Removal of Trace Contaminants from the Air - ACS Publications

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Introduction,

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P e s t i c i d e s in t h e

a n d F a t e of P e r s i s t e n t

Atmosphere

Downloaded by UNIV OF MELBOURNE on January 17, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0017.ch004

D. E. GLOTFELTY and J. H . CARO Agricultural Chemicals Management Laboratory, Agricultural Environmental Quality Institute, Agricultural Research Service, USDA, Beltsville, Md. 20705

As has been often r e p o r t e d , worldwide p r o d u c t i o n and use o f organic p e s t i c i d e s grew e x p l o s i v e l y in the quarter century f o l l o w i n g the i n t r o d u c t i o n o f DDT d u r i n g World War II. In the peak year o f 1968, annual p r o d u c t i o n in the United States reached more than 5.3 x 105 m e t r i c tons o f a c t i v e p e s t i c i d e i n g r e d i e n t s , i n c l u d i n g 2.64 x 10 , 1.83 x 10 , and 8.65 x 10 m e t r i c tons o f insecticides, h e r b i c i d e s , and f u n g i c i d e s , r e s p e c t i v e l y ( 1 ) . The current domestic market i n c l u d e s more than 420 biologically a c t i v e chemical compounds s o l d in over 6700 d i f f e r e n t formulations ( 2 ) . These m u l t i t u d i n o u s products have specific, r e g i s t e r e d uses: they are a p p l i e d p r i m a r i l y t o agri cultural crops, soils, livestock, f o r e s t s , bodies o f water, and the general environment, to a s s i s t in food and f i b e r p r o d u c t i o n and t o p r o t e c t p u b l i c h e a l t h . They a l s o are used commercially in food p r o c e s s i n g and p r o t e c t i o n o f s t o r e d p r o d u c t s , and are a p p l i e d in the home and the garden. Each o f these purposeful a p p l i c a t i o n s provides a source f o r entry o f the chemicals i n t o the environment. U n i n t e n t i o n a l sources a l s o exist, such as u n c o n t r o l l e d e f f l u e n t s from p e s t i c i d e manufacturing and f o r m u l a t i n g p l a n t s , chemical spills, structural fires i n v o l v i n g c h e m i c a l s , and improper d i s p o s a l o f surplus chemicals or used c o n t a i n e r s . Once a p p l i e d or r e l e a s e d , the p e s t i c i d e s are t r a n s p o r t e d from the original site by water or air. Considerable research has been conducted on the mechanisms and scope o f t r a n s p o r t and on the identification of terminal r e s i d u e s , yet broad gaps still e x i s t in our understanding o f the system. We examine here the current s t a t e o f knowledge as i t a p p l i e s to the i n t r o d u c t i o n and t r a n s p o r t o f p e s t i c i d e s in the atmosphere, present a unique approach to e v a l u a t i n g the relative t r a n s p o r t p o t e n t i a l of specific p e s t i c i d e s , d i s c u s s the processes by which the p e s t i c i d e s are removed from the atmosphere, and i n d i c a t e the most fertile d i r e c t i o n s f o r future r e s e a r c h . P e s t i c i d e residues in air are o f concern not o n l y because o f d i r e c t economic e f f e c t s such as the d e p o s i t i o n o f illegal 5

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In Removal of Trace Contaminants from the Air; Deitz, Victor R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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r e s i d u e s on nontarget organisms and i n j u r y t o s u s c e p t i b l e c r o p s , b u t p e r h a p s more s i g n i f i c a n t l y , b e c a u s e o f t h e p o s s i b l e l o n g r a n g e , s u b t l e a d v e r s e e f f e c t s o n human h e a l t h (3,, k). Of p r i m a r y importance on a g l o b a l s c a l e are t h e p e r s i s t e n t p e s t i c i d e s , n o t a b l y t h e o r g a n o c h l o r i n e i n s e c t i c i d e s a s t y p i f i e d b y DDT a n d its derivatives. A l t h o u g h u s e o f DDT i s now l a r g e l y f o r b i d d e n i n many c o u n t r i e s , i n c l u d i n g t h e U n i t e d S t a t e s , i t s l o n g a n d i n t e n s i v e a p p l i c a t i o n over broad areas o f l a n d i n recent y e a r s , i t s subsequent well-documented t r a n s p o r t t o u n t r e a t e d a r e a s , and i t s c o n t i n u i n g u s e i n t h e d e v e l o p i n g c o u n t r i e s f o r p u b l i c h e a l t h purposes have caused l o w l e v e l s o f i t s r e s i d u e s t o be u b i q u i t o u s o n t h e s u r f a c e o f t h e e a r t h a s w e l l a s i n t h e a i r (5)· These r e s i d u e s a r e o f p a r t i c u l a r c o n c e r n b e c a u s e o f t h e i r physicochemical p r o p e r t i e s , which are t y p i c a l o f those o f t h e general class o f c h l o r i n a t e d i n s e c t i c i d e s . They h a v e l o w w a t e r s o l u b i l i t y , h i g h s o l u b i l i t y i n f a t s , and h i g h r e s i s t a n c e t o chemical and b i o l o g i c a l decomposition. Consequently, they tend t o p e r s i s t a n d a c c u m u l a t e i n l i v i n g o r g a n i s m s a n d h a v e demons t r a t e d a b i l i t y t o c o n c e n t r a t e i n food c h a i n s , so t h a t v e r y low l e v e l s i n t h e environment c a n cause s e r i o u s adverse e f f e c t s at the higher trophic l e v e l s . T h i s b e h a v i o r was n o t g e n e r a l l y r e c o g n i z e d f o r many y e a r s a f t e r t h e y were f i r s t u s e d , a n d much a v o i d a b l e damage was w r o u g h t t h r o u g h t h e i r o v e r u s e a n d m i s u s e . I t i s now i m p e r a t i v e t h a t we g a i n a t h o r o u g h u n d e r s t a n d i n g o f t h e mechanisms o f t r a n s p o r t o f DDT a n d r e l a t e d p e s t i c i d e s away from t h e s i t e o f a p p l i c a t i o n and t h e i r e v e n t u a l f a t e i n t h e e n v i r o n m e n t s o t h a t we may more j u d i c i o u s l y u s e p e s t i c i d e s i n the f u t u r e . I n t r o d u c t i o n o f P e s t i c i d e s i n t o t h e Atmosphere A b r i e f , p r o b a b l y i n c o m p l e t e , i n v e n t o r y o f some o f t h e c o n t r i b u t i n g sources o f p e s t i c i d e s i n a i r i s given i n Table I . With t h e e x c e p t i o n o f t h e a g r i c u l t u r a l i n p u t s , we know v e r y l i t t l e about t h e s e s o u r c e s . Each s o u r c e d o e s , h o w e v e r , r e l e a s e t h e p e s t i c i d e s i n t o t h e atmosphere i n a spectrum o f p a r t i c l e s i z e s , so t h a t c o n s i d e r a t i o n s r e l a t e d t o p a r t i c l e s i z e , w h i c h a r e d e a l t w i t h i n t h e l a t e r s e c t i o n on a e r i a l t r a n s p o r t , a r e a p p l i c a b l e to a l l sources. Owing t o t h e l a r g e a c r e a g e s a n d amounts o f chemicals i n v o l v e d , a g r i c u l t u r a l use o f p e s t i c i d e s deserves i n d i v i d u a l a t t e n t i o n as a primary c o n t r i b u t o r t o atmospheric contamination. Inputs from a g r i c u l t u r e c o n s i s t o f (a) l o s s e s t o t h e a i r d u r i n g a p p l i c a t i o n o f t h e p e s t i c i d e , and (b) p o s t a p p l i c a t i o n l o s s e s , i n c l u d i n g wind e r o s i o n o f contaminated s u r f a c e dust and v o l a t i l i z a t i o n i n m o l e c u l a r vapor form o f p e s t i c i d e r e s i d u e s on t h e s u r f a c e s o f v e g e t a t i o n o r s o i l (Table I). D i s c u s s i o n s o f each o f t h e s e f o l l o w .



