Seasonal Variations of Terrestrial OC Sources in Aerosols over the East China Sea: The Influence of Long-Range Air Mass Transport

CHEN Qu, GUO Zhigang, YU Meng, JIN Gui’e, and ZHAO Meixun

Seasonal Variations of Terrestrial OC Sources in Aerosols over the East China Sea: The Influence of Long-Range Air Mass Transport

CHEN Qu1), 2), *, GUO Zhigang3), YU Meng1), 2), JIN Gui’e1), 2), and ZHAO Meixun1),2)

1)Frontiers Science Center for Deep Ocean Multispheres and Earth System, and Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China 2) Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237,China 3)Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention, Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China

Aerosols represent an important source of terrestrial organic carbon (OC) from the East Asian continent to the China marginal seas, thus their provenance and transport play important roles in the global carbon cycle. Fifty samples of total suspended particle were collected seasonally from the nearshore Huaniao Island (HNI) in East China Sea (ECS) from April 2018 to January 2019; and they were analyzed for total organic carbon (TOC) content and stable carbon isotope (δ13C), as well as terrestrial biomarkers including-alkanes (C20-C33),-alkanols (C20-C32) and-fatty acids (-FAs, C20-C30), to distinguish the seasonal variabilities of terrestrial OC sources and reveal the influence of the long-range air mass transport on these sources. The TOC-δ13C values (range from −27.3‰ to −24.3‰) and molecular distributions of terrestrial biomarkers both suggested that terrestrial OC contributions to aerosols had significant seasonal variations. The source indices of terrestrial biomarkers (.., Fossil% = 82.8% for-alkanes) revealed that the fossil fuel OC contributions, including coal burning and vehicular emission, were higher in winter, mainly because of the long-range air mass transport from the north of the East Asian continent. The terrestrial plant OC contributions were higher in summer (.., Wax% = 32.4% for-alkanes), likely due to local vegetation sources from HNI and East Asian continental air masses. Cluster analysis of air mass backward-trajectories clearly showed that transport pathway plays an important role in determining the organic constituents of aerosols in China marginal seas. A comparison of these terrestrial OC contributions from different air mass origins suggested that fossil fuel OC showed less variations among various air mass origins from northern China in winter, while terrestrial plant OC sources from northern and southern China in summer contributed more than that from the air masses transported through the ECS. These results provided a basis for future quantification of terrestrial OC from different origins in marine aerosols, by combining biomarker index and carbon isotopes.

organic aerosols; terrestrial biomarkers; fossil fuel OC; terrestrial plant OC; cluster analysis; air mass transport

1 Introduction

Marginal seas are the major areas of carbon sequestration with up to 90% of marine sediment organic carbon (OC) burial (Hedges and Keil, 1995). The large-scale riverine inputs and atmospheric deposition bring large amounts of terrestrial organic matter to these areas (Jurado., 2008; Tao., 2016). The eastern China marginal seas in the western Pacific Ocean, including the East China Sea (ECS), Yellow Sea and Bohai Sea, were sinks of terrestrial organic matter transported not only from the rivers but also from atmospheric deposition by the East Asian continental outflow. Atmospheric deposi-tions were a significant source of terrestrial fossil OC from continental outflows to the China marginal sea se- diment (Fang., 2015; Huang., 2016; Wang., 2017; Yu., 2018), while the terrestrial higher plant OC from the Asian continent has been also found in the Pacific Ocean sediments through atmospheric deposition (Kawamura., 2003; Zhang., 2017). The quantification of terrestrial OC sources, including both fossil fuel and terrestrial plant OC, has emerged as an important research focus of atmospheric deposition in recent years. In addition, previous studies revealed that terrestrial OC in marginal sea sediments could be brought by long-range air mass transports from Asian continent. Thus, in order to understand the connection of terrestrial OC between atmospheric transport and its deposition in sediments, it is needed to study the terrestrial OC sources brought from long-range air mass transport in organic aerosol samples over marginal seas (Fang., 1999; Guo., 2003; Feng., 2007; Tao., 2017).

Some lipid biomarkers are ubiquitous and important compounds of organic aerosols (Gagosian., 1981; Kawamura., 2003; Kang., 2016). Their chemical stability and long residence time in the environment make them useful as source indicators to apportion different OC sources in aerosols, although they are normally only a few percent of the TOC (Gagosian., 1981; Simoneit., 1991; Bush and Mcinerney, 2013). The-alkanes (C20-C33),-alkanols (C20-C32) and-fatty acids (-FAs, C20-C30) can be used as terrestrial biomarkers (Simoneit and Mazurek, 1982; Simoneit, 1986; Boreddy., 2018). Mid-chain-alkanes (MC-alkanes, C20-C25) mainly originate from fossil fuel OC sources including traffic emission and coal burning (Simoneit, 1984) while long-chain-alkanes (LC-alkanes, ≥C26) are directly emitted from terrestrial plant waxes (Fang.,1999; Yamamoto., 2011; Lyu., 2017). Mid- and long-chain-alkanols (MLC-alkanols, ≥C20) are mainly derived from epicuticular plant wax of terrestrial higher plants (Wang and Kawamura, 2005; Fu., 2008). Mid- and long-chain-FAs (MLC-FAs, ≥C20) mainly originated from surface of plant leaves and wood combustion (Rogge., 1993; Fine., 2001). Thus, studying these source-specific lipid biomarkers in organic aerosols can provide more detailed insights to estimate various terrestrial OC sources, including fossil fuel OC and terrestrial plant OC.

