance of primary organic
matter dominated by WIOM in marine aerosols (Facchini et al., 2008; Keene et
al., 2007). However, it should be noted that although bubble-bursting process
is a common source for both sea salt (sea salt = 3.2 × Na+,
where 3.2 is the conservative mass ratio of salinity to Na in seawater; data
obtained from Boreddy and Kawamura, 2015) and WIOM in marine aerosols, we
found a negative/no correlation (r=-0.22) between sea salt and
water-insoluble organic carbon (WIOC)
concentrations in summer (Fig. 4b). This result suggests an additional
source of organic matter (completely independent of sea salt production and
wind speed) which is evidenced by the higher
MSA- / nss-SO42- mass ratios (Boreddy and Kawamura, 2015)
and higher concentrations of azelaic acid (Boreddy et al., 2017) during
summer and autumn. MSA- / nss-SO42- mass ratios have been
suggested as an indicator for the relative contribution of oceanic dimethyl
sulfide (DMS) vs. anthropogenic sources to sulfate (Gondwe et al., 2004;
Savoie and Prospero, 1989). Higher ratios indicate that nss-SO42- is
derived from the atmospheric oxidation of oceanic DMS, while lower ratios
suggest the anthropogenic contribution of SO2. On the other hand,
azelaic acid is a specific photochemical oxidation product of unsaturated
fatty acids emitted from the ocean surface (Kawamura and Sakaguchi, 1999) and
also found in biomass burning plumes (Graham et al., 2002). Therefore, it is
worthy to note that, although marine biogenic sources are major contributors
to organic matter during summer to autumn, there are some influence from
non-marine sources (for example, transport of biomass burning products from
Southeast Asia as suggested by higher ratios of nss-K+ / EC in
summer), mixed with marine sources.
Annual trends (time series) in the concentrations (µg m-3) of
carbonaceous aerosol components, water-soluble ionic tracer compound (MSA-)
and some specific mass ratios during 2001–2012 over the western North
Pacific. The linear trend equation (y=mx+c) is also shown for the each annual
trend.
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Annual trends
Figure 5 shows the annual trends in the concentrations of EC, OC, TC
(EC+OC), WSOC, and WSOC / OC ratios during the period of 2001–2012
over the western North Pacific (see Fig. S2 for annual mean variations).
Table 3 summarizes the results of the statistical analyses. All the annual
trends of chemical species and WSOC / OC ratios seem to present clear
seasonal patterns with higher values in winter–spring and lower values in
summer. On the other hand, seasonal variations of the OC / EC and
nss-K+ / EC ratios showed higher values in summer.
As seen from Fig. 5a–b and Table 3, concentrations of EC, OC, and TC during
2001–2012 ranged from 0.001 to 0.36 µg m-3 (mean:
0.142 µg m-3), 0.25 to 1.7 µg m-3
(0.76 µg m-3) and 0.28 to 2.01 µg m-3
(0.90 µg m-3), respectively. The annual variations of EC
showed a decreasing trend (-0.007 % yr-1), while OC and TC trends
are continuously increasing (+0.16 and
+0.11 % yr-1, respectively) from 2001 to 2012 although the rates
were not significant (p>0.05). However, the annual trends of OC / EC
and OC / TC ratios increased significantly (p<0.05;
+0.46 and +0.06 % yr-1) from 2001 to 2012
(Fig. 5d and e), suggesting that the contribution of primary fossil fuel
combustion to carbonaceous aerosols has declined during the sampling period.
This point is supported by the annual trend of nss-K+ / EC mass
ratios, which showed a significant increase (p<0.05;
+0.33 % yr-1) during the sampling period (Fig. 5g). This
observation is consistent with the study of Verma et al. (2015), who observed
a significant enhancement of levoglucosan (a good biomass burning tracer,
e.g., Simoneit, 2002) during 2006–2013 over the sampling site. Therefore,
all these results demonstrate that the contributions of biomass burning
emissions to carbonaceous aerosols have increased significantly over the
western North Pacific whereas the contributions of fossil fuel combustion
have decreased.
