Dicarboxylic acids, oxoacids, benzoic acid,  -dicarbonyls, WSOC, OC, and ions in spring 1 aerosols from Okinawa Island in the western North Pacific Rim: Size distributions and 2 formation processes

Size-segregated aerosols (9-stages from <0.43 to >11.3 µm in diameter) were collected at Cape 10 Hedo, Okinawa in spring 2008 and analyzed for water-soluble diacids (C 2 -C 12 ),  -oxoacids (  C 2 - 11  C 9 ), pyruvic acid, benzoic acid and  -dicarbonyls (C 2 -C 3 ) as well as water-soluble organic carbon 12 (WSOC), organic carbon (OC) and major ions (Na + , NH 4+ , K + , Mg 2+ , Ca 2+ , Cl - , NO 3- , SO 42- , and 13 MSA - ). In all the size-segregated aerosols, oxalic acid (C 2 ) was found as the most abundant species 14 followed by malonic and succinic acids whereas glyoxylic acid (  C 2 ) was the dominant oxoacid 15 and glyoxal (Gly) was more abundant than methylglyoxal. Diacids (C 2 -C 5 ),  C 2 and Gly as well as 16 WSOC and OC peaked at 0.65-1.1 µm in fine mode whereas azelaic (C 9 ) and 9-oxononanoic (  C 9 ) 17 acids peaked at 3.3-4.7 µm in coarse mode. Sulfate and ammonium are enriched in fine mode 18 whereas sodium and chloride are in coarse mode. Strong correlations of C 2 -C 5 diacids,  C 2 and Gly 19 with sulfate were observed in fine mode ( r = 0.86-0.99), indicating a commonality in their 20 secondary formation. Their significant correlations with liquid water content in fine mode ( r = 0.82- 21 0.95) further suggest an importance of the aqueous-phase production in Okinawa aerosols. They 22 may have also been directly emitted from biomass burning in fine mode as suggested by strong 23 correlations with potassium ( r = 0.85-0.96), which is a tracer of biomass burning. The coarse mode 24 peaks of malonic and succinic acids were obtained in the samples with marine air masses, 25 suggesting that they may be associated with the reaction on sea salt particles. Bimodal size 26 distributions of longer-chain diacid (C 9 ) and oxoacid (  C 9 ) with a major peak in the coarse mode 27 suggest their production by photooxidation of biogenic unsaturated fatty acids via heterogeneous 28 reactions on sea salt particles.


Introduction
Tropospheric aerosol is an important environmental issue because it dramatically reduces the visibility (Jacobson et al., 2000;Kanakidou et al., 2005), affects the radiative forcing of climate (Seinfeld and Pandis, 1998), and causes a negative impact on human health (Pope and Dockery, 2006).All of these effects strongly depend on the abundances of aerosols and their chemical and physical properties in different sizes.Particles with diameters of 0.1-1.0µm are very active in scattering and absorbing incoming solar radiation and have a direct impact on climate (Ramanathan et al., 2001;Seinfeld and Pankow, 2003).The knowledge of size distributions of chemical components is thus essential to better understand their potential contributions to climate change and pollution control.Their size distribution also provides evidence for the sources and formation pathways of atmospheric particles.
The emission sources and multiple secondary formation pathways of organic aerosols are not well understood.Organic compounds account for up to 70 % of fine aerosol mass and potentially control the physicochemical properties of aerosol particles (Davidson et al., 2005;Kanakidou et al., 2005).Low-molecular-weight diacids are one of the most abundant organic compound classes in the at-Published by Copernicus Publications on behalf of the European Geosciences Union.D. K. Deshmukh et al.: Diacids and related compounds in spring aerosols from Okinawa Island mosphere (Kawamura and Ikushima, 1993;Kawamura et al., 1996;Kawamura and Bikkina, 2016).They are primarily derived from incomplete combustion of fossil fuel and biomass burning (Kawamura and Kaplan, 1987;Falkovich et al., 2005), and secondarily produced in the atmosphere via photooxidation of unsaturated fatty acids and volatile organic compounds (VOCs) from biogenic and anthropogenic sources (Kawamura and Gagosian, 1987;Kawamura et al., 1996;Sempéré and Kawamura, 2003).The ability of organic aerosols to act as cloud condensation nuclei seems to be closely related to their mass-based size distributions (Pradeep Kumar et al., 2003;Ervens et al., 2007).
The increasing atmospheric burden of organic aerosols is associated with natural and anthropogenic emissions in the continental regions.Organic aerosols are eventually transported to the oceanic regions.Rapid industrialization in East Asia is expected to have an important impact on global atmospheric chemistry over the next decades (Wang et al., 2013;Tao et al., 2013;Bian et al., 2014).Large amounts of coal and biomass burning in East Asia add more anthropogenic aerosols which alter the aerosol chemical composition in the remote Pacific atmosphere (Mochida et al., 2007;Miyazaki et al., 2010;Agarwal et al., 2010;Wang et al., 2011;Engling et al., 2013).Water-soluble diacids and related compounds as well as major ions have previously been studied for their size distributions in remote marine aerosols (Kawamura et al., 2007: Mochida et al., 2007;Miyazaki et al., 2010), whereas their size-segregated characteristics have not been studied in the western North Pacific Rim.
We collected size-segregated aerosol samples with nine size ranges in spring 2008 in Cape Hedo, Okinawa, in the western North Pacific Rim. Cape Hedo is located on the northern edge of Okinawa Island and can serve as a suitable site for the observation of atmospheric transport of East Asian aerosols with insignificant interference from local emission sources (Takami et al., 2007).The samples were analyzed for dicarboxylic acids (C 2 -C 12 ) and related compounds such as ω-oxoacids (ωC 2 -ωC 9 ), pyruvic acid (C 3 ), and α-dicarbonyls (C 2 -C 3 ) to better understand the sources and processing of water-soluble organic compounds at this marine receptor site.Size-segregated samples were also analyzed for water-soluble organic carbon (WSOC), organic carbon (OC), and major inorganic ions.The role of liquid water content of aerosol in the size distribution of diacids and related compounds is discussed.The potential factors responsible for their size distributions are also discussed.