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A p p l i c a t i o n Inputs. Losses to the a i r during p e s t i c i d e a p p l i c a t i o n o f t e n c o n s t i t u t e 50% o r more o f t h e t o t a l p e s t i c i d e u s e d (6). Part of the m a t e r i a l l o s t w i l l r e t u r n q u i c k l y to the s u r f a c e a s h o r t d i s t a n c e downwind ( d r i f t ) ; p a r t w i l l be d e p o s i t e d more s l o w l y f u r t h e r downwind; and p a r t w i l l become a i r b o r n e f o r a much l o n g e r t i m e , d i l u t i n g r a p i d l y i n t h e a t m o s p h e r e , and r e t u r n i n g t o t h e s u r f a c e v e r y s l o w l y . How much o f t h e t o t a l l o s s f a l l s i n t o each o f t h e s e c a t e g o r i e s depends on t h e p a r t i c l e s i z e and t h e a m b i e n t w e a t h e r c o n d i t i o n s . F o r l e g a l and e c o n o m i c r e a s o n s , d r i f t has r e c e i v e d much a t t e n t i o n , b u t i t i s t h e p e s t i c i d e s t h a t a r e s u b j e c t t o more p r o t r a c t e d a t m o s p h e r i c t r a n s p o r t t h a t may u l t i m a t e l y be more s i g n i f i c a n t . P e s t i c i d e s sprayed from h e l i c o p t e r s o r a i r p l a n e s g i v e subs t a n t i a l l y g r e a t e r l o s s e s t o t h e a i r t h a n when a p p l i e d as g r o u n d s p r a y s , because the g r e a t e r r e l e a s e h e i g h t allows a longer f a l l t i m e d u r i n g w h i c h downwind t r a n s p o r t and d r o p l e t e v a p o r a t i o n can o c c u r . O n - t a r g e t d e p o s i t s o f 5$ o r l e s s o f t h e n o m i n a l a p p l i c a t i o n h a v e b e e n r e p o r t e d f o r some a e r i a l t r e a t m e n t s (h). The g r e a t e r r e l e a s e h e i g h t i s n o t n e c e s s a r i l y due t o i n a b i l i t y o f t h e a i r c r a f t t o come s u f f i c i e n t l y c l o s e t o t h e s u r f a c e , b u t c a n be c a u s e d by t h e a i r t u r b u l e n c e c r e a t e d i n t h e wake o f t h e aircraft. P o s t a p p l i c a t i o n Inputs. T r a n s p o r t by w i n d e r o s i o n o f c o n t a m i n a t e d s u r f a c e d u s t t o h i g h a l t i t u d e s and f o r l o n g d i s t a n c e s has b e e n documented (7_, 8_) , b u t i t i s more i n f r e q u e n t and l e s s i m p o r t a n t t h a n v a p o r i z a t i o n as a mechanism o f p e s t i c i d e i n p u t t o t h e a t m o s p h e r e . Not o n l y h i g h l y v o l a t i l e p e s t i c i d e s , b u t a l s o t h e s o - c a l l e d " n o n v o l a t i l e " p e s t i c i d e s s u c h as t h e common o r g a n o c h l o r i n e i n s e c t i c i d e s , c a n e v a p o r a t e f r o m s o i l , w a t e r , and v e g e t a t i o n s u r f a c e s i n s u b s t a n t i a l q u a n t i t i e s (9.). Indeed, v a p o r i z a t i o n t o t h e a t m o s p h e r e may be a d o m i n a n t pathway i n t o t h e e n v i r o n m e n t f o r compounds o f l o w w a t e r s o l u b i l i t y and l o n g p e r s i s t e n c e , p r o p e r t i e s t h a t the organochlorine p e s t i c i d e s possess. E v a p o r a t i o n b e g i n s when t h e p e s t i c i d e i s a p p l i e d and c o n t i n u e s as l o n g as t h e compound i s p r e s e n t . The e v a p o r a t i o n r a t e o f a p e s t i c i d e i n t o a i r f r o m s o i l , w a t e r , o r v e g e t a t i o n i s d e t e r m i n e d s o l e l y by t h e e f f e c t i v e v a p o r p r e s s u r e o f t h e s u b s t a n c e a t t h e e v a p o r a t i n g s u r f a c e and t h e r a t e of d i f f u s i o n through the l a y e r of a i r immediately adjacent t o t h e s u r f a c e (10_, 11_). A i r c l o s e t o t h e e v a p o r a t i n g s u r f a c e does n o t move, and m o l e c u l a r d i f f u s i o n a c r o s s t h e l a m i n a r " s k i n " o f a i r controls the v o l a t i l i z a t i o n r a t e . V o l a t i l i z a t i o n i n c r e a s e s i f t h e d e p t h o f t h e l a m i n a r l a y e r , n o r m a l l y 2 t o 3 mm, decreases o r i f the concentration gradient across the l a y e r increases. Many v a r i a b l e s may a f f e c t v o l a t i l i z a t i o n by i n f l u e n c i n g t h e two b a s i c f a c t o r s . These v a r i a b l e s i n c l u d e temperature, c o n c e n t r a t i o n o f p e s t i c i d e at the s u r f a c e , degree o f a d s o r p t i o n , t y p e o f f o r m u l a t i o n , r a t e o f mass t r a n s p o r t t o t h e



