Measuring the Strong Acid Content of Atmospheric Aerosol Particles

8 Measuring the Strong Acid Content Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 28, 2016 | http://pubs.acs.org Publication Date: June 1, 1993 | doi: 10.1021/ba-1993-0232.ch008

of Atmospheric Aerosol Particles Roger L. Tanner Energy and Environmental Engineering Center, Desert Research Institute, Reno, NV 89506

Methods used to determine the strong acid content of aerosol particles in the ambient atmosphere are reviewed. These methods include those for generic determination of strong acid content and those in which the concentrations of individual strong acid species are determined. Difficulties in sampling these species due to their reactivity and occurrence under non-steady-state atmospheric conditions are discussed, and the methods currently used for resolving these difficulties are critically evaluated.

F U N D A M E N T A L QUESTION about the i n t e r p r e t a t i o n o f acidic aerosol data is w h e t h e r researchers can characterize past a n d c u r r e n t a t m o s p h e r i c c o n centrations a n d d i s t r i b u t i o n s (spatial a n d temporal) w i t h sufficient accuracy for studies of t h e i r effects o n ecosystems a n d h u m a n h e a l t h . Part o f the answer to this q u e s t i o n can b e p r o v i d e d b y a r e v i e w of the m e t h o d s that have b e e n u s e d to measure the strong a c i d content of aerosol particles c o l l e c t e d f r o m the atmosphere. T h i s chapter serves as s u c h a r e v i e w , a n d , i n e v a l u a t i n g analytical p r o c e d u r e s , it specifically assesses the a b i l i t y o f each p r o c e d u r e to o v e r c o m e s a m p l i n g artifacts, to d i s t i n g u i s h b e t w e e n strong a n d weak acids, to p r o p e r l y p a r t i t i o n strong a c i d i t y b e t w e e n gas-phase a n d aerosol-phase species, a n d to quantitate strong a c i d i t y at the levels at w h i c h i t is f o u n d i n the a m b i e n t atmosphere.

Definitions of Acids and Bases A c i d s a n d bases are d e f i n e d i n accordance w i t h B r 0 n s t e d - L o w r y t h e o r y i n terms of t h e i r p r o p e n s i t y to donate or accept h y d r a t e d protons i n aqueous 0065-2393/93/0232-0229$06.00/0 © 1993 American Chemical Society

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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s o l u t i o n , a d e f i n i t i o n that is r e a d i l y e x t e n d e d to nonaqueous solvents. A n acid is thus a substance w i t h a t e n d e n c y to dissociate i n t o a h y d r a t e d p r o t o n i n s o l u t i o n , a n d it is a strong a c i d i f that t e n d e n c y is q u a n t i t a t i v e l y large c o m p a r e d to that of water; that is, at e q u i l i b r i u m its p r o t o n - d o n o r reaction w i t h water lies far to the r i g h t . W e a k acids are, b y contrast, o n l y partially dissociated i n aqueous s o l u t i o n , a n d the degree of dissociation is q u a n t i t a t i v e l y expressed i n terms of the e q u i l i b r i u m constant for its p r o t o n - d o n o r reaction w i t h water. A base i n aqueous solution is d e f i n e d analogously i n t e r m s of its p r o t o n - a c c e p t i n g capacity relative to H 0 ; a strong base n e a r l y c o m p l e t e l y dissociates i n aqueous solution to form free h y d r o x i d e ions. S o l v e n t dissociation is the process d e t e r m i n i n g co-existing [ H ] a n d [ O H ] w h e n a solution is a c i d i c or basic, that is, w h e n [ H ] > [ O H ] or v i c e versa. T h e t e r m [ H ] signifies the m o l a r concentration of h y d r a t e d protons i n s o l u t i o n , (H 0) H .

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 28, 2016 | http://pubs.acs.org Publication Date: June 1, 1993 | doi: 10.1021/ba-1993-0232.ch008

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A l o g a r i t h m i c scale is u s e d to express [ H ] as p H , w h e r e p H = -log(a +) = - l o g 7 [ H ] ; 7 is the s i n g l e - i o n activity coefficient a n d a + is the H activity. O t h e r terms have also b e e n u s e d to describe a c i d levels i n e n v i r o n m e n t a l s a m p l e s — t i t r a t a b l e a c i d i t y , titratable strong a c i d i t y , a n d total acidity. T i t r a t a b l e a c i d i t y is the a c i d content i n aqueous s o l u t i o n that is n e u t r a l i z a b l e b y a d d i t i o n of strong base; this t e r m has m e a n i n g o n l y i f the p H at the e n d p o i n t of the titration is d e n n e d . If an e n d p o i n t p H is chosen at the l o w e n d of o b s e r v e d p H s of c l o u d - w a t e r or r a i n (e.g., p H = 2 - 4 ) , this q u a n t i t y b e c o m e s total strong a c i d i t y i n a t m o s p h e r i c samples. H o w e v e r , i f a h i g h e r p H is chosen (e.g., p H = 7), this q u a n t i t y is often r e f e r r e d to as total strong a n d weak a c i d i t y ; at an e v e n h i g h e r e n d p o i n t p H ( p H = 10), it b e c o m e s total a c i d i t y , because n o w such conjugate a c i d species as a m m o n i u m are t i t r a t e d . T h e d e g r e e of n e u t r a l i z a t i o n at a specified p H d e p e n d s on c o n c e n t r a t i o n of the acidic species i n solution as w e l l as o n the e q u i l i b r i u m constant. +

