Elevated nitrogen-containing particles observed in Asian dustaerosol samples collected at the marine boundary layer of the BohaiSea and the Yellow Sea
H. Geng1,2, Y. Park1, H. Hwang3, S. Kang1, and C.-U. Ro1
1Department of Chemistry, Inha University, Incheon, 402–751, Korea2Research Center of Environmental Science and Engineering, Shanxi University, Taiyuan, 030006, China3Korea Polar Research Institute, Incheon, 406–840, Korea
Received: 23 April 2009 – Published in Atmos. Chem. Phys. Discuss.: 22 June 2009Revised: 27 August 2009 – Accepted: 3 September 2009 – Published: 22 September 2009
Abstract. Low-Z particle electron probe X-ray microanal-ysis (low-Z particle EPMA) shows powerful advantages forthe characterization of ambient particulate matter in environ-mental and geological applications. By the application ofthe low-Z particle EPMA single particle analysis, an overallexamination of 1800 coarse and fine particles (aerodynamicdiameters: 2.5–10µm and 1.0–2.5µm, respectively) in sixsamples collected on 28 April–1 May 2006 in the marineboundary layer (MBL) of the Bohai Sea and Yellow Sea wasconducted. Three samples (D1, D2, and D3) were collectedalong the Bohai Bay, Bohai Straits, and Yellow Sea near Ko-rea during an Asian dust storm event while the other threesamples (N3, N2, and N1) were collected on non-Asian dust(NAD) days. Based on X-ray spectral and secondary elec-tron image data, 15 different types of particles were identi-fied, in which soil-derived particles were encountered withthe largest frequency, followed by (C, N, O)-rich droplets(likely the mixture of organic matter and NH4NO3), parti-cles of marine origin, and carbonaceous, Fe-rich, fly ash, and(C, N, O, S)-rich droplet particles. Results show that dur-ing the Asian dust storm event relative abundances of the(C, N, O)-rich droplets and the nitrate-containing secondarysoil-derived particles were markedly increased (on averageby a factor of 4.5 and 2, respectively in PM2.5−10 fractionand by a factor of 1.9 and 1.5, respectively in PM1.0−2.5 frac-tion) in the MBL of the Bohai Sea and Yellow Sea, implyingthat Asian dust aerosols in springtime are an important car-rier of gaseous inorganic nitrogen species, especially NOx(or HNO3) and NH3.
The Asian dust storm event is a seasonal meteorologicalphenomenon that mostly originates in the deserts of Mon-golia and Northern China and Kazakhstan (Sullivan et al.,2007). Nearly every spring, usually from March to May,Asian dust aerosols will be carried eastward by strong windsand pass over Eastern China, North and South Korea, Japan,open seas and oceans, and even arrive in the United States(Bishop, et al., 2002; Zhao, et al., 2008). During long-range transport, Asian dust aerosols can react with a diversityof chemical species, coagulate with other particles, and/orprovide reaction sites in the atmosphere, so that they po-tentially carry many chemical species along with the origi-nal soil components, and the physical and chemical proper-ties of the dust particles may change (Hwang et al., 2008).These change/coagulation processes affect the optical andhygroscopic properties of Asian dust particles, which are re-lated to important but uncertain direct impacts on climatechange, radiative budget, and possibly rainfall patterns (Ra-manathan, et al., 2001; Jickells et al., 2005; Su et al., 2008),and they also affect the cycles and chemical balances of im-portant trace gases (e.g. SO2, NOx, O3), acid deposition,and the pH of precipitation (Sullivan et al., 2007). Thus,increasing attention has been devoted to the study of mod-ification of the physicochemical properties of Asian dustparticles during long-range transport. It was observed thatAsian dust aerosols collected in Korea experienced reactionswith NOx and SO2 so that a number of nitrate- or sulfate-containing particles such as NaNO3, Na2SO4, Ca(NO3)2,and (NH4)2SO4 were encountered (Hwang and Ro, 2006;Ro et al., 2005). In Japan, Hong Kong, Taiwan, and even
Published by Copernicus Publications on behalf of the European Geosciences Union.
6934 H. Geng et al.: Elevated nitrogen-containing particles observed in Asian dust aerosol samples
mainland China (from the source area to Chinese coastal re-gions), Asian dust particles have been reported to undergosignificant chemical modifications through mixing with seasalts or reacting with anthropogenic air pollutants (Fang etal., 1999; Ma et al., 2001; Wang et al., 2007; Lin et al., 2008).
Through “bulk” and “single-particle” analyses, the chem-ical compositional characteristics of Asian dust aerosol par-ticles in the regions surrounding the Yellow Sea have alsobeen investigated (Lee et al., 2002; Hwang and Ro, 2006;Zhang et al., 2007), but few reports have been published oncompositional modification of aerosol particles in the marineboundary layer (MBL) of the Bohai Sea and Yellow Sea dur-ing Asian dust storm events. Owing to a lack of the directobservations, some conclusions on the interactions amongmineral Asian dust, sea salts, and anthropogenic air pollu-tants (e.g. ammonium, and sulfur or nitrogen oxides species)in the MBL had to rely on speculation to some extent (Jeongand Park, 2008; Nishikawa et al., 1991; Park et al., 2003).Jeong and Park (2008) suggested that the favorable areasfor gas (e.g. SO2, NOx) – aerosol interaction during Asiandust storm events were the downstream regions where rela-tive humidity is relatively high. Nishikawa et al. (1991) at-tributed the increase of concentrations of SO2−
4 and NO−
3 incoarse mode particles over Japan to the interactions betweengaseous acidic species and Asian dust particles over the ma-rine area. From the observations that SO2−
4 and NO−
3 wereproduced on the surface of Asian dust particles collected inJapan, but hardly formed on those particles over Beijing andQingdao, China (Zhang and Iwasaka, 1999; Zhang et al.,2003; Fan et al., 1996; Ma et al., 2001), Park et al. (2003)suggested the higher concentrations of particulate SO2−
4 andNO−
3 at Gosan, Korea, were generated during the transportof dust storms over the Yellow Sea. Therefore, it is criticaland necessary to investigate the morphology and chemicalcomposition of individual particles in the MBL of the Bo-hai Sea and Yellow Sea during Asian dust storm events inorder to better understand the compositional modification ofAsian dust aerosols in the atmosphere over China, Korea, andJapan.
