ozone chemistry and technology - ACS Publications

Measurement of Ozone in Terms of Its Optical Absorption RALPH STAIR

OZONE CHEMISTRY AND TECHNOLOGY Downloaded from pubs.acs.org by YORK UNIV on 12/14/18. For personal use only.

National Bureau of Standards, Washington 25, D. C.

The unique absorption spectrum of ozone provides an ideal physical basis for measuring its concentra tion in the atmosphere even in the presence of significant quantities of other atmospheric pollutants, whether of gaseous or particulate character. V a r i ous types of optical equipment have been consid ered, both for measurement of the total amount of ozone and for determination of its vertical distribu tion a n d horizontal concentration. Natural sunlight furnishes a suitable a n d convenient light source for measuring the total amount a n d vertical distribution of ozone. Special sources having high radiant intensity within the spectral region of 2500 to 3600 Α., where ozone has a high optical absorption, are desired for use in measuring the horizontal concen tration of ozone. The light source, whether the sun or some special source such as a mercury arc lamp, may be employed with a simple filter radiometer or with a more or less elaborate prism or grating spectroradiometer as desired. In most of the recent work at the National Bureau of Standards a double, quartz prism spectroradiometer has been used at Washington, D. C., Climax, Colo., Los Angeles, Calif., and Sunspot, Ν. M., in ozone studies.

Since a b o u t 1845 (44) t h e r e h a s b e e n a n i n t e r e s t i n ozone a n d i n i t s m e a s u r e m e n t a n d i t s effect, e i t h e r d i r e c t l y o r i n d i r e c t l y , u p o n l i f e a n d o t h e r a c t i v i t i e s . O f t e n t h e r e h a s b e e n a difference of o p i n i o n r e g a r d i n g i t s q u a n t i t y a n d effects u p o n l i f e processes. I n f a c t , t h e r e h a s o n o c c a s i o n b e e n a d o u b t as t o i t s existence. A t t i m e s i t has been cloaked w i t h unique h e a l t h - g i v i n g properties w h i c h are inconsistent w i t h present i n f o r m a t i o n . A l t h o u g h a l a r g e n u m b e r of m e a s u r e m e n t s h a v e b e e n m a d e , u s u a l l y b y some c h e m i c a l m e t h o d , m a n y of t h e m a r e t o d a y c o n s i d e r e d w o r t h l e s s because, i n m a n y cases, t h e c h e m i c a l processes i n v o l v e d a r e n o w k n o w n t o h a v e b e e n affected b y o t h e r atmospheric components. I t is o n l y d u r i n g t h e p a s t t w o o r t h r e e decades t h a t a c c u r a t e d a t a o n ozone h a v e b e e n o b t a i n e d t h r o u g h t h e use of i m p r o v e d m e t h o d s of measurement. B e c a u s e ozone is h i g h l y u n s t a b l e a n d u s u a l l y exists i n b u t s m a l l q u a n t i t i e s n e a r 269

