- Email: [email protected]
Aeolian transport and deposition of carbonaceous aerosols over the Northwest Pacific Ocean in spring
Atmospheric Environment xxx (xxxx) xxx
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: http://www.elsevier.com/locate/atmosenv
Aeolian transport and deposition of carbonaceous aerosols over the Northwest Pacific Ocean in spring Zilan Wu a, Limin Hu b, c, *, Tianfeng Guo d, Tian Lin e, Zhigang Guo b, d a
College of Resources and Environment, Shanxi University of Finance and Economics, Taiyuan, 030006, China Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao, 266061, China c Key Laboratory of Marine Sedimentology and Environmental Geology, First Institute of Oceanography, Ministry of Natural Resources, Qingdao, 266061, China d Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention, Institute of Atmospheric Sciences, Department of Environmental Science and Engineering, Fudan University, Shanghai, 200433, China e College of Marine Ecology and Environment, Shanghai Ocean University, Shanghai, 201306, China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Long-range transported carbonaceous aerosols from East Asia deeply perturb the atmospheric conditions over the NWP in spring. � Episodic exchange of n-alkanes at the air–sea interface occurs through windinduced marine emission or dry parti cle deposition. � Air-to-sea deposition of organic aerosols from East Asia acts as an important source of terrestrial lipids in the open NWP.
A R T I C L E I N F O
A B S T R A C T
Keywords: Northwest Pacific Ocean East Asia Carbonaceous aerosols n-Alkanes
The present study provides insight into the long-range transport of carbonaceous aerosols over the Northwest Pacific Ocean (NWP) based on marine aerosol samples collected onboard a research vessel in the spring of 2015. Organic carbon (OC) and elemental carbon (EC) concentrations showed maxima in proximity to land, with high levels also observed in advective air masses. The decoupled spatial variations in OC and EC levels in relatively pristine air masses from the Pacific, in which aerosol n-alkanes of marine origin were relatively high in abun dance, revealed the influence of heterogeneity in air masses and suggested the release of n-alkanes from local marine environments. This was confirmed by the enhanced particle-bound fraction of marine organic mattersourced n-alkanes, resulting in non-significant relationships between the gas–particle partition coefficient of nalkanes and the corresponding sub-cooled liquid vapor pressure in some marine air masses. High OC/EC ratios over the NWP indicated secondary organic aerosol formation, possibly as a result of marine emissions followed by gas-to-particle conversion and/or aerosol aging during long-range transport from East Asia. Similar molecular profiles of n-alkanes were found in air and seawater particles in regions dominated by either marine or conti nental influence, reflecting the strengths of continental and/or marine input and more importantly, suggesting
* Corresponding author. Key Laboratory of Marine Sedimentology and Environmental Geology, First Institute of Oceanography, Ministry of Natural Resources, Qingdao, 266061, China. E-mail address: [email protected] (L. Hu). https://doi.org/10.1016/j.atmosenv.2019.117209 Received 6 September 2019; Received in revised form 2 December 2019; Accepted 5 December 2019 Available online 9 December 2019 1352-2310/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Zilan Wu, Atmospheric Environment, https://doi.org/10.1016/j.atmosenv.2019.117209
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
the occurrence of air–sea exchange through wind-induced marine emissions or atmospheric deposition. Relative to the East China Sea, dry particle deposition after long-range transport acted as a more important source for terrestrial lipids in the open NWP.
1. Introduction
abundantly associated (Guo et al., 2004). Research on the molecular profiles of terrestrial biomarkers in dust aerosol and sedimentary ar chives of the NWP has suggested that aeolian dust fallout is a significant input pathway for terrestrial organic materials to the open ocean (Kawamura, 1995). However, the source fingerprints of n-alkanes in seawater particles of the NWP and their relationship with overlying marine aerosol remain to be determined. The air-sea interface of the NWP is positioned between two large pools of carbon-containing species (i.e., atmosphere and sediment), and is thus a significant pathway in the global carbon cycle, supplied by atmospheric and riverine inputs. The air-sea interface is believed to be an active layer of materials exchange between air and sea. However, previous studies have focused on the impact of continental organic aerosols on the marine atmosphere (Kang et al., 2017; Wang et al., 2015), with less attention given to the influence of marine setting. Previously, Barger and Garrett (1976) observed that atmospheric fatty acids over the Mediterranean and the eastern equatorial Pacific were of essentially marine origin. The R/V Dong Fang Hong 2 of the Ocean University of China undertook a cruise in the spring of 2015, during which samples of air and particulate matter in seawater were collected. The NWP, and particularly the affiliated marginal seas, is deeply affected by atmospheric dry deposition of nutrients and OM in spring, leading to high primary net productivity (Wang et al., 2017). Along with substantial continental outflow from East Asia, this provides a valuable opportunity to characterize the carbonaceous aerosols over the NWP and evaluate the relative importance of terrestrial and marine sources of aerosol lipids based on relating the molecular signatures preserved in air and seawater and the gas–particle partitioning of n-alkanes.
Globally, the East Asian region has been inferred to be a substantial and growing source of mineral dust, as well as naturally generated and anthropogenic aerosols (Koch et al., 2007). Transport of these conti nentally derived materials into the western and central North Pacific atmosphere (and even North America), especially during spring, when strong westerlies dominate, has been well established (Jaffe et al., 1999; Pan et al., 2015; Takemura et al., 2002; Uematsu et al., 2002). This phenomenon is corroborated by conspicuous combined dust and pollu tion signatures of East Asian origin at the receptor areas. Among at mospheric aerosols, carbonaceous particles, consisting of graphite-like and optically absorbing elemental carbon (EC) or black carbon (BC), as well as optically scattering/absorbing organic carbon (OC), are of particular concern in East Asia, an area of rapid economic growth and high population density. This is due to their high abundance in the regional atmosphere (Cao et al., 2007; Shimada et al., 2016) and rele vance for the radiation balance, adverse effects on human health and environmental pollution (Grandey et al., 2018; Lou et al., 2019; Taka hashi et al., 2014). The global emission inventories of carbonaceous aerosols indicate that East Asia is a major contributor of BC and primary organic aerosols (Chung and Seinfeld, 2002; Qin and Xie, 2012; Wang et al., 2012); China alone bears approximately 25% of the total burden (Streets et al., 2004). In the face of increasing anthropogenic emissions (Yang et al., 2015) and seasonally active wind systems in the East Asian-Northwest Pacific (NWP) region, the outflow and long-range at mospheric transport (LRAT) of carbonaceous species are of increasing importance (Boreddy et al., 2018; Miyakawa et al., 2019; Shimada et al., 2016; Wang et al., 2019). Accordingly, a number of studies have char acterized the extent and chemical composition of carbonaceous aerosol influx to the NWP atmosphere, along with the significance of long-range aerosol transport. However, these studies have been largely island-based; at present, there are no high spatial resolution records of the distribution of carbonaceous aerosols over the open NWP. This has resulted in difficulties characterizing the pollution outflow from East Asia and subsequent deposition. n-Alkanes, an important fraction of OC, are introduced to open ocean settings from adjacent continents through LRAT and oceanic contribu tion, after which a significant fraction of the more refractory n-alkanes settle downwards in deep waters and sediments (Bendle et al., 2007), whereas their short-chain counterparts may be accumulated through the marine food web or degraded (Colombo et al., 1989; Kniemeyer et al., 2007). Considering the role of the NWP as a “sink”, n-alkanes are excellent proxies for tracking the biogeochemical cycling of terrestrially derived carbon-containing substances (Hu et al., 2012; Zhu et al., 2008) and revealing changes in atmospheric transport in this region (Bendle et al., 2007; Kawamura et al., 2003), owing to their inherent chemical stability, ubiquitous occurrence in the environment, and well under stood sources. However, research on n-alkanes in the NWP has focused on the atmospheric boundary layer (Bendle et al., 2007; Yamamoto et al., 2013; Yamamoto and Kawamura, 2011) and bottom boundary layer (Amo and Minagawa, 2003; Mei et al., 2019; Zhu et al., 2008) rather than the sea-to-air interface. Existing research, has uncovered the abundance, spatial distribution, molecular nature, and long-range transport of terrestrial organic matter (OM), based on long-chain or wax-origin n-alkanes, shedding light on the fate of terrestrial OM from source-to-sink and controlling factors. More importantly, the imprinted geochemical information could be disentangled. It has been argued that the aeolian transport of terrestrial OC in East Asia is closely linked to transported dust, with which terrestrial plant-derived n-alkanes are
2. Materials and methods 2.1. Sampling Air and seawater samples were intensively acquired from the East China Sea (ECS) to the open NWP from March 29 to May 6, 2015, covering an area from 25� N to 38� N and 120� E to 152� E (Wu et al., 2017a, b). Briefly, on a daily basis, about 412 m3 of air was drawn though a quartz fiber filter, then through a polyurethane foam (PUF) plug below using a high-volume air sampler to trap airborne particles and gaseous organic compounds, respectively. To prevent shipborne contamination, air sampler was placed windward on the foredeck of the ship and pumped only when the winds were coming from the ocean sector, i.e., opposite the vessel exhausts. Surface seawater samples of about 60–100 L were simultaneously gathered en route and immediately filtrated through a glass fiber filter to obtain suspended particulate matter. Scrupulous precautions were taken against sampling artifacts, including adsorption of gaseous n-alkanes onto the filter and break through of gaseous n-alkanes, and to avoid any potential contamination from vessel exhaust or sample transportation and handling by applying field blanks. Further details on sampling position, as well as environ mental parameters such as temperature, salinity, and wind speed are summarized in Tables S1 and S2 in the Supporting Information (SI). 2.2. Chemical analysis and quantification n-Alkanes in integrated air samples and seawater filters were analyzed following previously described methods (Wang et al., 2015). In brief, filters and PUFs were Soxhlet extracted with dichloromethane for 48 h after adding deuterated polycyclic aromatic hydrocarbons (PAHs) as surrogates, after which solvent extracts were concentrated using a 2
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
rotary evaporator under vacuum. Solvent was then exchanged for n-hexane and extracts were further purified over an alumina/silica column. The aliphatic fraction was eluted with 50 mL of dichloromethane/n-hexane (1:1, V:V). A known quantity of hexame thylbenzene (HMB) was added as an internal standard prior to instru mental analysis. Determination of n-alkanes was performed with an Agilent 7890A gas chromatograph equipped with a capillary column (30 m � 0.25 mm � 0.25 μm, DB5-MS) coupled to an Agilent 5975C mass spectrometer with an electron impact ion source. The gas chro matograph operating procedure is reported elsewhere (Wang et al., 2015).