Asian dust particles are chemically and morphologicallyheterogeneous (Wang et al., 2009). Their “average” com-positions and aerodynamic diameters could be obtained by“bulk” analysis, but the exact aerosol mixing state and thechemical micro-processes in (or on) aerosol particles havebeen unambiguously determined by the application of singleparticle measurements (Tsuji et al., 2008). A recently devel-oped electron probe X-ray microanalysis (EPMA) technique,named low-Z particle EPMA, shows powerful advantagesfor the characterization of environmental and geological sin-gle particles (Ro et al., 2001a, 2005). By the application ofthe technique, the particle size distribution and the quantita-tive chemical compositions of individual particles have beenobtained without a complicated sample pretreatment process,and many environmentally important atmospheric particles,e.g. sulfates, nitrates, ammonium, and carbonaceous species
have been at least semi-quantitatively elucidated (Ro et al.,2000, 2005). Even the reaction process of Asian dust and seasalts with NOx or SO2 species were observed (Hwang andRo, 2006). Hence, herein, low-Z particle EPMA was utilizedto examine the morphologies and chemical compositions ofaerosol particles (including Asian dust aerosols) collected ona commercial ferryboat plying between Incheon, Korea andTianjin, China from 28 April to 1 May 2006.
The objective of the present study is to characterize sin-gle aerosol particles collected in the MBL of the Bohai Seaand Yellow Sea in the springtime, and to investigate the in-fluence of an Asian dust storm event on their compositionalcharacteristics and relative abundances.
2 Materials and methods
2.1 Samples
Six sets of aerosol samples were collected on a ferryboatplying between the seaports of Incheon, Korea, and Tianjin,China. The sampling locations along the navigation route areshown in Fig. 1. The first sampling location (for samples D1and N3) is on the Bohai Sea near the Tianjin seaport, rep-resenting the windward place where aerosols are influencedmostly by the continental outflow from mainland China; thesecond location (for samples D2 and N2) is at the demarca-tion area between the Bohai Sea and the Yellow Sea, whichis regarded as a relatively clean marine environment (Fenget al., 2007); the third location (for samples D3 and N1) ison the Yellow Sea near the Incheon port, which is a leewardplace where ambient aerosols are influenced by air massesfrom the Seoul-Incheon metropolitan area as well as frommainland China and the marine environment. Samples N1,N2, and N3 were collected when the ferryboat travelled fromKyonggi-man (near the Incheon port) to the Bohai Straits,and to the Bohai Bay (in the west of the Bohai Sea) on 28and 29 April 2006 (Table 1). The other three samples D1, D2,and D3 were collected on 30 April and 1 May 2006 when theferry returned from Tianjin to the Incheon seaport while anAsian dust storm event originating from Mongolia and north-east China was occurring (Fig. 2). In this work, Asian dustsamples are designated as D1, D2, and D3 and non-Asiandust (NAD) samples designated as N1, N2, and N3.
The sources of aerosol particles for different samples wereinferred through backward air-mass trajectories that werecalculated via the HYbrid Single Particle Lagrangian Inte-grated Trajectory (HYSPLIT) model at the National Oceano-graphic and Atmospheric Administration (NOAA) Air Re-sources Laboratory’s web server (http://www.arl.noaa.gov/ready/hysplit4.html), as shown in Fig. 3. The air masses forsamples D1, D2, and D3 mostly originated from Mongoliaand northeast China, unlike those for N1 (air masses stayedaround 36 h over the Yellow Sea), N2 (air masses at recep-tor heights of 300 m, 500 m, and 1000 m originated from
H. Geng et al.: Elevated nitrogen-containing particles observed in Asian dust aerosol samples 6935
Table 1. Sampling date and time, sampling sites, and geographical traits of sampling locations.
Sampling locations Geographical traits Sample Sampling time (KST)
Yellow Sea(near Incheon, Ko-rea)
The Yellow Sea, a semi-enclosedmarginal sea of the Northwest Pa-cific Ocean, is surrounded by main-land China and the Korean penin-sula. To the northwest is the Bo-hai Sea and to the south is the EastChina Sea.
N1D3
At 22:30∼01:30 on 28 April 2006At 07:50∼13:00 on 1 May 2006
The Bohai Straits is located be-tween Liaodong and Shandongpeninsula, connecting the YellowSea and Bohai Sea. Several chan-nels are formed as it is compartedby many small islands.
N2D2
At 10:20∼16:00 on 29 April 2006At 21:00∼22:40 on 30 April 2006
Bohai Bay(a part of the Bo-hai Sea near Tian-jin, China)
The Bohai Sea is a nearly enclosedinterior sea with 3784 km of coast-line and an average depth of 18 m. Itis the largest inland sea of China andis composed of Liaodong Bay (tothe north), Bohai Bay (to the west),Laizhou Bay (to the south), and themiddle sea.
N3D1
At 16:00∼19:00 on 29 April 2006At 14:00∼17:05 on 30 April 2006
Note: Samples N1, N2, and N3 were collected on non-Asian dust (NAD) days while D1, D2, D3 were collected during an Asian dust stormevent.
Figures (Fig. 1- Fig. 5): 778
779
780
781
782
Fig. 1. Map of the ferryboat sailing route and sampling locations
- 34 -
Fig. 1. Map of the ferryboat sailing route and sampling locations.
northeastern, eastern, and southeastern China, respectively),and N3 (air masses at heights of 300 m and 500 m originatedfrom central and southern China and those at 1000 m weremainly from western China).
Particles were collected on Ag foils by a three-stage cas-cade Dekati PM10 impactor with aerodynamic cut-off diam-eters of 10µm, 2.5µm, and 1.0µm at a 10 L/min of flowrate. The impactor was set at the highest location on theferryboat prow, about 15 m distant from sea level. For each
sample, sampling durations varied to obtain an appropriatenumber of particles without overloading. The collected sam-ples were put in plastic carriers, sealed, and stored beforeEPMA measurements. The stage 2 and 3 particles (2.5–10µm and 1.0–2.5µm size range, respectively), designatedas “PM2.5−10 fraction” and “PM1.0−2.5 fraction” for conve-nience, were measured and analyzed.
