Single-particle characterization of the Antarctic aerosols collected at King George Island, Chile

Transcription

1 Single-particle characterization of the Antarctic aerosols collected at King George Island, Chile Shila Maskey¹, Hae-Jin Jung¹, BoHwa Kim¹, Hyeok Jung², Kang-Ho Ahn², and Chul-Un Ro¹ ¹Department of Chemistry, Inha University, Korea ²Department of Mechanical Engineering, Hanyang University, Korea

2 Objective of the work * Single-particle characterization of the Antarctic aerosol samples based on secondary electron images (SEIs) and X-ray spectral data * Possible mechanism for the formation of sulfate-containing sea-salt particles. The Antarctic Introduction Best place for monitoring the background level of the earth s atmosphere as it is isolated from anthropogenic activities In the summertime, phytoplankton can grow rapidly due to longer daytime and warmer temperature.

3 Introduction Most of the analysis of the Antarctic aerosols were carried out by bulk analysis: * More than 70% of sea salt were found in the supermicron particles, and >90% of submicron particles showed the chloride depletion by sulfate, nitrate, and methane sulfonate. (Kerminen et al., 2000) * The sea spray contribution (Na +, Mg 2+, Cl -, and ss-so 2-4 ) is dominant (77% w/w) in the coarse fraction, whereas the biogenic source (methane sulfonate and nss-so 2-4 ) exist (67.5% w/w) in the fine fraction. (Fattori et al., 2005) * In the Antarctic, excess nss-so 2-4 dominated and the contribution of methyl sulfonic acid (MSA) to the chloride depletion was significant. (Virkkula et al., 2006) * Particulate sulfate, ammonium, and methane sulfonate were internally mixed and found predominantly in the submicron size range. (Teinila et al., 2000)

4 Single-particle analysis (SPA) for the Antarctic aerosol characterization * Majority of the particles consist of sea-salts (>94%) and some transformed particles include excess sulfate and methane sulfonate with sea-salt particles - by using laser microprobe mass spectrometry. (Wouters et al., 1990) * 92% of the particles were identified as sea-salts, and in almost all particles, at least three elements Na, Cl, and Mg were detected - by using Micro-PIXE. (Biancato et al., 2006) * Sea-salt particles were transformed to secondary sea-salts more in the fine fraction through heterogeneous reactions mainly with gaseous organosulfur species (e.g. DMS, MSA) in the summer and with reactive NOx in the winter - by using SEM/EDX. (Hara et al, 2005) * In the austral summer samples, sulfur-rich particles and reacted sea-salt particles were high, because the production of marine sulfur was enhanced - by using TEM/EDX. (Mouri et al., 1999)

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6 Samples : King George Island Sampling dates and times : sunrise: 6-7 AM and sunset: 6-7 PM (during March) Sample No. Date Sampling time Temperature C King Sejong station S1 12/3/ :08~15: S2 13/3/ :40~14: S3 14/3/ :34~10: S4 15/3/ :44~16: S5 16/3/ :12~09:17 0 Sampling site: the King Sejong Station, a Korean scientific research station, the King George Island, Antarctica, Chile (62⁰ 13 S, 58⁰ 47 W). Collecting substrate: Al foil 3 stage cascade impactor (PM-10 impactor, Dekati Ltd.) - flow rate 10 L/min Stage No Cut-off diameter (μm)

7 Low-Z particle EPMA (Electron Probe X-ray Microanalysis) for single particle analysis 1. SEM-EDX (Scanning Electron Microscopy Energy Dispersive X-ray Spectrometer) - Individual Particle Analysis * shape and size : secondary / backscattered electron images * chemical compositions : X-ray spectrum 2. Ultra-thin window EDX for low-z elements detection (e.g., C, N, O, F) 3. Metallic collecting substrates for minimizing charging effect (e.g., Ag, Al) 4. Monte Carlo calculation for Quantification 5. Chemical speciation of aerosol particles Expert System

8 Monte Carlo (MC) Calculation for Quantification Measurement Simulation Electron detector Electron beam with 10keV X-ray detector C O Ca CaCO3 measured data Simulated SEM metal foil Intensity(arb. units) X-ray Energy (kev) Measured and simulated spectra for a CaCO 3 standard particle on a Be substrate

9 Results and discussion: Typical secondary electron images(seis) of the Antarctic aerosol particles st2 NaCl/O st3 (Na,Mg,Ca)(SO 4,Cl) NaCl/O (Na,Mg,K,Ca)(SO 4,Cl) (Na,Mg,K,Ca)(SO 4,Cl) NaCl/O (Na,Mg)(SO 4,Cl) (Na,Mg)Cl/O (Na,Mg)(SO 4,Cl) NaCl NaCl NaCl/O Na(SO 4,Cl) NaCl/O (Na,Mg)(SO 4,Cl) Na(SO 4,Cl) NaCl NaCl/O (Na,Mg,Ca)(SO 4,Cl) Na 2 SO 4 Na(SO 4,Cl) (Na,Mg)(SO 4,Cl) 10µm NaCl NaCl (Na,Mg)(SO 4,Cl) 5µm Na(SO 4,Cl) Stage2 (2.5-10μm) Stage3 (1-2.5μm) Chemical speciation based on their X-ray spectral and chemical concentration data