3. Results and discussion 3.1. Mass loading of carbonaceous aerosols The mass concentrations of ambient atmospheric aerosol and bound OC and EC over the NWP averaged 36.7 � 19.4, 1.52 � 1.24 and 0.36 � 0.44 μg m 3, respectively. The general spatial distributions of ambient atmospheric aerosol and bound OC and EC clearly showed high values in nearshore regions of strong continental influence (Fig. 1), and decreased towards the open ocean with high amplitude variation. The total aerosol carbon concentrations (TC, sum of OC and EC) in this study (1.88 � 1.67 μg m 3) were comparable to those identified by island-based air moni toring in spring in the open NWP (average of 2.77 μg m 3) (Kunwar and Kawamura, 2014). However, they were slightly lower than the results from Huaniao Island during spring in the coastal ECS (average of 5 μg m 3) (Wang et al., 2015). This is probably due to the combined effects of dilution and atmospheric deposition scavenging on atmospherically-transported carbonaceous aerosols from the East Asia since Huaniao Island is basically free of local anthropogenic emissions and positioned downwind of the East Asian continental influx to the NWP (Wang et al., 2015). Bulk aerosol levels showed a linear dependence on wind speed (Fig. S2, R2 ¼ 0.36, P < 0.01), indicating sea-to-air ejection due to the interaction of wind activity with surface seawater as confirmed by Woodcock (1953). However, this positive relationship was not observed for aerosol OC, despite possible enrichment of OM in sea spay aerosols (P > 0.05). This, in conjunction with their independent distribution patterns (P > 0.05), suggests that high wind speed may facilitate the transfer of surface seawater-cultured materials such as mineral sea salts to the overlying air, but may effectively offset this kinetic gain by rapid dilution. The decoupled variation in the levels of OC and EC over the course of the cruise (Fig. 1) indicated episodic but possibly heteroge neous inputs of carbonaceous materials. Relative to the evident differ ences in EC in air masses of different origins, mass loading of OC exhibited less divergence, within a factor of two (Table 1). Unlike the incomplete combustion-sourced EC, aerosol OC over the NWP is concurrently impacted by oceanic primary emissions and/or secondary organic aerosol (SOA) precursors, resulting in less depletion in the absence of continental emissions from LRAT. There was no covariation in OC and EC in marine air masses (A5–A10) (P > 0.05), but covariation was present in the continentally derived air masses (r ¼ 0.85, P < 0.01). This may be attributable to additional OC sources in oceanic environ ments when the influence of continental emissions decreases, and therefore increased contributions from the local marine environment. However, despite the marine influence from the Pacific Ocean, a northerly to westerly airflow generally dominated (Fig. S1) (Wu et al., 2017a). Under the wind regime of westerlies during this sampling campaign, episodes of elevated EC mass fraction in samples A14, A18, and A19 (1.0–1.4%) over the remote NWP were still encountered. The corresponding air masses trajectories were mainly allocated to North and East China, and occasionally the Korean Peninsula and Japan, reflecting the strength of terrestrial sources (Fig. S1). To roughly assess the emission and transformation of carbonaceous aerosols during LRAT, OC to EC ratios (OC/EC) were calculated. Aerosol OC over the NWP was more than five times as abundant as EC on average, with an OC/EC ratio of 6.51 � 5.86. The OC/EC values of continentally influenced air masses (4.32 � 1.55) were lower than those of marine air masses with low EC contents (16.9 � 7.87), and both were higher than the records for North and South China based on the Asian emissions inventory (1–2 and 2–3, respectively) (Shimada et al., 2015) and the coastal region off the ECS (average of 2.8) (Wang et al., 2015). Additionally, in contrast to measurements from land to sea (Ding et al., 2019), OC/EC ratios from the ECS to the open NWP did not decrease with increasing distance from the shore. This may have been due to the continuous OM input from marine release and decreasing EC loading, especially for marine air masses. SOA formation may have contributed
2.3. Carbonaceous components in aerosols The amounts of OC and EC on aerosol filter punches were measured with the Desert Research Institute Model 2001 Thermal/Optical Carbon Analyzer in accordance with the IMPROVE thermal/optical reflectance method (Chow et al., 1993). The temperature and oxidation conditions for the targeted carbon fractions, i.e., OC (OC1, OC2, OC3, and OC4), EC (EC1, EC2, and EC3), and pyrolyzed carbon, are detailed elsewhere (Wang et al., 2015). 2.4. Quality control Every batch of ten samples was extracted alongside a laboratory blank (solvent only) and a spiked blank (n-alkane standard mixtures spiked into solvent) to control for targeted compound losses during extraction, concentration, and fractionation. Deuterated phenanthrened10 and chrysene-d12 were selected to monitor the recovery efficiency of n-alkanes during sample processing, assuming their similar chemical properties and equal molecular weight to the analytes, i.e., C14 and C33 n-alkanes (Guo et al., 2009), with recoveries of 94 � 7% and 82 � 9% in the gas phase, and 88 � 14% and 95 � 10% in the aerosol phase, and 96 � 15% and 103 � 15% in the seawater particulate phase, respectively (Wu et al., 2017b). The recoveries of spiked blanks were 81 � 8%, whereas all targeted compounds were undetectable in the laboratory blanks. The analysis of adsorption to a back-up filter and breakthrough of gaseous n-alkanes in the subjacent PUF absorbent indicated negligible influence of sampling artifacts on gas–particle partitioning, with n-alkane levels in back-up filters or PUFs lower than the method detection limits varying from 0.05 to 0.41 pg m 3 for air samples. The detection limits of aerosol OC and EC were determined based on three times the standard deviation of the field blanks, which were 0.08 and 0.02 μg m 3, respectively; the reported concentrations of OC and EC in this study were field blank-corrected. 2.5. Sources of n-alkanes in air and seawater Generally, n-alkanes are sourced from higher plant waxes and pet rogenic or combustion-related fossil fuel residues (Simoneit, 1986); the former can be estimated as the excess odd homolog deducted by the average of adjacent even homologs (Simoneit et al., 1991). The NWP ecosystem is characterized by high biological productivity (Obata et al., 1996), related to merged inputs from atmosphere and land runoff; consequently, the contribution of marine OM reservoirs (e.g., algae, phytoplankton, and bacteria) to particle-associated n-alkanes should be taken into account, based on the proportion of n-alkanes characteristic of particularly marine origin (i.e., C15, C17, and C19 homologs) (Cheva lier et al., 2015). The estimate herein is regarded as a conservative value, as some n-alkanes possess multiple fingerprints, including marine-origin fingerprints (e.g., C16 and C18 homologs) (Aloulou et al., 2010). Thus, non-inclusion of these homologs may underestimate the marine-sourced fraction of n-alkanes in the NWP. 3
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
Fig. 1. Aerosol particle concentrations and associated organic carbon (OC) and elemental carbon (EC) mass fractions over the Northwest Pacific Ocean (NWP).
(Fig. 3). This suggests a weak odd-to- even carbon number predomi nance, demonstrating the dominance of fossil fuel-derived n-alkanes (80 � 5%) and/or degradation during LRAT over the NWP (Simoneit, 1986), and different contributions from naturally biogenic sources (i.e., terrestrial higher plant epicuticular waxes and marine OM) with distinct air mass origins. Asian dust was not encountered during the cruise. However, the long-range transported aerosols from East Asia, where springtime biogenic emissions blossom, were overprinted with plant wax signals (12 � 6%), even in the samples collected furthermost over the NWP. This resulted in episodes of relatively high n-alkane content from leaf waxes in the time series at samples A14, A15, A18, A19, and A20 over the open NWP collectively composed of 15% of the cumulative wax-origin n-alkanes. Levels of n-alkanes in aerosol particles were significantly correlated with either aerosol EC (R2 ¼ 0.55, P < 0.01) or OC (R2 ¼ 0.35, P < 0.01) (Fig. S3), indicating their common sources and association of air n-alkanes with carbonaceous aerosols during LRAT. Based on the analysis of n-alkane homolog distribution, the compo sition of aerosol samples near the continent were consistent with those of offshore airborne particles of primarily continental origin (R2 ¼ 0.53–0.96, P < 0.05), hinting at the significance of East Asia as a source of continentally derived n-alkanes over the NWP. However, remarkable differences in distribution patterns were identified, with distinct air mass origins (Fig. 3). Compared with continentally derived air masses (12 � 6%), homolog patterns in the marine air masses (A5–A10) were clearly shifted toward lighter n-alkanes, with up to 41 � 3% accounted for homologs � n-C20. This resulted in a lower average chain length (ACL) for high molecular weight n-alkanes (C25–C33) (28.7 � 0.50) (Table 1). It is notable that marine air masses in the smooth surface regime at wind speeds of 5 � 2 m s 1 contained an increasing proportion of aerosol n-alkanes related to marine OM (19 � 3% for the sum of C15, C17, and C19), confirming effective sea-to-air release triggered by wave breaking. n-Alkanes with sufficient volatility can occur in the gas phase, and thus be incorporated into or attached onto aerosol particles (Gagosian, 1986). Gas–particle partitioning of semivolatile organic compounds is sensitive to aerosol chemical composition and environmental conditions (Arp et al., 2008). Owing to the multiplicity of n-alkane origins, atmo spheric partitioning of n-alkanes was investigated to elucidate the in fluence of continental and marine sources on atmospheric mechanisms involved in the LRAT of these compounds. The distribution of air
Table 1 Comparisons of occurrences of carbonaceous aerosols (μg m 3) and bound nalkanes (ng m 3) in continentally influenced and marine air masses. Compounds OC EC SOA Plant wax n-alkanes Marine-derived n-alkanes ACL for C25–C33 n-alkanes
Continentally influenced air mass
Marine air mass
mean
std
mean
std
1.73 0.47 0.83 5.76 1.36 29.2
1.36 0.47 0.53 6.87 0.31 0.41
0.89 0.04 0.32 0.25 1.92 28.7
0.21 0.04 0.27 0.14 0.33 0.50
to this result. The field-measured OC/EC over the NWP far exceeded the indicative split ratio of OC/EC 2 for SOA (Chow et al., 1996), and hence the contribution of SOA to aerosol OC was further derived based on the EC-tracing method (Turpin and Huntzicker, 1995). The SOA fraction of OC over the NWP averaged 49 � 16%, with insignificant differences among air masses of various origins (P > 0.05), illustrating the domi nance of SOA. As shown by the three-ring PAHs in the NWP (Wu et al., 2017b), the oceanic surface can act as a net source of these particle-bound organic compounds in the overlying air through gas ex change followed by binding to aerosols. Moreover, the lower ratios of more photoreactive to less reactive PAH isomer pairs (Wu et al., 2017b) implies that aerosol particles over the NWP were subject to the aging process. Therefore, the prevalence of SOA may have resulted partly from the volatilization of organic compounds, followed by gas-to-particle conversion and/or aging of East Asian continental aerosols during LRAT. 3.2. n-Alkanes over the NWP and gas–particle partitioning Normal n-alkanes in the range of C14 to C33, were detected in air, the sum of which (gaseous and aerosol phases) varied from 24.1 to 146 ng m 3 (Fig. 2 and Table S3). These n-alkanes were mainly accounted by the aerosol phase fraction in the continentally influenced air masses (53 � 9%), whereas the opposite was true of the marine air masses (63 � 6%). The Carbon Preference Index (CPI) for the aerosol-phase n-alkanes was calculated as the odd/even ratio in the C14–C33 range, which averaged 1.36 � 0.30, with carbon number maxima at C27, C29, or C31 for continentally derived air masses, and C17 or C19 for marine air masses 4
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
Fig. 2. Spatial distributions of n-alkanes in the gas (pink color) and aerosol phases (purple color) (upper panel) and particulate matter of surface seawater (lower panel). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. Relative abundance of aerosol n-alkanes in continentally influenced (a) and marine (b) air masses.
5
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
n-alkanes between phases is described with a partition coefficient (Kp, m3 μg 1), defined as the ratio of filter and PUF sorbent-retained n-alkane concentrations (ng m 3) normalized with aerosol particle mass con centrations (μg m 3). We performed linear regressions between the partition coefficients and the corresponding sub-cooled liquid vapor pressures (P0L ) of n-alkanes, with event-to-event changes in the fitting parameters given in Table S4. The P0L of n-alkanes from C16 to C30 rec ommended by Chickos and Hanshaw (2004) were applied and corrected for field ambient temperature based on Xie et al. (2014). The P0L -based model showed similar trendlines for n-alkanes in continentally influ enced air masses with relatively narrow variations in slopes (from 0.37 to 0.04) and intercepts (from 3.06 to 1.96). This suggested common sources between the carbonaceous particles and the compounds of in terest, and thus comparable molecular interactions between them. However, with respect to marine air masses, statistically significant re lationships were observed only in A6, A8, and A10 (P < 0.05), with shallower slopes (Fig. S4 and Table S4). These results were attributable to stronger association with particles for lighter n-alkanes. This comparison was underlain by different organic aerosol composi tions between these two categories of air masses. As mentioned above, the marine air masses had a lower abundance of EC and were rich in marine OM. This would be expected to decrease the sorption of com bustion- and/or plant wax-derived n-alkanes, but augment the associa tion with n-alkanes of marine origin. As such, lighter n-alkanes demonstrated stronger particle-bound tendencies in marine air masses. On average, logKp values for n-C16 to n-C20 homologs were 0.47–0.77 log units higher than those measured in the advective air masses from East Asia. Surprisingly, unlike the general observations (Qi et al., 2014; Tsapakis and Stephanou, 2005), logKp was negatively correlated with the reciprocal temperature for n-C16 to n-C19 homologs (R2 ¼ 0.15–0.48, P < 0.05, Fig. S5) rather than long-chain n-alkanes (n-C25 to n-C33). With a negligible influence of filter sorption artefact, the stronger tempera ture dependence of lighter n-alkanes may be related to gas-phase re actions with OH and NO3 radicals (Atkinson, 1997). Alternatively, and possibly more likely, it may indicate the release of gaseous lighter n-alkanes from surface seawater in the NWP, which would be consistent with the enhanced portion of marine-derived n-alkanes in gas phase in marine air masses (32 � 6%) relative to continentally influenced air masses (22 � 4%). These emitted marine source n-alkanes with rela tively high volatility and diffusivity are considered to re-equilibrate faster with ambient aerosols under warm conditions (air temperatures of 19 � 3 � C), thus elevating the corresponding aerosol-phase fraction. On the whole, the long-range-transported atmospheric particles over the NWP suggest that land-based particles enter a relatively pristine marine atmosphere, such that slow kinetics may lead to a clear non-equilibrium with slopes far deviating from the theoretical value of 1 owing to the equilibration of associated land-based n-alkanes to the ambient tem perature and local marine aerosols present. Log-log plots between Kp and P0L revealed shallower slopes than those obtained in urban areas (Cincinelli et al., 2007; Mandalakis et al., 2002).