Previous studies suggested that SOA is largely composed of water-soluble
organic matter (Weber et al., 2007; Kondo et al., 2007). In this study, the
annual trend of WSOC showed a significant increase (p < 0.05;
+0.18 % yr-1) from 2001 to 2012 (Fig. 5c). Generally, atmospheric
aging makes aerosols more water-soluble during long-range transport (Aggarwal
and Kawamura, 2009; Rudich et al., 2007; Robinson et al., 2007; Jimenez et
al., 2009; Kawamura et al., 2010), especially in the remote marine atmosphere
(Kawamura et al., 2003). This point is further supported by a decadal
increase (+0.08 % yr-1) in the WSOC / OC ratios (Fig. 5f).
These results may demonstrate that the increased concentrations of WSOC over
the western North Pacific are significantly linked with increased
photochemical aging of organic aerosols and oxidation of various VOCs during
long-range atmospheric transport (Zhang et al., 2007; Decesari et al., 2010).
An increasing trend of WSOC / TC (p < 0.05;
+0.15 % yr-1; Table 3) again suggests that photochemical
formation of WSOC and its contributions to SOA have increased over the
western North Pacific during 2001–2012. We observed an abrupt decrease in
the WSOC / OC ratios between 2007 and 2008 (Fig. 5f), probably due to
enhanced OC that may be caused by unknown sources. However, it should be
noted that an observed decline in the WSOC / OC ratios does not affect
the decadal trend even if those data are excluded from the trend analysis.
To better understand the contributions of photochemical oxidation of biogenic
VOCs to WSOC during long-range atmospheric transport, we present the annual
trend of water-soluble organic ion, i.e., MSA- (a biogenic tracer; see
Fig. 5h). In our previous study (Boreddy and Kawamura, 2015), we reported
that MSA- significantly correlates with continental pollutants such as
NH4+ (r=0.56), nss-K+ (0.52) and nss-SO42- (0.50) and
no correlation with Na+, suggesting that continentally derived MSA-
may be associated with the terrestrial higher plants and other biogenic
sources along with Asian pollutants during the long-range transport. However,
we should not ignore the oceanic biogenic emissions, especially in summer
(Bikkina et al., 2014), although it has less abundance compared to
continental biogenic emissions over the western North Pacific. In this study,
the annual trend of MSA- showed a significant increase (p < 0.05;
+0.14 % yr-1) during 2001–2012, implying that continental
transport of biogenic VOCs (BVOCs) over the western North Pacific have
increased significantly during 2001–2012.
Regression analyses between (a) WSOC and MODIS-derived cloud
condensation nuclei (CCN), (b) sea salt and CCN, (c) WSOC + sea salt and CCN,
and (d) WSOC and sea salt concentrations over the western North Pacific.
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Zhang et al. (2016) reported an increase (from 132 000 to
175 000 t yr-1) in the emission of isoprene in northern China during
1982–2010 using an emission model. Based on strong correlations (r>0.90) between isoprene and above-canopy temperature, they
suggested that oxidations of biogenic BVOCs from the terrestrial higher
plants are important in Asia (especially in China). Since Chichijima is an
outflow region of East Asia, long-range atmospheric transport of BVOCs may be
possible from terrestrial higher plants in Asia/China to the western North
Pacific by westerly winds, which may significantly contribute to the enhanced
trends of OC and WSOC during 2001–2012. We found significant (p<0.05)
increases in the annual trends of methylglyoxal and pyruvic acid, which are
tracers of aqueous-phase oxidation of biogenic isoprene (Carlton et al.,
2009), over the western North Pacific as shown in Fig. S3. We also found a
moderate correlation (r=0.40, p<0.01) between of MSA- and WSOC
concentrations (not shown as a figure). These results demonstrate that the
increase in WSOC is likely due to the increased photochemical oxidation of
BVOCs during long-range transport over the western North Pacific in addition
to the other emissions such as biomass burning.