ADVANCES

270

IN CHEMISTRY SERIES

t h e e a r t h ' s s u r f a c e r e l a t i v e t o i t s g r e a t e r presence a t h i g h e r a l t i t u d e s , s o m e m e t h o d of p r o d u c t i o n a t these h i g h e r levels m u s t b e p o s t u l a t e d . T h e a c t i o n o f s h o r t w a v e solar r a d i a t i o n i s g e n e r a l l y h e l d as t h e p r i n c i p a l source of e n e r g y f o r t h e p r o d u c t i o n of o z o n e . T h e effects of c o s m i c r a y s , l i g h t n i n g , p a r t i c l e r a d i a t i o n f r o m t h e s u n , o r o p e r a t i o n of p u r e l y c h e m i c a l r e a c t i o n s w i t h i n t h e a t m o s p h e r e a r e t h o u g h t t o be n e g l i g i b l e r e l a t i v e t o s h o r t - w a v e u l t r a v i o l e t i n t h e p r o d u c t i o n of ozone i n t h e u p p e r atmosphere. T h e p h o t o c h e m i c a l r e a c t i o n w h i c h p r o d u c e s ozone f r o m t h e a t m o s p h e r e r e q u i r e s r a d i a t i o n of w a v e l e n g t h s s h o r t e r t h a n a b o u t 2400 A . T h e s e r a d i a t i o n s a r e s u f f i c i e n t l y energetic t o b r e a k t h e c h e m i c a l b o n d of t h e o x y g e n m o l e c u l e a n d p r o d u c e free o x y g e n atoms. T h e oxygen atoms c a n react w i t h the oxygen i n the atmosphere to produce ozone. T h e p r i n c i p a l m o l e c u l a r c o m p o n e n t s of t h e a t m o s p h e r e a r e o x y g e n , n i t r o g e n , c a r bon dioxide, argon, a n d water vapor. These m a y v a r y somewhat w i t h altitude, b u t , e x c e p t f o r w a t e r v a p o r , t h e y e x t e n d t o h i g h levels i n r o u g h l y t h e same r e l a t i v e p r o p o r t i o n s as f o u n d n e a r t h e e a r t h ' s s u r f a c e . T h e y a n d t h e o t h e r m i n o r n o r m a l c o m p o n e n t s of a c l e a n a t m o s p h e r e a r e o p t i c a l l y h i g h l y t r a n s p a r e n t t h r o u g h o u t t h e v i s i b l e a n d u l t r a v i o l e t s p e c t r u m d o w n t o n e a r 2000 A . W h e n r a d i a n t e n e r g y of w a v e l e n g t h s b e l o w 2400 A . is a b s o r b e d , d i s s o c i a t i o n processes m a y be o p e r a t i v e . T h e p r o d u c t i o n of ozone is t h e m o s t n o t e w o r t h y of these processes, as t h e o x y g e n is b r o k e n u p a n d diffuses a m o n g t h e o t h e r c o m p o n e n t s of t h e e a r t h ' s u p p e r a t m o s p h e r e . A l t h o u g h c o m p l e t e l y d i s s o c i a t e d a t t h e h i g h e r levels, a t s o m e w h a t l o w e r levels n o t a l l t h e o x y g e n is d i s s o c i a t e d a n d b y some m o r e o r less u n k n o w n process r e c o m b i n a t i o n o c c u r s , r e s u l t i n g i n a m i x t u r e of 0 , 0 , a n d O W h e t h e r t h r e e a t o m s of Οχ c o m b i n e t o f o r m 0 o r a n O c o m b i n e s w i t h a n 0 is n o t k n o w n . P e n n d o r f (35) a n d B l a c e t (2) h a v e suggested t h a t a t h r e e - b o d y c o l l i s i o n i n v o l v i n g a f o r e i g n b o d y m a y p l a y a p a r t i n t h i s r e c o m b i n a t i o n . J u s t w h a t is t h e process o r j u s t h o w i t o p e r a t e s is n o t c l e a r . T h e ozone's b e i n g h i g h l y o p a q u e t o t h e s h o r t w a v e l e n g t h u l t r a v i o l e t e f f e c t i v e l y shields t h e gas a t s t i l l l o w e r levels f r o m s u c h r a d i a t i o n , so t h a t d e c o m p o s i t i o n o c c u r s (Jf). T h e e n d r e s u l t is t h e r e f o r e a n e q u i l i b r i u m c o n d i t i o n i n t h e u p p e r a t m o s p h e r e b e t w e e n t h e p r o d u c t i o n a n d d e c o m p o s i t i o n of ozone b y s u n l i g h t of t h e different w a v e l e n g t h s , w h i c h is e x e m p l i f i e d b y t h e b u i l d i n g u p of a m a x i m u m d e n s i t y of ozone a t a n i n t e r m e d i a t e l e v e l . T h i s d i s t r i b u t i o n of ozone h a s b e e n f o u n d b y different i n v e s t i g a t o r s a t v a r i o u s t i m e s a n d places t o l i e u s u a l l y b e t w e e n a b o u t 10 a n d 20 m i l e s h i g h (17, 24, 42, 50, 60). 3