Site description and aerosol collection
The geographical location of Okinawa Island (26.87 • N and 128.25 • E) and its surroundings in East Asia are shown in Fig. 1.Okinawa is located in the outflow region of con- tinental aerosols and on the pathways to the Pacific.Cape Hedo has been used as a supersite of the Atmospheric Brown Clouds project to study the atmospheric transport of Chinese aerosols and their chemical transformation during long-range transport from East Asia (Takiguchi et al., 2008;Kunwar and Kawamura, 2014).The sampling site at Cape Hedo is about 60 m a.s.l.
Size-segregated aerosol samples were collected at Cape Hedo Atmosphere and Aerosol Monitoring Station (CHAAMS) from 18 March to 13 April 2008.This period was characterized by westerly wind in the lower troposphere, which is the principal process responsible for the transport of both fossil fuel combustion and biomass burning aerosols in East Asia to the western North Pacific.9-Stage Andersen middle volume impactor (Tokyo Dylec Company, Japan; 100 L min −1 ) was used for the collection of size-segregated samples.The sampler was equipped with quartz fiber filters (QFFs, 80 mm in diameter) that were precombusted at 450 • C for 6 h in a furnace to eliminate the adsorbed organic compounds.A total of five sets (OKI-1 to OKI-5) of size-segregated aerosol samples were collected.Each sample set consists of nine filters for the sizes of < 0.43, 0.43-0.65,0.65-1.1,1.1-2.1, 2. 1-3.3, 3.3-4.7, 4.7-7.0, 7.0-11.3, and > 11.3 µm.The filter was placed in a preheated 50 mL glass vial with a Teflon-lined screw cap and stored in a freezer at the station.The samples were stored in darkness at −20 • C prior to analysis in Sapporo.One set of field blanks was collected by placing a precombusted QFF into the sampler for 30 s without sucking air before installing the real QFF.

Analytical procedures
Diacids and related compounds were determined by the method of Kawamura and Ikushima (1993), and Kawamura (1993).Aliquot of the filters was extracted with organic-free ultrapure water (specific resistivity > 18.2 M cm) under ultrasonication.The extracts were passed through a glass column packed with quartz wool to remove insoluble particles and filter debris.The extracts were concentrated using a rotary evaporator under vacuum and derivatized to dibutyl esters and dibutoxy acetals with 14 % BF 3 in n-butanol at 100 • C. Acetonitrile and n-hexane were added into the derivatized sample and washed with organicfree pure water.The hexane layer was further concentrated using a rotary evaporator under vacuum and dried to almost dryness by N 2 blowdown and dissolved in 100 µL of n-hexane.2 µL of the sample was injected into a capillary gas chromatograph (GC) (Hewlett-Packard HP6890) equipped with a flame ionization detector.Authentic diacid dibutyl esters were used as external standards for the peak identification and quantification.Identifications of diacids and related compounds were confirmed by GC-mass spectrometry.Recoveries of authentic standards spiked to a precombusted QFF were 85 % for oxalic acid (C 2 ) and more than 90 % for malonic to adipic (C 3 -C 6 ) acids.The detection limits of diacids and related compounds were ca.0.002 ng m −3 .The analytical errors in duplicate analyses are within 10 % for major species.
To measure water-soluble organic carbon (WSOC), a punch of 20 mm diameter of each QFF was extracted with organic-free ultrapure water in a 50 mL glass vial with a Teflon-lined screw cap under ultrasonication for 15 min.The water extracts were subsequently passed through a syringe filter (Millex-GV, Millipore; diameter of 0.22 µm).The extract was first acidified with 1.2 M HCl and purged with pure air in order to remove dissolve inorganic carbon and then WSOC was measured using a total organic carbon (TOC) analyzer (Shimadzu TOC-V CSH ) (Miyazaki et al., 2011).External calibration was performed using potassium hydrogen phthalate before analysis of WSOC.The sample was measured three times and the average value was used for the calculation of WSOC concentrations.The analytical error in the triplicate analysis was 5 % with a detection limit of 0.1 µgC m −3 .
Organic and elemental carbon (OC and EC) was determined using a Sunset Lab carbon analyzer following the Interagency Monitoring of Protected Visual Environments (IMPROVE) thermal evolution protocol as described by H. Wang et al. (2005).A filter disc of 1.5 cm 2 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/O 2 in the second ramp.The evolved CO 2 during the oxidation at each temperature step was measured with a non-dispersive infrared detector system.The detection limits of OC and EC were ca.0.05 and 0.02 µgC m −3 , respectively.The analytical errors in the triplicate analysis of the filter sample were estimated to be 5 % for OC and EC.EC was detected only in fine fractions.The concentration of total carbon (TC) was calculated by summing the concentrations of OC and EC in each size fraction.
For the determination of major ions, a punch of 20 mm diameter of each filter was extracted with organic-free ul-trapure water under ultrasonication.These extracts were filtered through a disc filter (Millex-GV, Millipore; diameter of 0.22 µm) and injected into an ion chromatograph (Compact IC 761; Metrohm, Switzerland) for measuring MSA − , Cl − , SO 2− 4 , NO − 3 , Na + , NH + 4 , K + , Ca 2+ , and Mg 2+ (Boreddy and Kawamura, 2015).Anions were separated on a SI-90 4E Shodex column (Showa Denko; Tokyo, Japan) using a mixture of 1.8 mM Na 2 CO 3 and 1.7 mM NaHCO 3 solution at a flow rate of 1.2 mL min −1 as an eluent and 40 mM H 2 SO 4 for a suppressor.A Metrosep C2-150 Metrohm column was used for cation analysis using a mixture of 4 mM tartaric acid and 1 mM dipicolinic acid solution as an eluent at a flow rate of 1.0 mL min −1 .The injected loop volume was 200 µL.The detection limits for anions and cations were ca.0.1 ng m −3 .The analytical error in duplicate analysis was about 10 %.
Field blanks were extracted and analyzed like the real samples.However, blank levels were 0.1-5 % of real samples.The reported concentrations of organic and inorganic species were corrected for the field blanks.All the chemicals including authentic standards were purchased from Wako Pure Chemical Co.(Japan), except for 14 % BF 3 /n-butanol (Sigma-Aldrich, USA).

Backward air mass trajectories and meteorology
The backward trajectories of air masses were computed for the sampling period using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model 4.0 developed by the National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory (ARL) (Draxler and Rolph, 2013).The 7-day trajectories at 500 m above the ground level for the samples collected on Okinawa are shown in Fig. 2. Typical air mass trajectories corresponding to 09:00 UTC for the samples collected on Okinawa are shown in Fig. S1 in the Supplement.