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a i r - s u r f a c e i n t e r f a c e , supply o f l a t e n t heat o f v a p o r i z a t i o n , r a t e o f a i r f l o w above t h e s u r f a c e , a n d a t m o s p h e r i c t u r b u l e n c e . M u l t i l a y e r e d s u r f a c e d e p o s i t s such as f o l i a r spray a p p l i c a t i o n s evaporate a t a constant r a t e u n t i l so l i t t l e remains t h a t t h e s u r f a c e i s no l o n g e r u n i f o r m l y c o v e r e d ( 1 1 ) . Evaporation r a t e f r o m p l a n t o r s o i l s u r f a c e s may b e v e r y h i g h s h o r t l y a f t e r application. I n one f i e l d e x p e r i m e n t , f o r e x a m p l e , a v a p o r c o n c e n t r a t i o n c o r r e s p o n d i n g t o s a t u r a t i o n a t the ambient t e m p e r a t u r e was o b s e r v e d f o r d i e l d r i n i n a i r 10 cm above a s h o r t stand o f orchardgrass during the f i r s t 2 hours a f t e r f o l i a r a p p l i c a t i o n ; t h e f i e l d e f f e c t i v e l y behaved as a s u r f a c e o f pure d i e l d r i n during t h i s period (12). The v a p o r i z a t i o n o f a p e s t i c i d e f r o m s o i l i s p a r t i c u l a r l y i n f l u e n c e d by i t s s o l u b i l i t y i n s o i l water and i t s degree o f a d s o r p t i o n on s o i l p a r t i c l e s . S o i l w a t e r p l a y s an extremely i m p o r t a n t r o l e ; i t competes w i t h t h e p e s t i c i d e f o r a d s o r p t i o n s i t e s and a l s o c a r r i e s s o l u b i l i z e d r e s i d u e s t o the s u r f a c e (10, 11, 13). The r a t e o f d e s o r p t i o n o f p e s t i c i d e r e s i d u e s a n d t h e i r movement t o t h e s u r f a c e a r e c o n t r o l l e d b y t e m p e r a t u r e , p e s t i c i d e c o n c e n t r a t i o n i n the s o i l , and s o i l moisture content and b u l k d e n s i t y . The l o n g - t e r m l o s s r a t e o f o r g a n o c h l o r i n e p e s t i c i d e s incorporated i n t o s o i l i s probably c l o s e l y r e l a t e d t o t h e mass f l o w o f w a t e r t o t h e s o i l s u r f a c e ( l l ) . D i f f e r i n g l o s s r a t e s from d i f f e r e n t s o i l types r e f l e c t the a d s o r p t i o n capacity o f the s o i l , which i s p r i m a r i l v ^ e l a t e d t o i t s organic matter content. Loss i s g e n e r a l l y g r e a t e r f r o m a sandy s o i l o f low o r g a n i c m a t t e r content t h a n from a h i g h - o r g a n i c s o i l . I n c o r p o r a t i o n i n t o s o i l w i l l sharply reduce v o l a t i l i z a t i o n l o s s e s ; some r e l a t i v e l y v o l a t i l e h e r b i c i d e s l o s e a c t i v i t y r a p i d l y by v o l a t i l i z a t i o n u n l e s s t h e y a r e i n c o r p o r a t e d (ik). V o l a t i l i z a t i o n o f p e s t i c i d e s from v e g e t a t i o n s u r f a c e s has n o t been s t u d i e d as e x t e n s i v e l y as t h a t from s o i l , a l t h o u g h i t can be high f o r surface d e p o s i t s . V o l a t i l i z a t i o n i s reduced by s o l u b i l i z a t i o n o f t h e s u r f a c e p e s t i c i d e s i n p l a n t waxes a n d o i l s , O e n e t r a t i o n t o i n t e r n a l p o r t i o n s o f the p l a n t , and t r a n s l o c a t i o n . The f o r m u l a t i o n a n d method o f a p p l i c a t i o n may b e i m p o r t a n t (15) , as a r e a n y c h e m i c a l changes i n t h e p e s t i c i d e c a u s e d b y t h e p l a n t o r b y s u n l i g h t . The e f f e c t o f p l a n t t r a n s p i r a t i o n o n p e s t i c i d e v o l a t i l i z a t i o n i s u n c l e a r , although water e v a p o r a t i o n from p l a n t l e a v e s i s known t o change t h e e n e r g y b u d g e t . P l a n t s w i t h adequate moisture are r e l a t i v e l y c o o l because the incoming s o l a r r a d i a t i o n i s d i s s i p a t e d as l a t e n t heat o f evaporation; water e v a p o r a t i o n i s o b v i o u s l y r e s t r i c t e d under dry c o n d i t i o n s and t h e p l a n t s become warmer t h r o u g h i n s o l a t i o n ( r e c e i p t o f i n c o m i n g solar radiation). C o o l e r p l a n t s u r f a c e s may r e s t r i c t p e s t i c i d e v o l a t i l i zation. The g r o s s f l u x o f p e s t i c i d e p a s s i n g i n t o t h e a t m o s p h e r e b y v o l a t i l i z a t i o n from a t r e a t e d area can be e s t i m a t e d i n the f i e l d , u s i n g a c c e p t e d m i c r o m e t e o r o l o g i c a l t e c h n i q u e s ( l 6 ) . A number o f



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s p e c i f i c p r o c e d u r e s a r e a v a i l a b l e , a l l o f w h i c h r e q u i r e measurement o f t h e p e s t i c i d e c o n c e n t r a t i o n i n a i r a t s e v e r a l h e i g h t s above t h e f i e l d . The b a s i c t e c h n i q u e depends upon c a l c u l a t i o n o f an a p p r o p r i a t e d i f f u s i v i t y c o e f f i c i e n t , w h i c h i s t h e n r e l a t e d to the p e s t i c i d e concentration gradient to give a value f o r pesticide flux. The v a l i d i t y o f t h e numbers so o b t a i n e d depends on how c l o s e l y t h e f i e l d c o n d i t i o n s a p p r o a c h t h e t h e o r e t i c a l l y i d e a l , h o r i z o n t a l l y uniform, two-dimensional surface. Aerodynamic p r o p e r t i e s o f a g i v e n t e r r a i n may make t h e method i m p r a c t i c a l i n some s i t u a t i o n s , b u t s u c c e s s f u l measurements have b e e n made u n d e r f i e l d c o n d i t i o n s (12, 1T_, l8_) . The r e s u l t s o f t h e s e e x p e r i m e n t s , c o n d u c t e d w i t h t h e i n s e c t i c i d e s d i e l d r i n and h e p t a c h l o r , showed t h a t p o s t a p p l i c a t i o n v o l a t i l i z a t i o n i s a s i g n i f i c a n t pathway o f l o s s e v e n when t h e chemicals are i n c o r p o r a t e d i n t o the s o i l . In a corn f i e l d , v o l a t i l i z a t i o n o f s o i l - i n c o r p o r a t e d d i e l d r i n and h e p t a c h l o r d u r i n g t h e c r o p s e a s o n t o t a l e d 3.6 and 6.8%, respectively, of the nominal a p p l i c a t i o n (l8). T h e s e l o s s e s were l a r g e r t h a n t h o s e by o t h e r m e a s u r e d pathways s u c h as s e d i m e n t movement, r u n o f f w a t e r t r a n s p o r t , and c r o p u p t a k e (19.). This i s s i g n i f i c a n t , because v o l a t i l i z a t i o n i s u n c o n t r o l l e d , continuous t r a n s p o r t , whereas sediment t r a n s p o r t , t h e o t h e r major r o u t e o f l o s s , c a n be l a r g e l y c o n t r o l l e d by common a g r i c u l t u r a l p r a c t i c e s . H e p t a c h l o r and d i e l d r i n a p p l i e d as a s u r f a c e s p r a y t o a f i e l d o f o r c h a r d g r a s s v o l a t i l i z e d f r o m t h e v e g e t a t i o n much more r a p i d l y t h a n when t h e y w e r e i n c o r p o r a t e d i n t o t h e s o i l (12). The d a t a showed t h a t 90% o f t h e r e s i d u e s o f b o t h compounds i n i t i a l l y o n t h e g r a s s v o l a t i l i z e d w i t h i n 3 w e e k s . Whether i n c o r p o r a t e d o r l e f t on t h e s u r f a c e , t h e p e s t i c i d e f l u x i n t o t h e a i r showed v e r y s t r o n g d i u r n a l v a r i a t i o n , w i t h t h e maximum f l u x o c c u r r i n g a r o u n d s o l a r noon. An o b v i o u s r e a s o n f o r t h i s i s t h e g r e a t e r s u r f a c e h e a t i n g c a u s e d by t h e more d i r e c t i n s o l a t i o n a t noon. T h i s , i n t u r n , p r o v i d e s l a t e n t heat o f v a p o r i z a t i o n , i n c r e a s e s t h e r m a l i n s t a b i l i t y and t u r b u l e n c e i n t h e a i r , and i n c r e a s e s t h e upward r a t e o f w a t e r movement t o t h e e v a p o r a t i n g s u r f a c e , b r i n g i n g w i t h i t a s s o c i a t e d p e s t i c i d e by b u l k movement. I n sum, t h e r e i s l i t t l e d o u b t t h a t a e r i a l t r a n s p o r t f r o m t r e a t e d l a n d s i s a m a j o r mechanism o f d i s s i p a t i o n o f the organochlorine i n s e c t i c i d e s i n t o the environment. E v a p o r a t i o n o f a p e s t i c i d e f r o m a w a t e r s u r f a c e depends upon t h e v a p o r p r e s s u r e o f t h e compound, i t s s o l u b i l i t y , and t h e amount of the residue that i s i n true s o l u t i o n . The v e r y l o w w a t e r s o l u b i l i t y of organochlorine insecticides y i e l d s only very d i l u t e , i d e a l s o l u t i o n s f o r w h i c h H e n r y s Law i s o b e y e d . H e n r y ' s Law s p e c i f i e s t h a t the vapor pressure over the s o l u t i o n i s prop o r t i o n a l t o b o t h t h e v a p o r p r e s s u r e o f t h e p u r e compound a t t h a t t e m p e r a t u r e and t h e r e l a t i v e s a t u r a t i o n o f t h e s o l u t i o n . V o l a t i l i z a t i o n r a t e s f r o m w a t e r c a n be v e r y h i g h , e s p e c i a l l y f o r l o w s o l u b i l i t y compounds (20). In a d d i t i o n to being i n true 1



4.