3

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of O z o n e

x

v

2

2

Evaluation

F i v e p r i n c i p a l m e t h o d s of ozone e v a l u a t i o n h a v e been e m p l o y e d b y different i n v e s t i g a t o r s a t different t i m e s a n d places. T h e s e m a y be r o u g h l y classified w i t h some v a r i a t i o n s as f o l l o w s : C h e m i c a l m e a s u r e m e n t of ozone i n t e r m s of t h e release of i o d i n e i n a b u f f e r e d potassium iodide solution. Recent improvements i n this m e t h o d p e r m i t the continuous r e c o r d i n g of ozone c o n c e n t r a t i o n i n t e r m s of c o l o r i m e t r y o r p o t e n t i o m e t e r r e a d i n g s (13, 27). C o l o r m e a s u r e m e n t of ozone i n t e r m s of o x i d a t i o n o f p h e n o l p h t h a l i n ( C H O ) t o p h e n o l p h t h a l e i n ( C H O ) . I n t h i s m e t h o d h y d r o g e n p e r o x i d e is u s e d t o d e v e l o p a s t a n d a r d c u r v e (29, 34). P h y s i c a l m e a s u r e m e n t of ozone i n t e r m s of t h e r a t e of c r a c k i n g of b e n t o r s t r e t c h e d r u b b e r (3). M e a s u r e m e n t of ozone i n t e r m s of t h e response of c e r t a i n p l a n t s , e s p e c i a l l y f o r d a m a g e t o u p p e r leaf surfaces (19, 54). O p t i c a l m e a s u r e m e n t of ozone i n t e r m s of i t s u l t r a v i o l e t , l u m i n o u s , o r i n f r a r e d absorption. 2 0