Estimation of liquid water content (LWC) of aerosol
LWC of aerosol was calculated for the size-segregated samples collected on Okinawa Island using the ISORROPIA II model (Fountoukis and Nenes, 2007).ISORROPIA II is a computationally efficient and rigorous thermodynamic equilibrium model that exhibits robust and rapid convergence under all aerosol types with high computational speed ( Nenes et al., 1998).ISORROPIA II implies the Zdanovskii-Stokes-Robinson equation and treats only the thermodynamics of K + -Ca 2+ -Mg 2+ -NH + 4 -Na + -SO 2− 4 -NO − 3 -Cl − -H 2 O aerosol system to estimate the LWC.Therefore, the measured organic species such as diacids and related compounds are not included in ISORROPIA II.The model was run as a "reverse problem", in which temperature, relative humidity, and aerosol phase concentrations of water-soluble inorganic ions were used as input for the estimation of aerosol LWC.

Size-segregated aerosol chemical characteristics
We use 2.1 µm as a split diameter between the fine-and coarse-mode particles.Table 1 presents the concentrations of inorganic and carbonaceous species in the fine-and coarsemode aerosols.Abundances of organic matter (OM) in the atmosphere are generally estimated by multiplying the measured OC mass concentrations with the conversion factor of 1.6 for urban aerosols and 2.1 for aged aerosols (Turpin and Lim, 2001).CHAAMS is located in the outflow region of East Asian aerosols and local anthropogenic activities are insignificant.Because the aerosols reaching Okinawa undergo the atmospheric oxidation during the long-range transport, the fraction of oxygenated organic species is often high (Takami et al., 2007;Irei et al., 2014;Kunwar and Kawamura, 2014).Therefore, we used the conversion factor of 2.1, instead of 1.6 for the calculation of OM.
OM was more enriched in fine-size fractions than in the coarse-size fractions (Table 1).The elevated level of OM in fine fractions on Okinawa suggests a substantial contribu- tion of organic aerosols primarily from combustion sources, and secondarily from photochemical processes during longrange atmospheric transport.The OM in fine-mode aerosol on Okinawa may consist of oxygenated organic compounds such as diacids, ω-oxoacids, and α-dicarbonyls.Okinawa was strongly affected by long-range transport of continental air masses from Siberia and Mongolia as well as North China and Korea (Fig. 2).It is difficult to specify the source regions of air masses for each sample set because the sampling duration was 3-5 days.Each sample contains mixed continental and oceanic air masses.Precipitation may have an insignificant effect on the transport of pollutants from the source region to Okinawa because air masses did not experience serious precipitation events during transport (Fig. S2a).Sulfate is the most abundant anion in fine mode, whereas chloride is the dominant anion in coarse mode.The cation budget is largely controlled by ammonium in fine mode, whereas sodium is the most abundant cation in coarse mode.The high abundance of SO 2− 4 in fine particles suggests a sig-nificant contribution of anthropogenic sources including industrial emissions in East Asia via long-range transport of aerosols over the western North Pacific Rim. SO 2− 4 is an anthropogenic tracer of industrial activities whereas NH + 4 is the secondary product of NH 3 that is largely derived from the agricultural usage of nitrogen-based fertilizers (Pakkanen et al., 2001) and volatilization from soils and livestock waste in East Asia (Huang et al., 2006).The dominant presences of Na + and Cl − in coarse mode suggest a substantial contribution from sea salt.Na + and Cl − are emitted from the ocean surface as relatively larger particles.A substantial amount of NO − 3 was detected in coarse mode, suggesting a formation of Ca(NO 3 ) 2 or NaNO 3 in coarse fractions through the reactive adsorption of gaseous HNO 3 onto preexisting alkaline particles.
The molecular distributions of detected diacids and related compounds in size-segregated aerosols are shown in Fig. 3. Table 2 presents the summarized concentrations of those compounds in fine and coarse modes.Oxalic acid (C 2 ) was  found to be the most abundant diacid followed by malonic (C 3 ) and succinic (C 4 ) acids in all size-segregated aerosols.The predominance of C 2 in size-segregated aerosols is due to the fact that it can be secondarily produced by the photooxidation of anthropogenic and biogenic organic precursors in gas and aqueous phase (Kawamura and Sakaguchi, 1999;Warneck, 2003;Carlton et al., 2006).C 2 can also be produced primarily from fossil fuel combustion (Kawamura and Kaplan, 1987) and biomass burning (Kundu et al., 2010) in East Asia and is long-range-transported to Okinawa.Phthalic (Ph) and adipic (C 6 ) acids are the next abundant diacids, whereas ketomalonic acid (kC 3 ) is more abundant than C 6 diacid in the size ranges of 0.43-0.65 to 0.65-1.1 µm (Fig. 3).Ph and C 6 diacids originate from various anthropogenic sources and thus they can be used as anthropogenic tracers.Ph primarily originates from coal burning and vehicular emissions, whereas photooxidation of aromatic hydrocarbons such as naphthalene and o-xylene derived from incomplete combustion of fossil fuel form Ph via secondary processes (Kawamura and Kaplan, 1987).Moreover, abundant presence of Ph may also be caused by enhanced emission of phthalates from plastics used in heavily populated and industrialized regions in China and the subsequent longrange atmospheric transport to Okinawa.Phthalic acid esters are used as plasticizers in resins and polymers (Simoneit et al., 2005).They can be released into the air by evaporation because they are not chemically bonded to the polymer.Kawamura and Usukura (1993) reported that C 6 diacid is an oxidation product through the reaction of cyclohexene with ozone (O 3 ).The high abundances of Ph and C 6 diacids on Okinawa suggest a significant influence of anthropogenic sources in East Asia via long-range transport of aerosols over the western North Pacific Rim.
Azelaic acid (C 9 ) is more abundant than adjacent suberic (C 8 ) and decanedioic (C 10 ) acids in all the sizesegregated aerosols (Fig. 3 and Table 2).Kawamura and Gagosian (1987) proposed that C 9 is a photooxidation product of biogenic unsaturated fatty acids such as oleic acid (C 18:1 ) containing a double bond at C-9 position.Unsaturated fatty acids can be emitted from sea surface microlayers and from local vegetation on Okinawa (Kunwar and Kawamura, 2014).Moreover, Okinawa was affected by long-range transport of air masses from Siberia and Mongolia as well as North China and Korea (Fig. 2).Such continental air masses can also deliver C 9 via atmospheric processing of unsaturated fatty acids during long-range transport.The abundant presence of C 9 indicates that atmospheric oxidation of unsaturated fatty acids also occurs in Okinawa aerosols during longrange transport.ω-Oxocarboxylic acids and α-dicarbonyls were detected in the Okinawa aerosols.Glyoxylic acid (ωC 2 ) was identified as the most abundant ω-oxoacid, whereas glyoxal (Gly) was more abundant than methylglyoxal (MeGly) in all the sizes.ωC 2 and Gly are the oxidation products of several anthropogenic and biogenic VOCs and are primarily generated by fossil fuel combustion and biomass burning (Zimmermann and Poppe, 1996;Volkamer et al., 2001), and are further oxidized to C 2 diacid (Myriokefalitakis et al.,

2011
).The predominance of ωC 2 and Gly indicates their importance as key precursors of C 2 in Okinawa aerosols.