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Atmospheric Strong and Weak Acids T h e s e definitions must b e a p p l i e d i n a clear a n d u n a m b i g u o u s w a y to at m o s p h e r i c samples. I n this chapter, strong a n d weak acids are a s s u m e d to be present i n the atmosphere i n b o t h c o n d e n s e d (solid particle a n d l i q u i d droplet) a n d gaseous forms. I n a d d i t i o n , fine particles ( < ~ 2 μπι i n diameter) as w e l l as w a t e r - s o l u b l e gaseous species are r e m o v e d i n large part b y scav e n g i n g into clouds a n d r a i n droplets. S t r o n g acids, w h e t h e r present as gases or i n aerosol particles, are somewhat a r b i t r a r i l y a s s u m e d to b e c o m p l e t e l y dissociated (>99%) into H a n d an anion i n a t m o s p h e r i c water samples w i t h p H ^ 4. T h i s d e f i n i t i o n is e q u i v a l e n t to i d e n t i f y i n g strong acids as those w i t h p K values < ~ 2 . T h i s d e f i n i t i o n w i l l be used w i t h the r e c o g n i t i o n that strong a c i d species m a y b e undissociated i n solid-particle or c o n c e n t r a t e d d r o p l e t phases i n aerosols b u t dissociated w h e n scavenged into the m o r e d i l u t e (by a factor of 1000) h y d r o m e t e o r phase. +

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T h e most c o m m o n l y o c c u r r i n g a t m o s p h e r i c strong acids are n i t r i c a n d sulfuric acids, w h i c h are d e r i v e d f r o m the oxidation of n i t r o g e n oxides a n d sulfur d i o x i d e , a n d are present p r e d o m i n a n t l y i n the gaseous a n d particulate phases, r e s p e c t i v e l y . H y d r o c h l o r i c a c i d a n d oxalic a c i d (the latter q u a l i f y i n g as a strong a c i d based o n its K b e i n g less than 2) m a y also be present occasionally i n the atmosphere ( J , 2). H P 0 , methanesulfonic a c i d , a n d h y d r o x y m e t h a n e s u l f o n i c a c i d c o u l d also be present, b u t the a c i d forms of these species have not b e e n specifically i d e n t i f i e d . T h e s e strong a c i d species are n e u t r a l i z e d , partially or i n toto, b y a t m o s p h e r i c gaseous a m m o n i a a n d to a lesser extent b y s o i l - d e r i v e d particulate matter; this n e u t r a l i z a t i o n p r o duces the o b s e r v e d c o m p o s i t i o n of sulfate- a n d n i t r a t e - c o n t a i n i n g aerosols (with s m a l l e r amounts of o t h e r anions) along w i t h the e q u i l i b r i u m - d e t e r m i n e d levels of gaseous n i t r i c a c i d (3, 4). A c i d i c species that are present i n a t m o s p h e r i c solid particles, c o n d e n s e d phases, or mixtures t h e r e o f may thus i n c l u d e sulfuric a c i d , a m m o n i u m bisulfate, a n d letovicite [ ( N H ) H ( S 0 ) ] m i x e d w i t h n e u t r a l species ( a m m o n i u m sulfate, a m m o n i u m nitrate, N a C l , N a N O . 3 , N a S 0 , a n d c e r t a i n m i x e d a m m o n i u m sulfate a n d nitrate salts). U n d e r h i g h e r h u m i d i t y conditions, aqueous solutions c o n t a i n i n g H , N H , N a , H S 0 ~ , S 0 " , N 0 ~ , a n d C I " m a y also coexist w i t h some of these s o l i d c o n d e n s e d phases, or the s o l i d phases may be absent altogether. d l


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W e a k a c i d species i n the atmosphere m a y i n c l u d e any or a l l of the f o l l o w i n g : inorganic species such as nitrous a c i d ; d i s s o l v e d S(IV); d i h y d r o g e n phosphates a n d h y d r o g e n phosphates; h y d r a t e d t r a n s i t i o n m e t a l species; organic species, i n c l u d i n g carboxylic acids a n d t h e i r h y d r o x y - a n d k e t o - a c i d relatives, p h e n o l s ; a n d some pesticide c o m p o u n d s . W e a k a c i d species, f o r m i c a n d acetic acids i n p a r t i c u l a r , c e r t a i n l y c o n t r i b u t e to the a c i d i t y of p r e c i p i tation, especially i n r e m o t e areas (5). A n i m p o r t a n t a n d u n r e s o l v e d q u e s t i o n is the extent to w h i c h these w e a k l y acidic species c o n t r i b u t e to the free acidity of aerosol particles. I n this chapter the discussion is r e s t r i c t e d to methods for d e t e r m i n i n g strong a c i d i t y i n aerosol particles a n d for d i s t i n g u i s h i n g strong a c i d species f r o m weak acids a n d acidic gases.

Measurement Methods D e s c r i p t i o n s of analytical m e t h o d s for strong a c i d a n d acidic sulfate content of a t m o s p h e r i c aerosols have b e e n r e v i e w e d (6-10). M e t h o d s for acidic aerosol d e t e r m i n a t i o n are r e v i e w e d i n this chapter a c c o r d i n g to the m e a s u r e m e n t p r i n c i p l e : e i t h e r filter c o l l e c t i o n a n d post-collection extraction, d e r i v atization o r t h e r m a l treatment, a n d analysis; or i n situ c o l l e c t i o n (real-time or stepwise) a n d analysis.