2 0

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4

1 6

4

STAIR-OPTICAL ABSORPTION

271

T h e first f o u r m e t h o d s f o r t h e m e a s u r e m e n t of ozone a r e n o t discussed h e r e , e x c e p t f o r n o t i n g t h a t e a c h of t h e m is m o r e o r less q u a l i t a t i v e i n c o m p a r i s o n w i t h t h e o p t i c a l m e t h o d . T h i s is e s p e c i a l l y t h e case u n d e r c o n d i t i o n s w h e r e i n t h e a t m o s p h e r e c o n t a i n s a n u m b e r of o t h e r p o l l u t a n t s s u c h as exist i n t h e L o s A n g e l e s a r e a . U n d e r c o n d i t i o n s of p o l l u t e d a t m o s p h e r e , d i s a g r e e m e n t o f t e n r e s u l t s a m o n g scientific w o r k e r s (28) as t o w h a t c o n s t i t u t e s a c e r t a i n i d e n t i f i c a t i o n a n d t r u e m e a s u r e of o z o n e . H a a g e n - S m i t (18) e a r l y c o n c l u d e d f r o m results of c h e m i c a l a n d p h o t o c h e m i c a l e x p e r i m e n t s t h a t t h e m a j o r p o r t i o n of t h e o x i d a n t c o m p o n e n t of t h e L o s A n g e l e s s m o g consisted of v a r i o u s o r g a n i c p e r o x i d e s r a t h e r t h a n ozone. O z o n e p l a n t d a m a g e i s o f t e n c o m p l i c a t e d b y t h e presence of o t h e r a t m o s p h e r i c o x i d a n t s (19). F u r t h e r m o r e , ozone p l a n t d a m a g e o c c u r s o n l y i n t h e presence of l a r g e a m o u n t s of ozone. C r a b t r e e a n d B i g g s (5) f o u n d t h a t c e r t a i n free r a d i c a l s m a y p r o d u c e r u b b e r c r a c k i n g of a t y p e w h i c h c a n n o t be d i s t i n g u i s h e d f r o m t h a t r e s u l t i n g f r o m e x p o s u r e t o h i g h c o n c e n t r a t i o n s of ozone. H e n c e , o n l y some o p t i c a l m e t h o d r e m a i n s as a possible reasonable m e a n s f o r t h e a c c u r a t e e v a l u a t i o n of ozone c o n c e n t r a t i o n s i n t h e a t m o s p h e r e , e s p e c i a l l y w h e n t h e ozone is associated w i t h o t h e r p o l l u t a n t s o r o x i d a n t s . O z o n e i n gaseous f o r m i n w h i c h i t is n o r m a l l y e n c o u n t e r e d e x h i b i t s selective a b s o r p t i o n i n s e v e r a l regions of t h e s p e c t r u m . W i t h i n t h e H a r t l e y b a n d , c e n t e r e d a t 2500 t o 2600 A . i n t h e s h o r t w a v e u l t r a v i o l e t , t h e a b s o r p t i o n is m o s t i n t e n s e . N e x t comes t h e H u g g i n s b a n d w h i c h is c e n t e r e d a t s l i g h t l y l o n g e r w a v e l e n g t h s a n d extends t o a b o u t 3400 A . D a t a o n t h e a b s o r b a n c e of ozone i n t h i s s p e c t r a l r e g i o n as g i v e n b y T s i - Z e a n d S h i n - P i a w (57, 58) a r e r e p r o d u c e d i n F i g u r e 1 (49). These or s i m i l a r d a t a h a v e been e m p l o y e d b y t h e v a r i o u s i n v e s t i g a t o r s w h o s e w o r k s a r e r e p o r t e d i n t h i s p a p e r i n t h e i r d e t e r m i n a t i o n of t h e a m o u n t of ozone i n t h e a t m o s p h e r e . [Recent d a t a , h o w e v e r , i n d i c a t e a p p r e c i a b l y l o w e r ozone a b s o r p t i o n coefficients t h r o u g h o u t m o s t of t h e u l t r a v i o l e t s p e c t r u m (21).1 T w o ozone b a n d s s i t u a t e d n e a r 9.5 m i c r o n s i n t h e i n f r a r e d (1) s h o w c o n s i d e r a b l e a b s o r p t i o n , n e a r 5 0 % f o r a single a t m o s p h e r e , b u t a r e m a r k e d l y t e m p e r a t u r e a n d p r e s s u r e s e n s i t i v e . T h i s p r o p e r t y is u s e f u l i n c e r t a i n w o r k as w i l l b e n o t e d b e l o w . F i n a l l y , ozone e x h i b i t s some a b s o r p t i o n i n t h e v i s i b l e s p e c t r u m , i n p a r t i c u l a r w i t h i n t h e y e l l o w - o r a n g e r e g i o n , r e s u l t i n g i n g i v i n g t h e gas a l i g h t b l u e c o l o r w h e n v i e w e d i n b u l k . T h i s a b s o r p t i o n is w e a k b u t m a y b e u s e d t o s u p p l e m e n t o r c h e c k e x p e r i m e n t a l work which employs the ultraviolet bands. T h e a b s o r p t i o n of ozone i n t h e v i s i b l e s p e c t r u m is e s p e c i a l l y u s e f u l a n d h a s been u s e d i n a n a l y z i n g some of t h e S m i t h s o n i a n d a t a , w h i c h a r e e x t e n s i v e b u t do n o t e x t e n d t o i n c l u d e a n a p p r e c i a b l e a m o u n t of t h e u l t r a v i o l e t r e g i o n . W u l f (61) a n d others (14, 25) h a v e m a d e c o n s i d e r a b l e use of these d a t a , d e d u c i n g ozone c o n c e n t r a t i o n s w h i c h a r e i n f a i r l y g o o d a g r e e m e n t w i t h those o b t a i n e d t h r o u g h t h e use of u l t r a violet measurements. T h e h i g h o p t i c a l o p a c i t y of ozone w i t h i n t h e H a r t l e y a n d H u g g i n s b a n d s , w i t h t h e a b s o r p t i o n c e n t e r e d a t 2500 t o 2600 A . a n d e x t e n d i n g t o a b o u t 3400 Α . , h a s f u r n i s h e d a u n i q u e p o s s i b i l i t y f o r i t s m e a s u r e m e n t i n t e r m s of i t s r e l a t i v e o r a b s o l u t e s p e c t r a l a b s o r p t i o n . S e v e r a l o p t i c a l m e t h o d s , e a c h b a s e d u p o n t h i s c h a r a c t e r i s t i c of ozone, h a v e been set u p a t t h e N a t i o n a l B u r e a u of S t a n d a r d s (46, 48-51)These d e a l t w i t h t h e m e a s u r e m e n t of t h e t o t a l a m o u n t , v e r t i c a l d i s t r i b u t i o n , a n d l o c a l c o n c e n t r a t i o n of ozone b y t h r e e g e n e r a l m e t h o d s . T h e first, e m p l o y e d i n t h e m e a s u r e m e n t o f t o t a l ozone, w a s b a s e d u p o n t h e r e l a t i v e a b s o r p t i o n of t h e u l t r a v i o l e t s o l a r r a d i a n t e n e r g y b y t h e t e r r e s t r i a l a t m o s p h e r e as t h e a i r m a s s c h a n g e d d u r i n g t h e d a y . T h e second m e t h o d w a s e m p l o y e d i n t h e e v a l u a t i o n o f t h e v e r t i c a l d i s t r i b u t i o n of t h e ozone a n d w a s b a s e d u p o n s i m i l a r s o l a r r a d i a t i o n changes as t h e i n s t r u m e n t s were c a r r i e d a l o f t b y b a l l o o n s . T h e t h i r d m e t h o d h a d t o d o w i t h t h e m e a s u r e m e n t of ozone c o n c e n t r a t i o n i n t h e l o w e r a t m o s p h e r e ( a t t h e e a r t h ' s s u r f a c e ) a n d w a s b a s e d u p o n t h e r e l a t i v e s p e c t r a l o p t i c a l a b s o r p t i o n of a h o r i z o n t a l b e a m f r o m a n a r t i f i c i a l u l t r a v i o l e t source.