Inorganic species
The particle size distributions of major ions are shown in Fig. 4. Pearson correlation coefficients (r) among the measured ions in different size modes are given in Table 3. Na + and Cl − are mainly derived from the ocean surface as sea salt particles in the marine atmosphere (Kumar et al., 2008;Geng et al., 2009).The size distributions of Na + and Cl − were found to be bimodal with two peaks in coarse mode (Fig. 4a and b).Their peaks at 2.1-3.3 or 3.3-4.7 µm and at > 11.3 µm suggest that they are of marine origin due to bubble bursting of surface seawater.Andreas (1998) suggested that the sea spray falls into two types that are defined as film and jet bubbles; film bubbles correspond to the size of 0.5-5 µm, whereas jet bubbles are produced the size of 5-20 µm.Their coarse-mode peaks at 2.1-3.3 µm or 3.3-4.7 and > 11.3 µm in Okinawa aerosols were associated with film and jet bubbles.We found that size distribution of Mg 2+ is similar to those of Na + and Cl − with a significant positive correlation to coarsemode Na + and Cl − (r = 0.98), suggesting their similar origin and sources.
A high concentration of Ca 2+ in coarse-mode particles demonstrates its contribution from soil dust (Kerminen et al., 1997a;Tsai and Chen, 2006).A lifting of soil dust in continental sites followed by subsequent long-range atmospheric transport to a remote marine site is also proposed as an important source of Ca 2+ (Y.Wang et al., 2005).Ca 2+ showed unimodal distribution with a peak at either 2.1-3.3 or 3.3-4.7 µm (Fig. 4c).The coarse-mode Ca 2+ is mostly derived from crustal CaCO 3 , which heterogeneously reacts with acidic gases (HNO 3 and SO 2 ) (Kerminen et al., 1997a).This formation mechanism is further supported by a strong correlation of coarse-mode Ca 2+ with NO − 3 (r = 0.98).There is no correlation between Ca 2+ and Na + or Cl − (r = −0.12 or −0.27), revealing that sea salt contribution of Ca 2+ is neg- ligible in Okinawa aerosols.This result suggests that longrange transport of soil dust is an important contributor of Ca 2+ in the marine aerosols from the western North Pacific Rim.
There are natural limestone caves formed by elevated coral reefs on Okinawa Island.Although local limestone dust may also be resuspended in the atmosphere by wind (Shimada et al., 2015), the local dust contribution to the ambient level of Ca 2+ on Okinawa may be small.This interpretation can be supported by the fact that Ca 2+ peaked in lower coarse size range of 2.1-3.3 or 3.3-4.7 µm.It has been suggested that Ca 2+ is likely associated with upper coarse size range when the contribution of locally produced soil particles is significant (Bian et al., 2014).Smaller coarse-mode Ca 2+ is likely associated with long-range-transported Asian dust to Okinawa.Moreover, concentrations of Ca 2+ in coarse mode were found to be much higher in OKI-1 (0.51 µg m −3 ) and OKI-2 (0.60 µg m −3 ) than that in the OKI-5 sample (0.15 µg m −3 ).Backward trajectories also indicated that the air masses which originated from Mongolia and Siberia were transported to Okinawa during the collection of OKI-1 and OKI-2 samples, whereas the OKI-5 sample has an influence of marine air masses.Such air mass origin again indicates the long-range transport of Asian dust from East Asia to the western North Pacific.
Potassium is enriched in biomass burning aerosols and therefore its abundances in fine particles can serve as a diagnostic tracer of biomass burning (Yamasoe et al., 2000).Moreover, contributions of K + from sea salt and dust sources are highly variable in regional case studies, with its dominance in coarse-mode particles.Fresh biomass burning particles mostly reside in the condensation mode at 0.1-0.5 µm in diameter (Kaufman and Fraser, 1997;Kleeman and Cass, 1999).A unimodal size distribution of K + was observed in most sample sets (OKI-1 to OKI-4), with a peak at 0.65-1.1 µm in diameter (Fig. 4e).The peak of K + at 0.65-1.1 µm suggests that biomass burning particles emitted in East Asia might have undergone growth to a relatively large size by absorbing water vapor from the atmosphere during longrange transport to Okinawa.This interpretation is supported by the fact that K + showed a positive correlation with LWC (r = 0.83) in fine mode.The fine-mode nss-K + accounted for 95 % of total K + in the OKI-2 sample set and 88 % of that in the OKI-3 sample set when air masses come from Siberia and Mongolia as well as North China.The abundant presence of fine-mode nss-K + in the OKI-2 and OKI-3 samples further indicates a long-range atmospheric transport of biomass burning aerosols from the Asian continent to the western North Pacific Rim.
NO x is a precursor of NO − 3 , which can be converted to HNO 3 and then react with NH 3 to form NH 4 NO 3 .A unimodal size distribution of NO − 3 was observed with a peak at 2.1-3.3 or 3.3-4.7 µm in diameter (Fig. 4f).It should also be noted that the NO − 3 concentration in coarse mode is much higher than that in fine mode (Table 1).This result suggests that either dust or sea salt particles are the source of coarse-mode NO − 3 on Okinawa.Coarse-mode NO − 3 is produced by heterogeneous reaction of gaseous NO 2 or HNO 3 with alkaline metals such as Na + and Ca 2+ as shown in Reactions (R1) and (R2) (Kouyoumdjian and Saliba, 2006;Seinfeld and Pandis, 2006).HNO 3(g) + NaCl (aq and s) → NaNO 3(aq and s) + HCl (g) (R1) As discussed earlier, the air masses which originated from Siberia are transported over Mongolia and North China.Asian dust can