10 10000 Intensity sity n te In Typical EDX spectra and atomic concentrations of the Antarctic aerosol particles C Na Al Diameter:2.91 μm Elemental concentration (at. %) Cl Na: 53.4 Cl: 46.6 NaCl kev Al O Na Mg Diameter:2.49 μm Elemental concentration (at. %) C: 9.9; S: 4.1 Cl O:29.5; Cl: 29.9 Na: 22.9; Mg: 3.9 S S/Na (at%)=0.18 (Na,Mg)(SO 4,Cl)/C kev sity n te In Intensity C O Na Al Mg Diameter:1.62 μm Elemental concentration (at. %) C: 24.7; S: 1.2 Cl O: 17.9; Cl: 24.8 Na: 26.8; K: 0.5 Mg: 4.1 S K S/Na (at%)=0.04 (Na,Mg)(SO 4,Cl)/C kev C Na Al O Mg Cl S S/Na (at%)= 0.16 Diameter:2.57 μm Elemental concentration (at. %) C: 6.5; S: 5.0 O: 24.2; Cl: 26.6 Na: 31.1; K: 0.6 Ca Mg: 4.1; Ca: 2.0 K (Na,Mg,Ca)(SO 4,Cl) kev Sea-water ratio of sulfur to sodium (S/Na) = 0.083

11 Relative abundances of major chemical species encountered in the five Antarctic aerosol samples Relative abundances (%) Genuine sea-salt SO4-containing sea-salt Fe-containing Others Relative abundances (%) Genuine sea-salt SO4-containing sea-salt Fe-containing Others 0.0 S1 S2 S3 S4 S5 Samples 0.0 S1 S2 S3 S4 S5 Samples Stage2 (2.5-10μm) Stage3 (1-2.5μm) Major particle types: genuine sea-salt (on average 33%) sulfate-containing sea-salt particles (on average 63%) Minor particle types: Fe-containing, CaCO 3, AlSi, carbonaceous

12 Relative abundances of different types of sea-salt particles encountered in the five Antarctic aerosol samples Sea-salt particle types Relative abundances in the coarse fraction Relative abundances in the fine fraction S1 S2 S3 S4 S5 ave S1 S2 S3 S4 S5 ave Sum Genuine sea-salts NaCl NaCl/C NaCl/C,O NaCl/O,Mg NaCl/C,O,Mg SO 4 -containing sea-salts Na(SO 4,Cl) Na(SO 4,Cl)/C (Na,Mg)(SO 4,Cl) (Na,Mg)(SO 4,Cl)/C (Na,Mg,Ca)(SO 4,Cl) (Na,Mg,Ca)(SO 4,Cl)/C (Na,Mg,K)(SO 4,Cl)/C (Na,Mg,K,Ca)(SO 4,Cl) (Na,Mg,K,Ca)(SO 4,Cl)/C (Na,K,Ca)(SO 4,Cl) Sum Total number of particles is 2,890 and ~ 96% are sea-salt containing particles.

13 Why elevated sulfate containing sea-salt particles observed in the Antarctic aerosol particles? - Backward trajectories: S1 S2 S3 S4 S5 During sampling period, all air masses came from ocean.

14 Observed average ratio of sulfur to sodium 0.28 (SWR= 0.083) Sulfur species is not only from sea water, but also from other sources. sea-salt particles : > 96% No anthropogenic influence (SO x and NO x ) backward trajectories All air masses came from the ocean. no nitrate-containing particles observed The sea-salt particles were probably modified by heterogeneous reaction between sea-salt and biogenic sulfur species (e.g., DMS, MSA)

15 Formation of non-sea-salt sulfate (nss-so 4 2- ) in marine boundary layer oceanic phytoplankton (CH 3 ) 2 S + CH 2 CH 2 COO DMSP (dimethylsulphoniopropionate) sun light (A compound synthesized by phytoplankton and several types of anaerobic bacteria as an internal cell component) (CH 3 ) 2 S(aq) DMS (dimethylsulfide) Volatile DMS CH 3 SO 3 Na (sodium methyl sulfonate, nss-so 4 2- ) Sea-salt + MSA OH OH CH 3 SO 3 H CH 3 SOOH MSA MSIA (methane sulfinic acid) (methane sulfonic acid) Sea-salt (CH 3 ) 2 SO DMSO (dimethyl sulfoxide) OH H 2 SO 4 SO 2 Na 2 SO 4 (nss-so 4 2- ) Addition pathway OH OH Oxidation Abstraction pathway (CH 3 ) 2 S (g)

16 Investigation of sulfate-containing sea-salt particles using ATR-FT-IR Technique a b (a) secondary electron image and (b) ATR-FT-IR image of the same aerosol particles

17 Typical ATR-FT-IR Spectra of sulfate-containing sea-salt particles as well as those for standard sulfur-containing compounds Dimethyl sulfoxide (DMSO) Transmission (%) Sodium sulfate Dimethyl sulfide (DMS) Sodium methyl sulfonate 1332 (CH 3 SO 3 Na) 1641(OH) 3392 (OH) Particle # 8 Particle # (SO 2-4 ) 1171 (SO 2-4 ) 1046 (SO 2-4 ) Wavenumber cm -1

18 Conclusions * Dominated by sea-salt (~33%) and sulfate-containing sea-salt particles (~63%) * Minor chemical species: Fe-containing, CaCO 3, carbonaceous, and aluminosilicate particles probably from local origin * No anthropogenic influence for these summertime Antarctic aerosol samples * Elevated sulfate-containing sea-salt particles resulted from heterogeneous reactions between biogenic sulfur species and sea-salt particles * Sulfate-containing particles were encountered more frequently in the fine fraction than in the coarse one (average values:71% vs. 56%). * Identification of sulfate-containing sea-salt particles is under investigation by using ATR-FT-IR technique.

19 the King Sejong Station (a Korean scientific research station ) Thank you!!!