Filter Collection Thermal Volatilization. T h e r m a l v o l a t i l i z a t i o n schemes have b e e n p o p u l a r for speciation of acidic sulfate c o m p o u n d s i n aerosols. B o t h filter-


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b a s e d a n d i n situ approaches are u s e d ; the f o r m e r are d i s c u s s e d i n this section. F i l t e r samples w e r e h e a t e d a n d H S 0 was c o l l e c t e d b y m i c r o d i f fusion (J J , 12) or d e t e r m i n e d d i r e c t l y b y flame p h o t o m e t r y after v o l a t i l i z a t i o n from poly(tetrafluoroethylene) (Teflon) filters (13,14). I n one m e t h o d , H S 0 was d i s t i n g u i s h e d f r o m o t h e r volatile sulfates (e.g., N H H S 0 and ( N H ) S 0 ) , a n d f r o m n o n v o l a t i l e sulfates (e.g., N a ^ C ^ ) b y h e a t i n g consecu t i v e l y to t w o different t e m p e r a t u r e s (15). P e r i m i d i n y l a m m o n i u m sulfate was f o r m e d f r o m acidic sulfates a n d d e c o m p o s e d to S 0 for W e s t - G a e k e analysis i n another a p p r o a c h (16). T h e s e methodologies, d e v e l o p e d i n the late 1960s a n d e a r l y 1970s, w e r e s t i m u l a t i n g attempts to analyze acidic sulfate species i n aerosols. H o w e v e r , because of serious r e c o v e r y p r o b l e m s (15) a n d l i m i t e d success i n d i s t i n g u i s h ing the two major aerosol species ( N H H S 0 a n d ( N H ) S 0 ) from each o t h e r (17), they have fallen i n t o disfavor i n the past decade. A few exceptions i n v o l v i n g h e a t e d dénuder c o l l e c t i o n a n d i n situ analysis are discussed i n the f o l l o w i n g sections. 2

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l e c t e d p a r t i c l e samples have b e e n a n a l y z e d for net strong a c i d c o n t e n t b y several r e l a t e d p r o c e d u r e s , a l l starting w i t h extraction of the s a m p l e i n t o w a t e r or d i l u t e m i n e r a l a c i d . T h e strong a c i d content o f particles m a y b e d e t e r m i n e d i n p r i n c i p l e , based o n the d e f i n i t i o n , b y a s i m p l e m e a s u r e m e n t o f the p H o f an aqueous extract. H o w e v e r , this p r o c e d u r e is n e a r l y always subject to serious e r r o r w h e n a t m o s p h e r i c b u f f e r i n g agents such as d i s s o l v e d C 0 , weak carboxylic acids, or h y d r a t e d forms o f transition m e t a l ions (18, 19) are present, because free a c i d i t y ( d e t e r m i n e d b y the p H m e a s u r e m e n t ) cannot t h e n b e e q u a t e d to strong a c i d content (the analyte of interest) i n these buffered solutions. H e n c e , most c u r r e n t p r o c e d u r e s n o w p r e s c r i b e filter extraction i n t o weak m i n e r a l a c i d (e.g., Ι Ο Ν H C 1 0 or H S 0 ) , w i t h m e a s u r e m e n t o f the free a c i d i t y as the difference b e t w e e n e x p ( - p H ) a n d the free a c i d concentration c o n t a i n e d i n the extractant (20, 21) ( p H is o b s e r v e d p H ) . T h e p r i n c i p a l differences b e t w e e n these p H - m e a s u r e m e n t approaches are i n the s a m p l i n g apparatus, that i s , the p r e t r e a t m e n t o f the a t m o s p h e r i c aerosol p r i o r to c o l l e c t i o n o n the filter. T h e s e p r o c e d u r a l dif ferences are discussed i n r e l a t i o n to s a m p l i n g artifacts. 2

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T i t r a t i o n p r o c e d u r e s for strong a c i d content of a t m o s p h e r i c aerosols that use a n e x p o n e n t i a l d i s p l a y of data points (titration a c c o r d i n g to G r a n , ref e r e n c e 22) w e r e r e p o r t e d b y J u n g e a n d S c h e i c h (19), w e r e p e r f e c t e d b y Brosset a n d co-workers (23, 24), a n d have since b e e n w i d e l y u s e d b y o t h e r groups (25, 26). E l e c t r o l y t i c generation of strong base i n G r a n t i t r a t i o n p r o c e d u r e s has b e e n u s e d b y several groups (25, 27, 28). A g a i n , d i s s o l u t i o n of filter samples i n ~ 0 . 1 m M m i n e r a l a c i d p r i o r to G r a n t i t r a t i o n , w i t h c o r r e c t i o n for the a c i d content o f the extracting s o l u t i o n , allows for titration