The ECS is located in the downwind of the East Asian continental outflow in autumn and winter when the nor- therly wind prevails, and receives a large amount of terrestrial OCatmospheric deposition from the land area surrounding Yangtze River Delta (hereinafter referred to as Yangtze River Delta) and northern China (Lin., 2013; Wang., 2015). Elucidating the characteristics of terrestrial OC in aerosols over the ECS would enhance the understanding of the significant role of air mass transport on the ECS and Northwest Pacific Ocean carbon cycle. However, previous studies of marine aerosols over ECS mainly focused on the occurrence and sources of nutrients, heavy metals and trace elements (Hsu., 2010; Zhang., 2010; Guo., 2014), rather than on their OC sources (Wang., 2015; Kang., 2017). In addition, the previous studies did not directly evaluate the influence of air mass trajectory on terrestrial OC sources.

In order to distinguish the terrestrial OC sources of aerosols over the ECS, and also to reveal the influences of long-range air mass transport on seasonal variabilities of these sources, fifty samples of total suspended particle (TSP) were collected at a nearshore island (Huaniao Island, HNI) in the ECS during four seasons. TOC based parameters (TOC content and δ13C),-alkanes (C20-C33),-alkanols (C20-C32), and-FAs (C20-C32) were analyzed to apportion seasonal variations of terrestrial OC sources,.., terrestrial plants and fossil fuel residues. Cluster analysis of air mass backward-trajectories was also used to estimate the impact from long-range air mass transport on seasonal variabilities of these sources.

2 Materials and Methods

2.1 Sampling Site and Sample Collection

The sampling site is on the rooftop of a three-story building on the northwest side of HNI (30.86˚N, 122.67˚ E; elev. About 50m.a.s.l.), approximately 66km east of ECS coast (Fig.1). This small island with 3.28km2land area has a residential population of less than 1000 people, and there is almost no industrial activity. It is an ideal receptor to assess the impacts of terrestrial OC transported from East Asian continental air masses. The air quality index was less than 100 over 340d in 2018, indicating the low air pollution in HNI. To reduce the influence of local human emissions, the sampling site is about 2km away from the population center.

A total of fifty TSP samples were collected in four sea- sons (2 April–2 May 2018, Spring; 20 August–15 September 2018, Summer; 23 October–22 November 2018, Autumn; 3 January, 2019–24 January 2019, Winter) by a high volume air sampler at a flow rate of 300mLmin−1.The sampling duration for each sample was 48 hours, starting at about 9:00am on the first day to about 9:00am of the third day. The quartz fiber filters (20cm×25cm, PALL, USA) were used as the collection substrates. And one operational blank sample was obtained in each season. Prior to sampling, the filters were baked at 450℃ for 4h to remove residual OC. Then they were equilibrated at constant temperature and humidity conditions (20℃, 40%) for 24h and weighed by a calibrated microbalance. After sampling, the pre-weighed filters wrapped with pre-baked aluminum foil were put into polyethylene sealed bags and stored at –20℃ prior to later analysis.The numbers of samples with the exception of blank ones collected in each season were listed in Table 1.

2.2 Air Mass Backward-Trajectory Analysis

Air mass backward-trajectory analysis is a common method to trace the air mass transport paths by using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model developed by the National Oceanic and Atmospheric Administration (NOAA) (https://ready. arl.noaa.Gov/hypub-bin/trajasrc.pl). Three-day backward-trajectories from the sampling site were calculated in 00:00, 06:00, 12:00, and 18:00 UTC per day at 500m above ground level during all sampling days. The backward-trajectories in each season were then clustered according to similarities in spatial variances by the TrajStat software to characterizethe origins of HNI air masses. The percentage of each cluster denoted the frequency of the backward-trajectories.

2.3 TOC and Biomarker Analysis

For each sample, the small punch area was taken to remove inorganic carbon by 6molL−1HCl, then dried in an oven at 50℃ for 6h before the measurement. TOC and the stable carbon ratio of TOC (TOC-δ13C) were determi- ned using an elemental analyzer (Thermo Flash 2000) and Stable Isotope Ratio Mass Spectrometer (Thermo Delta V). The corresponding standard deviations were 0.04% (standard samples,=5) and 0.2‰ (standard samples,= 6), respectively.

In this study, three kinds of lipid biomarkers (-alkanes,-alkanols and-FAs) were extracted from the TSP samples. Each freeze-dried filter sample was extracted six times with dichloromethane (DCM)/methanol(MeOH) (3:1, v/v) mixture by ultra-sonication. Then, the extracts were dried under pure N2and hydrolyzed with 6% KOH/ CH3OH mixture. After hydrolysis, a neutral fraction was liquid-liquid extracted with hexane, and the acid fraction was back-extracted with hexane/DCM (4:1, v/v) after acidification to pH=2. Theneutral fraction was further separated into two fractions (alkanes, alkanols) by using silica gel column with hexane and DCM/MeOH (95:5, v/v), respectively. The alkanol fraction was then derivatized with BSTFA and DCM at 70℃ for 1h. The fatty acid fraction was transesterified to the corresponding fatty acid methyl esters (FAMEs) with HCl/MeOH (5:95) mixture at 70℃ for 12h and then FAMEs were extracted with he- xane.