be transported from the Asian continent to Okinawa.Therefore, it is possible that the gaseous HNO 3 might already have reacted with CaCO 3 (mineral dust particle) to form NO − 3 before arriving to Okinawa through R-2.We found that coarse-mode Na + , which is derived from sea salts, is negatively correlated (r = −0.30)with coarse-mode NO − 3 .Although this correlation is not significant (p = 0.51), the negative correlation may indicate some reactive loss of NO − 3 from sea salt particles in coarse mode on Okinawa.NO − 3 peaked at the same particle size of Ca 2+ .Therefore, NO − 3 in Okinawa coarse-mode aerosols probably resulted from the uptake of HNO 3 gas by soil dust particles enriched with Ca 2+ via heterogeneous reactions near the source regions.This process is further supported by a good correlation between NO − 3 and Ca 2+ (r = 0.98) in coarse mode.The particle size distributions of SO 2− 4 , which is a major source of acid deposition (Pakkanen et al., 2001), have been the subject of numerous studies in the past few decades (Huang et al., 2006;Kouyoumdjian and Saliba, 2006).Condensation-mode SO 2− 4 arises from gas-phase oxidation of SO 2 followed by gas-to-particle conversion, whereas finemode SO 2− 4 is formed through aqueous-phase oxidation of SO 2 in aerosols and cloud droplets (Seinfeld and Pandis, 1998).SO 2− 4 on coarse mode can be attributed to a combination of sulfate and heterogeneous reactions of SO 2 on soil dust or sea salt particles (Seinfeld and Pandis, 1998;Pakkanen et al., 2001).A unimodal size distribution of SO 2− 4 was observed with a peak at 0.65-1.1 µm (Fig. 4g).Gao et al. (2012) suggested that an in-cloud process produces SO 2− 4 as larger particles by aqueous-phase oxidation of SO 2 in cloud droplets.Therefore, the peak of SO 2− 4 at 0.65-1.1 µm on Okinawa may be involved with aqueous-phase oxidation of SO 2 in aerosols.
Size distribution of methanesulfonate (MSA − ) is similar to that of SO 2− 4 (Fig. 4i) on Okinawa.MSA − showed a strong correlation with SO 2− 4 (r = 0.89) in fine mode, suggesting that MSA − should have similar origin with SO 2− 4 in fine mode.Although MSA − is produced by gas-to-particle conversion via the oxidation of dimethyl sulfide (DMS) emitted from the ocean (Quinn et al., 1993;Kerminen et al., 1997b), there is some indirect evidence that liquid-phase production might also be possible (Jefferson et al., 1998).Biomass burning also produces DMS in the atmosphere (Meinardi et al., 2003;Geng and Mu, 2006).MSA − showed high correlation with K + or NH + 4 (r = 0.92) in fine mode, indicating that an enhanced emission of DMS from biomass burning followed by the subsequent oxidation during long-range transport may have contributed significantly to fine-mode MSA − on Okinawa.Moreover, MSA − can also be produced in fine mode by the oxidation of DMS that is emitted from marine phytoplankton in the surrounding ocean.It is noteworthy that East Asian aerosols traveled over the marine regions including the East China Sea, Sea of Japan, and Pacific Ocean during long-range atmospheric transport.The size distribution of MSA − observed over Okinawa is consistent with previous studies from the China Sea by Gao et al. (1996), who suggested that MSA is produced through the oxidation of Scontaining species in the marine atmosphere.
NH + 4 in the Okinawa aerosols shows a unimodal size distribution with a peak at 0.65-1.1 µm (Fig. 4h), indicating that NH + 4 is mainly formed by gas-to-particle conversion via the reaction with H 2 SO 4 and HNO 3 .Interestingly, the size distribution of NH + 4 is similar to that of SO 2− 4 and diacids such as oxalic acid (Figs.4g and 5a).We also found a strong correlation between SO 2− 4 and NH + 4 on fine mode (r = 0.99).Ion balance calculations are commonly used to evaluate acidbase balance of aerosol particles.Average equivalent ratios of total cations (Na + , NH + 4 , K + , Mg 2+ , and Ca 2+ ) to anions (Cl − , NO − 3 , and SO 2− 4 ) in fine fractions varied from 0.75 for the size bin of 0.65-1.1 µm to 0.86 for the size bin of 1.1-2.1 µm, indicating that fine-mode aerosols on Okinawa were apparently acidic.
NH 3 is an alkaline gas that neutralizes the acidic particles in the atmosphere.Kerminen et al. (1997a) proposed that particulate NH + 4 is secondarily formed via heterogeneous reactions of gaseous NH 3 with acidic species (H 2 SO 4 and HNO 3 ).The reaction of NH 3 with H 2 SO 4 is favored over its reaction with HNO 3 .The average NH + 4 /SO 2− 4 equivalent ratios in fine-mode particles on Okinawa varied from 0.36 for the size bin of 1.1-2.1 µm to 0.81 for the size bin of 0.43- nawa aerosols.Therefore, NO − 3 reacts with coarse particles that contain alkaline species (Ca 2+ ) in Okinawa aerosols.
The size distribution of SO 2− 4 depends on the concentration of NH + 4 , richness of NH 3 in the air, and the presence of coarse-mode particles.SO 2− 4 and NH + 4 often coexist in fine mode because H 2 SO 4 condenses on this mode as fine particles that have more surface area (Jacobson, 2002).Although NH 3 was not abundant enough to neutralize all SO 2− 4 , most of SO 2− 4 might be neutralized by NH 3 in fine mode.Hence, SO 2− 4 is enriched in fine mode rather than being associated with dust particles.An enrichment of NO − 3 in the dust fraction in our study is supported by the laboratory studies of Hanisch and Crowley (2001a, b), who found a large and irreversible uptake between HNO 3 and various authentic dust samples including samples from the Chinese dust region.