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47

aqueous s o l u t i o n , p e s t i c i d e r e s i d u e s i n n a t u r a l w a t e r s may b e a s s o c i a t e d w i t h suspended p a r t i c u l a t e m a t t e r , i n c o r p o r a t e d i n t o l i v i n g o r dead o r g a n i s m s , o r p r e f e r e n t i a l l y d i s s o l v e d i n o r g a n i c films. A l l o f these serve t o lower the v o l a t i l i z a t i o n r a t e , w h i c h may t h e n depend o n t h e r a t e a t w h i c h t h e r e s i d u e i s a d d e d t o o r removed f r o m t r u e s o l u t i o n , o r o n t h e r a t e o f m i x i n g w h e r e v e r t h e p e s t i c i d e i n t h e s u r f a c e l a y e r "becomes d e p l e t e d . Bottom s e d i m e n t s , which o f t e n serve as a r e s e r v o i r o f p e s t i c i d e r e s i d u e s , may r e l e a s e t h e r e s i d u e s i n t o s o l u t i o n v e r y s l o w l y , so t h a t v o l a t i l i z a t i o n f r o m t h e w a t e r b o d y may r e f l e c t t h e s l o w d e s o r p t i o n and m i x i n g p r o c e s s e s . To c o u n t e r b a l a n c e these v o l a t i l i z a t i o n - r e d u c i n g p r o c e s s e s , a n enhancement p r o c e s s may a l s o take place. The l i p i d - s o l u b l e , h y d r o p h o b i c p e s t i c i d e s may p r e f e r e n t i a l l y d i s s o l v e i n the n a t u r a l organic surface f i l m s a s s o c i a t e d with a l l water bodies. They t h e n may become a i r b o r n e b y t h e a c t i o n o f spray o r b u r s t i n g bubbles a t the s u r f a c e , a t a r a t e e x c e e d i n g t h a t p r e d i c t e d b y H e n r y ' s Law f o r t h e s m a l l amounts i n t r u e s o l u t i o n (21, 22). Transport

o f P e s t i c i d e s i n the

Atmosphere

Two s p a t i a l s c a l e s o f t r a n s p o r t f r o m t h e p o i n t o f p e s t i c i d e application exist: l o c a l , w h i c h i n c l u d e s d i s t a n c e s up t o a b o u t 15 m i l e s , a n d l o n g r a n g e , w h i c h u l t i m a t e l y i n c l u d e s g l o b a l t r a n s p o r t . D r i f t s o f sprays and dusts caused by l o c a l a i r t r a n s p o r t can produce s u b s t a n t i a l a p p l i c a t i o n l o s s e s and r e s u l t i n adverse b i o l o g i c a l e f f e c t s o n n o n t a r g e t o r g a n i s m s ( i n c l u d i n g man) a n d e c o n o m i c l o s s e s f r o m damage t o c r o p s a n d o r n a m e n t a l p l a n t s . These, as w e l l as t h e d e p o s i t i o n o f i l l e g a l r e s i d u e s , have r e s u l t e d i n numerous l a w s u i t s a g a i n s t p e s t i c i d e a p p l i c a t o r s a n d m a n u f a c t u r e r s (]+_, 23^, 2k ). G l o b a l - s c a l e p e s t i c i d e t r a n s p o r t b y a i r i s l e s s w e l l u n d e r s t o o d because t h e tremendous d i l u t i o n produces v e r y l o w c o n c e n t r a t i o n s and because the d i v e r s e i n p u t and r e m o v a l mechanisms a r e n o t c l e a r l y d e f i n e d . O b v i o u s l y , such t r a n s p o r t does o c c u r , s i n c e DDT r e s i d u e s h a v e b e e n f o u n d i n many r e m o t e e n v i r o n m e n t s (6_, 2 5 , 2 6 , 27). I n c r e a s e d k n o w l e d g e o f t h e t r a n s p o r t process and the s i g n i f i c a n c e o f t r a n s p o r t e d r e s i d u e s has i m p o r t a n t i m p l i c a t i o n s f o r f u t u r e d e c i s i o n s c o n c e r n i n g p e s t i c i d e use. The e x t e n t t o w h i c h a p e s t i c i d e i s d i s p e r s e d i n t h e atmosp h e r e depends p r i m a r i l y o n i t s p a r t i c l e s i z e a n d i t s c o n s t i t u t i o n . I n s t u d y i n g a t m o s p h e r i c t r a n s p o r t , t h e b a s i c q u e s t i o n s t h a t must be a n s w e r e d i n c l u d e : ( l ) What i s t h e p a r t i c l e s i z e ? (2) I s t h e a i r b o r n e p a r t i c l e l i k e l y t o change i n s i z e ? (3) W i l l t h e c h e m i c a l n a t u r e o f t h e a i r b o r n e p e s t i c i d e change? F o r p u r p o s e s o f o r d e r l y d i s c u s s i o n , i t i s convenient t o c l a s s i f y the processes that r e l a t e t o p a r t i c l e s i z e , p a r t i c l e s i z e c h a n g e , a n d c h e m i c a l change a s primary, secondary, and t e r t i a r y , r e s p e c t i v e l y , as i n Table I I . Conceptually, a p a r t i c l e i s considered t o include spray d r o p l e t s , dust p a r t i c l e s , atmospheric a e r o s o l , wind-blown s o i l p a r t i c l e s ,


48

REMOVAL OF TRACE CONTAMINANTS F R O M

Table I .

Sources

Agriculture

Application losses, d r i f t , postapplication v o l a t i l i z a t i o n and w i n d e r o s i o n

Public

Spraying f o r c o n t r o l of insect vectors disease

health

Industrial

P e s t i c i d e m a n u f a c t u r e and f o r m u l a t i o n , e f f l u e n t s , fumes, v a p o r , d u s t

Commercial

Mothproofing, food

Home and g a r d e n

Local

Accidents

S p i l l s , improper

Table I I .

of

processing

application disposal

Processes A f f e c t i n g the Transport of P e s t i c i d e s i n A i r

Classification

Processes

Primary

Atmospheric: advection, convection, turbul e n c e , eddy d i f f u s i o n , f r o n t a l l i f t i n g Gravitational: f a l l o u t , sedimentation Small p a r t i c l e : impaction, deposition, molecular d i f f u s i o n

Secondary

E v a p o r a t i o n and c o n d e n s a t i o n Nucleation Coagulation Sorption Solution-vapor equilibrium

Tertiary

AIR

P r i n c i p a l Sources of P e s t i c i d e s i n A i r

Activity


THE

Chemical r e a c t i o n s : oxidation, hydrolysis, c a t a l y t i c decomposition Photochemical reactions: d i r e c t or i n d i r e c t , isomerization, oxidation



4.