These three fractions were finally dissolved in 50–100 μL of isooctane. Then each fraction was directly injected into gas chromatography mass spectrometer (GC-MS) for structural identification. Gas chromatograph (GC, Agilent model 7890A) was used for quantitative analysis. The GC with a flame ionization detector (GC-FID) was equipped with a HP-1 capillary column (50m×0.32mm×0.17μm, J&W Scientific).The sample was injected 1μL on a split-lessmode and the injector port temperature was 320℃ (40℃ before analysis). Besides, the temperature program for oven was reported in Wu(2016) and Yu(2019).

2.4 Quality Assurance and Quality Control

Oneparallel operational blank sample was collected in each season to identify and quantify any contamination during sample acquisition and handling. They were processedwith the same pretreatmentand instrumental analysis as TSP samples.Targeted compounds were all below detection limits in these samples.The standard deviationwas below 10% for GC analysis.

For quantification, deuterium-substituted-C24alkane,-C19alkanol and-C19FA of known concentrations were added to the samples as internal standard prior to extraction. The C20-C33-alkanes,C20-C32-alkanols (even number), andC20-C26and-C28, C30FAs were detected in > 90% of the aerosol samples, and-C27, C29FAs were detected in < 40% of the samples.

3 Results

3.1 Meteorological Conditions and Air Mass Backward-Trajectories

The seasonal average temperature, wind speed, relative humidity and precipitationranged from 7.4 to 26.5℃, 5.5 to 7.3ms−1, 72% to 84% and 39.3 to 76.7mm, respecti- vely (Table 1). The seasonal average temperature,pre- capitation and relative humidity were highest in summer, whereas the wind speed was highest in spring. Three-day air mass backward-trajectories during the sampling periods were calculated by HYSPLIT model. These backward-trajectories were classified to four clusters in each season to quantify the air mass transport directions by using TrajStat software (Fig.1) (Lai., 2015). During spring about half of air masses were transported from the continent, with only 12% from northern China and 39% mainly affected by Yangtze River Delta (Fig.1a). During summer some 41% air masses originated from northern China and southern China, but of which 24% passed over the ECS (Fig.1b). Most air masses were from North East Asia during autumn and winter (Fig.1c and Fig.1d), with 33% of the air masses in autumn originating from the northeast, passing over the marginal seas (Fig.1c).

Table 1 Seasonal meteorological data for HNI aerosol samples during sampling time

Fig.1 Cluster analysis of the three-day backward-trajectories at HNI during four seasons. Three-day backward-trajectories in each season were classified into four clusters.

3.2 Bulk Parameters

The concentrations of TSP and TOC as well as TOC- δ13C values are summarized in Table 2. The mean concen- trations during the four seasons ranged from 40.0± 17.1 (summer) to (66.4±44.2)μgm−3(autumn) for TSP and 4.1±3.1 (autumn) to (5.1±2.7)μgm−3(winter) for TOC, respectively.TSP concentrations in spring, autumn and winter were similar, and they were all higher than that in summer. The TOC-δ13C values ranged from −27.3‰ to −24.3‰ for all samples, with the highest mean value in winter (−25.1‰±0.5‰) and lowest mean value in summer (−26.1‰±0.8‰) (Table 2).

Table 2 Concentrations of TSP (μgm−3), TOC (μgm−3) and TOC-δ13C (‰) at HNI during four seasons

3.3 Terrestrial Biomarkers

3.3.1-alkanes (C20-C33)

Fig.2 and Table 3 present the concentrations and source indices of-alkanes for four seasons. The-alkanes were measured in a range of C20-C33. The seasonal mean concentration of-alkanes ranged from 6.9±5.6 (autumn) to (29.1±12.6)ngm−3(spring). The MC-alkanes in HNI showed no odd/even carbon number predominance, with the concentrations ranged from 1.3±0.8 (autumn) to (6.5±2.4)ngm−3(spring). In contrast, LC-alkanes displayed a strong odd/even carbon numberpredominance, with the concentrations ranged from 5.6±4.9 (autumn) to (22.7±10.1)ngm−3(spring) (Table 3). The MC/LC ratios varied from 0.25±0.08 (summer) to 0.99±0.17 (winter) (Table 3). The Cmaxof-alkanes (C20-C33) wascentered at C25in winter, C29in the other three seasons.

The carbon preference index (CPI) was the concentration ratio of odd-carbon-alkanes over even-carbon-alkanes (Simoneit andMazurek, 1982; Simoneit., 1991). The CPI for total-alkanes (CPI1), MC-alkanes (CPI2) and LC-alkanes (CPI3) varied from 1.35±0.03 (winter) to 2.13±1.07 (summer), 1.31±0.03 (winter) to 1.84±0.26 (spring) and 1.41±0.24 (winter) to 2.31± 0.92 (autumn) (Table 3). The plant wax ratio (Wax%) and fossil fuel ratio (Fossil%) were used to determine the relative contribution of plant wax and fossil fuel residue to-alkanes (Simoneit, 1986; Simoneit., 1991; Kang., 2016). The Wax% of-alkanes ranged from 17.2% ±3.9% (winter) to 33.7%±14.3% (autumn) (Table 3). The highestFossil% occurred in winter (82.8%±3.9%) (Table3).The Average Chain Length (ACL) is the weight-averaged number of carbon atoms for terrestrial higher plant biomarkers, which depends on environmental conditions (.., aridity, temperature) and plant type (.., grasses, woody plants) (Peltzer and Gagosian, 1989; Kawamura., 2003; Schreuder., 2018).The ACL for LC-alkanes ranged from 29.2±0.2 (winter) to 29.5±0.2 (summer) (Table 3).