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of l-μιηοΐ amounts o f strong a c i d (28), w i t h p r e c i s i o n a n d accuracy of the o r d e r o f ± 1 0 % (26, 29). E r r o r s m a y o c c u r i n the G r a n titration p r o c e d u r e i f w e a k l y a c i d i c species w i t h dissociation constants (expressed as p K ) i n the range of the extract p H are present. I n p a r t i c u l a r , c u r v a t u r e o r r e d u c t i o n (or both) of the slope o f the G r a n e x p o n e n t i a l plot results (24), because w e a k a c i d dissociation a n d titration of released free acidity take place d u r i n g the p o r t i o n of the titration u s e d for e n d - p o i n t d e t e r m i n a t i o n . F o r t u i t o u s l y , some of the c o m m o n , weak carboxylic acids (e.g., f o r m i c a n d acetic) are not stable t o w a r d m i c r o b i a l d e c o m p o s i t i o n w h e n c o l l e c t e d i n aerosol samples f r o m the a t m o s p h e r e , so m u c h o f the h i s t o r i c a l data base o n strong a c i d content o f aerosols does not suffer f r o m this positive e r r o r source, unless of course the m i c r o b i a l processes p r o d u c e a d d i t i o n a l strong acids.


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Specific Extraction of Atmospheric Acids. S e v e r a l efforts to spe cifically extract a t m o s p h e r i c acids from aerosol samples have b e e n r e p o r t e d , focusing o n sulfuric a c i d . B e n z a l d e h y d e has b e e n s h o w n to specifically extract H S 0 f r o m d r i e d acidic aerosol sulfate samples (15), w i t h analysis for sulfate m a d e i n aqueous "back-extracts" of the H S 0 - b e n z a l d e h y d e solutions. Isop r o p y l a l c o h o l q u a n t i t a t i v e l y extracts H S 0 from aerosol samples o n q u a r t z filters (30) b u t also removes a m m o n i u m bisulfate from samples as w e l l (15). D i s t r i b u t i o n coefficients of i s o p r o p y l alcohol w i t h respect to m i x e d n i t r a t e sulfate aerosol phases are still not w e l l k n o w n . I n a d d i t i o n , difficulties h a v e b e e n r e p o r t e d i n t w o separate investigations o f the q u a n t i t a t i v e r e m o v a l a n d selectivity of extraction techniques that use b e n z a l d e h y d e (31, 32). B e c a u s e free H S 0 is not a c o m m o n constituent of a m b i e n t aerosols, use of specific extractant methods has decreased i n recent years i n favor of g e n e r i c strong acid d e t e r m i n a t i o n . T e c h n i q u e s that specifically r e m o v e gaseous strong acids p r i o r to aerosol c o l l e c t i o n o n filters are discussed h e r e . 2

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p o r t e d (33) i n w h i c h filter-collected H S 0 is c o n v e r t e d to d i m e t h y l sulfate b y reaction w i t h d i a z o m e t h a n e , w i t h subsequent analysis b y gas e h r o m a tography-flame photometric detection ( G C - F P D ) . This method apparently does not specifically d e t e r m i n e sulfuric a c i d i n the presence o f a m m o n i u m bisulfate a n d m i x e d acidic s u l f a t e - n i t r a t e salts (Tanner a n d F a j e r , u n p u b l i s h e d data, 1981). A m e t h o d has b e e n p r o p o s e d for d e r i v a t i z a t i o n of c o l l e c t e d H S 0 b y reaction w i t h d i e t h y l a m i n e i n d r y a i r , f o l l o w e d b y r e a c t i o n w i t h C S a n d c u p r i c i o n to f o r m a c o l o r e d c o m p l e x for s p e c t r o p h o t o m e t r i c d e t e r m i n a t i o n (34). T h i s m e t h o d also suffers f r o m a large a m m o n i u m bisulfate interference. A r e l a t e d m e t h o d (35), i n w h i c h H S 0 a n d o t h e r aerosol strong acids are c o n v e r t e d to C - l a b e l e d b i s ( d i e t h y l a m m o n i u m ) sulfate a n d analogs, is a useful (and u n d e r u t i l i z e d ) t e c h n i q u e for d e t e r m i n a t i o n of l o w levels o f strong acids i n aerosols. 2

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In Situ Procedures Sulfuric acid

Continuous Sampling with Periodic Determination.

i n a t m o s p h e r i c aerosols m a y b e d e t e r m i n e d i n situ b y u s i n g a c o n t i n u o u s l y s a m p l i n g flame p h o t o m e t r i c detector ( F P D ) (36-38). T h e t e c h n i q u e uses a diffusion d é n u d e r i n l e t t u b e for S 0 r e m o v a l p r i o r to F P D d e t e r m i n a t i o n of aerosol, i n t h e same w a y that a n F P D i n s t r u m e n t has b e e n u s e d for c o n t i n u o u s aerosol sulfur analysis b y several groups (39-41). H o w e v e r , i n this adaptation the t e m p e r a t u r e o f the d é n u d e r t u b e o r a zone j u s t u p s t r e a m from it is c y c l e d b e t w e e n r o o m t e m p e r a t u r e a n d about 120 ° C . A t a m b i e n t t e m p e r a t u r e s , sulfuric a c i d remains i n t h e aerosol phase, b u t at about 120 °C i t is v o l a t i l i z e d a n d r e m o v e d to t h e walls o f t h e d é n u d e r t u b e . T h e difference b e t w e e n t h e signal at a m b i e n t t e m p e r a t u r e s a n d 120 ° C is p r o p o r t i o n a l to [ H S 0 ] . T h e m i n i m u m cycle t i m e , a n d thus the t i m e r e s o l u t i o n , is about 6 - 8 m i n . S e n s i t i v i t y o f the i n s t r u m e n t is b a r e l y adequate for h i g h a m b i e n t levels o f acidic sulfate, a n d t h e l i m i t o f d e t e c t i o n i n this difference m o d e is insufficient for t h e usual levels o f a t m o s p h e r i c H S 0 .