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and t h e l i k e , a n d f o r some o f t h e p h y s i c a l p r o c e s s e s d i s c u s s e d below, the behavior o f molecular vapor i s i n d i s t i n g u i s h a b l e from that o f other small p a r t i c l e s . P a r t i c l e Size o f Inputs. The p a r t i c l e s i z e s o f p e s t i c i d e s t h a t become a i r b o r n e d u r i n g a p p l i c a t i o n o f t h e c h e m i c a l v a r y w i t h the f o r m u l a t i o n and a p p l i c a t i o n equipment. G r a n u l a r f o r m u l a t i o n s c o n s i s t o f p a r t i c l e s t h a t a r e 200 t o 600 ym i n d i a m e t e r a n d h a v e no d i r e c t p o t e n t i a l f o r d i s p e r s i o n . D u s t s a r e s c r e e n e d d u r i n g m a n u f a c t u r i n g t o 80-90% l e s s t h a n 30 ym a n d a p a r t i c l e number m e d i a n d i a m e t e r o f 1-10 ym (2j3) . These f i n e p a r t i c l e s have h i g h transport p o t e n t i a l , an undesirable a t t r i b u t e that has contributed t o d e c r e a s e d use o f d u s t f o r m u l a t i o n s d e s p i t e t h e i r e f f i c i e n t c o v e r a g e o f p l a n t s u r f a c e s (29.). S p r a y s , b y f a r t h e most w i d e l y u s e d f o r m u l a t i o n s , may c o n s i s t o f a w a t e r o r o i l b a s e i n w h i c h the p e s t i c i d e i s d i s p e r s e d , an e m u l s i f i a b l e l i q u i d , o r a w e t t a b l e powder i n w h i c h t h e a c t i v e c h e m i c a l âs o n t h e s u r f a c e o f c l a y ^articles. A l l common s p r a y e q u i p m e n t p r o d u c e s a s p e c t r u m o f p a r t i c l e s i z e s r a n g i n g f r o m 1 t o o v e r 1+00 um. F o r a s p r a y e r s e t f o r 200-um d r o p l e t s , u p t o Q% o f t h e s p r a y volume i s i n d r o p l e t s f i n e r t h a n 200 ym (JO. T h i c k e n e d s p r a y s s u c h as w a t e r i n - o i l ( i n v e r t ) e m u l s i o n s may p r o d u c e l a r g e r s p r a y d r o p a v e r a g e s . S m a l l drops can not be e l i m i n a t e d w i t h p r e s e n t equipment, m e r e l y r e d u c e d i n number ( 2 9 ) . The p a r t i c l e s i z e s o f p o s t a p p l i c a t i o n i n p u t s i n c l u d e m o l e c u l a r v a p o r from v o l a t i l i z i n g p e s t i c i d e s and a wide range o f p a r t i c l e s i z e s from wind-eroded dust. Long-range a e r i a l t r a n s p o r t o f p e s t i c i d e s on dust i s probably a s s o c i a t e d w i t h p a r t i c l e s f i n e r t h a n 10 ym i n d i a m e t e r . Transport and P a r t i c l e S i z e . P a r t i c l e s descend through the atmosphere under the i n f l u e n c e o f g r a v i t y b y well-known p h y s i c a l laws t h a t govern the r e s i s t a n c e t o motion o f p a r t i c l e s moving i n v i s c o u s m e d i a . The d e s c e n t i s d e s c r i b e d b y t h e S t o k e s e q u a t i o n , commonly f o u n d i n p h y s i c s t e x t s . L a r g e r p a r t i c l e s , as i s e v i d e n t , t e n d t o s e t t l e t o t h e s u r f a c e r a t h e r r a p i d l y , b u t as p a r t i c l e s i z e d e c r e a s e s , t h e e f f e c t o f g r a v i t y becomes l e s s o w i n g t o b u o y a n c y , v i s c o u s f o r c e s , a n d t h e random m o t i o n o f t h e a i r i n w h i c h t h e p a r t i c l e s a r e s u s p e n d e d ( 3_, 30_, 3 1 ) . The t r a n s p o r t p o t e n t i a l t h e n depends o n t h e b a l a n c e a c h i e v e d b e t w e e n p a r t i c l e s i z e , g r a v i t y , a n d t h e s t r e n g t h o f t h e s e random m o t i o n s . The p h y s i c a l f o r c e s i n v o l v e d a r e shown i n T a b l e I I a s p r i m a r y processes. P a r t i c l e s l a r g e r t h a n 100 ym i n d i a m e t e r f a l l r e l a t i v e l y r a p i d l y and are l a r g e l y u n a f f e c t e d by a i r t u r b u l e n c e . Smaller p a r t i c l e s may h a v e a p o s i t i v e f a l l v e l o c i t y , b u t t h e i r m o t i o n i s d e p e n d e n t upon t h e s t r e n g t h o f t h e t u r b u l e n c e . Particles of 10 ym o r l e s s a r e d i s p e r s e d p r i m a r i l y b y t u r b u l e n c e u n d e r a l l m e t e o r o l o g i c a l c o n d i t i o n s (31). Molecular vapor i s returned t o the s u r f a c e w i t h d i f f i c u l t y , and l o n g - r a n g e t r a n s p o r t i s i n d e e d very probable. Estimates o f atmospheric residence time based on


50

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r a t e s o f removal o f s u r f a c e dust o r atmospheric a e r o s o l are not adequate i n d e s c r i b i n g the f a t e o f molecular vapor i n a i r . E s t i m a t e s o f t h e amount o f t i m e t h a t m o l e c u l a r v a p o r may p e r s i s t i n a i r a r e l a r g e l y s p e c u l a t i v e and depend upon t h e r a t e o f s e c o n d a r y and t e r t i a r y p r o c e s s e s ( T a b l e I I ) , c o n c e r n i n g w h i c h l i t t l e experimental information i s a v a i l a b l e . T r a n s p o r t and P a r t i c l e S i z e Change. Once p e s t i c i d e p a r t i c l e s become a i r b o r n e , t h e y may change i n s i z e . The p h y s i c a l f o r c e s i n v o l v e d a r e shown i n T a b l e I I as s e c o n d a r y p r o c e s s e s . The p a r t i c l e s c a n become e i t h e r l a r g e r o r s m a l l e r . F o r e x a m p l e , a spray d r o p l e t can e v a p o r a t e t o a s m a l l e r s i z e , o r m o l e c u l a r v a p o r may s o r b t o an a e r o c o l l o i d a l p a r t i c l e o r p a r t i t i o n i n t o a r a i n d r o p , s o t h a t t h e r e s u l t i n g p a r t i c l e i s l a r g e r . The t r a n s p o r t p o t e n t i a l , o f c o u r s e , changes w i t h t h e change i n p a r t i c l e size. T r a n s p o r t and C h e m i c a l Change. The t e r t i a r y p r o c e s s e s l i s t e d i n T a b l e I I a r e t h o s e t h a t change t h e c h e m i c a l n a t u r e o f t h e a i r b o r n e p e s t i c i d e . These p r o c e s s e s a r e d o u b l y i m p o r t a n t , i n t h a t they not o n l y profoundly a f f e c t the t r a n s p o r t p o t e n t i a l but a l s o change t h e a n a l y t i c a l and t o x i c o l o g i c a l p r o p e r t i e s o f t h e pesticide. U n f o r t u n a t e l y , we know t h e l e a s t a b o u t t h e s e p r o c e s s e s as t h e y o p e r a t e i n t h e a t m o s p h e r e , l a r g e l y b e c a u s e o f u n c e r t a i n t y a b o u t t h e p h y s i c a l s t a t e o f a i r b o r n e p e s t i c i d e s and a b o u t how l o n g one may r e a s o n a b l y e x p e c t a p e s t i c i d e t o r e m a i n a i r b o r n e . C o n s i d e r i n g t h a t t h e a t m o s p h e r e i s a h i g h l y r e a c t i v e medium and t h a t many p e s t i c i d e s u n d e r g o s u n l i g h t - i n d u c e d p h o t o c h e m i c a l r e a c t i o n s under l a b o r a t o r y c o n d i t i o n s , the e f f e c t o f t e r t i a r y c h a n g e s c a n n o t be n e g l e c t e d when d e s c r i b i n g p e s t i c i d e t r a n s p o r t by a i r . Meteorological Conditions A f f e c t i n g Transport. The m i x i n g c h a r a c t e r i s t i c s o f t h e a t m o s p h e r e a r e g o v e r n e d i n a m a j o r way by i t s v e r t i c a l t e m p e r a t u r e g r a d i e n t , b u t a l s o depend upon s u r f a c e r o u g h n e s s , w h i c h c a n c r e a t e t u r b u l e n c e due t o w i n d s h e a r . The turbulent mixing process i s a complicated s t r u c t u r e of eddies, a random m o t i o n t h a t g e n e r a l l y f o l l o w s t h e m a t h e m a t i c a l l a w s o f d i f f u s i o n , b u t w i t h a d i f f u s i o n c o e f f i c i e n t 1 0 t o 10 times greater than that of molecular d i f f u s i o n . S i n c e a i r f l o w above the surface i s p r a c t i c a l l y never laminar, molecular d i f f u s i o n c a n u s u a l l y be n e g l e c t e d , e x c e p t v e r y c l o s e t o an e v a p o r a t i n g o r adsorbing surface. The d i f f u s i o n c o e f f i c i e n t f o r a c o l l e c t i o n o f p a r t i c l e s i n t h e a t m o s p h e r e may a c t u a l l y show enormous v a r i a t i o n s , r a n g i n g f r o m 0.2 c m / s e c f o r m o l e c u l a r d i f f u s i o n to 1 0 cm / s e c f o r l a r g e - s c a l e c y c l o n i c s t o r m s ( 3 0 ) . The m a g n i t u d e o f eddy d i f f u s i o n may change d i u r n a l l y , o w i n g t o d i f f e r e n c e s b e t w e e n s u r f a c e and a i r t e m p e r a t u r e t h a t a r e c r e a t e d by s o l a r h e a t i n g and n o c t u r n a l r a d i a t i o n a l c o o l i n g o f 2

