Trace Characterization of Individual Aerosol Particles using an X-ray Microprobe

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T h e e l e m e n t a l c o m p o s i t i o n s o f a t m o s p h e r i c a e r o s o l s a r e r e l a t e d b o t h t o t h e i r o r i g i n ( s o u r c e ) a n d w i th p h y s i c a l a n d c h e m i c a l p r o c e s s e s d u r i n g transportation. Therefore, the analysis of individual aerosol particles has been received great interest in t e r m s o f s o u r c e a p p o r t i o n m e n t a n d e s t i m a t i o n o f tr an sf or ma ti on re ac ti on s, et c . An el ec tr on pr ob e m i c r o - a n a l y z e r ( E P M A ) h a s b e e n u t i l i z e d f o r t h i s p u r p o s e [ 1 ] . H o w e v e r , o w i n g t o t h e s i g n i f i c a n t b a c k g r o u n d b e n e a t h t h e s i g n a l , t h e d e t e c t a b l e ele men ts are lim ite d to the maj or com pon ent s. A combi natio n of a newly insta lled X-ray micro probe a n d X - r a y f l u o r e s c e n c e ( X R F ) d e t e c t i o n h a s realized trace quantification down to fg ( femtogram ) l e v e l a n d c h e m i c a l s t a t e d i a g n o s i s u s i n g X A N E S (X-ray absorption near edge structure) spectra has b e c o m e p o s s i b l e . S o m e n e w p o s s i b i l i t i e s f o r d i s c o v e r i n g t h e h i s t o r y o f i n d i v i d u a l a e r o s o l particles are under discussion. Trace Characterization of Individual Aerosol Particles using an X-ray Microprobe Figure 1 shows the experimental layout around th e sp ec im en . A pa ir of el li pt ic al mi rr or s (K ir kp at ri ck a n d B a e z m i r r o r , K B m i r r o r ) w e r e i n s t a l l e d a t b e a m l i n e B L 3 9 X U . I t s e f f e c t i v e a p e r t u r e i s a p p r o x i m a t e l y 1 5 0 × 1 5 0 μ m 2 , a n d R h c o a t e d mirrors cover X-rays up to 18 keV. The preliminary e x p e r i m e n t a l r e s u l t s s h o w a f o c a l s p o t d o w n t o 2 μ m (V) × 4 μ m (H) with the photon flux more than 1 × 10 10 pho ton s/s for mon och rom ati zed 10 keV X- r a y s [ 2 ] . A s a m p l e w a s m o u n t e d o n t h e X Y s c a n n i n g s t a g e , a n d X R F f r o m t h e s a m p l e w a s d e t e c t e d w i t h a c o n v e n t i o n a l S i ( L i ) d e t e c t o r . T o a v o i d a n e x c e s s i v e c o u n t i n g r a t e , a s a m p l e - d e t e c t o r d i s t a n c e o f 1 0 0 m m w a s u s e d . T h e minimum detection limit ( MDL ) of the XRF analysis w a s e v a l u a t e d w i t h a s e r i e s o f t h i n m e t a l f i l m s d e p o s i t e d o n t o t h e K a p t o n f o i l s o f 1 0 μ m i n thickness, and the evaluated MDL was 0.3 fg for Ni using 10 keV X-ray microbeam. C o a r s e a n d f i n e f r a c t i o n s o f a e r o s o l s w e r e collected separately in two-stage filterpacks (8 and 0.4 μ m Nuclepore filters) with a 50% effective cutoff diameter of 1 μ m. Sampling was conducted at the U j i c a m p u s o f K y o t o U n i v e r s i t y d u r i n g t h e A s i a n dust storm ( Kosa ) event. A coarse fraction of the Fig. 1. Experimental setup for the X-ray microprobe. Si(Li) M1 KB mirror M2 Sample Beam 68 c o l l e c t e d a e r o s o l p a r t i c l e s w a s s u b j e c t e d t o X R F measurement. Figure 2 shows an Fe XRF image of the aerosol particles obtained with a 10 keV X-ray microbeam. Blac k poin ts repr esen t part icl es whi ch giv e stro ng Fe si gn al s. Th e si ze s of th e la rg e pa rt ic le s we re e v a l u a t e d f r o m a c r o s s s e c t i o n o f t h e F e X R F image, and the sizes of the particles less than the b e a m s i z e w e r e e s t i m a t e d f r o m t h e F e s i g n a l o n t h e a s s u m p t i o n t h a t t h e p o r t i o n o f t h e F e composition was the same among the particles. Figure 3 shows micro-XANES (X-ray absorption n e a r e d g e s t r u c t u r e ) s p e c t r a o f a e r o s o l p a