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O n e adaptation o f this approach uses a f u r t h e r t e m p e r a t u r e c y c l e to 2 2 0 ° C , w h i c h volatilizes a m m o n i u m sulfate a n d bisulfate i n aerosols b u t n o t n o n v o l a t i l e sulfates (e.g., N a S 0 ) (37, 40). A m m o n i u m bisulfate a n d a m m o n i u m sulfate are n o t differentiated b y this approach. A n o t h e r adaptation collects the aerosol sulfuric a c i d o n a h e a t e d dénuder t u b e (38) for ~ 1 5 m i n , t h e n removes i t t h e r m a l l y for F P D analysis w h i l e c o l l e c t i n g a n o t h e r sample o n a second dénuder t u b e . Because t h e sample is p r e c o n c e n t r a t e d i n this a p p r o a c h , t h e m o r e sensitive v e r s i o n o f the F P D is n o t r e q u i r e d . 2

4

A c i d i c sulfates, i n c l u d i n g sulfuric a c i d aerosol, m a y b e differentiated b y u s i n g t h e h u m i d o g r a p h t e c h n i q u e o f C h a r l s o n et a l . (42) w i t h t h e i m p r o v e d speciation o f m o r e r e c e n t l y d e v e l o p e d t h e r m i d o g r a p h variations (43, 44). T h e latter t e c h n i q u e involves h e a t i n g t h e air stream c o n t a i n i n g t h e aerosol from 20 to 3 8 0 °C i n 5 - m i n cycles, r a p i d l y c o o l i n g i t to t h e d r y b u l b t e m p e r a t u r e , a n d m e a s u r i n g t h e l i g h t scattering at 6 5 to 7 0 % r e l a t i v e h u m i d i t y (RH) w i t h a n e p h e l o m e t e r . B y c o m p a r i n g actual t h e r m i d o g r a m s w i t h those o f test aerosols, t h e fractional a c i d i t y (neq H / n e q S 0 ~ ; n e q denotes n a n o equivalents) c a n b e m e a s u r e d a n d t h e approximate l e v e l o f H S 0 d e t e r m i n e d . M o r e recent w o r k (45) demonstrates t h e u t i l i t y o f this t e c h n i q u e for d e t e r m i n i n g e v e n b a c k g r o u n d levels o f a c i d i c sulfate ( < 1 0 n e q / m ) . +

4

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O n e other i n situ t e c h n i q u e can b e u s e d to d e t e r m i n e fractional a c i d i t y i n a t m o s p h e r i c aerosols b y means o f F o u r i e r transform i n f r a r e d ( F T I R ) spectroscopy (46). O r i g i n a l l y , i m p a c t o r samples w e r e c o l l e c t e d a n d w e r e p r e s s e d i n t o a K B r m a t r i x , a n d t h e n t h e I R s p e c t r u m was t a k e n b y attenuated total reflectance (ATR) F T I R spectroscopy to d e t e r m i n e r e l a t i v e a c i d i t y , b a s e d o n differences i n absorption bands for sulfate a n d bisulfate species. A e r o s o l s w i t h [ H ] / [ S 0 ~ ] ratios greater t h a n 1 c o u l d also b e q u a l i t a t i v e l y i d e n t i f i e d . M o r e recent innovations i n t h e F T I R t e c h n i q u e (47, 48) have m a d e possible +

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the q u a n t i t a t i o n of sulfuric a c i d ; f u r t h e r , w i t h the use of A T R plates as substrates i n i m p a c t i v e c o l l e c t i o n o f a t m o s p h e r i c aerosols, the t e c h n i q u e can be u s e d for i n situ d e t e r m i n a t i o n of acidic sulfates a n d o t h e r aerosol c o n stituents w i t h d i s t i n g u i s h a b l e I R spectra (e.g., nitrates a n d silicates). A m o d ification o f this t e c h n i q u e has also b e e n a p p l i e d to cascade i m p a c t o r samples b y B r o w n et a l . (49), albeit not i n a n i n situ m o d e . T h e f o r m e r t e c h n i q u e deserves f u r t h e r study a n d m i g h t serve as a standard m e t h o d for a c i d i c aerosol species.

Continuous Sampling and Determination.