Fig.2 Seasonal variations of concentrations and molecular distributions of the n-alkanes (a, C20-C33), n-alkanols (b, C20-C32), and n-FAs (c, C20-C32) from HNI aerosols.

Table 3 Concentrations (ng m−3), Cmax and other source indices of n-alkanes, n-alkanols and n-FAs in four seasons

3.3.2-alkanols (C20-C32)

The concentrations, Cmaxand ACL of even number MLC-alkanols are summarized in Table 3 and shown in Fig.2. The seasonal mean concentration of MLC-alkanols varied from (8.5±4.7)ngm−3(winter) to (36.3±31.6)ngm−3(spring) (Table 3and Fig.2). For all the samples, the most abundant-alkanol was generally C24, C28or C30but some winter aerosol samples showed a Cmaxat C20. Accor- ding to the seasonal-alkanol abundance, Cmaxwas C28or C30in four seasons (Fig.2b). The ACL of MLC-alkanols showed a range of 25.1±0.6 (winter) to 26.4±0.9 (summer) (Table 3).

3.3.3-FAs (C20-C32)

The concentrations andthe associated indices (CPI, Cmaxand ACL) of MLC-FAs are listed in Table 3. The-FAs (

4 Discussion

4.1 Identification of the Terrestrial OC Sources Using TOC-δ13C

The TOC-δ13C values in aerosols have been widely used to identify the aerosol OC sources (Cao., 2011; Kundu and Kawamura, 2014). The comparison of TOC-δ13C in aerosols during four the seasons would provide some important information about the terrestrial OC sources in HNI aerosols. Previous studies reported that δ13C values of C3and C4plants varied from −29.2‰ to −24.6‰ and −13.8‰ to −12.3‰, δ13C values of coal varied from −24.9‰ to −22.5‰, and δ13C value of liquid fuel varied from −28.6‰ to −23.8‰ (Ballentine., 1998; Ho., 2006; Widory, 2006; Das., 2010; Ancelet., 2011). Thus, the lowest δ13C value in summer (−26.1‰) likely reflected more OC inputs from C3 plants and traffic emissions (.., ship emissions), whereas the highest δ13C value in winter (−25.1‰) can be explained by the more coal burning in northern China for heating and by long-range air mass transport (Fig. 1b). The δ13C value (−27.3‰ to −24.3‰) during four seasons in this study was lower than the marine aerosols in Cheju Island (−24.6‰ to −22.5‰) (Kundu and Kawamura, 2014) and Okinawa Island (−24.2‰ to −19.5‰) (Kunwar., 2016), possibly because HNI as a nearshore island received more traffic emissions under the influence of East Asian monsoon (Cao., 2011). And δ13C values in winter (−25.8‰ to −24.3‰) were consistent with those of PM 2.5 samples in winter from northern Chinese cities (−25.3‰ to −23.2‰) (Cao., 2011), suggesting that aerosols at HNI were largely influenced by northern China in winter. Besides,the δ13C value at HNI in winter was higher than that in Shanghai and Hangzhou (average: −25.95‰) belong to Yangtze River Delta. While in summer, the δ13C value at HNI (−26.1‰±0.5‰) was similar with that in these two cities (average: −25.8‰) (Cao., 2011).

However, the terrestrial plant and fossil fuel sources of HNI aerosol samples are not easy to specify by TOC-δ13C values, because of the overlapping δ13C values among C3 plants, coal and liquid fuel sources. Thus, on the basis of TOC-δ13C values, liquid biomarkers of aerosol samples were used to further evaluate the seasonal variations of terrestrial OC sources (terrestrial plant and fossil fuel OC) and the influence of long-range air mass transport.

4.2 Identification of the Terrestrial OC Sources Using Biomarkers

MC-alkanes are usually considered to be mainly from fossil fuel residues, while LC-alkanes, MLC-alkanols and MLC-FAs are commonly attributed to terrestrial plants (Simoneit, 1984; Rogge., 1993; Fang., 1999; Wang and Kawamura, 2005). In order to assess the seasonal contributions of fossil fuel OC and terrestrial plant OC sources to organic aerosols, the concentrations and source indices of terrestrial biomarkers were shown in Table 3. The concentration of terrestrial biomarkers not only depended on the contribution of OC sources, but also on the total aerosol and TOC concentrations, and they tended to covary with the concentration of TSP and TOC (Chen., 2021). Thus, instead of concentrations, the source indices such as MC/LC ratios, CPI, Cmax, Wax%, Fossil% and ACL were used to avoid misleading information about aerosol OC sources and transport path- ways.