Water-soluble organic carbon (WSOC) and organic carbon (OC)
The mass-based size distribution of WSOC is characterized by a major peak at 0.65-1.1 µm in fine mode and by a small peak at 3.3-4.7 µm in coarse mode (Fig. 6a and Table 1).Huang et al. (2006) observed that fine-mode WSOC was primarily derived from combustion sources and secondarily produced in the atmosphere by the photochemical oxidation of VOCs.The WSOC concentrations showed a strong correlation with fine-mode SO 2− 4 (r = 0.96).Because production of SO 2− 4 is closely linked to photochemical activity, this result suggests an important secondary production of WSOC in fine-mode particles during long-range atmospheric transport from East Asia.WSOC concentrations also showed high correlation with K + (r = 0.93) and NH + 4 (r = 0.91) in fine mode.This result suggests that direct emission from biomass burning or fast oxidation of biomass-burning-derived precursors significantly contributes to the formation of fine-mode WSOC in Okinawa aerosols during long-range transport.
The mass size distribution pattern of OC is similar to that of WSOC with a major peak in the size range of 0.65-1.1 µm, whereas a small peak appeared in the size range of 3.3-4.7 µm in diameter (Fig. 6b).Primary emission from biomass burning and/or photooxidation of biomass-burning-derived precursors might be a dominant source of fine-mode OC in Okinawa aerosols.This interpretation is supported by the fact that OC showed a strong correlation (r = 0.95) with K + in fine mode.The fine-mode OC showed significant positive correlations with SO 2− 4 (r = 0.93) and NH + 4 (0.91), suggesting a secondary photochemical formation of OC in the fine mode of Okinawa aerosols.
A significant portion of OC may be oxidized to WSOC during the atmospheric transport from East Asia to the western North Pacific.The mass ratio of WSOC / OC has been proposed as a measure of photochemical processing or aging of organic aerosols especially in long-range-transported aerosols (Aggarwal and Kawamura, 2009).The WSOC / OC ratios varied from 0.51 to 0.76 with an average of 0.67 ± 0.09 in the fine mode and from 0.43 to 0.63 with an average of 0.55 ± 0.09 in the coarse mode.The higher WSOC / OC ratio in fine mode suggests that organics are significantly subjected to photochemical processing in fine aerosols during long-range transport from the Asian continent to Okinawa compared to coarse-mode aerosols.
Source contributions and secondary processes that may convert VOCs to more soluble forms on the surface area of fine particles could cause higher WSOC / OC ratios in fine mode.Biomass-burning-derived OC is highly watersoluble and usually resides in fine mode, whereas coarsemode OC contains high molecular weight organic compounds emitted by soil resuspension and emissions of pollen and fungal spores, which are less water-soluble (Wang et al., 2011;Mkoma et al., 2013).Biomass burning significantly contributed to fine-mode WSOC on Okinawa as discussed above.Moreover, accumulation of gas-phase precursors of WSOC may occur preferentially in the particle size, with the greatest surface area (Kanakidou et al., 2005).It has been proposed that fine particles offer more surface area, and thus the reaction rate is more on the surface of fine particles than coarse particles (Kanakidou et al., 2005).The higher WSOC / OC ratio in fine particles than coarse parti-cles has also been observed in long-range-transported East Asian aerosols over northern Japan (Agarwal et al., 2010).
The WSOC / OC ratio in fine mode showed a weak positive correlation with downward solar radiation flux (r = 0.39).This weak correlation is probably due to the fact that fine-mode WSOC can be produced in the aqueous phase of aerosols during long-range transport.Based on the yearround measurements of total suspended aerosols from Okinawa Island, Kunwar and Kawamura (2014) documented higher WSOC / OC ratio in winter (ave.0.60) and spring (ave.0.45) than summer (ave.0.28).These observations demonstrate that WSOC can be produced from OC under a weak solar radiation condition on the transport pathway from the source region to Okinawa possibly via aqueous-phase processing.
Calculated LWC for each sample from Okinawa and average LWC in size-segregated aerosols are shown in Fig. 7.The highest LWC was found at the size of 0.65-1.1 µm in the fine mode in Okinawa samples.WSOC can also contribute to aerosol LWC, although its ability to absorb water is significantly less than that of inorganics (Ansari and Pandis, 2000;Speer et al., 2003;Engelhart et al., 2011).Moreover, organic species are not taken into account in ISOR-ROPIA II for the calculation of LWC.It is noteworthy that WSOC / OC ratio and LWC in fine mode significantly correlate with r = 0.87, whereas negative correlation was found in coarse mode (r = −0.19),suggesting a possible production of WSOC from OC in aerosol aqueous phase in the fine mode of Okinawa aerosols.There may also be another important source of fine-mode WSOC in Okinawa aerosols such as primary emission from biomass burning and secondary formation via gas-phase photochemical reactions during longrange atmospheric transport (Hagler et al., 2007;Lim et al., 2010).This result may indicate that shorter-chain diacids and related polar compounds can contribute more to fine-mode WSOC via oxidation of various organic precursors during long-range transport (Carlton et al., 2007;Kawamura et al., 2005Kawamura et al., , 2007;;Miyazaki et al., 2010).