r t i c l e s marked on an Fe image, (a) and (b) in Fig. 2 . The X R F y i e l d m e t h o d w a s u s e d t o o b t a i n t h e s e s p e c t r a . T h e X A N E S s p e c t r u m o b t a i n e d f o r p a r t i c l e ( b ) i s i d e n t i c a l t o t h a t o f h e m a t i t e F e 2 O 3 , and most of the other particles gave similar spectra. Chemical shifts in the absorption edge between the s p e c t r a o f t h e a e r o s o l p a r t i c l e s a n d t h o s e o f a r e f e r e n c e , F e t h i n f i l m s h o w t h e d i f f e r e n c e i n v a l e n c e s t a t e o f F e , a n d t h e F e i n t h e a e r o s o l particle (a) might be in a divalent state. Figure 4 shows XRF spectra measured from an i n d i v i d u a l a e r o s o l p a r t i c l e s . T h e d i a m e t e r o f t h e particle (a) is estimated to be 10 μ m from the cross section of the Fe XRF image. Considering that the m a t r i x o f t h e p a r t i c l e i s q u a r t z , t h e m a s s o f t h e p a r t i c l e c a n b e e s t i m a t e d t o b e 1 . 2 n g f r o m t h e d e n s i t y o f t h e q u a r t z ( 2 . 2 g / c m 3 ) . O w i n g t o t h e a t m o s p h e r i c e n v i r o n m e n t a n d t h e r a t h e r p o o r e x c i t a t i o n e f f i c i e n c i e s f o r l i g h t e l e m e n t s , n o p e a k was observed for S and Cl. However, Ca, Ti, Cr, Mn, Fe, Ni, Cu and Zn wer e det ect ed. For sma ll particles attenuation of XRF inside the sample can b e n e g l e c t e d , a n d s e m i - q u a n t i t a t i v e i n f o r m a t i o n c a n b e o b t a i n e d f r o m a c o m p a r i s o n o f t h e X R F s i g n a l s b e t w e e n t h e s a m p l e a n d r e f e r e n c e t h i n films previously measured. The absolute amounts o f C a , T i , M n a n d N i i n t h e p a r t i c l e s w e r e estimated to be 220, 120, 110, 17 fg for the particle ( a ) a n d 4 7 0 , 1 0 0 , 3 0 , 3 f g f o r t h e p a r t i c l e ( b ) , r e s p e c t i v e l y [ 3 ] . S i m i l a r l y , s e m i - q u a n t i t a t i v e a n a l y s e s o f t h e p a r t i c l e s w e r e c a r r i e d o u t f o r 1 6 p a r t i c l e s o n t h i s s a m p l e , a n d t h e d i f f e r e n c e o f e l e m e n t a l c o m p o s i t i o n s ( e l e m e n t a l p r o f i l e s ) w a s Fig. 2. Fe XRF image of aerosol particles on a Nuclepore filter. 10 keV X-rays were used for excitation. 200 150 100 50 0 200 150 100 50 0 Displacement ( μ μ m ) (a) (b) 69 5 4 3 2 1 0 12 10 8 6 4 2 ( a ) XRF Yield ( cps ) X-ray Energy ( keV ) ( b ) XRF Yield ( cps ) 5 4 3 2 1 0 Ca Ti Cr Ni Cu Zn Ar Ti Mn FeK α α K β β Zn Ca References [1] K. R. Spurny eds., “Analytical Chemistry of Aerosols”, Lewis Publishers (1999). [2] S. Hayakawa et al. , J. Synchrotron Rad. 8 (2001) 328. [3] S. Hayakawa, S. Tohno, K. Takagawa, A. Hamamoto, Y. Nishida, M. Suzuki, Y. Sa to an d T. Hi ro ka wa , An al . Sc i. 1 7s ( 20 01 ) i1 15 . [4] S. Tohno et al. , J. Aerosol Sci. 32 (2001) S873. f o u n d b e t w e e n t h e p a r t i c l e s . Further interpretations of elemental p r o f i l e s a r e b e i n g c a r r i e d o u t t o r e v e a l t h e o r i g i n a n d t h e tra nsp ort ati on pro ces s of ind ivi dua l a e r o s o l p a r t i c l e s . U t i l i z i n g t h i s un iq ue ch ar ac te ri st ic s of th e X- ra y mic rop rob e of ult ra- hig h sen siti vity , analyses of a single rain drop and a s i n g l e f o g d r o p l e t [ 4 ] a r e n o w i n progress. Fig. 3. Micro XANES spectra from individual aerosol particles marked on the Fe image (see Fig. 2). The reference metal spectrum is imposed in the figure. 1.0 0.8 0.6 0.4 0.2 0.0 XRF Yield ( arb. unit) 7.15 7.14 7.13 7.12 7.11 7.10 7.09 X-ray Energy ( keV ) Metal (a) (b) F i g . 4 . X R F s p e c t r u m f r o m i n d i v i d u a l aerosol particles (a) and (b) marked in Fig. 2. Shinjiro Hayakawa a and Susumu Tohno b (a) Hiroshima University (b) Kyoto University E-mail: hayakawa @ hiroshima-u.ac.jp 70