4.2.1 Higher fossil fuel OC contributions in winter

The MC/LC ratios, CPI, Cmax, Fossil% and ACL for-alkanes have been widely used to identify the fossil fuel OC source strengths in aerosols (Simoneit., 2004; Jeng, 2006; Chen., 2014; Ren., 2016; Boreddy., 2018). The highest MC/LC ratio, Fossil% and the lowest CPI (CPI1, CPI2), ACL in winter all indicate that fossil fuel OC in winter aerosols exceeded that in the other three season aerosols. The MC/LC ratio in winter was four-fold greater than in summer, while the Fossil% in winter was about 25% higher than in autumn (Table 3). And the CPI in winter is close to 1, which is normally the value for the lipids emitted from fossil fuels (Rogge., 1993; Simoneit, 2004). Total-alkane Cmaxvalues of C25in winter, compared to C29in spring, summer and autumn, lend further support to the larger contribution of fossil fuel OC to HNI aerosols in winter, because previous studies showed that the Cmaxwas C25for sub-bitu- minous coal, lignite and gasoline vehicles (Oros and Simoneit, 2000; Schauer., 2002). The semi-volatile MC-alkanes can partition between gas and particulate phases during atmospheric dispersion processes, the- alkane Cmaxvalues can vary depending on temperature (Van Vaeck., 1984). Thus, Cmaxvalues must be interpreted cautiously as OC source indices.

Seasonal variations of fossil fuel OC contributions to HNI aerosols were likely caused by the traffic emissions and coal usage for domestic heating which was transported from East Asian continent in winter (Wang., 2015). Coal combustion for heating remains prevalent in winter throughout northern China (Feng., 2007). The lowest temperature (Table 1) likely also contributed to enhanced fossil fuel OC in winter aerosols, which is due to the reduced volatilization of MC-alkanes (Lyu., 2017). All the air masses in winter originated from the northwest directions, including Mongolia, Russia and northern China, where coal-burning is common in winter (Fig.1d).Meanwhile, the higher fossil fuel contribution in winter was less affected by the ocean pathway over the Yellow Sea and ECS because the oceanic atmosphere in winter could contain continental outflow and pollutant emissions from northern China (Feng., 2006).

4.2.2 Higher terrestrial plant OC contributions in summer

The seasonal variations of terrestrial plant OC contributions to aerosols can be estimated by the Wax%, CPI, Cmaxand ACL values of terrestrial biomarkers (Simoneit., 2004; Jeng, 2006; Chen., 2014; Ren., 2016; Boreddy., 2018). The higher values of source indices: Wax%, CPI for LC-alkanes (CPI3) and MLC-FAs (CPI4), ACL for LC-alkanes, MLC-alkanols and MLC-FAs, Cmaxfor MLC-alkanes in summer suggested higher contribution of terrestrial plant OC to summer HNI aerosols. The Wax% in summer was about two times higher than in winter (Table 3). The higher contribution may be attributed to higher temperature, humidity and precipitation in summer (Table 3), which promote the vegetation growth (Yadav., 2013). Almost half of air masses originated from southern China in summer provided potential source of terrestrial plant OC to aerosols (Fig.1b). Therefore, higher terrestrial plant OC contributions in summer were both attributed to local vegetation sources and long-range air mass transport from China. The Wax% in summer (average: 32.4%) were all lower than that in these three cities (Shanghai: 50.0%; Qingdao: 39.8%, Beijing: 43.0%) (Guo., 2003; Duan., 2010; Wang., 2016). This is probably because HNI was affected by more marine contributions and the ship emissions (fossil fuel contributions) transported from the Yangtze River Delta and Yangshan Port in summer compared with the Chinese cities (Wang., 2016; Yu., 2018).

4.3 Cluster Analysis of Terrestrial OC Sources in Winter and Summer

Cluster analysis of backward-trajectories was used to characterize the fossil fuel and terrestrial plant OC brought from different air mass transport paths in HNI aerosols. The indices of fossil fuel OC and terrestrial plant OC sources in each cluster were calculated by the proportion of air mass backward-trajectories from different samples. Winter and summer samples were selected for cluster analysis of terrestrial OC sources (Fig.1), because higher fossil fuel OC contribution occurred in winter and higher terrestrial plant OC contribution occurred in summer. The concentrations and source indices (.., CPI, Wax%, ACL) of-alkanes,-alkanols and-FAs in each cluster were summarized in Fig.3 and Table 4.

Fig.3 Source indices for backward-trajectory clusters in summer and winter.

Table 4 Concentrations (ng m−3) of n-alkanes, n-alkanols and n-FAs for backward-trajectory clusters in summer and winter

In summer, the air masses of Clusters 3 and 4 from China brought more terrestrial OC, compared with Cluster 1 and 2 originated from the ocean. The terrestrial biomarkers (MC-alkanes, LC-alkanes, MLC-alkanols and MLC-FAs) concentrations in Cluster 3 were 0.6 to 1.9- fold greater than those in Cluster 2 (Table 4). CPI1, CPI3, Wax% and ACL (LC-alkanes) were all higher in Clusters 3 and 4, which supported the supposition that the contribution of terrestrial plant wax in HNI aerosol OC mainly came from China, not the air masses transported though ECS (Fig.3). However, higher MC/LC, Fossil% and lower CPI1, CPI3, ACL occurred in Clusters 1 and 2, suggesting significant contribution of fossil fuel emissions brought from marine air masses over ECS (Fig.3). This is likely because fossil fuel OC in Clusters 1 and 2 are not only from continental outflows, but also from ship emissions around Yangtze River Delta (Wang., 2016). Thus, comparing with the continental air masses in summer, the input of marine air mass reduced the contribution of terrestrial plant and increased the contribution of fossil fuel OC sources to HNI aerosols.