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Ozone

T h e m e a s u r e m e n t of t h e t o t a l a m o u n t of ozone has r e c e i v e d greatest a t t e n t i o n a n d m a y p r o b a b l y b e s t be m a d e b y m e a n s of a d o u b l e , q u a r t z p r i s m s p e c t r o r a d i o m e t e r or spectrophotometer. T h e d e t e c t o r m a y be a t h e r m o p i l e (36), p h o t o e l e c t r i c c e l l (48, 49, 52), o r t h e p h o t o g r a p h i c p l a t e (11, 17, 20, 22, 32, 42). T h e t h e r m o p i l e has the a d v a n t a g e of m e a s u r i n g r a d i a n t e n e r g y of t h e d i f f e r e n t w a v e l e n g t h s d i r e c t l y i n a b s o l u t e u n i t s w h e n once c a l i b r a t e d . H o w e v e r , i t s l o w s e n s i t i v i t y rules a g a i n s t i t s use i n r a p i d w o r k i n t h e u l t r a v i o l e t s p e c t r u m w h e r e e n e r g y v a l u e s are l o w . A d v a n t a g e s s u c h as h i g h s e n s i t i v i t y , d i r e c t r e c o r d i n g , a n d f r e e d o m f r o m t h e d e v e l o p m e n t of p l a t e s a n d t h e i r v a r i a t i o n i n s e n s i t i v i t y g e n e r a l l y r u l e i n f a v o r of p h o t o e l e c t r i c d e t e c t i o n . D a t a o n t o t a l ozone o b t a i n e d b y t h e N a t i o n a l B u r e a u of S t a n d a r d s a t C l i m a x , C o l o . , a n d a t S u n s p o t , Ν . M . (48, φ, 52) w i t h a d o u b l e , q u a r t z p r i s m s p e c t r o r a d i o m e t e r e m p l o y i n g a p h o t o e l e c t r i c d e t e c t o r are i l l u s t r a t e d i n F i g u r e s 2, 3, a n d 4. S m a l l differences i n a t m o s p h e r i c s c a t t e r i n g b e t w e e n t h e t h r e e sets of d a t a r e s u l t f r o m differences i n a l t i t u d e of t h e t w o s t a t i o n s a n d f r o m clearness of t h e a t m o s p h e r e a t t h e t i m e s t h e m e a s u r e m e n t s were i n p r o g r e s s . C l o s e a g r e e m e n t b e t w e e n t h e ozone v a l u e s is s i g n i f i c a n t . T h e e q u i p m e n t e m p l o y e d i n these i n v e s t i g a t i o n s w a s designed p r i m a r i l y f o r t h e p u r p o s e of e v a l u a t i n g t h e s p e c t r a l d i s t r i b u t i o n of t h e r a d i a n t energy f r o m the sun. S p e c t r a l energy d a t a obtained at Sunspot, Ν . M . , d u r i n g J u n e


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Figure 2. Atmospheric transmittance at Climax, Colo, (altitude, 11,190 feet), a n d determination of total ozone a b o v e observing station Mean of data for 4 dear days in September 1951

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Figure 3. Atmospheric transmittance at Sacramento Peak (Sunspot), Ν . M . (altitude, 9200 feet), a n d determination of total ozone above observing station Mean of data for 4 dear days in July 1953