suggesting that flocculation occurred in the river-sea mixing zone, affecting n-alkanes in the ECS. Plant wax n-alkanes accounted for a lower proportion of total n-alkanes at stations in the open NWP (7 � 4%) compared with ECS stations (13 � 3%), owing to the lower efficiency of long-range riverine transport. Meanwhile, fossil fuel-derived n-alkanes in particulate phase were predominant from the ECS to the NWP (72 � 12%). As shown in Fig. 4, n-alkanes followed a typical unimodal distribu tion pattern at stations WP1, WP2, and WC1 at the ECS, in which the nC15 and n-C17 homologs alone represented 18–31% of the total n-alkane pool. This indicated the importance of marine-origin OM rather than terrestrial wax for biogenic n-alkanes. This molecular profile differed from that preserved in the overlying aerosols (A2, A3, A25, and A26), in which n-alkanes displayed higher abundances of plant wax signatures (Tables S3 and S5). Based on Spearman correlation analysis, the mo lecular distributions of n-alkanes in these paired samples in the ECS were decoupled. This may be ascribed to unequal weighting of inputs from marine OM and terrestrial wax to air and seawater, except for stations WP3–WP5 which were more weakly influenced by marine OM (r ¼ 0.56–0.75, P < 0.05). Attenuated continental outflow was observed at stations WK2–WB9 in the open NWP, the homolog profiles of which bore a close resemblance to those of the respective marine aerosols (A5–A10) (r ¼ 0.56–0.81, P < 0.05). This indicated that seawater serves as a source of n-alkanes for the overlying aerosols, which is consistent the results of atmospheric partitioning. However, in the presence of clear advective air masses over the open NWP and low-strength marine sources, rela tively conformable composition profiles between air and seawater were found at stations WB1, WB5, WA6, and WA8 (r ¼ 0.63–0.84, P < 0.01). Leaf wax signatures were pronounced in seawater particles at these stations, indicating the effects of LRAT and deposition. Because most river-transported continental materials are trapped in estuaries and continental shelf areas in the NWP, LRAT is a crucial link between the East Asian continent and the open NWP (Ohkouchi et al., 1997). Although the air samples collected over the NWP represent snapshots in time, the relative abundance of autochthonously/allochthonously biogenic n-alkanes in airborne and coupled seawater particles deter mined by marine and continental source strength reflects the episodic exchange of OM at the air–sea interface through wind-induced marine emissions or dry particle deposition when westerlies prevail. 3.4. Dry deposition of carbonaceous aerosols Dry deposition fluxes of OC and EC in aerosols were estimated from the ECS to the NWP for the first time as a product of the aerosol-phase concentration and particle deposition velocity (Vd, cm s 1). No previ ous studies have undertaken field measurements of the particle size distribution of OC and EC aerosols and corresponding Vd over the NWP. Based on the Huaniao Island air monitoring positioned in the transport pathway from East Asia to the NWP (Wang et al., 2019, 2015), more than 80% of OC and EC were fine particulate matter (PM2.5), with an average Vd of approximately 0.02 cm s 1 denoting non-seasonal varia tion. As estimated by Slinn and Slinn (1980), the Vd of aerosol particles with radii of 0.1–0.5 μm at wind speeds of 5–10 m s 1 over water sur faces ranged from 0.02 to 0.5 cm s 1. Ganzeveld et al. (1998) assumed a Vd of 0.1 and 0.025 cm s 1 for hydrophilic and hydrophobic aerosols over the ocean surface, respectively. Although the present study iden tified marine emissions from the NWP, their influence on aerosol par ticle nature and growth cannot be fully assessed. Thus, the constant Vd value of 0.02 cm s 1 was applied for aerosol particles. The estimated deposition fluxes of OC aerosols over the NWP ranged from 26.2 to 123 μg m 2 d 1, with one to two orders of magnitude higher than those for EC (6.30 � 7.67 μg m 2 d 1). These estimations were comparable to the modeling of dry OC and EC aerosol deposition to the oceans over 30–60� N, which averaged 110 and 20 μg m 2 d 1, respectively (Jurado et al., 2008). The sea-to-sediment fluxes of plant wax n-alkanes in the ECS were estimated to be 2–69 μg m 2 d 1, based
3.3. Particulate n-alkanes in surface seawater and implications for air–sea exchange The sum of C14–C33 n-alkanes in seawater particles in the NWP ranged from 23.4 to 149 ng L 1, most of which consisted of the longchain homologs (57 � 13%). Spatially, the stations at the ECS were burdened by higher n-alkane levels than in the open NWP (Fig. 2 and Table S5). This was consistent with the appreciable anthropogenic im pacts of urban and industrial activities through riverine inputs or surface runoff (Hu et al., 2012). Interestingly, salinity exerted an influence on n-alkane spatial distribution; from the low salinity ECS to the high salinity open NWP, n-alkane levels decreased then showed a small rise before reaching a consistent level (Fig. S6). However, the marine OM-derived n-alkanes were not subject to this influence, probably 6
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
Fig. 4. Relative abundance of particulate n-alkanes in surface seawater at stations P1, P2, B3, and A1 (a); C1 (b); and the rest of the stations (c).
on the products of region-wide wax-origin n-alkane concentrations (Hu et al., 2012), sediment dry density (Liu et al., 2006), and sedimentation rate (Liu et al., 2007). This was substantially higher than the dry deposition fluxes into the ECS (0.28 � 0.15 μg m 2 d 1). This result suggested that there was minimal contribution of atmospheric inputs to terrestrial n-alkane abundance present in deep-sea sediments in the ECS. Rather, wax n-alkanes preserved in the bottom boundary layer of the ECS were more characteristic of riverine-transported sediment. How ever, this situation was reversed in the open NWP, where air-to-sea fluxes of aerosol n-alkanes derived from higher plant leaf waxes (0.04 � 0.03 μg m 2 d 1) were one to two orders of magnitude higher than the deposition fluxes of pelagic sediments (Amo and Minagawa, 2003; Kawamura, 1995). Although the deposition fluxes estimated herein were subject to uncertainty especially lack of determination of ocean-wide occurrences of sediment n-alkanes in the open NWP, this finding, combined with the conspicuously terrestrial wax-derived fin gerprints in aerosol, surface seawater, and sediment particles based on Kawamura (1995) and the present study, highlights the role of aeolian transport from East Asia for terrestrial lipids present in the NWP.
interests or personal relationships that could have appeared to influence the work reported in this paper.
4. Conclusions
References
This study of carbonaceous aerosols combined with particulate matter in seawater was performed on a vessel traveling from the ECS to the open NWP. The relatively consistent variation in OC and EC aerosols and n-alkane homolog compositions in advective air masses suggests that long-range transported carbonaceous aerosols deeply perturb the atmospheric background conditions over the NWP in spring. Relatively high marine OM signatures were observed in aerosol n-alkanes, along with stronger particle associations among lighter n-alkanes of marine origin when the Pacific influence was dominant. This revealed the enhanced marine local releases induced by wave breaking. Relative to the ECS, air-to-sea deposition of transported organic aerosols from East Asia plays an important role in oceanic geochemistry as a source of terrestrial lipids in the open NWP, as evidenced by significant atmo spheric inputs of wax n-alkanes and conformable terrestrial wax signa tures in air and seawater particles.
Aloulou, F., Kallel, M., Dammak, M., Elleuch, B., Saliot, A., 2010. Even-numbered nalkanes/n-alkenes predominance in surface sediments of Gabes Gulf in Tunisia. Environ. Earth Sci. 61, 1–10. Amo, M., Minagawa, M., 2003. Sedimentary record of marine and terrigenous organic matter delivery to the Shatsky Rise, western North Pacific, over the last 130 kyr. Org. Geochem. 34, 1299–1312. Arp, H.P.H., Schwarzenbach, R.P., Goss, K.-U., 2008. Ambient gas/particle partitioning. 2: the influence of particle source and temperature on sorption to dry terrestrial aerosols. Environ. Sci. Technol. 42, 5951–5957. Atkinson, R., 1997. Gas-phase tropospheric chemistry of volatile organic compounds: 1. Alkanes and alkenes. J. Phys. Chem. Ref. Data 26, 215–290. Barger, W.R., Garrett, W.D., 1976. Surface active organic material in air over the Mediterranean and over the eastern equatorial Pacific. J. Geophys. Res. 81, 3151–3157. Bendle, J., Kawamura, K., Yamazaki, K., Niwai, T., 2007. Latitudinal distribution of terrestrial lipid biomarkers and n-alkane compound-specific stable carbon isotope ratios in the atmosphere over the western Pacific and Southern Ocean. Geochem. Cosmochim. Acta 71, 5934–5955. Boreddy, S.K.R., Haque, M.M., Kawamura, K., 2018. Long-term (2001–2012) trends of carbonaceous aerosols from a remote island in the western North Pacific: an outflow region of Asian pollutants. Atmos. Chem. Phys. 18, 1291–1306. Cao, J.J., Lee, S.C., Chow, J.C., Watson, J.G., Ho, K.F., Zhang, R.J., Jin, Z.D., Shen, Z.X., Chen, G.C., Kang, Y.M., Zou, S.C., Zhang, L.Z., Qi, S.H., Dai, M.H., Cheng, Y., Hu, K., 2007. Spatial and seasonal distributions of carbonaceous aerosols over China. J. Geophys. Res. Atmos. 112. Chevalier, N., Savoye, N., Dubois, S., Lama, M.L., David, V., Lecroart, P., le M� enach, K., Budzinski, H., 2015. Precise indices based on n-alkane distribution for quantifying sources of sedimentary organic matter in coastal systems. Org. Geochem. 88, 69–77. Chickos, J.S., Hanshaw, W., 2004. Vapor pressures and vaporization enthalpies of the nalkanes from C21 to C30 at T ¼ 298.15 K by correlation gas chromatography. J. Chem. Eng. Data 49, 77–85. Chow, J.C., Watson, J.G., Lu, Z., Lowenthal, D.H., Frazier, C.A., Solomon, P.A., Thuillier, R.H., Magliano, K., 1996. Descriptive analysis of PM2.5 and PM10 at regionally representative locations during SJVAQS/AUSPEX. Atmos. Environ. 30, 2079–2112.
Acknowledgements This work was supported by the National Key Research and Devel opment Program of China (No: 2016YFA0601903), the National Basic Research Program of China (No: 2014CB953701), the Natural Science Foundation of China (NSFC) (No: 41876115) and Basic Scientific Fund for National Public Research Institutes of China (2017S01). We wish to thank the crew of R/V of Dong Fang Hong 2 of the Ocean University of China for collecting samples. The anonymous reviewers should be sincerely appreciated for their constructive comments that greatly improved this work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.atmosenv.2019.117209.