Atmospheric implications
It is well known that atmospheric aerosols play a key role in the climate
system as they can act as cloud condensation nuclei (CCN) and impact on cloud
formation and thus radiative forcing (RF) (IPCC, 2013). The RF of aerosol is
generally estimated by using the aerosol optical depth (AOD), single-scattering albedo (SSA) and asymmetry parameter (Pani et al., 2016a). EC
scatters the shortwave incoming solar radiation less than OC, although it
strongly absorbs the shortwave solar radiation as well as longwave outgoing
terrestrial radiation in the atmosphere (Charlson et al., 1992; Ramanathan et
al., 2001; Magi, 2009, 2011). The SSA, defined as
the ratio of scattering to the extinction coefficient of aerosols, is an
important property for determining the direct RF (Pani et al., 2016a, b). The
SSA is highly sensitive to the nature (scattering and/or absorption) of
aerosols in the atmosphere. Therefore, although OC has certain uncertainty
because of light-absorbing brown carbon, OC / EC ratios can be used to
understand the relative contributions of scattering or absorbing aerosols in
the atmosphere.
Further, a good knowledge of the OC / EC ratios in aerosols (for example,
biomass burning) may also help to improve model representation of the
absorption caused by organic compounds constituting so-called brown carbon,
which contributes to the aerosol RF (Chung et al., 2012; Saleh et al., 2014;
Kirchstetter and Thatcher, 2012). In this study, atmospheric aging may make
OC more scattering during long-range transport over the western North
Pacific. An increasing trend of OC / EC ratios suggests that scattering
aerosols are significantly increased over the western North Pacific. In
contrast, absorbing aerosols may be decreased during the study period. This
result may provide an important implication for radiative forcing because
scattering and absorption coefficients are playing crucial role in the
radiative forcing calculations as mentioned above.
Novakov and Corrigan (1996) found that pure organic components from biomass
smoke emissions can form cloud condensation nuclei (CCN) without the presence
of sulfate (SO42-) and other inorganic compounds. Roberts et
al. (2002) showed that biomass-burning-derived organic aerosols do serve as
CCN. Further, large loadings of CCN in continental air masses were observed
over the western North Pacific (Matsumoto et al., 1997; Boreddy et al.,
2015). In this study, the enhanced WSOC concentrations and WSOC / OC
ratios in continental air masses suggest an important role of WSOC in CCN
activity over the western North Pacific in addition to other aerosol
constituents such as SO42- and sea salts. To better understand the
impact of WSOC on cloud-forming potential, we performed regression analyses
between WSOC, sea salt and CCN concentrations as shown in Fig. 6. CCN data
were downloaded from the MODIS satellite over the region
(140–145∘ E, 25–30∘ N) in the western North Pacific for
the period of July 2002 to December 2012.
The above results showed significantly good correlations (r=0.61 and 0.64,
p<0.05) between WSOC versus CCN and sea salt versus CCN concentrations
(Fig. 6a and b), suggesting the importance of WSOC for the formation of CCN
over the western North Pacific in addition to sea salt. Further, the
correlation coefficient between sea salt and CCN concentrations was slightly
increased (r=0.68; p<0.05) when WSOC was added to the sea salt as shown
in Fig. 6c. Likely, the slope of the regression line between WSOC + sea
salt and CCN was little higher (2.21E7) than the slope between sea salt and
CCN (2.19E7). These results indicate that WSOC may slightly enhance the cloud-forming potential of sea salt, although it has less concentration over the
western North Pacific. All these results suggest that a significant
uncertainty exists in RF due to the contribution of water-soluble organic
matter to cloud forming. Therefore, climate modelers should consider WSOC in
addition to other factors (sea salts, sulfate, etc.), while calculating RF
over the western North Pacific. This point is consistent with the previous
studies, which explain the contribution of water-soluble organic matter to
CCN (Matsumoto et al., 1997; Zhao et al., 2016).
It should be noted that all these ratios are applicable to organic fractions
that are derived from the bulk parameters only; however, the size of particles also plays a role in RF as well as their morphology, chemical
composition and mixing state (Jacobson, 2001; Lohmann and Feichter, 2005;
R. Zhang et al., 2008). Although fine particles are important for CCN
activation, physico-chemical processes (coagulation, condensation and other
heterogeneous reactions) can make the particles from fine to coarse mode in
aqueous phase, particularly over the marine atmosphere. Thus, bulk parameters
of organic matter and its role in CCN activation are important in the remote
marine atmosphere. Sea spray is not a major source of WSOC as inferred from
Fig. 6d, which showed a moderate correlation (r=0.42; p>0.05) between
WSOC and sea salt during the study period. In this study, atmospheric
processes or chemical aging makes OC more water-soluble during long-range
transport over the western North Pacific as discussed in Sect. 3.2.