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MILLIMICRONS

Figure 4 . Atmospheric transmittance at Sacramento Peak (Sunspot), Ν . M., a n d determination of total ozone above observing station Mean of data for 4 clear days in June 1955

1955 a r e g i v e n i n F i g u r e 5. A p h o t o e l e c t r i c d e t e c t o r w a s e m p l o y e d a n d , b y use of a synchronous electric motor, wave length d r i v e , a n d a s t r i p recorder, the observed d a t a were a u t o m a t i c a l l y p l o t t e d o n a p a p e r c h a r t . T h e i n s t r u m e n t w a s e i t h e r p o i n t e d d i r e c t l y a t t h e s u n (52) o r else a d i r e c t b e a m of l i g h t f r o m t h e s u n w a s reflected o n t o t h e e n t r a n c e s l i t b y m e a n s of a h e l i o s t a t (48, 53). This arrangement p e r m i t t e d t h e i n t e g r a t i o n of t h e r a d i a t i o n f r o m t h e e n t i r e s o l a r d i s k w h i l e i t e l i m i n a t e d almost completely the scattered radiation f r o m the s k y . A b o u t 1930, D o b s o n (7) set u p a d o u b l e , q u a r t z p r i s m i n s t r u m e n t , t h e D o b s o n ozone m e t e r , f o r use i n m e a s u r i n g ozone o n l y b y i s o l a t i n g t w o w a v e l e n g t h s i n t h e ultraviolet solar s p e c t r u m ; f r o m their relative intensities the absorption caused b y ozone c o u l d b e d e t e r m i n e d a n d , hence, i t s a m o u n t . A n y t w o w a v e l e n g t h s m a y be e m p l o y e d s u c h t h a t one of t h e m lies w e l l w i t h i n t h e ozone a b s o r p t i o n b a n d w h i l e t h e o t h e r is j u s t o u t s i d e , o r a l m o s t o u t s i d e , t h e b a n d . T h i s i n s t r u m e n t is l i k e w i s e p h o t o e l e c t r i c so t h a t ozone m e a s u r e m e n t s m a y be m a d e r a p i d l y . H o w e v e r , since t h e u l t r a v i o l e t s o l a r s p e c t r u m is m o d i f i e d g r e a t l y b y t h o u s a n d s of F r a u n h o f e r l i n e s o r b a n d s a n d since t h e i r a p p a r e n t s t r u c t u r e is g r e a t l y affected b y i n s t r u m e n t a l s l i t w i d t h , e r r o r s m a y b e i n t r o d u c e d t h r o u g h s l i t w i d t h changes o r w a v e l e n g t h shifts r e s u l t i n g f r o m t e m p e r a t u r e changes w i t h i n t h e i n s t r u m e n t . I n t h i s respect t h e s c a n n i n g of t h e c o m p l e t e s p e c t r u m a n d t h e use of m a n y w a v e l e n g t h s i n ozone e v a l u a t i o n is t o be p r e f e r r e d . A n e a r l y m e t h o d d e v e l o p e d a t t h e N a t i o n a l B u r e a u of S t a n d a r d s (45, 46> 51) a n d r e c e n t l y e m p l o y e d b y M i y a k e a n d K a w a m u r a (30) f o r t h e m e a s u r e m e n t of t h e t o t a l

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Spectral distribution of radiant energy from the sun at Sunspot, Ν . M . Mean for 4 clear days in June 1955