2.2 EPMA measurement
The low-Z particle EPMA measurement was carried outon a Hitachi S-3500N environmental scanning electronmicroscope equipped with an Oxford Link SATW ultra-thinwindow energy-dispersive X-ray spectrometry (EDX) detec-tor. The resolution of the detector was 133 eV for Mn–Kα
X-ray. The X-ray spectra were recorded under the controlof EMAX Hitachi software. A 10 kV accelerating voltage,1.0 nA of beam current, and a typical measuring time of 10 swere employed to ensure a low background level of spectra,a good sensitivity for low-Z element analysis, and statisti-cally sufficient counts in the X-ray spectra (Ro et al., 1999).The cold stage of the electron microprobe at liquid nitro-gen temperature (∼−193◦C) minimized contamination andlowered beam-damage effects on sensitive particles. Over-all, 1800 particles were analyzed manually (150 particles foreach stage sample).
6936 H. Geng et al.: Elevated nitrogen-containing particles observed in Asian dust aerosol samples783
784
785
786
787
788
789
(a) 23:19 p.m. on 30 Apr. 2006 (KST) (b) 07:02 a.m. on 1 May 2006 (KST)
Fig. 2. GOES-9 Satellite images of the Asian dust storm event (provided by Nation
al Oceanic and Atmospheric Administration (NOAA), the United States). (Orange
areas in GOES images denote high-density dust, and blue areas indicate dust dis
persion areas. Red dots denote cities.)
- 35 -
Fig. 2. GOES-9 Satellite images of the Asian dust storm event (provided by National Oceanic and Atmospheric Administration (NOAA),the United States). (Orange areas in GOES images denote high-density dust, and blue areas indicate dust dispersion areas. Red dots denotecities.)
2.3 Data analysis
The size and shape of each individual particle were input inthe quantification procedure and the net X-ray intensities ofthe elements were obtained by non-linear least-squares fittingof the collected spectra through the AXIL program (Veke-mans et al., 1994). A Monte Carlo calculation combinedwith reverse successive approximations was applied to de-termine the particles’ elemental concentrations on the basisof the X-ray intensities (Ro et al., 2003). For standard par-ticles, the quantification procedure provided results accuratewithin 12% relative deviations between the calculated andnominal elemental concentrations, except for C and K wherethe characteristic X-rays overlap with those from the Ag sub-strate (Ro et al., 2001b). The formula concentrations and thegroup distributions were rapidly and reliably determined bythe “expert system” program (Ro et al., 2004).
3 Results and discussion
3.1 Classification of individual particles based on theirX-ray spectra and secondary electron image (SEI)
A brief explanation of the classification rules used to clas-sify all the measured particles follows below. First, a particleis regarded as being composed of just one chemical speciesif this species accounts for at least a 90% atomic fraction,in which case the particle with one chemical species is re-garded as a “pure” particle. Second, efforts were made tospecify chemical species even for particles internally mixedwith two or more species. Since many different types ofinternally mixed particles were identified, mixture particleswere grouped on the basis of all the chemical species with a
>10% formula fraction. Third, elements with an atomic con-centration of less than 1.0% are not included in the procedureof chemical speciation because elements at trace levels can-not be reliably investigated (Ro et al., 2000, 2001a). On thebasis of the above criteria, all the measured particles wereclassified into 15 types based on their X-ray spectral and SEIdata, as shown in Fig. 4 and Table 2. The characteristics ofeach particle type are described below.
(1) Primary and secondary soil-derived particles
Primary soil-derived particles are those emitted directly fromsources such as quarrying, agricultural harvesting, and en-trainment of soils by the wind. Aluminosilicate (AlSi)-containing, quartz (SiO2), and calcite (CaCO3)/dolomite(CaMg(CO3)2) particles are the representative types, whichusually appeared as irregular and bright on their SEIs(Fig. 4). The primary soil-derived particles can readilyinteract airborne sulfur and nitrogen oxides, especially inthe presence of moisture (Lan et al., 2005; Hwang and Ro,2006). These reaction or mixture products of primary parti-cles with “secondary acids” were termed as secondary soil-derived particles (Sullivan et al., 2007), and mainly included“reacted CaCO3/CaMg(CO3)2” and “aluminosilicate + (N,S)”. (N, S) notation represents compounds containing eithernitrates, sulfates, or both. The secondary aluminosilicate par-ticles can be formed either through the reaction of cation ionsin aluminosilicates with H2SO4 and/or HNO3, or from ad-sorption of NH4NO3 or NH4HSO4/(NH4)2SO4 on particlesurfaces that act as a reaction place for airborne NH3 andH2SO4 (or HNO3) (Sullivan et al., 2007), e.g. particles #3,#7, #12, #21, #28, #39, #44, #48, #51, #58, #84, and #88 inFig. 4. The secondary CaCO3/CaMg(CO3)2 particles contain
Fig. 3. 36 h backward air mass trajectories for different samples. The backward air mass trajectories were calculated via the HYbrid Lagrangian Integrated Trajectory (HYSPLIT) model at the NOAA Air Resources Laboratory’s web server (http://www.arl.noaa.gov/ready/hysplit4.html).
- 36 -
Fig. 3. 36 h backward air mass trajectories for different samples. The backward air mass trajectories were calculated via the HYbrid SingleParticle Lagrangian Integrated Trajectory (HYSPLIT) model at the NOAA Air Resources Laboratory’s web server (http://www.arl.noaa.gov/ready/hysplit4.html).
CaSO4, Ca(NO3)2, or their mixture compounds, e.g. parti-cles #20, #26, #36, #42, #70, and #77 in Fig. 4.
For SiO2 and CaCO3 particles, the calculated atomic con-centration ratios are [Si]:[O]≈1:2 and [Ca]:[C]:[O]≈1:1:3,very close to their respective stoichiometry (Table 2), indi-cating the validity of the quantification procedure applied inthis study. As far as aluminosilicates, they have many kindsof groups (such as feldspar, mica, kaoline, zeolites, or otherminerals containing Al and Si) that accommodate a wide va-
riety of cations, such as Na+, K+, Ca2+, Mg2+, and Fe2+,and thus the atomic concentration ratios of O, Si, and Alare not constant. The ratios of [Si]/[Al] and [Si+Al]/[O] areshown in Table 3. It was found that the ratio of [Si+Al]/[O]in PM1.0−2.5 fraction of Asian dust particles was significantlylower than those in PM2.5−10 fraction of Asian dust and NADparticles while the ratio of [Si]/[Al] had no statistically sig-nificant difference between the Asian dust and NAD par-ticles, indicating that there were elevated oxygen contents
6938 H. Geng et al.: Elevated nitrogen-containing particles observed in Asian dust aerosol samples
799
800
801
(a) Typical PM2.5-10 and PM1.0-2.5 particles for a non-Asian dust sample
802
803
804
805
806
807
808
(b) Typical PM2.5-10 and PM1.0-2.5 particles for an Asian dust sample
Fig. 4. Typical particle types for non-Asian dust and Asian dust samples. (Fourteen
types of particles are listed here, although the droplet particle rich in (C, N, O, S) is not
shown. The elemental concentration data for each particle is shown in Table 2. (N, S)
notation represents the compounds containing either nitrates, sulphates, or both.)