Author contribution section Zilan Wu and Limin Hu conceived the idea of the study; Zilan Wu and Tianfeng Guo conducted the analyses; all authors were involved in discussion of the results and revision of the manuscript and refining the ideas. Declaration of competing interest The authors declare that they have no known competing financial 7
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
Chow, J.C., Watson, J.G., Pritchett, L.C., Pierson, W.R., Frazier, C.A., Purcell, R.G., 1993. The dri thermal/optical reflectance carbon analysis system: description, evaluation and applications in U.S. Air quality studies. Atmos. Environ. Part A. Gen. Top. 27, 1185–1201. Chung, S.H., Seinfeld, J.H., 2002. Global distribution and climate forcing of carbonaceous aerosols. J. Geophys. Res. Atmos. 107. AAC 14-1-AAC 14-33. Cincinelli, A., Bubba, M. Del, Martellini, T., Gambaro, A., Lepri, L., 2007. Gas-particle concentration and distribution of n-alkanes and polycyclic aromatic hydrocarbons in the atmosphere of Prato (Italy). Chemosphere 68, 472–478. Colombo, J.C., Pelletier, E., Brochu, C., Khalil, M., Catoggio, J.A., 1989. Determination of hydrocarbon sources using n-alkane and polyaromatic hydrocarbon distribution indexes. Case study: Rio de la Plata Estuary, Argentina. Environ. Sci. Technol. 23, 888–894. Ding, X., Qi, J., Meng, X., 2019. Characteristics and sources of organic carbon in coastal and marine atmospheric particulates over East China. Atmos. Res. 228, 281–291. Gagosian, R.B., 1986. The air-sea exchange of particulate organic matter: In: BuatM�enard, P. (Ed.), The Sources and Long-Range Transport of Lipids in Aerosols BT the Role of Air-Sea Exchange in Geochemical Cycling. Springer Netherlands, Dordrecht, pp. 409–442. Ganzeveld, L., Lelieveld, J., Roelofs, G.-J., 1998. A dry deposition parameterization for sulfur oxides in a chemistry and general circulation model. J. Geophys. Res. Atmos. 103, 5679–5694. Grandey, B.S., Yeo, L.K., Lee, H.-H., Wang, C., 2018. The equilibrium climate response to sulfur dioxide and carbonaceous aerosol emissions from East and southeast Asia. Geophys. Res. Lett. 45 (11), 311–318, 325. Guo, Z., Lin, T., Zhang, G., Hu, L., Zheng, M., 2009. Occurrence and sources of polycyclic aromatic hydrocarbons and n-alkanes in PM2.5 in the roadside environment of a major city in China. J. Hazard Mater. 170, 888–894. Guo, Z.G., Feng, J.L., Fang, M., Chen, H.Y., Lau, K.H., 2004. The elemental and organic characteristics of PM2.5 in Asian dust episodes in Qingdao, China, 2002. Atmos. Environ. 38, 909–919. Hu, L., Shi, X., Yu, Z., Lin, T., Wang, H., Ma, D., Guo, Z., Yang, Z., 2012. Distribution of sedimentary organic matter in estuarine–inner shelf regions of the East China Sea: implications for hydrodynamic forces and anthropogenic impact. Mar. Chem. 142–144, 29–40. Jaffe, D., Anderson, T., Covert, D., Kotchenruther, R., Trost, B., Danielson, J., Simpson, W., Berntsen, T., Karlsdottir, S., Blake, D., Harris, J., Carmichael, G., Uno, I., 1999. Transport of Asian air pollution to North America. Geophys. Res. Lett. 26, 711–714. Jurado, E., Dachs, J., Duarte, C.M., Sim� o, R., 2008. Atmospheric deposition of organic and black carbon to the global oceans. Atmos. Environ. 42, 7931–7939. Kang, M., Yang, F., Ren, H., Zhao, W., Zhao, Y., Li, L., Yan, Y., Zhang, Yingjie, Lai, S., Zhang, Yingyi, Yang, Y., Wang, Z., Sun, Y., Fu, P., 2017. Influence of continental organic aerosols to the marine atmosphere over the East China Sea: insights from lipids, PAHs and phthalates. Sci. Total Environ. 607–608, 339–350. Kawamura, K., 1995. Land-derived lipid class compounds in the deep-sea sediments and marine aerosols from North Pacific. In: Sakai, H., Nozaki, Y. (Eds.), Biogeochemical Processes and Ocean Flux in the Western Pacific. Terra Scientific Publishing Co., Tokyo, pp. 31–51. Kawamura, K., Ishimura, Y., Yamazaki, K., 2003. Four years’ observations of terrestrial lipid class compounds in marine aerosols from the western North Pacific. Glob. Biogeochem. Cycles 17, 3–19. Kniemeyer, O., Musat, F., Sievert, S.M., Knittel, K., Wilkes, H., Blumenberg, M., Michaelis, W., Classen, A., Bolm, C., Joye, S.B., Widdel, F., 2007. Anaerobic oxidation of short-chain hydrocarbons by marine sulphate-reducing bacteria. Nature 449, 898. Koch, D., Bond, T.C., Streets, D., Unger, N., van der Werf, G.R., 2007. Global impacts of aerosols from particular source regions and sectors. J. Geophys. Res. Atmos. 112. Kunwar, B., Kawamura, K., 2014. One-year observations of carbonaceous and nitrogenous components and major ions in the aerosols from subtropical Okinawa Island, an outflow region of Asian dusts. Atmos. Chem. Phys. 14, 1819–1836. Liu, J.P., Li, A.C., Xu, K.H., Velozzi, D.M., Yang, Z.S., Milliman, J.D., DeMaster, D.J., 2006. Sedimentary features of the Yangtze river-derived along-shelf clinoform deposit in the east China sea. Cont. Shelf Res. 26, 2141–2156. Liu, J.P., Xu, K.H., Li, A.C., Milliman, J.D., Velozzi, D.M., Xiao, S.B., Yang, Z.S., 2007. Flux and fate of Yangtze river sediment delivered to the east China sea. Geomorphology 85, 208–224. Lou, S., Yang, Y., Wang, H., Smith, S.J., Qian, Y., Rasch, P.J., 2019. Black carbon amplifies haze over the North China plain by weakening the east Asian winter monsoon. Geophys. Res. Lett. 46, 452–460. Mandalakis, M., Tsapakis, M., Tsoga, A., Stephanou, E.G., 2002. Gas–particle concentrations and distribution of aliphatic hydrocarbons, PAHs, PCBs and PCDD/Fs in the atmosphere of Athens (Greece). Atmos. Environ. 36, 4023–4035. Mei, X., Li, X., Wang, Z., Zhang, C., Zhang, Y., 2019. Cross shelf transport of terrigenous organic matter in surface sediments from outer shelf to Okinawa Trough in East China Sea. J. Mar. Syst. 199, 103224. Miyakawa, T., Komazaki, Y., Zhu, C., Taketani, F., Pan, X., Wang, Z., Kanaya, Y., 2019. Characterization of carbonaceous aerosols in Asian outflow in the spring of 2015: importance of non-fossil fuel sources. Atmos. Environ., 116858 Obata, A., Ishizaka, J., Endoh, M., 1996. Global verification of critical depth theory for phytoplankton bloom with climatological in situ temperature and satellite ocean color data. J. Geophys. Res. Ocean 101, 20657–20667.
Ohkouchi, N., Kawamura, K., Kawahata, H., Taira, A., 1997. Latitudinal distributions of terrestrial biomarkers in the sediments from the Central Pacific. Geochem. Cosmochim. Acta 61, 1911–1918. Pan, X., Uno, I., Hara, Y., Kuribayashi, M., Kobayashi, H., Sugimoto, N., Yamamoto, S., Shimohara, T., Wang, Z., 2015. Observation of the simultaneous transport of Asian mineral dust aerosols with anthropogenic pollutants using a POPC during a longlasting dust event in late spring 2014. Geophys. Res. Lett. 42, 1593–1598. Qi, H., Li, W.-L., Liu, L.-Y., Song, W.-W., Ma, W.-L., Li, Y.-F., 2014. Brominated flame retardants in the urban atmosphere of Northeast China: concentrations, temperature dependence and gas–particle partitioning. Sci. Total Environ. 491–492, 60–66. Qin, Y., Xie, S.D., 2012. Spatial and temporal variation of anthropogenic black carbon emissions in China for the period 1980–2009. Atmos. Chem. Phys. 12, 4825–4841. Shimada, K., Shimada, M., Takami, A., Hasegawa, S., Fushimi, A., Arakaki, T., Izumi, W., Hatakeyama, S., 2015. Mode and place of origin of carbonaceous aerosols transported from East Asia to cape hedo, Okinawa, Japan. Aerosol Air Qual. Res. 15, 799–813. Shimada, K., Takami, A., Kato, S., Kajii, Y., Hasegawa, S., Fushimi, A., Shimizu, A., Sugimoto, N., Chan, C.K., Kim, Y.P., Lin, N.H., Hatakeyama, S., 2016. Characteristics of carbonaceous aerosols in large-scale Asian wintertime outflows at Cape Hedo, Okinawa, Japan. J. Aerosol Sci. 100, 97–107. Simoneit, B.R.T., 1986. Characterization of organic constituents in aerosols in relation to their rigin and transport: a review. Int. J. Environ. Anal. Chem. 23, 207–237. Simoneit, B.R.T., Crisp, P.T., Mazurek, M.A., Standley, L.J., 1991. Composition of extractable organic matter of aerosols from the blue mountains and southeast coast of Australia. Environ. Int. 17, 405–419. Slinn, S.A., Slinn, W.G.N., 1980. Predictions for particle deposition on natural waters. Atmos. Environ. 14, 1013–1016. Streets, D.G., Bond, T.C., Lee, T., Jang, C., 2004. On the future of carbonaceous aerosol emissions. J. Geophys. Res. Atmos. 109. Takahashi, K., Nansai, K., Tohno, S., Nishizawa, M., Kurokawa, J., Ohara, T., 2014. Production-based emissions, consumption-based emissions and consumption-based health impacts of PM2.5 carbonaceous aerosols in Asia. Atmos. Environ. 97, 406–415. Takemura, T., Uno, I., Nakajima, T., Higurashi, A., Sano, I., 2002. Modeling study of long-range transport of Asian dust and anthropogenic aerosols from East Asia. Geophys. Res. Lett. 29, 11–14. Tsapakis, M., Stephanou, E.G., 2005. Occurrence of gaseous and particulate polycyclic aromatic hydrocarbons in the urban atmosphere: study of sources and ambient temperature effect on the gas/particle concentration and distribution. Environ. Pollut. 133, 147–156. Turpin, B.J., Huntzicker, J.J., 1995. Identification of secondary organic aerosol episodes and quantitation of primary and secondary organic aerosol concentrations during SCAQS. Atmos. Environ. 29, 3527–3544. Uematsu, M., Yoshikawa, A., Muraki, H., Arao, K., Uno, I., 2002. Transport of mineral and anthropogenic aerosols during a Kosa event over East Asia. J. Geophys. Res. Atmos. 107. AAC 3-1-AAC 3-7. Wang, F., Feng, T., Guo, Z., Li, Y., Lin, T., Rose, N.L., 2019. Sources and dry deposition of carbonaceous aerosols over the coastal East China Sea: implications for anthropogenic pollutant pathways and deposition. Environ. Pollut. 245, 771–779. Wang, F., Guo, Z., Lin, T., Hu, L., Chen, Y., Zhu, Y., 2015. Characterization of carbonaceous aerosols over the East China Sea: the impact of the East Asian continental outflow. Atmos. Environ. 110, 163–173. Wang, F.J., Chen, Y., Guo, Z.G., Gao, H.W., Mackey, K.R., Yao, X.H., Zhuang, G.S., Paytan, A., 2017. Combined effects of iron and copper from atmospheric dry deposition on ocean productivity. Geophys. Res. Lett. 44, 2546–2555. Wang, R., Tao, S., Wang, W., Liu, J., Shen, H., Shen, G., Wang, B., Liu, X., Li, W., Huang, Y., Zhang, Y., Lu, Y., Chen, H., Chen, Y., Wang, C., Zhu, D., Wang, X., Li, B., Liu, W., Ma, J., 2012. Black carbon emissions in China from 1949 to 2050. Environ. Sci. Technol. 46, 7595–7603. Woodcock, A.H., 1953. Salt nuclei in marine air as a function of altitude and wind force. J. Meteorol. 10, 362–371. Wu, Z., Lin, T., Li, Z., Jiang, Y., Li, Y., Yao, X., Gao, H., Guo, Z., 2017. Air–sea exchange and gas–particle partitioning of polycyclic aromatic hydrocarbons over the northwestern Pacific Ocean: role of East Asian continental outflow. Environ. Pollut. 230, 444–452. Wu, Z., Lin, T., Li, Z., Li, Y., Guo, T., Guo, Z., 2017. Atmospheric occurrence, transport and gas–particle partitioning of polychlorinated biphenyls over the northwestern Pacific Ocean. Atmos. Environ. 167, 487–495. Xie, M., Hannigan, M.P., Barsanti, K.C., 2014. Gas/particle partitioning of n-alkanes, PAHs and oxygenated PAHs in urban Denver. Atmos. Environ. Times 95, 355–362. Yamamoto, S., Kawamura, K., 2011. Stable hydrogen isotope ratios of n-alkanes in atmospheric aerosols from Okinawa, Japan. Res. Org. Geochem. 27, 81–89. Yamamoto, S., Kawamura, K., Seki, O., Kariya, T., Lee, M., 2013. Influence of aerosol source regions and transport pathway on δD of terrestrial biomarkers in atmospheric aerosols from the East China Sea. Geochem. Cosmochim. Acta 106, 164–176. Yang, Y., Liao, H., Lou, S., 2015. Decadal trend and interannual variation of outflow of aerosols from East Asia: roles of variations in meteorological parameters and emissions. Atmos. Environ. 100, 141–153. Zhu, C., Xue, B., Pan, J., Zhang, H., Wagner, T., Pancost, R.D., 2008. The dispersal of sedimentary terrestrial organic matter in the East China Sea (ECS) as revealed by biomarkers and hydro-chemical characteristics. Org. Geochem. 39, 952–957.