In winter, the air masses of Clusters 1 to 4 all originated fromthe north of East Asian continent. The MC-alkanes, LC-alkanes, MLC-alkanols and MLC-FAs concentrations in Cluster 4 were 0.5–1.4 times higher than those in Cluster 2, suggesting that terrestrial OC abundance decreased due to the air masses passed over Bohai Sea and Yellow Sea (Fig.1d). Compared with the concentrations of terrestrial biomarkers, the source indices (MC/LC, Fossil%, CPI, ACL) in winter showed less variations probably because all air mass in winter originated from northern East Asia.

In total, the variations of fossil fuel and terrestrial plant OC contributions to HNI aerosols among four clusters in summer were larger than those in winter, respectively. Because 59% air masses (Clusters 1 and 2) originated from the ocean in summer, but air masses were almost exclusively originated from the continent in winter (Fig.1). Although the marine air mass in summer reduced the contribution of terrestrial plant, Wax% from marine air masses in summer (Cluster 1: 24.4%; Cluster 2: 21.7%) were also higher than continental air masses in winter (Clusters 1–4: 14.6%–21.4%) (Fig.3). Wax% and Fossil% from continental origins in summer (Clusters 3 and 4) were 2.6 and 0.7 times as much as that in winter (Clusters 1 to 4), respectively (Fig.3). However, the mean Wax% and Fossil% in summer was about 1.9 and 0.8 times as much as that in winter, respectively (Fig.3). Thus, the dilution of the terrestrial plant and fossil fuel OC contributions by marine air mass transport confirmed that the origins of long-range air mass transport can influence the OC sources in aerosols.

5 Conclusions

The seasonal TOC-δ13C values suggested that the terrestrial OC contributions were different in summer and winter, which was also supported by source indices of terrestrial biomarkers. Higher fossil fuel OC contributions, including coal burning and vehicular emission in winter, were mainly caused by the long-range air mass transport originated from north of the East Asian continent. Higher terrestrial plant OC contributions in summer were likely attributed to local vegetation sources and continental air masses from Northeast China and Zhejiang Province. According to the cluster analysis of air mass backward-trajectories, winter terrestrial OC showed less variations among different air mass origins in northern China. In summer, the ECS contributed more fossil fuel OC, while northern and southern China contributed more terrestrial plant OC. The long-range marine air mass transport would dilute the differences of terrestrial OC contributions between winter and summer.

This study proposed an approach to evaluate the influence of the long-range air mass transports on seasonal terrestrial OC sources, and provided a basis for the quantification of terrestrial OC from different origins in marine aerosols. To further quantify aerosol terrestrial OC sources and the contributions from long-range air mass transports, carbon isotopes of both TOC and source- specific biomarkers are needed in the future study.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. U1706219). This is MCTL (Key Laboratory of Marine Chemistry Theory and Technology) contribution #237.

Ancelet, T., Davy, P. K., Trompetter, W. J., Markwitz, A., and Weatherburn, D. C., 2011. Carbonaceous aerosols in an urban tunnel., 45 (26): 4463-4469.

Ballentine, D. C., Macko, S. A., and Turekian, W. C., 1998. Variability of stable carbon isotopic compositions in individual fatty acids from combustion of C4and C3plants: Implications for biomass burning., 152 (1-2): 151- 161.

Boreddy, S. K. R., Haque, M. M., Kawamura, K., Fu, P. Q., and Kim, Y., 2018. Homologous series of n-alkanes (C19-C35), fatty acids (C12-C32) and n-alcohols (C8-C30) in atmospheric aerosols from central Alaska: Molecular distributions, seasonality and source indices., 184: 87-97.

Bush, R. T., and Mcinerney, F. A., 2013. Leaf wax-alkane distributions in and across modern plants: Implications for paleoecology and chemotaxonomy., 117: 161-179.

Cao, J. J., Chow, J. C., Tao, J., Lee, S. C., Watson, J. G., Ho, K. F.,., 2011. Stable carbon isotopes in aerosols from Chinese cities: Influence of fossil fuels., 45 (6): 1359-1363.

Chen, Y., Cao, J. J., Zhao, J., Xu, H. M., Arimoto, R., Wang, G. H.,., 2014.-alkanes and polycyclic aromatic hydrocarbons in total suspended particulates from the southeastern Tibetan Plateau: Concentrations, seasonal variations, and sources., 470-471 (2): 9-18.

Chen, Q., Guo, Z. G., Yu, M., Sachs, J. P., Hou, P. F., Li, L.,., 2021. Lipid biomarker estimates of seasonal variations of aerosol organic carbon sources in coastal Qingdao, China., 151: 104148.

Das, O., Wang, Y., and Hsieh, Y. P., 2010. Chemical and carbon isotopic characteristics of ash and smoke derived from burning of C3and C4grasses., 41 (3): 263- 269.

Duan, F. K., He, K. B., and Liu, X. D., 2010. Characteristics and source identification of fine particulate-alkanes in Beijing, China., 22 (7): 998-1005.

Fang, M., Zheng, M., Wang, F., Chim, K. S., and Kot, S. C., 1999. The long-range transport of aerosols from northern China to Hong Kong–A multi-technique study., 33 (11): 1803-1817.

Fang, Y., Chen, Y., Tian, C., Lin, T., Hu, L., Huang, G.,., 2015. Flux and budget of BC in the continental shelf seas adjacent to Chinese high BC emission source regions., 29 (7): 957-972.