- 37 -
Fig. 4. Typical particle types for non-Asian dust and Asian dustsamples. (Fourteen types of particles are listed here, although thedroplet particle rich in (C, N, O, S) is not shown. The elemental con-centration data for each particle is shown in Table 2. (N, S) notationrepresents the compounds containing either nitrates, sulphates, orboth.)
in smaller AlSi-containing particles for Asian dust samples,possibly attributable to their reactions with air pollutants (e.g.SO2, NOx).
(2) Genuine and reacted sea-salt particles
The sea is also a major source of primary particles. Sea saltaerosols (SSA) mainly produced by wave action are ubiq-uitous in the MBL and can significantly impact particulatematter concentrations in coastal regions (Oum et al., 1998;Athanasopoulou et al., 2008). The genuine sea salt parti-cles, e.g. particles #6, #8, and #10 in Fig. 4, are regarded tobe generated from the fresh marine-derived particles with-out experiencing chemical reactions after being emitted intothe air by the bursting of air bubbles entrained in breakingwaves (i.e.,bubble burstingor sea sprayprocess) (de Hooget al., 2005). Genuine sea salt particles can react with ni-trogen and sulfur oxides species in the atmosphere to formreacted (or aged) sea salts, resulting in chlorine loss (some-times without remaining chloride if they are completely re-acted) (McInnes et al., 1994; Gard et al., 1998; Laskin et al.,
2003), e.g. particles #2, #18, #60, and #67 in Fig. 4. Often, itwas observed that the reacted sea salts were mixed with pri-mary or secondary soil-derived particles (e.g. particles #19,#59, #61, #64, #71, and #82 in Fig. 4). They were formed bytwo possible mechanisms, i.e., (a) collisions of reacted sea-salt and soil-derived particles in the atmosphere (Andreaeet al., 1986); (b) in-cloud processes through droplet coa-lescence (Fan et al., 1996). These particles were classifiedinto the group of “reacted sea salts & mixture,” which pos-sibly included components such as NaCl, NaNO3, Na2SO4,MgSO4, MgCl2, and Mg(NO3)2, with minor inclusions ofaluminosilicate, CaSO4, or Ca(NO3)2. A substantial portionof sea salt particles were collected as water droplets whichcan be inferred from the morphology. Some sea salt parti-cles appear as a group of particles scattered over a circulardark pattern, e.g. the genuine sea salt particles such as par-ticles #6, #8, #10 and reacted sea salt (& mixture) particlessuch as particles #2, #18, #59, #61, #64 in Fig. 4. These seasalt particles were possibly collected as a fresh droplet fromsea spray, spread over the collecting substrate, and dried outinto small crystalline particles. Some sea salt particles havetypical cubic shape of sea salts, e.g., the reacted sea salt (&mixture) particles #23, #67, and #71 in Fig. 4, indicating thatthese particles were likely collected as single particles andthe crystalline shape was well preserved.
It is worth noting that the majority of genuine sea saltparticles contained some oxygen that perhaps came from theNaOH shell around the NaCl (Laskin et al., 2003). SinceNaOH is very hygroscopic, the sea salt particles will form analkaline hygroscopic coating that has liquid-like propertiesas they dry out. NaOH is generated at the air-solutioninterface by photolysis of O3, followed by its reaction withwater vapor. The surface reaction of ozone is expressed asthe following equation (Oum et al., 1998):
O3 + H2O + 2Cl− → Cl2 + 2OH− + O2
(3) Carbonaceous particles
The carbonaceous particles are divided into two types basedon SEI and the contents of C, N, and O. One is carbon-richparticles, which are defined as those particles wherein theC and O atomic concentrations are more than 90% and thecontent of C is 3 times larger than that of O (Ro et al., 2000).They have complex morphologies such as fractal-like chainstructures (e.g. soot aggregates – one of the most abundantforms in urban aerosols), separate spherules (e.g. tar balls),and irregular-shaped carbons (e.g. char) (Chen et al., 2005a).In the present study, no identifiable, separated particles ofsoot agglomerate and tar ball were encountered, suggestingthat the aerosols in the MBL had quite different character-istics from those in an urban environment. Alternatively, itmay have been because the soot and tar balls were normallybelow 1µm in diameter so that the Dekati PM10 impactor
Note: (1) Particle classification was based both on SEI and element contents from X-ray data. Elements with atomic concentration less than1.0 % were neglected.(2) (N, S) notation represents the compounds containing nitrates, sulfates, or both.
6940 H. Geng et al.: Elevated nitrogen-containing particles observed in Asian dust aerosol samples
Table 3. Atomic concentration ratios of Al/Si and (Al+Si)/O in AlSi-containing particles (including aluminosilicate and “aluminosili-cate + (N, S)” particles).
Particles Ratio of [Al]/[Si] Ratio of [Al+Si]/[O]
PM2.5−10 PM1.0−2.5 PM2.5−10 PM1.0−2.5
Non-Asian dustsamples(N3, N2, N1)
0.534±0.188(n = 211)
0.556±0.183(n = 225)
0.428±0.188(n = 211)
0.396±0.152(n = 225)
Asian dust samples(D1, D2, D3)
0.520±0.182(n = 246)
0.542±0.184(n = 214)
0.453±0.151(n = 246)
0.333±0.142#∗
(n = 214)
Note: (1) The results are expressed as mean± sd. “n” represents the number of particles.(2) Comparison between particle size (#:p≤0.05) and between samples (*:p≤0.05) was tested by two-way ANOVA. There is statisticalsignificance whenp value≤0.05.
failed to collect them. It was reported that a large numberof small secondary aerosol particles (aerodynamic diameter<1µm) in ambient aerosol samples were soot inclusions andabout 90% of the soot inclusions had a size of 20-250 nm(Vester et al., 2007). Only a handful of char particles wereobserved (11 of 900 in PM2.5−10 fraction and 3 of 900 inPM1.0−2.5 fraction), e.g. particles #16, #35, and #79 in Fig. 4.They were probably derived from the pyrolysis and oxida-tion of fuel particles, and had compact and irregular-shapedmorphologies that were related to the nature of the fuel, thefuel/air ratio, and the combustion temperature (Chen et al.,2005a, b).