Dicarboxylic acids and related compounds
The size distributions of selected diacids and related compounds are shown in Fig. 5. Based on the sources and formation processes, their size distributions fall into two groups: a group with a dominant fine mode and a group with a dominant coarse mode, as discussed in the following sections.

C 2 , ωC 2 , Gly, Ph, and benzoic acid
The first group, including C 2 , ωC 2 , Gly, Ph, and benzoic acid, showed similar size distributions to maxima in fine mode.C 2 showed a peak at 0.65-1.1 µm in fine mode (Fig. 5a).The size distribution of C 2 on Okinawa is different from that observed off the coast of East Asia by Mochida et al. (2003aMochida et al. ( , 2007)), who found a strong bimodal pattern of C 2 with a peak in the coarse mode.They suggested that the coarse-mode peak of C 2 emerged by the uptake of gaseous diacids or heterogeneous oxidations of organic precursors on the dust and sea salt particles during long-range transport.The unimodal distribution of C 2 on Okinawa with maxima in fine mode suggests that the heterogeneous uptake of C 2 on dust and sea salt particles did not occur.
The condensation mode of C 2 is likely produced photochemically in the gas phase followed by condensation onto preexisting particles at 0.1-0.5 µm (Huang et al., 2006).The fine-mode peak of C 2 at the size of 0.65-1.1 µm in Okinawa aerosols suggests a preferential production of C 2 via the oxidation of precursors in aerosol aqueous phase during long-range atmospheric transport.We found that size distribution of C 2 diacid is similar to that of SO 2− 4 (Figs.4g and 5a), suggesting a secondary formation of C 2 , possibly in aerosol aqueous phase.The good correlations of C 2 with SO 2− 4 (r = 0.92) and NH + 4 (0.89) in fine mode further supports that C 2 is a secondary photochemical product.Finemode C 2 can also be produced primarily from fossil fuel combustion and biomass burning in East Asia and longrange-transported to Okinawa.C 2 diacid showed a significant positive correlation with fine-mode K + (r = 0.85), indicating that biomass burning contributed significantly to fine-mode C 2 in Okinawa aerosols.Lim et al. (2005) and Legrand et al. (2007) reported the formation of diacids in aqueous phase.Here we investigate the impact of LWC on the formation of diacids in Okinawa aerosols.LWC of a particle can influence the production of C 2 via the changes in gas/particle partitioning of organic precursors and subsequent heterogeneous reactions in aerosol aqueous phase.A strong positive correlation (r = 0.92) of C 2 with LWC was found in fine mode, whereas the correlation was negative in coarse mode (r = −0.29),indicating a possible aqueous-phase production of C 2 via the oxidation of C 2 precursors in fine mode.Several secondary for- C 2 is produced by the decay of its higher homologues (C 3 -C 5 diacids) or oxidation of unsaturated fatty acids such as oleic acid (C 18:1 ) followed by the degradation to shorterchain diacids in aqueous phase (Kawamura and Ikushima, 1993;Kawamura and Sakaguchi, 1999;Pavuluri et al., 2015).C 2 can also be produced by the aqueous-phase oxidation of ωC 2 , which can be formed by aqueous oxidation of Gly and MeGly, produced by the oxidation of various VOCs including toluene, ethene, and isoprene (Zimmermann and Poppe, 1996;Volkamer et al., 2001;Lim et al., 2005;Carlton et al., 2006;Ervens et al., 2008).
The scatter plots of C 2 with C 3 -C 5 diacids in fine and coarse modes are shown in Fig. S3.The robust correlations of C 2 with C 3 -C 5 diacids (r = 0.89-0.92)were found in fine mode, indicating that they might have similar sources and origin or C 2 may be produced via the decay of its higher homologues (C 3 -C 5 diacids) during long-range transport.The differences in the slopes of linear regression of C 2 with C 3 and C 4 diacids between fine and coarse modes are not significant but slopes are slightly higher in fine mode than the coarse mode (Fig. S3a-d and Table S1).Interestingly, a significantly higher slope was observed for the regression line between C 2 and glutaric (C 5 ) acid in fine mode than coarse mode (Fig. S3e-f and Table S1).It is also noteworthy that the slope of the regression line of C 2 with C 5 diacid is significantly higher than that for C 3 and C 4 diacids in fine mode (Fig. S3a, c, e and Table S2).These results indicate that finemode oxalic acid may be produced from oxidation of glutaric acid during long-range transport via succinic and malonic acids as intermediates.The laboratory studies of Hatakeyama et al. (1985) and Kalberer et al. (2010) have documented that glutaric acid is produced by the oxidation of cyclohexene by O 3 , which can be further oxidized in aqueous phase to result in oxalic acid (Kawamura and Sakaguchi, 1999;Legrand et al., 2007).This interpretation is further supported by the fact that C 3 -C 5 diacids were enriched in the fine mode of most samples (Fig. 5b-d) and showed good correlations with LWC (r = 0.82-0.89),possibly due to the enhanced secondary production by the oxidation of its precursor compounds in aerosol aqueous phase.
The size distribution of ωC 2 and Gly is similar to that of C 2 diacid in the Okinawa samples (Fig. 5e and f).The enrichment of ωC 2 and Gly in fine mode may be associated with enhanced secondary formation via aqueous-phase processing of their precursors during long-range transport.This interpretation is evidenced by the fact that strong correlations of ωC 2 and Gly were found with SO 2− 4 (r = 0.96 and 0.86, respectively) and LWC (0.95) in fine mode.The fine-mode ωC 2 and Gly can also be produced primarily from biomass burning in East Asia and be long-range-transported to Okinawa.Significant positive correlations between ωC 2 and K + (r = 0.90), and Gly and K + (0.86) suggest that biomass burning contributed significantly to the fine-mode ωC 2 and Gly in Okinawa aerosols.Gly is a well-known precursor of ωC 2 and C 2 in atmospheric aerosols (Lim et al., 2005;Ervens and Volkamer, 2010;Myriokefalitakis et al., 2011).The preferential enrichment of Gly and ωC 2 in fine mode can form C 2 in Okinawa aerosols by aqueous-phase processing.
High correlations among C 2 , ωC 2 , and Gly in fine mode (r = 0.92-0.99)also indicate their similar sources and formation processes and that C 2 diacid may be produced by the oxidation of ωC 2 and Gly in fine mode.There is no significant difference in the slope of regression line of C 2 with ωC 2 between the fine and coarse modes (Fig. S3g-h and Table S1), whereas the slope of the regression line of C 2 with Gly is significantly higher in fine mode than coarse mode (Fig. S3i-j and Table S1).It is also remarkable that the slope of linear regression of C 2 with Gly is significantly higher than that with ωC 2 in fine mode (Fig. S3g-i and Table S2).This result may indicate a possible formation of fine-mode oxalic acid from glyoxal via glyoxylic acid as an intermediate during long-range atmospheric transport in the western North Pacific.
The enrichment of C 2 , ωC 2 , and Gly in fine mode on Okinawa was probably due to the enhanced oxidation of anthropogenic precursors emitted in East Asia during long-range transport because their size distributions are consistent with that of Ph and benzoic acid (Fig. 5g and h), which are tracers of anthropogenic sources.The strong correlations of finemode C 2 , ωC 2 , and Gly with Ph (r = 0.85-0.93)and benzoic acid (r = 0.90-0.96)further suggest that anthropogenic precursors are their important sources in fine mode.Ph and benzoic acid are directly emitted from combustion sources and secondarily produced in the atmosphere by the photooxidation of aromatic hydrocarbons emitted from the incomplete combustion of fossil fuel (Kawamura et al., 1985;Kawamura and Kaplan, 1987;Ho et al., 2006).
Aromatic hydrocarbons such as naphthalene and toluene have been suggested as major precursors of Ph and benzoic acid, respectively (Schauer et al., 1996;Kawamura and Yasui, 2005).Based on the high levels of naphthalene and toluene in China (Liu et al., 2007;Tao et al., 2007;Duan et al., 2008), Ho et al. (2015) recently suggested that oxidation of naphthalene and toluene in the atmosphere is one of the major sources of Ph and benzoic acid, respectively.High levels of precursors in the source regions might favor the significant secondary production of Ph and benzoic acid during long-range transport in the western North Pacific.It may be possible that their precursors emitted in East Asia were taken up by aqueous-phase aerosol and oxidized to result in Ph and benzoic acid in fine mode during long-range transport.Moreover, enrichment of Ph and benzoic acid in fine mode further suggests that these species are associated with combustion sources either by primary emission and/or secondary production from the precursor compounds, being consistent with other anthropogenic SO 2− 4 , NH + 4 , and K + .Fine-mode Ph can also be produced from evaporation of phthalates from plastics used in populated and industrialized regions in East Asia and long-range-transported to Okinawa as discussed earlier.This explanation is consistent with the enrichment of terephthalic acid (tPh) in fine mode (Fig. 5i), which is a tracer of plastic burning (Kawamura and Pavuluri, 2011).