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the surface. I n s o l a t i o n r a i s e s the surface temperature so t h a t t h e r e l a t i v e l y c o o l o v e r l y i n g a i r i s most u n s t a b l e i n t h e e a r l y afternoon. The t e m p e r a t u r e l a p s e r a t e , a n d t h e r e b y t u r b u l e n t m i x i n g , i s much l e s s a t n i g h t f o r a g i v e n w i n d s p e e d , a n d n o c t u r n a l i n v e r s i o n s (warm a i r o v e r l y i n g c o o l a i r ) a r e common. C r o p I r r i g a t i o n m o d i f i e s t h e s e t e m p e r a t u r e c o n d i t i o n s b y t r a n s f o r m i n g more o f t h e i n c o m i n g s o l a r e n e r g y i n t o l a t e n t h e a t b y évapotranspirat i o n , and t h e a i r i s n o t as u n s t a b l e o v e r an i r r i g a t e d crop as over dry l a n d . Surface roughness, a l s o a f a c t o r i nc r e a t i n g a i r turbulence, may depend o n t h e c r o p s p e c i e s a n d w i n d s p e e d . F o r e x a m p l e , a t a l l p l i a n t c r o p s u c h a s wheat may b e n d i n t h e b r e e z e , s e a l i n g the t o p o f the crop and p r e v e n t i n g wind p e n e t r a t i o n c l o s e t o t h e surface. A r e l a t i v e l y r i g i d c r o p , s u c h a s c o r n , may a l l o w b e t t e r w i n d p e n e t r a t i o n a n d , t h e r e b y , i n c r e a s e d t r a n s p o r t away f r o m t h e s u r f a c e (2*0. A i r moves p e s t i c i d e away f r o m t h e t r e a t e d a r e a b y a comb i n a t i o n o f h o r i z o n t a l ( a d v e c t i o n ) and v e r t i c a l t r a n s p o r t (convection). The e x t e n t o f e a c h o f t h e s e i s d e t e r m i n e d p r i m a r i l y by a i r t u r b u l e n c e . Advection i s favored by l o w t u r b u l e n c e caused by t h e r m a l s t a b i l i t y a n d l o w w i n d s h e a r . Under t h e s e c o n d i t i o n s , which are t y p i c a l o f i n v e r s i o n s i t u a t i o n s , l o n g periods o f lowl y i n g h o r i z o n t a l transport occur. P e s t i c i d e concentration along t h e downwind s u r f a c e r e m a i n s h i g h a n d l o c a l t r a n s p o r t i n t o a d jacent areas w i t h r a p i d r e t u r n t o the s u r f a c e , p r i m a r i l y t y p i f i e d by f i n e p a r t i c l e " d r i f t " f r o m a p p l i c a t i o n o p e r a t i o n s , i s f a v o r e d (2k, 3 1 ) . Windy c o n d i t i o n s , a s most f a r m e r s know, a l s o i n c r e a s e d r i f t , because they f a v o r i n c r e a s e d a d v e c t i o n o f l a r g e r p a r t i c l e s t h a t n o r m a l l y s e t t l e r a p i d l y t o t h e s u r f a c e . The a p p l i c a t i o n o f f i n e p a r t i c l e s , dust o r s p r a y , under c o n v e c t i v e c o n d i t i o n s o f t h e r m a l i n s t a b i l i t y a n d m o d e r a t e w i n d s may l e s s e n l o c a l d r i f t problems by r a p i d l y d i l u t i n g a i r b o r n e r e s i d u e s and m a i n t a i n i n g l o w c o n c e n t r a t i o n s a t t h e downwind s u r f a c e . However, t h e i n c r e a s e d v e r t i c a l component i n c r e a s e s t h e p r o b a b i l i t y o f l o n g - r a n g e t r a n s p o r t a n d , a l t h o u g h s u r f a c e r e s i d u e s i m m e d i a t e l y downwind may be r e d u c e d , a p p l i c a t i o n l o s s e s may i n c r e a s e . I f t h e c o n v e c t e d residues e v e n t u a l l y r e t u r n t o the surface, t h e d r i f t has, i n r e a l i t y , o n l y been d i l u t e d . Removal o f P e s t i c i d e s f r o m t h e A t m o s p h e r e P e s t i c i d e s a r e removed f r o m t h e a t m o s p h e r e b y f o u r b a s i c comp e t i t i v e mechanisms: d r y d e p o s i t i o n , p r e c i p i t a t i o n s c a v e n g i n g , c h e m i c a l d e g r a d a t i o n , a n d p h o t o c h e m i c a l d e g r a d a t i o n . The processes t h a t a f f e c t atmospheric t r a n s p o r t o f p e s t i c i d e s (Table I I ) a l s o a f f e c t t h e i r r e m o v a l , s i n c e r e m o v a l , a s does t r a n s p o r t , depends p r i m a r i l y o n t h e s i z e a n d t h e n a t u r e o f t h e p e s t i c i d e particle. Discussions o f the i n d i v i d u a l f a c t o r s i n f l u e n c i n g t h e removal process f o l l o w .