The second type of carbonaceous particles is organic car-bons, which have high C and O contents, sometimes withminor N, P, S, K and/or Cl, e.g. particles #37, #75, and #91in Fig. 4. They are mostly from combustion or of biogenicorigins (Jurado et al., 2008). The simultaneous presence ofminor N, K, P, S, and/or Cl with C and O is considered tobe a “biogenic fingerprint” (Ro et al., 2002), e.g. particle#75. Relatively few biogenic particles were encountered inthe collected samples. It is likely that the majority of organiccompounds came from the products of combustion (result-ing from biomass burning, automotive or industrial sources)in the surrounding area, or resulted from the oxidation ofvolatile organic compounds (Jurado et al., 2008). Besides,crude oils spilled on the surface of the sea from tanker shipsor cargo vessels could not be excluded (Wang et al., 2008).
(4) Droplet particles rich in (C, N, O) and (C, N, O, S)
Two types of droplet particles have to be paid particular at-tention. One is rich in (C, N, O) and the other is rich in(C, N, O, S). They have similar morphology (dark shades)in the SEI due to low secondary and backscattered electronyields. The droplet particles rich in (C, N, O), likely themixture of organic matter and NH4NO3, often include mi-nor (less than 6 at.%) Na and Mg that possibly come from
seawater, but without S, e.g. particles #32, #45, #50, #55,#56, #57, #63, #68, #72, #78, #89, #90, and #92 in Fig. 4 andTable 2. They are abundant in nearly all the samples. How-ever, the droplet particles rich in (C, N, O, S), which seem tobe a mixture of organic matter and NH4HSO4/(NH4)2SO4(also sometimes with minor Na and Mg), were observedonly in PM1.0−2.5 fraction of samples N1 and N2. SinceNH4NO3 and NH4HSO4/(NH4)2SO4 are hygroscopic, theycan be readily mixed with seawater droplets containing Naand Mg in the MBL, making the particles become droplets.The reaction of NH3 with HNO3 produces NH4NO3 par-ticles with Dp<1µm, and the thermodynamic equilibriumbetween HNO3 and NH4NO3 strongly depends on ambienttemperature and relative humidity (Miyazaki et al., 2005).However, the (C, N, O)-rich droplet particles observed in thepresent study often had sizes larger than 1µm, suggestingthat the relative humidity in the ambient air was relativelyhigh, causing the particles to be collected as droplets.
Carbon-rich or organic matter contained in these parti-cles likely originated from crude oils spilled on the surfaceof the sea, marine organisms, or the “humic-like substances(HULIS)” in seawater. HULIS, which mostly come frombiomass burning or are of biogenic origin, have high con-tents of carbon and oxygen (Krivacsy et al., 2008). Vester etal. (2007) reported that the significant fraction of aerosol par-ticles in the size range of 0.1–10µm aerodynamic diametercollected at an urban background station in Germany is com-posed of secondary aerosol particles in droplet shape mainlycontaining ammonium, nitrate, sulfate, and organics. Thedroplet particles rich in (C, N, O) or (C, N, O, S) that we ob-served here are likely a part of the complex secondary aerosolparticles as they reported.
(5) Fe-rich, fly ash, and other species particles
Fe-rich particles have irregular shapes on the SEI and usu-ally contain more than 20 at.% of Fe element species, e.g.
H. Geng et al.: Elevated nitrogen-containing particles observed in Asian dust aerosol samples 6941
particles #11, #41, #66, and #85 in Fig. 4 and Table 2. Theirmajor type is iron oxide/hydroxides, the main sources ofwhich are metallurgical processes, traffic, and coal combus-tion (Ro et al., 2002). These Fe-rich particles possibly comefrom the surrounding cities and steel factories near the coast.
Fly ash is a powdery material made up of tiny glass spheresthat consists primarily of silicon, aluminum, iron, and/or cal-cium oxides. It is formed by mineral transformation in high-temperature combustion processes and has longer residenceperiods in the atmosphere. The fly ash particle tends to besmall and spherical, has a bright and circular shape on SEIimages, and is easily distinguished, e.g. particles #4, #47,#80, and #86 in Fig. 4. Large quantities of fly ash are gener-ated by coal combustion for electric utilities and power plants(Chen et al., 2004).
The particles that could not be classified into any of theabove fourteen types were put in the category of “otherspecies.” They included TiOx, CuOx, MgO, and (O, Cl)- and(C, N, Cl)-rich particles with low encounter frequencies, e.g.particles #24, #29, #38, #69 and #74 in Fig. 4. It is possi-ble that the particle rich in (O, Cl) is AgCl or AgO in whichAg+ failed to be distinguished due to the disturbance of thesubstrate. The particle rich in (C, N, Cl) is the mixture oforganic matter and NH4Cl, which is probably the reactionproduct of NH3 with HCl. The major sources of NH3 areanimal waste, fertilizer, soil emissions, and industrial emis-sions (Sorooshian et al., 2008), and HCl is likely a result ofthe reaction of NaCl from sea salts with HNO3 (or H2SO4)(Sullivan et al., 2007).