Feng, J. L., Chan, C. K., Fang, M., Hu, M., He, L. Y., and Tang, X. Y., 2006. Characteristics of organic matter in PM2.5 in Shanghai., 64 (8): 1393-1400.

Feng, J. L., Guo, Z. G., Chan, C. K., and Fang, M., 2007. Properties of organic matter in PM2.5 at Changdao Island, China–A rural site in the transport path of the Asian continental outflow., 41 (9): 1924-1935.

Fine, P. M., Cass, G. R., and Simoneit, B. R. T., 2001. Chemical characterization of fine particle emissions from fireplace combustion of woods grown in the northeastern United States., 35 (13): 2665-2675.

Fu, P. Q., Kawamura, K., Okuzawa, K., Aggarwal, S. G., Wang, G., Kanaya, Y.,., 2008. Organic molecular compositions and temporal variations of summertime mountain aerosols over Mt. Tai, North China Plain., 113 (19): 1429-1443.

Gagosian, R. B., Peltzer, E. T., and Zafiriou, O. C., 1981. Atmospheric transport of continentally derived lipids to the tropical North Pacific., 291: 312-314.

Guo, L., Chen, Y., Wang, F. J., Meng, X., Xu, Z. F., and Zhuang, G. S., 2014. Effects of Asian dust on the atmospheric input of trace elements to the East China Sea., 163: 19-27.

Guo, Z. G., Sheng, L. F., Feng, J. L., and Fang, M., 2003. Seasonal variation of solvent extractable organic compounds in the aerosols in Qingdao, China., 37 (13): 1825-1834.

Hedges, J. I., and Keil, R. G., 1995. Sedimentary organic matter preservation: An assessment and speculative synthesis., 49 (2-3): 123-126.

Ho, K. F., Lee, S. C., Cao, J. J., Li, Y. S., Chow, J. C., Watson, J. G.,., 2006. Variability of organic and elemental carbon, water soluble organic carbon, and isotopes in Hong Kong., 6 (3): 4569- 4576.

Hsu, S. C., Wong, G., Gong, G., Shiah, F., Huang, Y., Kao, S.,., 2010. Sources, solubility, and dry deposition of aerosol trace elements over the East China Sea., 120 (1-4): 116-127.

Huang, L., Zhang, J., Wu, Y., and Wang, J., 2016. Distribution and preservation of black carbon in the East China Sea sediments: Perspectives on carbon cycling at continental margins., 124: 43-52.

Jeng, W. L., 2006. Higher plant-alkane average chain length as an indicator of petrogenic hydrocarbon contamination in marine sediments., 102 (3/4): 242-251.

Jurado, E., Dachs, J., Duarte, C. M., and Simó, R., 2008. Atmospheric deposition of organic and black carbon to the global oceans., 42 (34): 7931-7939.

Kang, M. J., Fu, P. Q., Aggarwal, S. G., Kumar, S., Zhao, Y., Sun, Y.,., 2016. Size distributions of-alkanes, fatty acids and fatty alcohols in springtime aerosols from New Delhi, India., 219: 957-966.

Kang, M. J., Yang, F., Ren, H., Zhao, W. Y., Zhao, Y., Li, L. J.,., 2017.Influence of continental organic aerosols to the marine atmosphere over the East China Sea: Insights from lipids, PAHs and phthalates., 607-608: 339-350.

Kawamura, K., Ishimura, Y., and Yamazaki, K., 2003. Four years’ observations of terrestrial lipid class compounds in marine aerosols from the western North Pacific., 17 (1): 1-19.

Kundu, S., and Kawamura, K., 2014. Seasonal variations of stable carbon isotopic composition of bulk aerosol carbon from Gosan site, Jeju Island in the East China Sea., 94: 316-322.

Kunwar, B., Kawamura, K., and Zhu, C. M., 2016. Stable carbon and nitrogen isotopic compositions of ambient aerosols collected from Okinawa Island in the western North Pacific Rim, an outflow region of Asian dusts and pollutants., 131: 243-253.

Lai, S., Xie, Z., Song, T., Tang, J., Zhang, Y., and Mi, W., 2015. Occurrence and dry deposition of organophosphate esters in atmospheric particles over the northern South China Sea., 127: 195-200.

Lin, T., Hu, L. M., Guo, Z. G., Zhang, G., and Yang, Z. S., 2013. Deposition fluxes and fate of polycyclic aromatic hydrocarbons in the Yangtze River Estuarine-inner shelf in the East China Sea., 27 (1): 77-87.

Lyu, Y., Xu, T. T., Yang, X., Chen, J. M., Cheng, T. T., and Li, X., 2017. Seasonal contributions to size-resolved-alkanes (C8–C40) in the Shanghai atmosphere from regional anthropogenic activities and terrestrial plant waxes., 579: 1918-1928.

Oros, D. R., and Simoneit, B. R. T., 2000. Identification and emission rates of molecular tracers in coal smoke particulate matter., 79 (5): 515-536.

Peltzer, E. T., and Gagosian, R. B., 1989. Organic geochemistry of aerosols over the Pacific Ocean., 10: 281-338.

Ren, L. J., Fu, P. Q., He, Y., Hou, J. Z., Chen, J., Pavuluri, C. M.,., 2016. Molecular distributions and compound-specific stable carbon isotopic compositions of lipids in wintertime aerosols from Beijing., 6 (1): 27481.