T h e r e are no truly c o n -

tinuous t e c h n i q u e s for the d i r e c t d e t e r m i n a t i o n o f s u l f u r i c a c i d or o t h e r strong a c i d species i n a t m o s p h e r i c aerosols. T h e closest c a n d i d a t e m e t h o d is a f u r t h e r modification of the s e n s i t i v i t y - e n h a n c e d , flame p h o t o m e t r i c d e tector, i n w h i c h t w o detectors are u s e d , one w i t h a r o o m - t e m p e r a t u r e d é n u d e r a n d one w i t h a d é n u d e r t u b e heated to about 120 ° C . S u l f u r i c a c i d is potentially d e t e r m i n e d as the difference b e t w e e n the t w o channels. I n fact, a d e v i c e based o n this approach d i d not p e r f o r m w e l l i n a m b i e n t a i r s a m p l i n g (Tanner a n d S p r i n g s t o n , u n p u b l i s h e d data, 1990). E v e n w i t h the S F - d o p e d H f u e l gas for e n h a n c e d sensitivity, the l i m i t o f d e t e c t i o n is u n s u i t a b l y h i g h (5 μ g / m or greater) because o f the difficulty i n c a l i b r a t i n g the t w o separate F P D channels w i t h aerosol sulfates. 6

2

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Pitfalls of Sampling for Acidic Aerosols General Observations. M e a s u r e m e n t t e c h n i q u e s for traces o f s t r o n g acids i n a t m o s p h e r i c aerosols have b e e n p l a g u e d w i t h difficulties, often r e f e r r e d to as s a m p l i n g anomalies or artifacts. O f t e n , these difficulties have d e r i v e d f r o m the use of materials o n c o l l e c t i o n surfaces (e.g., filters a n d i m p a c t o r slides) that w e r e d e s i g n e d for o t h e r purposes. T h e s e difficulties a n d means o f c i r c u m v e n t i n g t h e m are discussed i n this section. Artifacts are of t w o general types: (1) r e v e r s i b l e or i r r e v e r s i b l e s o r p t i o n losses to filter o r i m p a c t o r surfaces i n integrative s a m p l i n g or to the s a m p l i n g l i n e s (e.g., n i t r i c acid o n T e f l o n lines) i n continuous, i n situ t e c h n i q u e s ; a n d (2) e q u i l i b r i u m d r i v e n loss or gain of species d u e to p a r t i c l e - p a r t i c l e reactions o n the c o l l e c t i o n surface or non-steady-state conditions i n the s a m p l e d a t m o s p h e r e d u r i n g the t i m e p e r i o d of sample c o l l e c t i o n . T h e s e types of s a m p l i n g artifacts are s u m m a r i z e d i n terms of surface, g a s - a e r o s o l , a n d a e r o s o l - a e r o s o l i n t e r actions. Surface Interactions. Loss o f strong a c i d content o f a t m o s p h e r i c aerosols was o b s e r v e d a n d a t t r i b u t e d to reaction w i t h basic sites i n the glass or cellulose filter matrices c o m m o n l y u s e d for h i g h - v o l u m e s a m p l i n g o f at m o s p h e r i c aerosols (46, 50). T h e s e filter materials, a n d glass fiber filters o f



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a l l types i n p a r t i c u l a r , are unsuitable for c o l l e c t i o n of acidic aerosol particles for subsequent extraction a n d analysis. E v e n i f t h e y are p r e t r e a t e d w i t h a c i d a n d fired to a h i g h t e m p e r a t u r e , subsequent r i n s i n g exposes a d d i t i o n a l free basic sites that n e u t r a l i z e acidic particles (28). H i g h - p u r i t y q u a r t z filters can b e p r e t r e a t e d w i t h a c i d to r e m o v e basic sites p r i o r to s a m p l i n g , a n d Teflon filter m e d i a are generally i n e r t to acidic particles. T h u s , Teflon a n d treated q u a r t z filter m e d i a have generally r e p l a c e d glass or cellulose filters for s a m p l i n g acidic aerosols. I n a d d i t i o n , t h e i r use eliminates a positive source of e r r o r i n sulfate measurements d u e to the base-catalyzed c o n v e r s i o n of s o r b e d , gaseous S 0 to sulfuric a c i d , a n a l y z e d as sulfate after extraction ( 5 0 52). 2

Gas-Aerosol Interactions. R e - e q u i l i b r a t i o n i n v o l v i n g gas a n d p a r ticulate-phase species m a y l e a d to errors i n s a m p l i n g a n d analysis of aerosol strong acids a n d other aerosol species, b o t h u n d e r steady-state a n d n o n steady-state conditions. I n p a r t i c u l a r , transport or exposure of c o n d e n s e d phase species o n particles to e n v i r o n m e n t s w i t h l o w e r gas-phase c o n c e n t r a tions m a y l e a d to loss of particulate species i n o r d e r to reestablish p e r t i n e n t e q u i l i b r i a . T h e example of greatest significance for a t m o s p h e r i c a c i d d e t e r minations is the e q u i l i b r i u m b e t w e e n volatile a m m o n i u m salts a n d gaseous acid a n d a m m o n i a levels. A l t h o u g h the s i m p l e evaporation of n e u t r a l salts from c o l l e c t e d particles s h o u l d not d i r e c t l y i n t r o d u c e errors i n strong acid d e t e r m i n a t i o n s , the m o r e subtle effects of e q u i l i b r a t i o n processes o n accurate d e t e r m i n a t i o n of aerosol strong a c i d levels s h o u l d b e c o n s i d e r e d . S e v e r a l studies have d e m o n s t r a t e d that o b s e r v e d , a t m o s p h e r i c levels of gaseous a m m o n i a a n d n i t r i c a c i d (or H C l ) are i n o r d e r - o f - m a g n i t u d e agreem e n t w i t h calculated e q u i l i b r i u m values u n d e r a m b i e n t t e m p e r a t u r e a n d relative h u m i d i t i e s , based o n t h e r m o d y n a m i c p r o p e r t i e s , w h e n e v e r a s o l i d a m m o n i u m n i t r a t e - c h l o r i d e aerosol phase (or m i x e d s u l f a t e - n i t r a t e - c h l o r i d e phase) is present (53-57). A t h e r m o d y n a m i c m o d e l has b e e n d e v e l o p e d a n d r e f i n e d for aerosol formation a n d g a s - a e r o s o l p a r t i t i o n i n g of these phases (58, 59). T h e s e a n d other studies d e a l i n g w i t h the effects of u s i n g averaged aerosol compositions, w i t h l o n g s a m p l i n g t i m e c o m p a r e d w i t h the m e a n t i m e for changes i n t h e r m o d y n a m i c a l l y significant variables, have b e e n r e v i e w e d b y T a n n e r a n d H a r r i s o n (60). T h e l i k e l i h o o d of k i n e t i c l i m i t a t i o n s i n the a c h i e v e m e n t of gas-aerosol e q u i l i b r i a has also b e e n evaluated as a f u n c t i o n of aerosol particle size a n d t e m p e r a t u r e , a n d the effect of s u c h l i m i t a t i o n s o n strategies for s a m p l i n g aerosol strong acids needs to b e c o n s i d e r e d . T h e p r i n c i p a l i m p l i c a t i o n of these studies for the accurate m e a s u r e m e n t of strong acids i n gaseous a n d aerosol phases is that s a m p l i n g m u s t b e p e r f o r m e d i n a w a y that does not d i s t u r b the e q u i l i b r i u m significantly i n the process. T h i s necessity has l e d to the d e v e l o p m e n t of the diffusion d é n u d e r s a m p l e r (61), w h i c h measures n i t r i c a c i d after its r e m o v a l to the walls of a