3.2 Effects of an Asian dust storm event on particleabundances
3.2.1 Effects on relative abundance of soil-derived par-ticles
As shown in Table 4, the primary soil-derived particles inwhich AlSi-containing particles greatly outnumber SiO2 andCaCO3 have higher relative abundance in PM2.5−10 fractionthan in PM1.0−2.5 fraction (on average, 43.8% vs. 14.0% forAsian dust samples, and 46.9% vs. 26.2% for NAD samples),in contrast to the secondary soil-derived particles (24.2% inPM2.5−10 fraction vs. 46.9% in PM1.0−2.5 fraction for Asiandust samples, and 19.2% vs. 37.3% for NAD samples), in-dicating that the fine soil-derived particles in the atmosphereare more prone to reacting or mixing with air pollutants or“secondary acids” than are the coarse ones, which agreeswith the results from Table 3. Along the Bohai Bay, the Bo-hai Straits, and Yellow Sea, the relative abundances of pri-mary soil-derived particles in both PM2.5−10 and PM1.0−2.5fractions gradually decrease, suggesting the presence of a de-position or composition-modifying process affecting theseparticles during transport. Based on the observation thatthe relative abundances of secondary soil-derived particlesfor Asian dust samples on the Yellow Sea (D3: 30.0% in
809
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811 (a) Secondary soil-derived particles
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(b) Reacted sea salt (& mixture) particles
Fig. 5. Relative abundances of secondary soil- or marine-derived particles
containing nitrates, sulfates, or both. (The secondary soil-derived particles
included “aluminosilicates + (N, S)” and “reacted CaCO3/(Ca,Mg)CO3”; and
the secondary marine-derived particles included “reacted sea salts” and
“reacted sea salts & mixture”.)
- 38 -
Fig. 5. Relative abundances of secondary soil- or marine-derivedparticles containing nitrates, sulfates, or both. (The secondary soil-derived particles included “aluminosilicates + (N, S)” and “reactedCaCO3/(Ca,Mg)CO3”; and the secondary marine-derived particlesincluded “reacted sea salts” and “reacted sea salts & mixture”.)
PM2.5−10 and 62.0% in PM1.0−2.5 fraction) outweigh thosefor NAD samples (N1: 12.0% in PM2.5−10 and 32.0% inPM1.0−2.5 fraction), the Asian dust storm event seems to in-crease secondary soil-derived particles in the MBL of theYellow Sea. The possible reasons are: on one hand, an-thropogenic SO2 and NOx in the MBL of the Yellow Seaare abundant, providing more chances for these chemicals toreact with the mineral dust; on the other hand, air massespassing over northeast China (Fig. 3) carry mineral duststhat have already reacted with “secondary acids,” since alarge amount of SO2 and NOx are emitted in that region(Hatakeyama et al., 2005).
Nitrogen-containing particles provide essential nutrientsfor ocean life through wet and dry atmospheric depositions(Zhang et al., 2007). However, excessive supply of nitrogennutrition (e.g., the atmospheric deposition of organic nitro-gen, ammonium, and nitrate) leads to eutrophication in theoceanic, coastal, and shelf regions (Bergstrom and Jansson,2006). Fig. 5a shows that in nearly all the samples, the sec-ondary soil-derived particles containing nitrates are the mostfrequently encountered, followed by those containing bothnitrates and sulfates, and then those containing sulfates. Thissuggests that a large amount of nitrogen-containing particlesare deposited into the Bohai Sea and Yellow Sea.
It was reported that Asian dust storm events provide aremoval mechanism for NOx (or HNO3) and perhaps con-tribute to nitrogen deposition in the Yellow Sea (Wu andOkada, 1994). In the present work, the amount of nitrate-containing, soil-derived particles is found to be higher in theAsian dust samples than in the NAD samples. Moreover,
the amount of such particles increases from the Bohai Seato the Yellow Sea, peaking at 44.7% in PM1.0−2.5 fractionand 26.7% in PM2.5−10 fraction (sample D3), whereas theamount of such particles in the NAD samples tends to de-crease (Fig. 5a). The results suggest that airborne NOx (orHNO3) rather than SO2 (or H2SO4) is prone to react with theprimary supermicron alminosilicate, SiO2, and CaCO3 par-ticles; or, the mass concentration of NOx in the MBL of theYellow Sea might be higher than the mass concentration ofSO2. In addition, photochemistry, sink processes, and a num-ber of other factors also play important roles in the forma-tion/decomposition process of nitrate- or sulfate-containingparticles (Sullivan et al., 2007; Hwang et al., 2006). Thedecreasing trend for the sulfate-containing secondary soil-derived particles both in the Asian dust and in the NAD sam-ples from the Bohai Sea to the Yellow Sea (Fig. 5a) indicatesthat the impact of NOx on soil-derived particles is larger thanthat of SO2 in the MBL near the Seoul-Incheon metropolis.
3.2.2 Effects on the relative abundance of marine-derived particles
The Asian dust storm event reduced relative abundances ofcoarse sea salt particles. In PM2.5−10 fraction, not only thegenuine sea salt particles but also the reacted sea salt (& mix-ture) particles exhibited lower encounter frequencies in theAsian dust samples than in the NAD samples (Table 4). InPM1.0−2.5 fraction, genuine sea salt particles were not ob-served for either type of sample, and there was no significantdifference in the average abundances of reacted sea salt (&mixture) particles between the Asian dust and NAD samples.This indicates that the Asian dust storm event had a greaterimpact on big sea salt particles than on small ones.
Marine particles can react with SO2 and NOx to produceNa2SO4 and NaNO3 particles, respectively (Laskin et al.,2003). In our previous studies, it has been convincinglydemonstrated that the low-Z particle EPMA technique canclearly distinguish “genuine” (either in the form of NaCl or amixture of NaCl and MgCl2) from “reacted” sea salts (mostlynitrate and/or sulfate of sodium and/or magnesium) (Hwangand Ro, 2006). Herein, the reacted sea salts (& mixture) wereclassified into 3 types, i.e. nitrate (N)-containing, sulfate (S)-containing, and both nitrate and sulfate (N+S)-containingtypes. Figure 5b shows that the reacted sea salt (& mixture)particles in PM1.0−2.5 fraction have a similar distribution atthe three sampling locations for the Asian dust and NAD par-ticles. Namely, (N+S)- but (S)-containing particles are moreabundant in the samples collected in the Bohai Straits thanin those collected on Bohai Bay and the Yellow Sea, whichis in contrast to the case for (N)-containing particles, whichare more abundant in the samples collected on the Bohai Bayand the Yellow Sea. (N+S)-containing particles from the Bo-hai Straits greatly outnumber (N)- or (S)-containing particlesfrom the Bohai Straits. In PM2.5−10 fraction, the relativeabundances of (N+S)-containing particles for the Asian dustsamples tend to decrease from the Bohai Sea to the YellowSea, whereas those for the NAD samples tend to increase,suggesting that the Asian dust storm event reduced the coarse(N+S)-containing sea salts (& mixture) in the MBL of theYellow Sea.