8
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: http://www.elsevier.com/locate/atmosenv
Aeolian transport and deposition of carbonaceous aerosols over the Northwest Pacific Ocean in spring Zilan Wu a, Limin Hu b, c, *, Tianfeng Guo d, Tian Lin e, Zhigang Guo b, d a
College of Resources and Environment, Shanxi University of Finance and Economics, Taiyuan, 030006, China Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao, 266061, China c Key Laboratory of Marine Sedimentology and Environmental Geology, First Institute of Oceanography, Ministry of Natural Resources, Qingdao, 266061, China d Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention, Institute of Atmospheric Sciences, Department of Environmental Science and Engineering, Fudan University, Shanghai, 200433, China e College of Marine Ecology and Environment, Shanghai Ocean University, Shanghai, 201306, China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Long-range transported carbonaceous aerosols from East Asia deeply perturb the atmospheric conditions over the NWP in spring. � Episodic exchange of n-alkanes at the air–sea interface occurs through windinduced marine emission or dry parti cle deposition. � Air-to-sea deposition of organic aerosols from East Asia acts as an important source of terrestrial lipids in the open NWP.
A R T I C L E I N F O
A B S T R A C T
Keywords: Northwest Pacific Ocean East Asia Carbonaceous aerosols n-Alkanes
The present study provides insight into the long-range transport of carbonaceous aerosols over the Northwest Pacific Ocean (NWP) based on marine aerosol samples collected onboard a research vessel in the spring of 2015. Organic carbon (OC) and elemental carbon (EC) concentrations showed maxima in proximity to land, with high levels also observed in advective air masses. The decoupled spatial variations in OC and EC levels in relatively pristine air masses from the Pacific, in which aerosol n-alkanes of marine origin were relatively high in abun dance, revealed the influence of heterogeneity in air masses and suggested the release of n-alkanes from local marine environments. This was confirmed by the enhanced particle-bound fraction of marine organic mattersourced n-alkanes, resulting in non-significant relationships between the gas–particle partition coefficient of nalkanes and the corresponding sub-cooled liquid vapor pressure in some marine air masses. High OC/EC ratios over the NWP indicated secondary organic aerosol formation, possibly as a result of marine emissions followed by gas-to-particle conversion and/or aerosol aging during long-range transport from East Asia. Similar molecular profiles of n-alkanes were found in air and seawater particles in regions dominated by either marine or conti nental influence, reflecting the strengths of continental and/or marine input and more importantly, suggesting
* Corresponding author. Key Laboratory of Marine Sedimentology and Environmental Geology, First Institute of Oceanography, Ministry of Natural Resources, Qingdao, 266061, China. E-mail address: [email protected] (L. Hu). https://doi.org/10.1016/j.atmosenv.2019.117209 Received 6 September 2019; Received in revised form 2 December 2019; Accepted 5 December 2019 Available online 9 December 2019 1352-2310/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Zilan Wu, Atmospheric Environment, https://doi.org/10.1016/j.atmosenv.2019.117209
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
the occurrence of air–sea exchange through wind-induced marine emissions or atmospheric deposition. Relative to the East China Sea, dry particle deposition after long-range transport acted as a more important source for terrestrial lipids in the open NWP.
1. Introduction
abundantly associated (Guo et al., 2004). Research on the molecular profiles of terrestrial biomarkers in dust aerosol and sedimentary ar chives of the NWP has suggested that aeolian dust fallout is a significant input pathway for terrestrial organic materials to the open ocean (Kawamura, 1995). However, the source fingerprints of n-alkanes in seawater particles of the NWP and their relationship with overlying marine aerosol remain to be determined. The air-sea interface of the NWP is positioned between two large pools of carbon-containing species (i.e., atmosphere and sediment), and is thus a significant pathway in the global carbon cycle, supplied by atmospheric and riverine inputs. The air-sea interface is believed to be an active layer of materials exchange between air and sea. However, previous studies have focused on the impact of continental organic aerosols on the marine atmosphere (Kang et al., 2017; Wang et al., 2015), with less attention given to the influence of marine setting. Previously, Barger and Garrett (1976) observed that atmospheric fatty acids over the Mediterranean and the eastern equatorial Pacific were of essentially marine origin. The R/V Dong Fang Hong 2 of the Ocean University of China undertook a cruise in the spring of 2015, during which samples of air and particulate matter in seawater were collected. The NWP, and particularly the affiliated marginal seas, is deeply affected by atmospheric dry deposition of nutrients and OM in spring, leading to high primary net productivity (Wang et al., 2017). Along with substantial continental outflow from East Asia, this provides a valuable opportunity to characterize the carbonaceous aerosols over the NWP and evaluate the relative importance of terrestrial and marine sources of aerosol lipids based on relating the molecular signatures preserved in air and seawater and the gas–particle partitioning of n-alkanes.
Globally, the East Asian region has been inferred to be a substantial and growing source of mineral dust, as well as naturally generated and anthropogenic aerosols (Koch et al., 2007). Transport of these conti nentally derived materials into the western and central North Pacific atmosphere (and even North America), especially during spring, when strong westerlies dominate, has been well established (Jaffe et al., 1999; Pan et al., 2015; Takemura et al., 2002; Uematsu et al., 2002). This phenomenon is corroborated by conspicuous combined dust and pollu tion signatures of East Asian origin at the receptor areas. Among at mospheric aerosols, carbonaceous particles, consisting of graphite-like and optically absorbing elemental carbon (EC) or black carbon (BC), as well as optically scattering/absorbing organic carbon (OC), are of particular concern in East Asia, an area of rapid economic growth and high population density. This is due to their high abundance in the regional atmosphere (Cao et al., 2007; Shimada et al., 2016) and rele vance for the radiation balance, adverse effects on human health and environmental pollution (Grandey et al., 2018; Lou et al., 2019; Taka hashi et al., 2014). The global emission inventories of carbonaceous aerosols indicate that East Asia is a major contributor of BC and primary organic aerosols (Chung and Seinfeld, 2002; Qin and Xie, 2012; Wang et al., 2012); China alone bears approximately 25% of the total burden (Streets et al., 2004). In the face of increasing anthropogenic emissions (Yang et al., 2015) and seasonally active wind systems in the East Asian-Northwest Pacific (NWP) region, the outflow and long-range at mospheric transport (LRAT) of carbonaceous species are of increasing importance (Boreddy et al., 2018; Miyakawa et al., 2019; Shimada et al., 2016; Wang et al., 2019). Accordingly, a number of studies have char acterized the extent and chemical composition of carbonaceous aerosol influx to the NWP atmosphere, along with the significance of long-range aerosol transport. However, these studies have been largely island-based; at present, there are no high spatial resolution records of the distribution of carbonaceous aerosols over the open NWP. This has resulted in difficulties characterizing the pollution outflow from East Asia and subsequent deposition. n-Alkanes, an important fraction of OC, are introduced to open ocean settings from adjacent continents through LRAT and oceanic contribu tion, after which a significant fraction of the more refractory n-alkanes settle downwards in deep waters and sediments (Bendle et al., 2007), whereas their short-chain counterparts may be accumulated through the marine food web or degraded (Colombo et al., 1989; Kniemeyer et al., 2007). Considering the role of the NWP as a “sink”, n-alkanes are excellent proxies for tracking the biogeochemical cycling of terrestrially derived carbon-containing substances (Hu et al., 2012; Zhu et al., 2008) and revealing changes in atmospheric transport in this region (Bendle et al., 2007; Kawamura et al., 2003), owing to their inherent chemical stability, ubiquitous occurrence in the environment, and well under stood sources. However, research on n-alkanes in the NWP has focused on the atmospheric boundary layer (Bendle et al., 2007; Yamamoto et al., 2013; Yamamoto and Kawamura, 2011) and bottom boundary layer (Amo and Minagawa, 2003; Mei et al., 2019; Zhu et al., 2008) rather than the sea-to-air interface. Existing research, has uncovered the abundance, spatial distribution, molecular nature, and long-range transport of terrestrial organic matter (OM), based on long-chain or wax-origin n-alkanes, shedding light on the fate of terrestrial OM from source-to-sink and controlling factors. More importantly, the imprinted geochemical information could be disentangled. It has been argued that the aeolian transport of terrestrial OC in East Asia is closely linked to transported dust, with which terrestrial plant-derived n-alkanes are
2. Materials and methods 2.1. Sampling Air and seawater samples were intensively acquired from the East China Sea (ECS) to the open NWP from March 29 to May 6, 2015, covering an area from 25� N to 38� N and 120� E to 152� E (Wu et al., 2017a, b). Briefly, on a daily basis, about 412 m3 of air was drawn though a quartz fiber filter, then through a polyurethane foam (PUF) plug below using a high-volume air sampler to trap airborne particles and gaseous organic compounds, respectively. To prevent shipborne contamination, air sampler was placed windward on the foredeck of the ship and pumped only when the winds were coming from the ocean sector, i.e., opposite the vessel exhausts. Surface seawater samples of about 60–100 L were simultaneously gathered en route and immediately filtrated through a glass fiber filter to obtain suspended particulate matter. Scrupulous precautions were taken against sampling artifacts, including adsorption of gaseous n-alkanes onto the filter and break through of gaseous n-alkanes, and to avoid any potential contamination from vessel exhaust or sample transportation and handling by applying field blanks. Further details on sampling position, as well as environ mental parameters such as temperature, salinity, and wind speed are summarized in Tables S1 and S2 in the Supporting Information (SI). 2.2. Chemical analysis and quantification n-Alkanes in integrated air samples and seawater filters were analyzed following previously described methods (Wang et al., 2015). In brief, filters and PUFs were Soxhlet extracted with dichloromethane for 48 h after adding deuterated polycyclic aromatic hydrocarbons (PAHs) as surrogates, after which solvent extracts were concentrated using a 2
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
rotary evaporator under vacuum. Solvent was then exchanged for n-hexane and extracts were further purified over an alumina/silica column. The aliphatic fraction was eluted with 50 mL of dichloromethane/n-hexane (1:1, V:V). A known quantity of hexame thylbenzene (HMB) was added as an internal standard prior to instru mental analysis. Determination of n-alkanes was performed with an Agilent 7890A gas chromatograph equipped with a capillary column (30 m � 0.25 mm � 0.25 μm, DB5-MS) coupled to an Agilent 5975C mass spectrometer with an electron impact ion source. The gas chro matograph operating procedure is reported elsewhere (Wang et al., 2015).