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P e s t i c i d e - A e r o s o l I n t e r a c t i o n s . The a t m o s p h e r e c o n t a i n s an a e r o s o l c o n s i s t i n g o f an enormous v a r i e t y o f m i n u t e , s u s p e n d e d s o l i d and l i q u i d - n a r t i c l e s t h a t c o n t i n u a l l y c i r c u l a t e t h r o u g h o u t the a i r space. A l t h o u g h c o n s i d e r a b l e v a r i a t i o n can o c c u r , t h e number d e n s i t y o f t h e s e p a r t i c l e s i s g e n e r a l l y a b o u t 1 t o 10 p e r c c o f a i r f o r t h e i m p o r t a n t p a r t i c l e s i z e r a n g e b e t w e e n 0.1 and 10 μη (32, 33.). P e s t i c i d e p a r t i c l e s , i n c l u d i n g molecular vapor, t h a t a r e i n t r o d u c e d i n t o t h i s a e r o s o l may i n t e r a c t w i t h i t , r e s u l t i n g i n e f f e c t i v e g r o w t h i n p e s t i c i d e p a r t i c l e s i z e and thereby h a s t e n i n g removal o f the p e s t i c i d e from the g e n e r a l circulation. The l a r g e s u r f a c e - t o - v o l u m e r a t i o o f a t m o s p h e r i c a e r o s o l s , t h e i r demonstrated a b i l i t y t o absorb o r r e a c t w i t h t r a c e g a s e s , and t h e i r r e l a t i v e l y s h o r t r e s i d e n c e t i m e i n t h e a t m o s p h e r e have l e d t o s p e c u l a t i o n t h a t a d s o r p t i o n o f p e s t i c i d e vapor onto a e r o s o l p a r t i c l e s may be a m a j o r r e m o v a l mechanism ( 3 1 , 3]0 . The p r e s e n c e o f p a r t i c u l a t e p e s t i c i d e s i n t h e a t m o s p h e r e has b e e n c o n f i r m e d (26_, 27, 35) , b u t c o n c l u s i v e e v i d e n c e i s l a c k i n g t h a t t h e y f o r m by p e s t i c i d e v a p o r - a e r o s o l i n t e r a c t i o n as d i s t i n g u i s h e d from wind-eroded dust o r p a r t i c u l a t e s from p e s t i c i d e application operations. The i m p o r t a n c e o f p e s t i c i d e - a e r o s o l i n t e r a c t i o n as a r e m o v a l mechanism c o u l d be b e t t e r e v a l u a t e d i f more i n f o r m a t i o n w e r e a v a i l a b l e on t h e p r o b a b i l i t y o f c o l l i s i o n , t h e a d h e s i v e f o r c e , and t h e c o m p o s i t i o n and p r o p e r t i e s o f t h e a e r o s o l (36_, 37). In a d d i t i o n t o i n c r e a s i n g the e f f e c t i v e p a r t i c l e s i z e , a e r o s o l a d s o r p t i o n c o u l d a l s o p r o m o t e i m p o r t a n t t e r t i a r y changes s u c h as c a t a l y z e d chemical r e a c t i o n s o r s e n s i t i z e d photochemical con v e r s i o n s . S t r o n g a d s o r p t i o n o n t o an a e r o s o l may e v e n change t h e w a v e l e n g t h o f maximum a b s o r p t i o n i n d i r e c t p h o t o c h e m i c a l r e a c t i o n s (38). However, e f f e c t s on t e r t i a r y p r o c e s s e s i n t h e atmosphere have not been i n v e s t i g a t e d d i r e c t l y because o f t h e i n h e r e n t experimental complexities. The f a c t t h a t a l l a t m o s p h e r i c a e r o s o l s c o n t a i n o r g a n i c m a t t e r a n d t h a t t h i s o r g a n i c m a t t e r i s known t o r e a c t c h e m i c a l l y c o n s t i t u t e s evidence t h a t aerosols might w e l l absorb organic p e s t i c i d e s and c o n t r o l t h e i r f a t e . The s o u r c e and c o m p o s i t i o n o f some o f t h e o r g a n i c m a t t e r i n a e r o s o l s h a v e b e e n i d e n t i f i e d . For e x a m p l e , m a r i t i m e a e r o s o l s c o n t a i n o r g a n i c m a t t e r h a v i n g a compo s i t i o n v e r y s i m i l a r t o t h a t o f n a t u r a l l y o c c u r r i n g o i l s l i c k s on t h e s u r f a c e o f t h e o c e a n (21, 22). Aerosols over land contain a v a r i e t y o f r e a c t i v e o r g a n i c compounds, e s p e c i a l l y t e r p e n e s , t h a t a r e r e l e a s e d by v e g e t a t i o n (39). Aerosols over urban areas c o n t a i n o r g a n i c matter t h a t i s p a r t i a l l y i n the form o f c o m b u s t i o n - d e r i v e d p o l y n u c l e a r a r o m a t i c h y d r o c a r b o n s (32) , w h i c h h a v e p h y s i c o c h e m i c a l p r o p e r t i e s s i m i l a r t o t h o s e o f many o r g a n i c pesticides. They a l s o h a v e b e e n shown t o c o n t a i n d e t e c t a b l e q u a n t i t i e s o f DDT (35)> so t h a t p e s t i c i d e a s s o c i a t i o n w i t h a e r o s o l o r g a n i c m a t t e r i s documented. R e a c t i o n s i n a e r o s o l s g e n e r a l l y i n v o l v e o x y g e n a t i o n s (39); t h e r e f o r e , t h e p r o d u c t s o f any



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p e s t i c i d e d e c o m p o s i t i o n t h a t w o u l d o c c u r w o u l d l i k e l y be p o l a r , w a t e r - s o l u b l e compounds. B e c a u s e p r e s e n t a n a l y t i c a l methods f o r o r g a n o c h l o r i n e i n s e c t i c i d e s are based p r i m a r i l y on the c o n s i s t e n t l i p i d s o l u b i l i t y o f t h e compounds, w a t e r - s o l u b l e p r o d u c t s w i l l l i k e l y be o v e r l o o k e d . T h i s c o u l d e x p l a i n some o f t h e u n a c c o u n t e d f o r l o s s e s o f DDT i n t h e e n v i r o n m e n t . P h o t o c h e m i c a l D e g r a d a t i o n . The e x t e n s i v e r e c e n t l i t e r a t u r e o n t h e p h o t o c h e m i s t r y o f p e s t i c i d e s h a s shown t h a t many p e s t i c i d e s r e a c t p h o t o c h e m i c a l l y i n t h e l a b o r a t o r y , o f t e n y i e l d i n g complex mixtures o f products. F r e q u e n t l y , the products i s o l a t e d i n t h e l a b o r a t o r y can i n d e e d be found i n the environment, and s u n l i g h t i n d u c e d p h o t o c h e m i c a l r e a c t i o n s have b e e n d e m o n s t r a t e d i n t h e field. However, t h e i m p o r t a n c e o f p h o t o c h e m i c a l r e a c t i o n s i n t h e atmosphere has n o t y e t been v a l i d l y a s s e s s e d , p r i m a r i l y because b a s i c i n f o r m a t i o n i s l a c k i n g . F o r example, a p h o t o c h e m i c a l r e a c t i o n c a n be i m p o r t a n t o n l y i f i t i s f a s t enough t o compete w i t h removal processes t h a t r e t u r n p e s t i c i d e s t o the s u r f a c e . T h i s r e q u i r e s k n o w l e d g e o f t h e mean a t m o s p h e r i c r e s i d e n c e t i m e o f the p e s t i c i d e and o f the q u a n t i t a t i v e r e l a t i o n between photoc h e m i c a l r e a c t i o n v e l o c i t y and the i n t e n s i t y and wavelength o f s u n l i g h t i n t h e l o w e r a t m o s p h e r e . These q u a n t i t i e s a r e u n c e r t a i n b e c a u s e t h e y depend upon t h e c o m p o s i t i o n o f t h e p a r t i c l e s c o n t a i n i n g the p e s t i c i d e . Thus f a r , work o n t h e p h o t o c h e m i c a l d e g r a d a t i o n o f p e s t i c i d e s has y i e l d e d i n f o r m a t i o n i m p o r t a n t t o t h e u n d e r s t a n d i n g o f p o s s i b l e atmospheric r e a c t i o n s . L a b o r a t o r y s t u d i e s h a v e shown, f o r example, t h a t i n t r a m o l e c u l a r e l i m i n a t i o n s and i s o m e r i z a t i o n s t e n d t o d o m i n a t e when p e s t i c i d e s i n m o l e c u l a r v a p o r f o r m a r e i r r a d i a t e d , whereas o t h e r r e a c t i o n s , such as o x i d a t i o n o r condensation, can occur with p e s t i c i d e s d i s s o l v e d i n water o r organic solvents. A p e s t i c i d e s t r o n g l y adsorbed t o a surface can e x h i b i t d r a m a t i c s h i f t s i n i t s a b s o r p t i o n s p e c t r u m (38). A p h o t o s e n s i t i z e r c a n s t r o n g l y a f f e c t r e a c t i o n r a t e , so t h a t the c o m p o s i t i o n of the atmospheric p a r t i c l e s c o n t a i n i n g the p e s t i c i d e i s very i m p o r t a n t (38, U p ) . The p h o t o c h e m i c a l r e a c t i v i t y o f many o f t h e organochlorine i n s e c t i c i d e s has been demonstrated i n the l a b o r a t o r y . The c y c l o d i e n e i n s e c t i c i d e s , such as d i e l d r i n , h e p t a c h l o r , o r a l d r i n , t y p i c a l l y undergo i s o m e r i z a t i o n o r d e c h l o r i n a t i o n . Although d i e l d r i n shows n e g l i g i b l e a b s o r p t i o n a t s o l a r w a v e l e n g t h s t h a t reach the s u r f a c e o f the e a r t h , i t r e a d i l y r e a c t s under n a t u r a l o r a r t i f i c i a l s u n l i g h t , e v e n i n t h e a b s e n c e o f any a p p a r e n t photosensitizer. Pure molecular vapor o f d i e l d r i n i n a i r o r i n e r t gas y i e l d s o n l y a s i n g l e p r o d u c t , p h o t o d i e l d r i n , r e s u l t i n g f r o m i s o m e r i z a t i o n (Ul^, k2). P h o t o d i e l d r i n has been found i n the environment, forming on v e g e t a t i o n and s o i l s u r f a c e s , i n n a t u r a l w a t e r s , a n d p r e s u m a b l y i n a i r . I t i s more t o x i c t h a n d i e l d r i n a n d much l e s s v o l a t i l e u n d e r f i e l d c o n d i t i o n s . I f d i e l d r i n o c c u r s i n p o l l u t e d atmospheres c o n t a i n i n g ozone o r