On normal (non-Asian dust) days, air pollutants from East-ern China take 1–2 days to arrive at the Korean peninsula(the width of the Yellow Sea between Eastern China and theKorean peninsula is 350 km–700 km), with the air massespassing through the Yellow Sea. This means that there isenough time for SO2 to be oxidized into H2SO4 or SO2−
H. Geng et al.: Elevated nitrogen-containing particles observed in Asian dust aerosol samples 6943
to interact with sea salt aerosol particles, resulting in the el-evation of reacted sea salt (& mixture) concentrations in theMBL of the Yellow Sea. However, during the Asian duststorm event, higher wind speeds and lower relative humiditypossibly hindered the transformation of SO2 into H2SO4 orSO2−
4 , therefore leading to fewer (S)-containing and (N+S)-containing particles. In addition, the presence of dimethyl-sulfide (DMS) in the seawater might complicate the discus-sion regarding the reaction of sea salt particles with anthro-pogenic NOx and SO2 in the MBL. DMS, which is producedby marine microbiota and is the most abundant volatile sul-fur compound emitted into the atmosphere from the ocean,can be oxidized to form various sulfur containing products,such as SO2, methanesulfonic acid (MSA), and non-sea saltSO2−
4 (nss-SO2−
4 ) (Yang et al., 2009). An investigation of thespatial distributions of atmospheric DMS and the sea-to-airflux of DMS in the North Yellow Sea, China, reported thereduction of the concentrations of DMS from inshore to off-shore sites in the surface water, revealing the indirect effectsof DMS on the aerosol compositions in the MBL (Yang etal., 2009).
3.2.3 Effects on the relative abundances of droplet par-ticles rich in (C, N, O) and (C, N, O, S)
The (C, N, O)-rich droplet particles were frequently encoun-tered both in PM2.5−10 and PM1.0−2.5 fractions, and greatlyoutnumbered the (C, N, O, S)-rich droplet particles that wereonly encountered in PM1.0−2.5 fraction (Table 4). For theNAD samples, the relative abundances of (C, N, O)-richdroplets increased from Bohai Bay to the Yellow Sea. This isnot unexpected, given that emissions of anthropogenic pollu-tant precursors are plentiful around the Yellow Sea, and thatthe relative humidity in the MBL is high, therefore provid-ing favourable conditions for the gas–aerosol interaction andfor the nitrate-ammonia interaction to form NH4NO3 on theaerosol (Pandey et al., 2008).
Moreover, the relative abundances of (C, N, O)-richdroplets in the Asian dust samples were significantly ele-vated over those of the NAD samples (on average 20.0%vs. 4.4% in PM2.5−10 fraction and 20.4% vs. 10.2% inPM1.0−2.5 fraction), suggesting that the NH4NO3-containingparticles increased in number when the Asian dust passedover the MBL. The observation that no (C, N, O, S)-richdroplets were encountered in the Asian dust samples im-plies that the Asian dust storm event reduced the formationof NH4HSO4/(NH4)2SO4-containing droplets. It is likelythat, on one hand, NH4HSO4/(NH4)2SO4 was more diffi-cult to form than NH4NO3 did during the Asian dust stormevent, or the formation of ammonium sulfate/bisufate mightbe suppressed as sulfuric acid was taken up by mineral par-ticles (Zhang et al., 1999, 2003), and that, on the other hand,NH4HSO4/(NH4)2SO4 particles were too small in diameter(<1µm) to be collected by the Dekati PM10 impactor.
3.2.4 Effects on the relative abundances of carbona-ceous, Fe-rich, fly ash, and other species particles
Overall, the Asian dust storm event reduced the relative abun-dances of carbonaceous particles (1.1% for the Asian dustsamples vs. 4.0% for the NAD samples in PM2.5−10 fractionand 0.9% vs. 4.5% in PM1.0−2.5 fraction) (Table 4). A possi-ble explanation is that the Asian dust mostly originated fromthe desert and Gobi, carrying a high abundance of mineraldust but not organic compounds (Hwang et al., 2008).
Fe-rich and fly ash particles are not frequently encounteredin either the NAD or the Asian dust samples in the MBL(Table 4). The Asian dust samples had higher abundances ofboth coarse and fine fly ash particles and lower abundancesof coarse Fe-rich particles than did the NAD samples. The flyash particles in the Asian dust samples may have come fromeast and northeast China (Fig. 3), where there exist severallarge coal-fired power plants.
In all the samples, only 14 (C, N, Cl)-rich particles wereencountered. They had low relative abundances except inPM2.5−10 fraction of sample D2, which was collected in theMBL of the Bohai Straits. This type of particle has highatomic concentrations of C, N, and Cl, and is likely the mix-ture of NH4Cl and organic matter. The release of HCl(g)
from acidified sea salt particles (caused by reaction withH2SO4 or HNO3) could result in elevated concentrations ofHCl(g) available for reaction with dust particles or NH3 (Sul-livan et al., 2007).
3.3 Possible mechanisms for the elevation of nitrogen-containing particles during an Asian dust stormevent
Geographical traits, meteorological conditions, and air massorigins exert significant influences on the chemical compo-sitions of aerosol particles. In this study, it was observedthat the Asian dust storm event that occurred on 30 Apriland 1 May 2006 particularly elevated the relative abundancesof secondary soil-derived particles containing nitrates anddroplet particles rich in (C, N, O). Overall, the relative abun-dances of these particles gradually increased in the MBLfrom Bohai Bay to the Bohai Straits and the Yellow Sea.In conclusion, inorganic nitrogen-containing particles in theMBL of the Bohai Sea and the Yellow Sea were enhancedduring the Asian dust storm event. The possible reasons aredescribed below.
The Yellow Sea, a semi-enclosed marginal sea of theNorthwest Pacific Ocean, joins the Bohai Sea to the norththrough the Bohai Straits. The aerosols in the MBL of theBohai Sea and the Yellow Sea are influenced by air massesmostly from mainland China and the Korean peninsula. In-evitably, these aerosols will be also affected by mineral dustand air pollutants carried by Asian dust aerosols. The trans-port route of a dust storm plays an important role in the effect.In the present study, it is noticeable that the pathway of air
6944 H. Geng et al.: Elevated nitrogen-containing particles observed in Asian dust aerosol samples
masses for sample D3 was different from that of air massesfor samples D1 and D2, although they all originated fromMongolia and northeast China (Fig. 3). The air masses atheights of 300 m, 500 m, and 1000 m for sample D3 passedover North Korea and the Seoul-Incheon metropolis (whilethose for samples D1 and D2 just passed over Northern Chinaand the Bohai Sea) and stayed for a shorter period on the sea.Consequently, sample D3 seems to have been more stronglyinfluenced by landmass and metropolis environments thanby marine environments, in contrast to samples D1 and D2.This may be why sample D3 contained more secondary soil-derived species and (C, N, O)-rich droplets.