3. Results and discussion 3.1. Mass loading of carbonaceous aerosols The mass concentrations of ambient atmospheric aerosol and bound OC and EC over the NWP averaged 36.7 � 19.4, 1.52 � 1.24 and 0.36 � 0.44 μg m 3, respectively. The general spatial distributions of ambient atmospheric aerosol and bound OC and EC clearly showed high values in nearshore regions of strong continental influence (Fig. 1), and decreased towards the open ocean with high amplitude variation. The total aerosol carbon concentrations (TC, sum of OC and EC) in this study (1.88 � 1.67 μg m 3) were comparable to those identified by island-based air moni toring in spring in the open NWP (average of 2.77 μg m 3) (Kunwar and Kawamura, 2014). However, they were slightly lower than the results from Huaniao Island during spring in the coastal ECS (average of 5 μg m 3) (Wang et al., 2015). This is probably due to the combined effects of dilution and atmospheric deposition scavenging on atmospherically-transported carbonaceous aerosols from the East Asia since Huaniao Island is basically free of local anthropogenic emissions and positioned downwind of the East Asian continental influx to the NWP (Wang et al., 2015). Bulk aerosol levels showed a linear dependence on wind speed (Fig. S2, R2 ¼ 0.36, P < 0.01), indicating sea-to-air ejection due to the interaction of wind activity with surface seawater as confirmed by Woodcock (1953). However, this positive relationship was not observed for aerosol OC, despite possible enrichment of OM in sea spay aerosols (P > 0.05). This, in conjunction with their independent distribution patterns (P > 0.05), suggests that high wind speed may facilitate the transfer of surface seawater-cultured materials such as mineral sea salts to the overlying air, but may effectively offset this kinetic gain by rapid dilution. The decoupled variation in the levels of OC and EC over the course of the cruise (Fig. 1) indicated episodic but possibly heteroge neous inputs of carbonaceous materials. Relative to the evident differ ences in EC in air masses of different origins, mass loading of OC exhibited less divergence, within a factor of two (Table 1). Unlike the incomplete combustion-sourced EC, aerosol OC over the NWP is concurrently impacted by oceanic primary emissions and/or secondary organic aerosol (SOA) precursors, resulting in less depletion in the absence of continental emissions from LRAT. There was no covariation in OC and EC in marine air masses (A5–A10) (P > 0.05), but covariation was present in the continentally derived air masses (r ¼ 0.85, P < 0.01). This may be attributable to additional OC sources in oceanic environ ments when the influence of continental emissions decreases, and therefore increased contributions from the local marine environment. However, despite the marine influence from the Pacific Ocean, a northerly to westerly airflow generally dominated (Fig. S1) (Wu et al., 2017a). Under the wind regime of westerlies during this sampling campaign, episodes of elevated EC mass fraction in samples A14, A18, and A19 (1.0–1.4%) over the remote NWP were still encountered. The corresponding air masses trajectories were mainly allocated to North and East China, and occasionally the Korean Peninsula and Japan, reflecting the strength of terrestrial sources (Fig. S1). To roughly assess the emission and transformation of carbonaceous aerosols during LRAT, OC to EC ratios (OC/EC) were calculated. Aerosol OC over the NWP was more than five times as abundant as EC on average, with an OC/EC ratio of 6.51 � 5.86. The OC/EC values of continentally influenced air masses (4.32 � 1.55) were lower than those of marine air masses with low EC contents (16.9 � 7.87), and both were higher than the records for North and South China based on the Asian emissions inventory (1–2 and 2–3, respectively) (Shimada et al., 2015) and the coastal region off the ECS (average of 2.8) (Wang et al., 2015). Additionally, in contrast to measurements from land to sea (Ding et al., 2019), OC/EC ratios from the ECS to the open NWP did not decrease with increasing distance from the shore. This may have been due to the continuous OM input from marine release and decreasing EC loading, especially for marine air masses. SOA formation may have contributed
2.3. Carbonaceous components in aerosols The amounts of OC and EC on aerosol filter punches were measured with the Desert Research Institute Model 2001 Thermal/Optical Carbon Analyzer in accordance with the IMPROVE thermal/optical reflectance method (Chow et al., 1993). The temperature and oxidation conditions for the targeted carbon fractions, i.e., OC (OC1, OC2, OC3, and OC4), EC (EC1, EC2, and EC3), and pyrolyzed carbon, are detailed elsewhere (Wang et al., 2015). 2.4. Quality control Every batch of ten samples was extracted alongside a laboratory blank (solvent only) and a spiked blank (n-alkane standard mixtures spiked into solvent) to control for targeted compound losses during extraction, concentration, and fractionation. Deuterated phenanthrened10 and chrysene-d12 were selected to monitor the recovery efficiency of n-alkanes during sample processing, assuming their similar chemical properties and equal molecular weight to the analytes, i.e., C14 and C33 n-alkanes (Guo et al., 2009), with recoveries of 94 � 7% and 82 � 9% in the gas phase, and 88 � 14% and 95 � 10% in the aerosol phase, and 96 � 15% and 103 � 15% in the seawater particulate phase, respectively (Wu et al., 2017b). The recoveries of spiked blanks were 81 � 8%, whereas all targeted compounds were undetectable in the laboratory blanks. The analysis of adsorption to a back-up filter and breakthrough of gaseous n-alkanes in the subjacent PUF absorbent indicated negligible influence of sampling artifacts on gas–particle partitioning, with n-alkane levels in back-up filters or PUFs lower than the method detection limits varying from 0.05 to 0.41 pg m 3 for air samples. The detection limits of aerosol OC and EC were determined based on three times the standard deviation of the field blanks, which were 0.08 and 0.02 μg m 3, respectively; the reported concentrations of OC and EC in this study were field blank-corrected. 2.5. Sources of n-alkanes in air and seawater Generally, n-alkanes are sourced from higher plant waxes and pet rogenic or combustion-related fossil fuel residues (Simoneit, 1986); the former can be estimated as the excess odd homolog deducted by the average of adjacent even homologs (Simoneit et al., 1991). The NWP ecosystem is characterized by high biological productivity (Obata et al., 1996), related to merged inputs from atmosphere and land runoff; consequently, the contribution of marine OM reservoirs (e.g., algae, phytoplankton, and bacteria) to particle-associated n-alkanes should be taken into account, based on the proportion of n-alkanes characteristic of particularly marine origin (i.e., C15, C17, and C19 homologs) (Cheva lier et al., 2015). The estimate herein is regarded as a conservative value, as some n-alkanes possess multiple fingerprints, including marine-origin fingerprints (e.g., C16 and C18 homologs) (Aloulou et al., 2010). Thus, non-inclusion of these homologs may underestimate the marine-sourced fraction of n-alkanes in the NWP. 3
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
Fig. 1. Aerosol particle concentrations and associated organic carbon (OC) and elemental carbon (EC) mass fractions over the Northwest Pacific Ocean (NWP).
(Fig. 3). This suggests a weak odd-to- even carbon number predomi nance, demonstrating the dominance of fossil fuel-derived n-alkanes (80 � 5%) and/or degradation during LRAT over the NWP (Simoneit, 1986), and different contributions from naturally biogenic sources (i.e., terrestrial higher plant epicuticular waxes and marine OM) with distinct air mass origins. Asian dust was not encountered during the cruise. However, the long-range transported aerosols from East Asia, where springtime biogenic emissions blossom, were overprinted with plant wax signals (12 � 6%), even in the samples collected furthermost over the NWP. This resulted in episodes of relatively high n-alkane content from leaf waxes in the time series at samples A14, A15, A18, A19, and A20 over the open NWP collectively composed of 15% of the cumulative wax-origin n-alkanes. Levels of n-alkanes in aerosol particles were significantly correlated with either aerosol EC (R2 ¼ 0.55, P < 0.01) or OC (R2 ¼ 0.35, P < 0.01) (Fig. S3), indicating their common sources and association of air n-alkanes with carbonaceous aerosols during LRAT. Based on the analysis of n-alkane homolog distribution, the compo sition of aerosol samples near the continent were consistent with those of offshore airborne particles of primarily continental origin (R2 ¼ 0.53–0.96, P < 0.05), hinting at the significance of East Asia as a source of continentally derived n-alkanes over the NWP. However, remarkable differences in distribution patterns were identified, with distinct air mass origins (Fig. 3). Compared with continentally derived air masses (12 � 6%), homolog patterns in the marine air masses (A5–A10) were clearly shifted toward lighter n-alkanes, with up to 41 � 3% accounted for homologs � n-C20. This resulted in a lower average chain length (ACL) for high molecular weight n-alkanes (C25–C33) (28.7 � 0.50) (Table 1). It is notable that marine air masses in the smooth surface regime at wind speeds of 5 � 2 m s 1 contained an increasing proportion of aerosol n-alkanes related to marine OM (19 � 3% for the sum of C15, C17, and C19), confirming effective sea-to-air release triggered by wave breaking. n-Alkanes with sufficient volatility can occur in the gas phase, and thus be incorporated into or attached onto aerosol particles (Gagosian, 1986). Gas–particle partitioning of semivolatile organic compounds is sensitive to aerosol chemical composition and environmental conditions (Arp et al., 2008). Owing to the multiplicity of n-alkane origins, atmo spheric partitioning of n-alkanes was investigated to elucidate the in fluence of continental and marine sources on atmospheric mechanisms involved in the LRAT of these compounds. The distribution of air
Table 1 Comparisons of occurrences of carbonaceous aerosols (μg m 3) and bound nalkanes (ng m 3) in continentally influenced and marine air masses. Compounds OC EC SOA Plant wax n-alkanes Marine-derived n-alkanes ACL for C25–C33 n-alkanes
Continentally influenced air mass
Marine air mass
mean
std
mean
std
1.73 0.47 0.83 5.76 1.36 29.2
1.36 0.47 0.53 6.87 0.31 0.41
0.89 0.04 0.32 0.25 1.92 28.7
0.21 0.04 0.27 0.14 0.33 0.50
to this result. The field-measured OC/EC over the NWP far exceeded the indicative split ratio of OC/EC 2 for SOA (Chow et al., 1996), and hence the contribution of SOA to aerosol OC was further derived based on the EC-tracing method (Turpin and Huntzicker, 1995). The SOA fraction of OC over the NWP averaged 49 � 16%, with insignificant differences among air masses of various origins (P > 0.05), illustrating the domi nance of SOA. As shown by the three-ring PAHs in the NWP (Wu et al., 2017b), the oceanic surface can act as a net source of these particle-bound organic compounds in the overlying air through gas ex change followed by binding to aerosols. Moreover, the lower ratios of more photoreactive to less reactive PAH isomer pairs (Wu et al., 2017b) implies that aerosol particles over the NWP were subject to the aging process. Therefore, the prevalence of SOA may have resulted partly from the volatilization of organic compounds, followed by gas-to-particle conversion and/or aging of East Asian continental aerosols during LRAT. 3.2. n-Alkanes over the NWP and gas–particle partitioning Normal n-alkanes in the range of C14 to C33, were detected in air, the sum of which (gaseous and aerosol phases) varied from 24.1 to 146 ng m 3 (Fig. 2 and Table S3). These n-alkanes were mainly accounted by the aerosol phase fraction in the continentally influenced air masses (53 � 9%), whereas the opposite was true of the marine air masses (63 � 6%). The Carbon Preference Index (CPI) for the aerosol-phase n-alkanes was calculated as the odd/even ratio in the C14–C33 range, which averaged 1.36 � 0.30, with carbon number maxima at C27, C29, or C31 for continentally derived air masses, and C17 or C19 for marine air masses 4
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
Fig. 2. Spatial distributions of n-alkanes in the gas (pink color) and aerosol phases (purple color) (upper panel) and particulate matter of surface seawater (lower panel). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. Relative abundance of aerosol n-alkanes in continentally influenced (a) and marine (b) air masses.
5
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
n-alkanes between phases is described with a partition coefficient (Kp, m3 μg 1), defined as the ratio of filter and PUF sorbent-retained n-alkane concentrations (ng m 3) normalized with aerosol particle mass con centrations (μg m 3). We performed linear regressions between the partition coefficients and the corresponding sub-cooled liquid vapor pressures (P0L ) of n-alkanes, with event-to-event changes in the fitting parameters given in Table S4. The P0L of n-alkanes from C16 to C30 rec ommended by Chickos and Hanshaw (2004) were applied and corrected for field ambient temperature based on Xie et al. (2014). The P0L -based model showed similar trendlines for n-alkanes in continentally influ enced air masses with relatively narrow variations in slopes (from 0.37 to 0.04) and intercepts (from 3.06 to 1.96). This suggested common sources between the carbonaceous particles and the compounds of in terest, and thus comparable molecular interactions between them. However, with respect to marine air masses, statistically significant re lationships were observed only in A6, A8, and A10 (P < 0.05), with shallower slopes (Fig. S4 and Table S4). These results were attributable to stronger association with particles for lighter n-alkanes. This comparison was underlain by different organic aerosol composi tions between these two categories of air masses. As mentioned above, the marine air masses had a lower abundance of EC and were rich in marine OM. This would be expected to decrease the sorption of com bustion- and/or plant wax-derived n-alkanes, but augment the associa tion with n-alkanes of marine origin. As such, lighter n-alkanes demonstrated stronger particle-bound tendencies in marine air masses. On average, logKp values for n-C16 to n-C20 homologs were 0.47–0.77 log units higher than those measured in the advective air masses from East Asia. Surprisingly, unlike the general observations (Qi et al., 2014; Tsapakis and Stephanou, 2005), logKp was negatively correlated with the reciprocal temperature for n-C16 to n-C19 homologs (R2 ¼ 0.15–0.48, P < 0.05, Fig. S5) rather than long-chain n-alkanes (n-C25 to n-C33). With a negligible influence of filter sorption artefact, the stronger tempera ture dependence of lighter n-alkanes may be related to gas-phase re actions with OH and NO3 radicals (Atkinson, 1997). Alternatively, and possibly more likely, it may indicate the release of gaseous lighter n-alkanes from surface seawater in the NWP, which would be consistent with the enhanced portion of marine-derived n-alkanes in gas phase in marine air masses (32 � 6%) relative to continentally influenced air masses (22 � 4%). These emitted marine source n-alkanes with rela tively high volatility and diffusivity are considered to re-equilibrate faster with ambient aerosols under warm conditions (air temperatures of 19 � 3 � C), thus elevating the corresponding aerosol-phase fraction. On the whole, the long-range-transported atmospheric particles over the NWP suggest that land-based particles enter a relatively pristine marine atmosphere, such that slow kinetics may lead to a clear non-equilibrium with slopes far deviating from the theoretical value of 1 owing to the equilibration of associated land-based n-alkanes to the ambient tem perature and local marine aerosols present. Log-log plots between Kp and P0L revealed shallower slopes than those obtained in urban areas (Cincinelli et al., 2007; Mandalakis et al., 2002).