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tube coated w i t h a m a t e r i a l that reacts i r r e v e r s i b l y w i t h H N 0 . T h e residence t i m e i n the tube is short e n o u g h ( 0 . 2 - 2 s) that particles are not m e a s u r a b l y r e m o v e d b y diffusion, n o r do they evaporate significantly t h e r e i n to reestablish gas-aerosol e q u i l i b r i u m .


3

Is this r e - e q u i l i b r a t i o n p h e n o m e n o n i m p o r t a n t for the m e a s u r e m e n t of aerosol strong acid content, as it is for gaseous n i t r i c acid? G e n e r a l l y , n i t r i c acid is t a k e n u p i n t o aerosol particles (solid or l i q u i d droplets) o n l y i f the particles have b e e n n e a r l y c o m p l e t e l y n e u t r a l i z e d b y a m b i e n t a m m o n i a , because the n i t r a t e - n i t r i c a c i d e q u i l i b r i u m favors the gas phase i n the p r e s ence of significant particulate strong a c i d (62). M o s t c o n c e r n , h o w e v e r , has b e e n expressed c o n c e r n i n g s a m p l i n g of acidic aerosols i n the presence of ambient ammonia. T h e existence of t h e r m o d y n a m i c e q u i l i b r i u m b e t w e e n gaseous N H a n d H N 0 a n d aerosol particles i n a m b i e n t air s h o u l d p r e c l u d e the uptake of a m m o n i a d u r i n g s a m p l i n g except u n d e r two circumstances. T h e first c o n d i t i o n involves post-collection n e u t r a l i z a t i o n of fine, acidic particles b y i m p a c t i n g , coarse, basic particles; populations of these particles w e r e not steady-state i n the atmosphere. T h i s c o n d i t i o n is discussed i n the f o l l o w i n g section. T h e second circumstance is w h e n s a m p l i n g m u s t p r o c e e d for a p e r i o d of m a n y hours i n o r d e r to acquire e n o u g h m a t e r i a l for analysis. I n this case the a m b i e n t conditions m a y change d u r i n g s a m p l i n g ; for e x a m p l e , a m b i e n t [ N H ] m a y increase, a n d already c o l l e c t e d acidic aerosols m a y b e n e u t r a l i z e d b y exposure to air of h i g h e r a m m o n i a levels t h a n those extant d u r i n g s a m p l i n g . I f the a m b i e n t [ N H ] decreases b u t still exceeds gaseous [ H N 0 ] , net a m m o n i a m i g h t b e released from c o l l e c t e d a m m o n i u m a c i d salts, i n c r e a s i n g the [ H ] / [ a n i o n ] ratio (and the apparent [ H * ] ) i n the r e m a i n i n g deposit. E x p e r i m e n t a l c o n f i r m a t i o n of this p h e n o m e n o n has a p p a r e n t l y not b e e n reported. 3