In East Asia, due to rapidly growing agricultural,livestock-farming, and industrial activities, particularly inChina, the concentrations of NH3 and NOx species haverapidly increased, leading to a large amount of NH4NO3 be-ing formed in the East Asian atmosphere (e.g., over the Yel-low Sea) (Kim et al., 2006). Large amounts of NH3 are emit-ted from regions of high agricultural activity and livestockfarming (e.g., Shandong, Anhui, Jiangsu, Henan, Sichuan,and Hebei provinces, China), whereas NOx is largely emittedfrom urban areas (e.g., the Seoul, Beijing, and Shanghai ar-eas) (Kim et al., 2006). The mean NO2 values at roadsides inSeoul (60 ppb or so) and Incheon (45.4 ppb) were higher thanthose in other cities such as Greater Manchester in the UK(33.2 ppb), Rotterdam in the Netherlands (20.8 ppb), Munichin Germany (17.2 ppb), and Stockholm in Sweden (15.1 ppb)(Pandey et al., 2008), providing favorable atmospheric con-ditions for particulate nitrate formation in the atmosphere.Song et al. (2009) reported that NH3 concentrations in theSeoul atmosphere were also high, ranging from 0.7 ppbv to13.6 ppbv with an average of (4.7±2.1) ppbv, and they at-tributed high concentrations of NH3 to the catalytic convert-ers of automobiles. Hence, it is not unexpected that the airmasses passing over the Seoul-Incheon metropolis can carrya large amount of NH4NO3. Indeed, Miyazaki et al. (2005)found that particulate nitrate transported from the Asian con-tinent to the western Pacific region was mostly in the form ofNH4NO3. The conversion of NH3 from gas phase to aerosolphase will reduce its dry deposition rate, allowing long-rangetransport of this nitrogen-containing species.
The anthropogenic NOx emitted from mainland China andthe Korean peninsula may outweigh the SO2 in the MBL ofthe Bohai Sea and the Yellow Sea, or it may be that NH4NO3is more favorably generated and observed in the spring-time than (NH4)2SO4/NH4HSO4, as inferred from theob-servations of many (C, N, O)-rich but not (C, N, O, S)-rich droplet particles and of many nitrate-containing but notsulfate-containing secondary soil-derived particles. Aerosolscollected in Asia are different in their chemical compositionsfrom those in Europe and North America because in Asiamore coal and biomass is burned, leading to a sharp increasein NOx emissions and an excess of NO2 concentrations, es-pecially in the atmosphere over China (Richter et al., 2005).NOx emissions are expected to continue to increase (Aki-
moto, 2003). At the same time, due to a reduction in indus-trial coal use and the execution of a series of strict environ-mental protection policies in China, SO2 emissions are ex-pected to decline, resulting in a reduction in acid depositionin China and even in Japan (Carmichael, et al., 2002). In thespring, Asian dust storms originating in the deserts of Mon-golia and China make their way to populated cities, addingdust components to the urban aerosols. In the Asian Pa-cific Regional Aerosol Characterization Experiment (ACE-Asia), it was observed that there were few “pure” particlesand a lot of calcium-rich particles coated with secondarynitrates; such nitrate-coated particles unexpectedly outnum-bered those coated with secondary sulfates, indicating thatthe Asian dust storm studied in that experiment carried a va-riety of nitrogen-containing pollutants along with the min-erals (Erickson, 2002). Tang et al. (2004) also reported thathigher concentrations of sulfate were prevalent in the interiorof China, while nitrate and ammonium were maximal at theeast coast. The higher NOx and lower SO2 emissions in thecoastal region might lead to the formation of a large amountof inorganic nitrogen-containing particles.
4 Conclusions
Six sets of aerosol samples were collected in the MarineBoundary Layer (MBL) of the Bohai Sea and the YellowSea from 28 April 2006 to 1 May 2006 using a three-stagecascade Dekati PM10 impactor. Samples D1, D2, and D3were collected during an Asian dust storm event whereassamples N1, N2, and N3 were collected on non-Asian dust(NAD) days. Overall, 1800 coarse and fine particles (aero-dynamic cutoff diameters: 2.5–10µm and 1.0–2.5µm, re-spectively) were analyzed using a quantitative single parti-cle analytical technique called low-Z particle EPMA. Basedon the X-ray spectral and SEI data, 15 types of particleswere characterized. For all the samples, soil-derived parti-cles (alminosilicates, SiO2, and CaCO3) were the most abun-dantly encountered, followed by (C, N, O)-rich droplet parti-cles (likely the mixture of organic matter and NH4NO3), ma-rine particles, and then carbonaceous, Fe-rich, fly ash, and(C, N, O, S)-rich droplet (likely the mixture of organic mat-ter and NH4HSO4/(NH4)2SO4) particles. During the Asiandust storm event, primary soil-derived particles decreasedand secondary particles, especially those containing nitrates,increased. Moreover, the relative abundance of the (C, N,O)-rich droplet particles was elevated in both PM2.5−10 andPM1.0−2.5 fractions, and the abundances of carbonaceous and(C, N, O, S)-rich droplet particles were reduced. The resultsimply that Asian dust aerosols were an important carrier ofgaseous inorganic nitrogen-containing species (within 1.0–10µm), especially NOx (or HNO3) and NH3, in the MBL ofthe Bohai Sea and the Yellow Sea. Considering the poten-tial increases in the emissions of NOx and NH3 in the coastalcities of mainland China and Korea, the influences of nitrate
H. Geng et al.: Elevated nitrogen-containing particles observed in Asian dust aerosol samples 6945
and ammonium on the biogeochemistry of the Bohai Sea andYellow Sea are expected to be further enhanced.
Acknowledgements.This work was supported by the KoreaScience and Engineering Foundation (KOSEF) grant funded by theKorea government (MOST) (ROA-2007-000-20030-0).
Edited by: N. Mihalopoulos
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