suggesting that flocculation occurred in the river-sea mixing zone, affecting n-alkanes in the ECS. Plant wax n-alkanes accounted for a lower proportion of total n-alkanes at stations in the open NWP (7 � 4%) compared with ECS stations (13 � 3%), owing to the lower efficiency of long-range riverine transport. Meanwhile, fossil fuel-derived n-alkanes in particulate phase were predominant from the ECS to the NWP (72 � 12%). As shown in Fig. 4, n-alkanes followed a typical unimodal distribu tion pattern at stations WP1, WP2, and WC1 at the ECS, in which the nC15 and n-C17 homologs alone represented 18–31% of the total n-alkane pool. This indicated the importance of marine-origin OM rather than terrestrial wax for biogenic n-alkanes. This molecular profile differed from that preserved in the overlying aerosols (A2, A3, A25, and A26), in which n-alkanes displayed higher abundances of plant wax signatures (Tables S3 and S5). Based on Spearman correlation analysis, the mo lecular distributions of n-alkanes in these paired samples in the ECS were decoupled. This may be ascribed to unequal weighting of inputs from marine OM and terrestrial wax to air and seawater, except for stations WP3–WP5 which were more weakly influenced by marine OM (r ¼ 0.56–0.75, P < 0.05). Attenuated continental outflow was observed at stations WK2–WB9 in the open NWP, the homolog profiles of which bore a close resemblance to those of the respective marine aerosols (A5–A10) (r ¼ 0.56–0.81, P < 0.05). This indicated that seawater serves as a source of n-alkanes for the overlying aerosols, which is consistent the results of atmospheric partitioning. However, in the presence of clear advective air masses over the open NWP and low-strength marine sources, rela tively conformable composition profiles between air and seawater were found at stations WB1, WB5, WA6, and WA8 (r ¼ 0.63–0.84, P < 0.01). Leaf wax signatures were pronounced in seawater particles at these stations, indicating the effects of LRAT and deposition. Because most river-transported continental materials are trapped in estuaries and continental shelf areas in the NWP, LRAT is a crucial link between the East Asian continent and the open NWP (Ohkouchi et al., 1997). Although the air samples collected over the NWP represent snapshots in time, the relative abundance of autochthonously/allochthonously biogenic n-alkanes in airborne and coupled seawater particles deter mined by marine and continental source strength reflects the episodic exchange of OM at the air–sea interface through wind-induced marine emissions or dry particle deposition when westerlies prevail. 3.4. Dry deposition of carbonaceous aerosols Dry deposition fluxes of OC and EC in aerosols were estimated from the ECS to the NWP for the first time as a product of the aerosol-phase concentration and particle deposition velocity (Vd, cm s 1). No previ ous studies have undertaken field measurements of the particle size distribution of OC and EC aerosols and corresponding Vd over the NWP. Based on the Huaniao Island air monitoring positioned in the transport pathway from East Asia to the NWP (Wang et al., 2019, 2015), more than 80% of OC and EC were fine particulate matter (PM2.5), with an average Vd of approximately 0.02 cm s 1 denoting non-seasonal varia tion. As estimated by Slinn and Slinn (1980), the Vd of aerosol particles with radii of 0.1–0.5 μm at wind speeds of 5–10 m s 1 over water sur faces ranged from 0.02 to 0.5 cm s 1. Ganzeveld et al. (1998) assumed a Vd of 0.1 and 0.025 cm s 1 for hydrophilic and hydrophobic aerosols over the ocean surface, respectively. Although the present study iden tified marine emissions from the NWP, their influence on aerosol par ticle nature and growth cannot be fully assessed. Thus, the constant Vd value of 0.02 cm s 1 was applied for aerosol particles. The estimated deposition fluxes of OC aerosols over the NWP ranged from 26.2 to 123 μg m 2 d 1, with one to two orders of magnitude higher than those for EC (6.30 � 7.67 μg m 2 d 1). These estimations were comparable to the modeling of dry OC and EC aerosol deposition to the oceans over 30–60� N, which averaged 110 and 20 μg m 2 d 1, respectively (Jurado et al., 2008). The sea-to-sediment fluxes of plant wax n-alkanes in the ECS were estimated to be 2–69 μg m 2 d 1, based
3.3. Particulate n-alkanes in surface seawater and implications for air–sea exchange The sum of C14–C33 n-alkanes in seawater particles in the NWP ranged from 23.4 to 149 ng L 1, most of which consisted of the longchain homologs (57 � 13%). Spatially, the stations at the ECS were burdened by higher n-alkane levels than in the open NWP (Fig. 2 and Table S5). This was consistent with the appreciable anthropogenic im pacts of urban and industrial activities through riverine inputs or surface runoff (Hu et al., 2012). Interestingly, salinity exerted an influence on n-alkane spatial distribution; from the low salinity ECS to the high salinity open NWP, n-alkane levels decreased then showed a small rise before reaching a consistent level (Fig. S6). However, the marine OM-derived n-alkanes were not subject to this influence, probably 6
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
Fig. 4. Relative abundance of particulate n-alkanes in surface seawater at stations P1, P2, B3, and A1 (a); C1 (b); and the rest of the stations (c).
on the products of region-wide wax-origin n-alkane concentrations (Hu et al., 2012), sediment dry density (Liu et al., 2006), and sedimentation rate (Liu et al., 2007). This was substantially higher than the dry deposition fluxes into the ECS (0.28 � 0.15 μg m 2 d 1). This result suggested that there was minimal contribution of atmospheric inputs to terrestrial n-alkane abundance present in deep-sea sediments in the ECS. Rather, wax n-alkanes preserved in the bottom boundary layer of the ECS were more characteristic of riverine-transported sediment. How ever, this situation was reversed in the open NWP, where air-to-sea fluxes of aerosol n-alkanes derived from higher plant leaf waxes (0.04 � 0.03 μg m 2 d 1) were one to two orders of magnitude higher than the deposition fluxes of pelagic sediments (Amo and Minagawa, 2003; Kawamura, 1995). Although the deposition fluxes estimated herein were subject to uncertainty especially lack of determination of ocean-wide occurrences of sediment n-alkanes in the open NWP, this finding, combined with the conspicuously terrestrial wax-derived fin gerprints in aerosol, surface seawater, and sediment particles based on Kawamura (1995) and the present study, highlights the role of aeolian transport from East Asia for terrestrial lipids present in the NWP.
interests or personal relationships that could have appeared to influence the work reported in this paper.
4. Conclusions
References
This study of carbonaceous aerosols combined with particulate matter in seawater was performed on a vessel traveling from the ECS to the open NWP. The relatively consistent variation in OC and EC aerosols and n-alkane homolog compositions in advective air masses suggests that long-range transported carbonaceous aerosols deeply perturb the atmospheric background conditions over the NWP in spring. Relatively high marine OM signatures were observed in aerosol n-alkanes, along with stronger particle associations among lighter n-alkanes of marine origin when the Pacific influence was dominant. This revealed the enhanced marine local releases induced by wave breaking. Relative to the ECS, air-to-sea deposition of transported organic aerosols from East Asia plays an important role in oceanic geochemistry as a source of terrestrial lipids in the open NWP, as evidenced by significant atmo spheric inputs of wax n-alkanes and conformable terrestrial wax signa tures in air and seawater particles.
Aloulou, F., Kallel, M., Dammak, M., Elleuch, B., Saliot, A., 2010. Even-numbered nalkanes/n-alkenes predominance in surface sediments of Gabes Gulf in Tunisia. Environ. Earth Sci. 61, 1–10. Amo, M., Minagawa, M., 2003. Sedimentary record of marine and terrigenous organic matter delivery to the Shatsky Rise, western North Pacific, over the last 130 kyr. Org. Geochem. 34, 1299–1312. Arp, H.P.H., Schwarzenbach, R.P., Goss, K.-U., 2008. Ambient gas/particle partitioning. 2: the influence of particle source and temperature on sorption to dry terrestrial aerosols. Environ. Sci. Technol. 42, 5951–5957. Atkinson, R., 1997. Gas-phase tropospheric chemistry of volatile organic compounds: 1. Alkanes and alkenes. J. Phys. Chem. Ref. Data 26, 215–290. Barger, W.R., Garrett, W.D., 1976. Surface active organic material in air over the Mediterranean and over the eastern equatorial Pacific. J. Geophys. Res. 81, 3151–3157. Bendle, J., Kawamura, K., Yamazaki, K., Niwai, T., 2007. Latitudinal distribution of terrestrial lipid biomarkers and n-alkane compound-specific stable carbon isotope ratios in the atmosphere over the western Pacific and Southern Ocean. Geochem. Cosmochim. Acta 71, 5934–5955. Boreddy, S.K.R., Haque, M.M., Kawamura, K., 2018. Long-term (2001–2012) trends of carbonaceous aerosols from a remote island in the western North Pacific: an outflow region of Asian pollutants. Atmos. Chem. Phys. 18, 1291–1306. Cao, J.J., Lee, S.C., Chow, J.C., Watson, J.G., Ho, K.F., Zhang, R.J., Jin, Z.D., Shen, Z.X., Chen, G.C., Kang, Y.M., Zou, S.C., Zhang, L.Z., Qi, S.H., Dai, M.H., Cheng, Y., Hu, K., 2007. Spatial and seasonal distributions of carbonaceous aerosols over China. J. Geophys. Res. Atmos. 112. Chevalier, N., Savoye, N., Dubois, S., Lama, M.L., David, V., Lecroart, P., le M� enach, K., Budzinski, H., 2015. Precise indices based on n-alkane distribution for quantifying sources of sedimentary organic matter in coastal systems. Org. Geochem. 88, 69–77. Chickos, J.S., Hanshaw, W., 2004. Vapor pressures and vaporization enthalpies of the nalkanes from C21 to C30 at T ¼ 298.15 K by correlation gas chromatography. J. Chem. Eng. Data 49, 77–85. Chow, J.C., Watson, J.G., Lu, Z., Lowenthal, D.H., Frazier, C.A., Solomon, P.A., Thuillier, R.H., Magliano, K., 1996. Descriptive analysis of PM2.5 and PM10 at regionally representative locations during SJVAQS/AUSPEX. Atmos. Environ. 30, 2079–2112.
Acknowledgements This work was supported by the National Key Research and Devel opment Program of China (No: 2016YFA0601903), the National Basic Research Program of China (No: 2014CB953701), the Natural Science Foundation of China (NSFC) (No: 41876115) and Basic Scientific Fund for National Public Research Institutes of China (2017S01). We wish to thank the crew of R/V of Dong Fang Hong 2 of the Ocean University of China for collecting samples. The anonymous reviewers should be sincerely appreciated for their constructive comments that greatly improved this work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.atmosenv.2019.117209.