3

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+

O n e p r o c e d u r e that is w i d e l y u s e d to c i r c u m v e n t these complications is to r e m o v e a m b i e n t a m m o n i a from the s a m p l e d air w i t h o u t r e m o v i n g particles b y i n s e r t i n g one of several types of diffusion d e n u d e r s u p s t r e a m from the filter(s). I n fact, i n a recent E n v i r o n m e n t a l P r o t e c t i o n A g e n c y ( E P A ) - s p o n s o r e d i n t e r c o m p a r i s o n of methods for d e t e r m i n a t i o n of strong acid content of aerosols, a l l b u t one p r o t o c o l u t i l i z e d a n a m m o n i a d é n u d e r (63), a n d a l l u s e d an i m p a c t o r or cyclone to r e m o v e coarse particles. T h e presence of this dénuder clearly prevents n e u t r a l i z a t i o n of a c i d i c aerosols b y a m m o n i a b u t also disturbs the gas-aerosol e q u i l i b r i u m b e t w e e n s u l f a t e nitrate aerosols a n d gaseous species. A m m o n i a a n d n i t r i c a c i d are released from the d e p o s i t i n g particles (64, 65) a n d m u s t b e c o l l e c t e d d o w n s t r e a m i f accurate particulate a m m o n i u m a n d nitrate d e t e r m i n a t i o n s are to b e m a d e . If e q u a l amounts o f a m m o n i a a n d n i t r i c a c i d are released, t h e n the absolute [ H ] ( n e q / m ) w i l l not b e a l t e r e d . N o specific e v i d e n c e is available i n the literature to demonstrate alteration of the o b s e r v e d [ H ] as the result of r e e q u i l i b r a t i o n , b u t this area deserves f u r t h e r study. +

3

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Particle-Particle Interactions. L o s s of strong a c i d content of aerosol particles can also occur because o f reactions b e t w e e n co-collected a c i d i c a n d basic particles i m p a c t e d together o n the c o l l e c t i o n surface. T h i s p h e n o m e n o n most f r e q u e n t l y occurs as t h e result o f i n t e r a c t i o n o f coarse ( > 2 . 5 μπι d i ameter), a l k a l i n e , s o i l - d e r i v e d particles w i t h fine ( < 2 . 5 μπι diameter) acidic sulfate particles (66). P a r t i c l e - p a r t i c l e interactions w i t h n e t n e u t r a l i z a t i o n can b e r e d u c e d i n m a n y cases b y s a m p l i n g w i t h a v i r t u a l i m p a c t o r o r a cyclone to r e m o v e coarse particles, a l t h o u g h this p r o c e d u r e does not p r e v e n t the effect i f external m i x t u r e s o f fine particles o f different a c i d contents are s a m p l e d . I n situ m e t h o d s w i t h shorter s a m p l i n g t i m e s can b e u s e d s u c h that these t o p o c h e m i c a l reactions are less l i k e l y to occur.

Summary and Conclusions M o d i f i e d filter s a m p l i n g methods that are available w i l l measure a m b i e n t levels o f strong a c i d i n a m b i e n t aerosol samples, a n d these m e t h o d s d o so w i t h acceptable p r e c i s i o n a n d accuracy [as i n d i c a t e d b y the balance b e t w e e n m e a s u r e d anions a n d cations (56, 57)] i n the absence o f significant levels o f particulate weak acids. A d d i t i o n a l intercomparisons i n v o l v i n g i n t r i n s i c a l l y different techniques for particulate strong acidity [e.g., I R spectroscopy (48), t h e r m a l speciation (38, 45), a n d filter methods (28)] are n e e d e d . F u r t h e r i n f o r m a t i o n o n the o c c u r r e n c e o f various weak acids i n airborne particles is n e e d e d , along w i t h f u r t h e r studies o f techniques for t h e i r specific d e t e r m i n a t i o n i n a t m o s p h e r i c aerosol samples. If l o n g s a m p l i n g times are r e q u i r e d to collect sufficient sample for a n a l ysis, i t is desirable to r e m o v e a m b i e n t a m m o n i a (by diffusional r e m o v a l to the walls o f a coated dénuder tube) because increases i n the c o n c e n t r a t i o n of this gas d u r i n g s a m p l i n g may result i n post-collection n e u t r a l i z a t i o n o f acidic aerosol particles. N o e v i d e n c e o f changes i n absolute [ H ] caused b y this p r o c e d u r e is r e p o r t e d , b u t further research is n e e d e d i n this area. +

Sulfuric acid m a y b e d e t e r m i n e d i n the presence o f other acidic aerosol constituents, b u t n o m e t h o d for the separation a n d d e t e r m i n a t i o n o f b i s u l fate, t h e most c o m m o n acidic species i n a t m o s p h e r i c aerosols, has b e e n successfully d e v e l o p e d . R e m o v a l o f coarse particles from t h e air stream is d e s i r a b l e f o r d e t e r m i n a t i o n o f fine particulate acidity because a m b i e n t coarse particles m a y contain basic substances. F u r t h e r study is n e e d e d o f the p h e n o m e n o n o f k i n e t i c l i m i t a t i o n s to the neutralization o f acidic aerosols. S i m u l t a n e o u s occurrences of a c i d i c aerosols at gaseous [ N H ] w e l l above the e q u i l i b r i u m values have b e e n r e p o r t e d (56, 67), a n d i t is still u n c l e a r w h e t h e r k i n e t i c l i m i t s to microscale n e u t r a l i z a t i o n or b o u n d a r y layer m i x i n g (macroscale) k i n e t i c s (or both) are responsible for these limitations. A n u n d e r s t a n d i n g of the extent of h u m a n exposure to a c i d i c aerosols, as w e l l as o f the availability o f acidic aerosols for w e t scavenging 3


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into h y d r o m e t e o r s , m a y d e p e n d o n the results of f u r t h e r studies of n e u tralization kinetics i n a m b i e n t air.


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for review March 20, 1991.

ACCEPTED

revised manuscript August 10,


Measuring the Strong Acid Content of Atmospheric Aerosol Particles

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