Rogge, W. F., Hildemann, L. M., Mazurek, M. A., and Cass, G. R., 1993. Sources of fine organic aerosol. 4. Particulate abrasion products from leaf surfaces of urban plants., 27 (13): 2700-2711.

Schauer, J. J., Kleeman, M. J., Cass, G. R., and Simoneit, B. R. T., 2002. Measurement of emissions from air pollution sources. 5. C1-C32organic compounds from gasoline-powered motor vehicles., 36 (6): 1169- 1180.

Schreuder, L. T., Stuut, J. B. W., Korte, L. F., Sinninghe Damsté, J. S., and Schouten, S., 2018. Aeolian transport and deposition of plant wax-alkanes across the tropical North Atlantic Ocean., 115: 113-121.

Simoneit, B. R. T., 1984. Organic matter of the troposphere–III. Characterization and sources of petroleum and pyrogenic residues in aerosols over the western United States., 18 (1): 51-67.

Simoneit, B. R. T., 1986. Characterization of organic constituents in aerosols in relation to their origin and transport: A review., 23 (3): 207-237.

Simoneit, B. R. T., and Mazurek, M. A., 1982. Organic matter of the troposphere–II. Natural background of biogenic lipid matter in aerosols over the rural western United States., 16 (19): 2139-2159.

Simoneit, B. R. T., Kobayashi, M., Mochida, M., Kawamura, K., and Huebert, B. J., 2004. Aerosol particles collected on aircraft flights over the northwestern Pacific region during the ACE–Asia campaign: Composition and major sources of the organic compounds., 109 (19): 159-172.

Simoneit, B. R. T., Sheng, G. Y., Chen, X. J., Fu, J. M., Zhang, J., and Xu, Y. P., 1991. Molecular marker study of extractable organic matter in aerosols from urban areas of China., 25 (10): 2111-2129.

Tao, S. Q, Eglinton, T. I., Montluçon, D. B., McIntyre, C., and Zhao, M. X., 2016. Diverse origins and pre-depositional histories of organic matter in contemporary Chinese marginal sea sediments., 191: 70- 88.

Tao, S. Q., Yin, X. J., Jiao, L. P., Zhao, S. H., and Chen, L. Q., 2017. Temporal variability of source-specific solvent-extra- ctable organic compounds in coastal aerosols over Xiamen, China., 8: 33.

Van Vaeck, L., Van Cauwenberghe, K., and Janssens, J., 1984. The gas-particle distribution of organic aerosol constituents: Measurement of volatilization artifact in HI-VOL cascade impactor sampling., 18: 417-430.

Wang, C., Zou, X., Zhao, Y., Li, Y., Song, Q., Wang, T., and Yu, W., 2017. Distribution pattern and mass budget of sedimentary polycyclic aromatic hydrocarbons in shelf areas of the Eastern China marginal seas., 112 (6): 4990-5004.

Wang, G. H., and Kawamura, K., 2005. Molecular characteristics of urban organic aerosols from Nanjing: A case study of a mega-city in China., 39 (19): 7430-7438.

Wang, F. W., Guo, Z. G., Lin, T., and Rose, N. L., 2016. Seasonal variation of carbonaceous pollutants in PM2.5 at an urban ‘supersite’ in Shanghai, China., 146: 238- 224.

Wang, F. W., Guo, Z. G., Lin, T., Hu, L. M., Chen, Y. J., and Zhu,Y. F., 2015. Characterization of carbonaceous aerosols over the East China Sea: The impact of the East Asian continental outflow., 110: 163-173.

Widory, D., 2006. Combustibles, fuels and their combustion products: A view through carbon isotopes., 10 (5): 831-841.

Wu, P., Bi, R., Duan, S. S., Jin, H. Y., Chen, J. F., and Hao, Q., 2016. Spatiotemporal variations of phytoplankton in the East China Sea and the Yellow Sea revealed by lipid biomarkers., 121 (1): 109-125.

Yadav, S., Tandon, A., and Attri, A. K., 2013. Monthly and seasonal variations in aerosol associated-alkane profiles in relation to meteorological parameters in New Delhi, India., 13 (1): 287-300.

Yamamoto, S., Kawamura, K., and Seki, O., 2011. Long-range atmospheric transport of terrestrial biomarkers by the Asian winter monsoon: Evidence from fresh snow from Sapporo, northern Japan., 45 (21): 3553- 3560.

Yu, M., Guo, Z. G., Wang, X. C., Eglinton, T. I., Yuan, Z. N., Xing, L., Zhang, H. L., and Zhao, M. X., 2018. Sources and radiocarbon ages of aerosol organic carbon along the east coast of China and implications for atmospheric fossil carbon contributions to China marginal seas., 619: 957-965.

Yu, M., Timothy, I. E., Negar, H., Daniel, B. M., Lukas, W., Hou, P. F., Zhang, H. L., and Zhao, M. X., 2019. Impacts of natural and human-induced hydrological variability on particulate organic carbon dynamics in the Yellow River., 53 (3): 1119-1129.

Zhang, H. L., Xing, L., and Zhao, M. X., 2017. Origins of terrestrial organic matter in surface sediments of the East China Sea shelf., 16: 793-802.

Zhang, Y., Yu, Q., Ma, W. C.,and Chen, L. M., 2010. Atmospheric deposition of inorganic nitrogen to the eastern China Seas and its implications to marine biogeochemistry., 115: D00K10.

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© Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2021

(Edited by Ji Dechun)