Author contribution section Zilan Wu and Limin Hu conceived the idea of the study; Zilan Wu and Tianfeng Guo conducted the analyses; all authors were involved in discussion of the results and revision of the manuscript and refining the ideas. Declaration of competing interest The authors declare that they have no known competing financial 7
Z. Wu et al.
Atmospheric Environment xxx (xxxx) xxx
Chow, J.C., Watson, J.G., Pritchett, L.C., Pierson, W.R., Frazier, C.A., Purcell, R.G., 1993. The dri thermal/optical reflectance carbon analysis system: description, evaluation and applications in U.S. Air quality studies. Atmos. Environ. Part A. Gen. Top. 27, 1185–1201. Chung, S.H., Seinfeld, J.H., 2002. Global distribution and climate forcing of carbonaceous aerosols. J. Geophys. Res. Atmos. 107. AAC 14-1-AAC 14-33. Cincinelli, A., Bubba, M. Del, Martellini, T., Gambaro, A., Lepri, L., 2007. Gas-particle concentration and distribution of n-alkanes and polycyclic aromatic hydrocarbons in the atmosphere of Prato (Italy). Chemosphere 68, 472–478. Colombo, J.C., Pelletier, E., Brochu, C., Khalil, M., Catoggio, J.A., 1989. Determination of hydrocarbon sources using n-alkane and polyaromatic hydrocarbon distribution indexes. Case study: Rio de la Plata Estuary, Argentina. Environ. Sci. Technol. 23, 888–894. Ding, X., Qi, J., Meng, X., 2019. Characteristics and sources of organic carbon in coastal and marine atmospheric particulates over East China. Atmos. Res. 228, 281–291. Gagosian, R.B., 1986. The air-sea exchange of particulate organic matter: In: BuatM�enard, P. (Ed.), The Sources and Long-Range Transport of Lipids in Aerosols BT the Role of Air-Sea Exchange in Geochemical Cycling. Springer Netherlands, Dordrecht, pp. 409–442. Ganzeveld, L., Lelieveld, J., Roelofs, G.-J., 1998. A dry deposition parameterization for sulfur oxides in a chemistry and general circulation model. J. Geophys. Res. Atmos. 103, 5679–5694. Grandey, B.S., Yeo, L.K., Lee, H.-H., Wang, C., 2018. The equilibrium climate response to sulfur dioxide and carbonaceous aerosol emissions from East and southeast Asia. Geophys. Res. Lett. 45 (11), 311–318, 325. Guo, Z., Lin, T., Zhang, G., Hu, L., Zheng, M., 2009. Occurrence and sources of polycyclic aromatic hydrocarbons and n-alkanes in PM2.5 in the roadside environment of a major city in China. J. Hazard Mater. 170, 888–894. Guo, Z.G., Feng, J.L., Fang, M., Chen, H.Y., Lau, K.H., 2004. The elemental and organic characteristics of PM2.5 in Asian dust episodes in Qingdao, China, 2002. Atmos. Environ. 38, 909–919. Hu, L., Shi, X., Yu, Z., Lin, T., Wang, H., Ma, D., Guo, Z., Yang, Z., 2012. Distribution of sedimentary organic matter in estuarine–inner shelf regions of the East China Sea: implications for hydrodynamic forces and anthropogenic impact. Mar. Chem. 142–144, 29–40. Jaffe, D., Anderson, T., Covert, D., Kotchenruther, R., Trost, B., Danielson, J., Simpson, W., Berntsen, T., Karlsdottir, S., Blake, D., Harris, J., Carmichael, G., Uno, I., 1999. Transport of Asian air pollution to North America. Geophys. Res. Lett. 26, 711–714. Jurado, E., Dachs, J., Duarte, C.M., Sim� o, R., 2008. Atmospheric deposition of organic and black carbon to the global oceans. Atmos. Environ. 42, 7931–7939. Kang, M., Yang, F., Ren, H., Zhao, W., Zhao, Y., Li, L., Yan, Y., Zhang, Yingjie, Lai, S., Zhang, Yingyi, Yang, Y., Wang, Z., Sun, Y., Fu, P., 2017. Influence of continental organic aerosols to the marine atmosphere over the East China Sea: insights from lipids, PAHs and phthalates. Sci. Total Environ. 607–608, 339–350. Kawamura, K., 1995. Land-derived lipid class compounds in the deep-sea sediments and marine aerosols from North Pacific. In: Sakai, H., Nozaki, Y. (Eds.), Biogeochemical Processes and Ocean Flux in the Western Pacific. Terra Scientific Publishing Co., Tokyo, pp. 31–51. Kawamura, K., Ishimura, Y., Yamazaki, K., 2003. Four years’ observations of terrestrial lipid class compounds in marine aerosols from the western North Pacific. Glob. Biogeochem. Cycles 17, 3–19. Kniemeyer, O., Musat, F., Sievert, S.M., Knittel, K., Wilkes, H., Blumenberg, M., Michaelis, W., Classen, A., Bolm, C., Joye, S.B., Widdel, F., 2007. Anaerobic oxidation of short-chain hydrocarbons by marine sulphate-reducing bacteria. Nature 449, 898. Koch, D., Bond, T.C., Streets, D., Unger, N., van der Werf, G.R., 2007. Global impacts of aerosols from particular source regions and sectors. J. Geophys. Res. Atmos. 112. Kunwar, B., Kawamura, K., 2014. One-year observations of carbonaceous and nitrogenous components and major ions in the aerosols from subtropical Okinawa Island, an outflow region of Asian dusts. Atmos. Chem. Phys. 14, 1819–1836. Liu, J.P., Li, A.C., Xu, K.H., Velozzi, D.M., Yang, Z.S., Milliman, J.D., DeMaster, D.J., 2006. Sedimentary features of the Yangtze river-derived along-shelf clinoform deposit in the east China sea. Cont. Shelf Res. 26, 2141–2156. Liu, J.P., Xu, K.H., Li, A.C., Milliman, J.D., Velozzi, D.M., Xiao, S.B., Yang, Z.S., 2007. Flux and fate of Yangtze river sediment delivered to the east China sea. Geomorphology 85, 208–224. Lou, S., Yang, Y., Wang, H., Smith, S.J., Qian, Y., Rasch, P.J., 2019. Black carbon amplifies haze over the North China plain by weakening the east Asian winter monsoon. Geophys. Res. Lett. 46, 452–460. Mandalakis, M., Tsapakis, M., Tsoga, A., Stephanou, E.G., 2002. Gas–particle concentrations and distribution of aliphatic hydrocarbons, PAHs, PCBs and PCDD/Fs in the atmosphere of Athens (Greece). Atmos. Environ. 36, 4023–4035. Mei, X., Li, X., Wang, Z., Zhang, C., Zhang, Y., 2019. Cross shelf transport of terrigenous organic matter in surface sediments from outer shelf to Okinawa Trough in East China Sea. J. Mar. Syst. 199, 103224. Miyakawa, T., Komazaki, Y., Zhu, C., Taketani, F., Pan, X., Wang, Z., Kanaya, Y., 2019. Characterization of carbonaceous aerosols in Asian outflow in the spring of 2015: importance of non-fossil fuel sources. Atmos. Environ., 116858 Obata, A., Ishizaka, J., Endoh, M., 1996. Global verification of critical depth theory for phytoplankton bloom with climatological in situ temperature and satellite ocean color data. J. Geophys. Res. Ocean 101, 20657–20667.
Ohkouchi, N., Kawamura, K., Kawahata, H., Taira, A., 1997. Latitudinal distributions of terrestrial biomarkers in the sediments from the Central Pacific. Geochem. Cosmochim. Acta 61, 1911–1918. Pan, X., Uno, I., Hara, Y., Kuribayashi, M., Kobayashi, H., Sugimoto, N., Yamamoto, S., Shimohara, T., Wang, Z., 2015. Observation of the simultaneous transport of Asian mineral dust aerosols with anthropogenic pollutants using a POPC during a longlasting dust event in late spring 2014. Geophys. Res. Lett. 42, 1593–1598. Qi, H., Li, W.-L., Liu, L.-Y., Song, W.-W., Ma, W.-L., Li, Y.-F., 2014. Brominated flame retardants in the urban atmosphere of Northeast China: concentrations, temperature dependence and gas–particle partitioning. Sci. Total Environ. 491–492, 60–66. Qin, Y., Xie, S.D., 2012. Spatial and temporal variation of anthropogenic black carbon emissions in China for the period 1980–2009. Atmos. Chem. Phys. 12, 4825–4841. Shimada, K., Shimada, M., Takami, A., Hasegawa, S., Fushimi, A., Arakaki, T., Izumi, W., Hatakeyama, S., 2015. Mode and place of origin of carbonaceous aerosols transported from East Asia to cape hedo, Okinawa, Japan. Aerosol Air Qual. Res. 15, 799–813. Shimada, K., Takami, A., Kato, S., Kajii, Y., Hasegawa, S., Fushimi, A., Shimizu, A., Sugimoto, N., Chan, C.K., Kim, Y.P., Lin, N.H., Hatakeyama, S., 2016. Characteristics of carbonaceous aerosols in large-scale Asian wintertime outflows at Cape Hedo, Okinawa, Japan. J. Aerosol Sci. 100, 97–107. Simoneit, B.R.T., 1986. Characterization of organic constituents in aerosols in relation to their rigin and transport: a review. Int. J. Environ. Anal. Chem. 23, 207–237. Simoneit, B.R.T., Crisp, P.T., Mazurek, M.A., Standley, L.J., 1991. Composition of extractable organic matter of aerosols from the blue mountains and southeast coast of Australia. Environ. Int. 17, 405–419. Slinn, S.A., Slinn, W.G.N., 1980. Predictions for particle deposition on natural waters. Atmos. Environ. 14, 1013–1016. Streets, D.G., Bond, T.C., Lee, T., Jang, C., 2004. On the future of carbonaceous aerosol emissions. J. Geophys. Res. Atmos. 109. Takahashi, K., Nansai, K., Tohno, S., Nishizawa, M., Kurokawa, J., Ohara, T., 2014. Production-based emissions, consumption-based emissions and consumption-based health impacts of PM2.5 carbonaceous aerosols in Asia. Atmos. Environ. 97, 406–415. Takemura, T., Uno, I., Nakajima, T., Higurashi, A., Sano, I., 2002. Modeling study of long-range transport of Asian dust and anthropogenic aerosols from East Asia. Geophys. Res. Lett. 29, 11–14. Tsapakis, M., Stephanou, E.G., 2005. Occurrence of gaseous and particulate polycyclic aromatic hydrocarbons in the urban atmosphere: study of sources and ambient temperature effect on the gas/particle concentration and distribution. Environ. Pollut. 133, 147–156. Turpin, B.J., Huntzicker, J.J., 1995. Identification of secondary organic aerosol episodes and quantitation of primary and secondary organic aerosol concentrations during SCAQS. Atmos. Environ. 29, 3527–3544. Uematsu, M., Yoshikawa, A., Muraki, H., Arao, K., Uno, I., 2002. Transport of mineral and anthropogenic aerosols during a Kosa event over East Asia. J. Geophys. Res. Atmos. 107. AAC 3-1-AAC 3-7. Wang, F., Feng, T., Guo, Z., Li, Y., Lin, T., Rose, N.L., 2019. Sources and dry deposition of carbonaceous aerosols over the coastal East China Sea: implications for anthropogenic pollutant pathways and deposition. Environ. Pollut. 245, 771–779. Wang, F., Guo, Z., Lin, T., Hu, L., Chen, Y., Zhu, Y., 2015. Characterization of carbonaceous aerosols over the East China Sea: the impact of the East Asian continental outflow. Atmos. Environ. 110, 163–173. Wang, F.J., Chen, Y., Guo, Z.G., Gao, H.W., Mackey, K.R., Yao, X.H., Zhuang, G.S., Paytan, A., 2017. Combined effects of iron and copper from atmospheric dry deposition on ocean productivity. Geophys. Res. Lett. 44, 2546–2555. Wang, R., Tao, S., Wang, W., Liu, J., Shen, H., Shen, G., Wang, B., Liu, X., Li, W., Huang, Y., Zhang, Y., Lu, Y., Chen, H., Chen, Y., Wang, C., Zhu, D., Wang, X., Li, B., Liu, W., Ma, J., 2012. Black carbon emissions in China from 1949 to 2050. Environ. Sci. Technol. 46, 7595–7603. Woodcock, A.H., 1953. Salt nuclei in marine air as a function of altitude and wind force. J. Meteorol. 10, 362–371. Wu, Z., Lin, T., Li, Z., Jiang, Y., Li, Y., Yao, X., Gao, H., Guo, Z., 2017. Air–sea exchange and gas–particle partitioning of polycyclic aromatic hydrocarbons over the northwestern Pacific Ocean: role of East Asian continental outflow. Environ. Pollut. 230, 444–452. Wu, Z., Lin, T., Li, Z., Li, Y., Guo, T., Guo, Z., 2017. Atmospheric occurrence, transport and gas–particle partitioning of polychlorinated biphenyls over the northwestern Pacific Ocean. Atmos. Environ. 167, 487–495. Xie, M., Hannigan, M.P., Barsanti, K.C., 2014. Gas/particle partitioning of n-alkanes, PAHs and oxygenated PAHs in urban Denver. Atmos. Environ. Times 95, 355–362. Yamamoto, S., Kawamura, K., 2011. Stable hydrogen isotope ratios of n-alkanes in atmospheric aerosols from Okinawa, Japan. Res. Org. Geochem. 27, 81–89. Yamamoto, S., Kawamura, K., Seki, O., Kariya, T., Lee, M., 2013. Influence of aerosol source regions and transport pathway on δD of terrestrial biomarkers in atmospheric aerosols from the East China Sea. Geochem. Cosmochim. Acta 106, 164–176. Yang, Y., Liao, H., Lou, S., 2015. Decadal trend and interannual variation of outflow of aerosols from East Asia: roles of variations in meteorological parameters and emissions. Atmos. Environ. 100, 141–153. Zhu, C., Xue, B., Pan, J., Zhang, H., Wagner, T., Pancost, R.D., 2008. The dispersal of sedimentary terrestrial organic matter in the East China Sea (ECS) as revealed by biomarkers and hydro-chemical characteristics. Org. Geochem. 39, 952–957.
8