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n i t r o g e n o x i d e s , o t h e r more p o l a r p r o d u c t s o f i r r a d i a t i o n may f o r m i n a d d i t i o n t o p h o t o d i e l d r i n (k3)* DDT decomposes s l o w l y u n d e r d i r e c t p h o t o l y s i s a t s o l a r w a v e l e n g t h s , c o n v e r t i n g t o DDD by an o x y g e n - i n h i b i t e d p r o c e s s . However, l i k e o t h e r c h l o r i n a t e d compounds, DDT a l s o u n d e r g o e s a p h o t o c h e m i c a l r e a c t i o n i n i t i a t e d by a r o m a t i c amines t h a t i s n o t o x y g e n i n h i b i t e d , b u t y i e l d s o x y g e n a t e d p r o d u c t s (kh). DDT has e v e n b e e n shown t o c o n v e r t t o p o l y c h l o r i n a t e d b i p h e n y l (PCB) u n d e r l o n g i r r a d i a t i o n t i m e s (h5). As m e n t i o n e d , t h e u l t i m a t e i m p o r t a n c e o f t h e s e p h o t o c h e m i c a l r e a c t i o n s i n t h e d e g r a d a t i o n o f a i r b o r n e r e s i d u e s c a n n o t now be ascertained. Dry D e p o s i t i o n . The d e p o s i t i o n o f p a r t i c l e s f r o m t h e atmos phere onto exposed s u r f a c e s i n the absence o f r a i n or s n o w f a l l , s o - c a l l e d d r y d e p o s i t i o n , o c c u r s as a l o n g - t e r m p r o c e s s f o r v i r t u a l l y a l l airborne materials. I t w i l l c o n t i n u e as l o n g as t h e a i r mass c o n t a i n i n g t h e m a t e r i a l r e m a i n s i n c o n t a c t w i t h t h e g r o u n d . T h u s , s u r f a c e d e p o s i t i o n may be a v e r y e f f e c t i v e mechanism f o r r e m o v i n g a i r b o r n e r e s i d u e s e v e n t h o u g h p a r t i c l e f a l l v e l o c i t y i s n e g l i g i b l e (31). E m p i r i c a l observations allow the r a t e o f p a r t i c l e r e m o v a l by s u r f a c e d e p o s i t i o n t o be q u a n t i f i e d w i t h o u t a c t u a l l y s p e c i f y i n g t h e p r o c e s s e s i n v o l v e d . The q u a n t i f i c a t i o n depends u p o n t h e p r o p o r t i o n a l i t y b e t w e e n t h e r a t e o f s u r f a c e a c c u m u l a t i o n s p e r u n i t a r e a and t h e s u r f a c e a i r b o r n e c o n c e n t r a t i o n . T h i s r a t i o o f f l u x t o c o n c e n t r a t i o n has dimensional u n i t s o f v e l o c i t y , and t h i s d e p o s i t i o n " v e l o c i t y " can be u s e d t o calculate depletion rates of a c o l l e c t i o n of airborne p a r t i c l e s . D e p o s i t i o n v e l o c i t y depends p r i m a r i l y upon p a r t i c l e t r a n s p o r t and hence i s a f u n c t i o n b a s i c a l l y o f p a r t i c l e s i z e r a t h e r t h a n o f the composition o f the p a r t i c l e . M o l e c u l a r v a p o r i s an e x c e p t i o n , i n t h a t d e p o s i t i o n v e l o c i t i e s , w h i c h may v a r y w i d e l y , depend upon the r e a c t i v i t y o f the molecule. ! t

n

The d e p o s i t i o n v e l o c i t y o f 100-μπι p a r t i c l e s as m e a s u r e d by d e p o s i t s on v e g e t a t i o n a p p r o x i m a t e s 50 cm/sec, and d e c r e a s e s w i t h d e c r e a s i n g p a r t i c l e s i z e t o a b o u t 0.03 cm/sec f o r n o n r e a c t i v e m o l e c u l a r v a p o r h a v i n g a p a r t i c l e d i a m e t e r o f a b o u t 0.001 μτη. By c o n t r a s t , a r e a c t i v e v a p o r s u c h as m o l e c u l a r i o d i n e moves t o t h e s u r f a c e a t a d i f f u s i o n - c o n t r o l l e d r a t e o f a b o u t 2 cm/sec. The o r g a n o c h l o r i n e i n s e c t i c i d e s e x h i b i t d e p o s i t i o n v e l o c i t i e s approaching that of nonreactive molecular vapor, i n d i c a t i n g that attachment at the s u r f a c e , r a t h e r than d i f f u s i o n t o the s u r f a c e , is the r a t e - c o n t r o l l i n gstep (U6). Large p a r t i c l e d e p o s i t i o n v e l o c i t y approximates the t e r m i n a l f a l l v e l o c i t y as d e f i n e d by t h e S t o k e s e q u a t i o n . The d e p o s i t i o n v e l o c i t y o f s m a l l e r p a r t i c l e s exceeds t h e i r t e r m i n a l f a l l ve l o c i t y , r e f l e c t i n g the increased importance o f t u r b u l e n t mixing i n b r i n g i n g small p a r t i c l e s to the s u r f a c e . Except f o r r e a c t i v e vapor, the d e p o s i t i o n v e l o c i t y i s approximately constant f o r


4.

GLOTFELTY

Persistent

A N D CARO

Pesticides

in the

Atmosphere

55

p a r t i c l e s b e l o w 1 ym. T h i s i s b e c a u s e b o t h v e r y f i n e p a r t i c l e s and u n r e a c t i v e m o l e c u l a r v a p o r a r e t r a n s p o r t e d a c r o s s t h e l a m i n a r l a y e r s u r r o u n d i n g exposed v e g e t a t i o n s u r f a c e s by Brownian m o t i o n , w i t h i m p a c t i o n becoming l e s s i m p o r t a n t as p a r t i c l e i n e r t i a decreases. In using d e p o s i t i o n v e l o c i t y data, i t i s convenient t o s p e c i f y a plume h e i g h t o r m i x i n g h e i g h t H_ t h r o u g h o u t w h i c h r a p i d , u n i f o r m m i x i n g i s assumed, and t h e n t o i n t e g r a t e t h e r e l a t i o n


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H

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N

Removal of Trace Contaminants from the Air - ACS Publications

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