C 9 and ωC 9
The second group of organic compounds, including C 9 and ωC 9 , showed bimodal size distribution with a major peak on coarse mode at 3.3-4.7 µm and minor peak on fine mode at 0.65-1.1 µm (Fig. 5j and k).The strong correlations were found between C 9 and Na + (r = 0.85), and ωC 9 and Na + (0.83) in coarse mode, indicating that C 9 and ωC 9 may be emitted into the atmosphere from the sea surface microlayers together with sea salt particles on Okinawa.Kawamura and Gagosian (1987) suggested that C 9 and ωC 9 are also derived from the photooxidation of unsaturated fatty acids such as oleic acid (C 18:1 ) that are produced by phytoplankton and emitted from sea surface microlayers as sea salt particles.The laboratory experiments also documented the formation of C 9 and ωC 9 due to photooxidation of C 18:1 (Matsunaga et al., 1999;Huang et al., 2005;Ziemann, 2005;Tedetti et al., 2007).Sea surface microlayers in the surroundings of Okinawa can also emit unsaturated fatty acids together with sea salts.Therefore, the major peaks of C 9 and ωC 9 on the coarse mode may be derived from heterogeneous oxidation of unsaturated fatty acids of marine phytoplankton origin on the sea salt particles.Wang et al. (2011) suggested that unsaturated fatty acids can be directly emitted as fine particles from food cooking emissions in urban areas in China and be oxidized to C 9 diacid in fine mode.The minor peak of C 9 and ωC 9 in fine mode can be explained by the oxidation of fine-mode unsaturated fatty acids derived from food cooking or gaseous unsaturated fatty acids during long-range transport to the western North Pacific.Kawamura and Ikushima (1993) proposed that the malonic to succinic acid ratio (C 3 / C 4 ) is a tracer to evaluate the extent of photochemical processing of organic aerosols.Because C 4 is oxidized to C 3 , an increase in the C 3 / C 4 ratio indicates an increased photochemical processing.The average C 3 / C 4 ratio in sum of all the size fractions was found to be 1.5 ± 0.1 in Okinawa aerosols.This result suggests that the extent of photochemical processing is much greater on Okinawa than Los Angeles (0.35) (Kawamura and Kaplan, 1987) but similar to that of urban Tokyo (1.5) (Kawamura and Ikushima, 1993), whereas it is lower than those of marine aerosols at Chichijima Island in the western North Pacific (2.0) (Mochida et al., 2003b) and the remote Pacific including the tropics (3.9) (Kawamura and Sakaguchi, 1999).Figure 8a shows changes in the C 3 / C 4 ratios as a function of particle size.The C 3 / C 4 ratios exhibit higher values at 1.1-2.1 µm in fine mode and at 2.1-3.3 and 3.3-4.7 µm in coarse mode.This result suggests that C 3 production via C 4 decomposition occurs more efficiently at these size ranges by aqueous-phase processing.

Ratios of selected diacids
Ph diacid originates from various anthropogenic sources, whereas C 9 diacid is specifically produced by the oxidation of biogenic unsaturated fatty acids (Kawamura and Gagosian, 1987;Kawamura and Ikushima, 1993).Therefore, Ph / C 9 ratio is most likely used as a tracer to understand the source strength of anthropogenic vs. biogenic sources of diacids.A higher Ph / C 9 ratio shows more influence of anthropogenic sources, whereas a lower ratio shows more influence of biogenic sources.Figure 8b presents changes in the ratios of Ph / C 9 as a function of particle sizes.The higher Ph / C 9 ratios were obtained on fine-mode particles rather than coarse-mode particles.These results suggest that fine aerosols on Okinawa are significantly influenced by anthropogenic sources whereas the coarse aerosols are more influenced by biogenic sources.A significant contribution of Ph on fine mode further supports that anthropogenic sources are an important source of diacids and related compounds in the fine mode of Okinawa aerosols.

Summary and conclusions
Nine-stage atmospheric particles from < 0.43 to > 11.3 µm in diameter, collected in spring 2008 at Cape Hedo, Okinawa, in the western North Pacific Rim, were analyzed for watersoluble diacids and related compounds as well as watersoluble organic carbon (WSOC), organic carbon (OC), and inorganic ions.The molecular distributions of diacids were

Figure 1 .
Figure 1.A map of East Asia with the location of Okinawa Island (26.87 • N and 128.25 • E) and Asian countries.

Figure 2 .
Figure 2. Seven-day backward air mass trajectories (NOAA HYSPLIT) at 500 m a.g.l.(09:00 UTC) for the aerosol samples (OKI-1 to OKI-5) collected on Okinawa Island.The dates given in each panel are the starting and ending times of the collection of aerosol samples on Okinawa Island.The color scale shows the altitude of the air parcel.

Figure 3 .
Figure 3. Average molecular distributions of water-soluble dicarboxylic acids and related compounds in size-segregated aerosols collected on Okinawa Island.

Figure 4 .
Figure 4. Size distributions of water-soluble inorganic ions in the aerosol samples collected on Okinawa Island.

Figure 5 .
Figure 5. Size distributions of selected water-soluble dicarboxylic acids and related compounds in the aerosol samples collected on Okinawa Island.

Figure 6 .
Figure 6.Size distributions of water-soluble organic carbon (WSOC) and organic carbon (OC) in the aerosol samples collected on Okinawa Island.

Figure 7 .
Figure 7. Aerosol liquid water contents for each sample in sizesegregated aerosols and average liquid water contents in sizesegregated aerosols on Okinawa Island.

Figure 8 .
Figure 8. Mass concentration ratios of malonic to succinic acid and phthalic to azelaic acid in size-segregated aerosols collected on Okinawa Island.

Table 1 .
Concentrations (µg m −3 ) of major inorganic ions and carbonaceous species in the fine-and coarse-mode aerosols on Okinawa Island in the western North Pacific.
a Fine mode represents aerosol size of D p < 2.1 µm.b Coarse mode represents aerosol size of D p > 2.1 µm.c Standard deviation.d Minimum.e Maximum.

Table 2 .
Summarized concentrations (ng m −3 ) of water-soluble dicarboxylic acids and related polar compounds in the fine-and coarse-mode aerosols from Okinawa Island in the western North Pacific Rim.
a Fine mode represents aerosol size of D p < 2.1 µm.b Coarse mode represents aerosol size of D p > 2.1 µm.c Standard deviation.d Minimum.e Maximum.

Table 3 .
Pearson correlation coefficients a (r) matrix among the selected chemical species/components measured in the fine-and coarse-mode aerosols from Okinawa Island in the western North Pacific Rim.
aCorrelation is significant at 0.05 level for the values where r is > 0.80.b Fine mode represents aerosol size of D p < 2.1 µm.c Coarse mode represents aerosol size of D p > 2.1 µm.