a m o u n t of ozone, i n c o r p o r a t e d a single p h o t o t u b e h a v i n g a s e n s i t i v i t y c o n f i n e d t o t h e s p e c t r a l r e g i o n of w a v e l e n g t h s s h o r t e r t h a n a b o u t 3600 A . T h e p h o t o t u b e w a s u s e d i n c o n j u n c t i o n w i t h a s i m p l e a m p l i f i e r a n d g r o u p s of t w o t o f o u r glass filters h a v i n g effective t o t a l t r a n s m i t t a n c e s f o r s u n l i g h t of a b o u t 20 t o 7 5 % . T h i s p e r m i t s t h e b r e a k i n g u p of t h e s h o r t w a v e s o l a r s p e c t r u m i n s u c h a w a y t h a t ozone changes are r e a d i l y e v a l u a t e d i n t e r m s of changes i n t h e o b s e r v e d filter t r a n s m i t t a n c e s . T h i s m e t h o d requires a knowledge of the relative spectral i n t e n s i t y of the u l t r a violet solar r a d i a n t energy outside the terrestrial atmosphere w i t h i n t h e spectral r a n g e of 3000 t o a b o u t 3400 Α . , i n a d d i t i o n t o t r a n s m i t t a n c e d a t a o n t h e filters a n d r e l a t i v e s p e c t r a l response d a t a f o r t h e p h o t o t u b e . F u r t h e r m o r e , i t is assumed t h a t t h e s o l a r r a d i a n t e n e r g y (outside t h e a t m o s p h e r e ) r e m a i n s c o n s t a n t d u r i n g t h e course of t h e ozone m e a s u r e m e n t s . A s a n a l t e r n a t i v e , t h e i n s t r u m e n t m a y b e c a l i b r a t e d b y c o m p a r i s o n w i t h a n o t h e r t y p e of ozone m e t e r — f o r e x a m p l e , a d o u b l e , q u a r t z p r i s m s p e c t r o r a d i o m e t e r o r a D o b s o n ozone m e t e r . T h i s p h o t o e l e c t r i c i n s t r u m e n t has t h e a d v a n t a g e of b e i n g s i m p l e , l i g h t i n w e i g h t , l o w i n cost, a n d r a p i d i n o p e r a t i o n . It t h u s l e n d s i t s e l f t o use i n field w o r k i n t h e a c c u m u l a t i o n of e x t e n s i v e d a t a o v e r w i d e areas o r t i m e i n t e r v a l s . D a t a o b t a i n e d a t S u n s p o t , Ν . M . , d u r i n g t h e s u m m e r of 1949 (45) w i t h t h e s i m p l e p h o t o e l e c t r i c - f i l t e r i n s t r u m e n t a r e g i v e n i n F i g u r e s 6 a n d 7. S m a l l changes i n

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the t o t a l a m o u n t of ozone a r e a p p a r e n t f r o m a n e x a m i n a t i o n of these c h a r t s . S o m e result f r o m u n c o r r e c t e d e r r o r s because of s c a t t e r i n g b y d u s t o r c l o u d s ; o t h e r changes a r e r e a l . T h e l a r g e r changes f r o m d a y t o d a y a r e i m p o r t a n t a n d i l l u s t r a t e t h e u s e fulness of a n i n s t r u m e n t of t h i s t y p e i n c o n t i n u a l l y m o n i t o r i n g t h e a m o u n t of a t m o s p h e r i c ozone. T h e m e a n values for day-to-day measurements made at W a s h i n g t o n , D . C , over a p e r i o d of 9 y e a r s (47) a r e g i v e n i n F i g u r e 8. E a c h p l o t t e d p o i n t r e p r e s e n t s t h e m e a n v a l u e f o r a single d a y . G r e a t e s t w e i g h t w a s g i v e n t o m e a s u r e m e n t s m a d e n e a r t h e n o o n h o u r , w h e n t h e s u n l i g h t w a s o b s e r v e d t h r o u g h t h e least a t m o s p h e r e . Similar

A D V A N C E S IN CHEMISTRY SERIES

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Seasonal variation of ozone at Washington, D. C. (altitude, 300 feet), using phototube a n d filter ozone meters

d a t a t a k e n b y F r i t z (15) a t W a s h i n g t o n , D . C , u s i n g a D o b s o n ozone m e t e r , o v e r a p e r i o d of 2 5 m o n t h s a r e i n close a g r e e m e n t . H i s w o r k s u b s t a n t i a t e d h i g h ozone values i n the s p r i n g w i t h m u c h lower values d u r i n g the late summer a n d fall. O z o n e d a t a o b t a i n e d a t h i g h l a t i t u d e s b y T 0 n s b e r g a n d O l s e n (56) a t T r o m s o , N o r w a y , f o l l o w a s l i g h t l y different seasonal v a r i a t i o n b u t a r e l i k e w i s e h i g h i n t h e e a r l y s p r i n g m o n t h s a n d l o w i n l a t e s u m m e r a n d f a l l . M e a s u r e m e n t s m a d e a t 12 E u r o p e a n s t a t i o n s a n d r e c e n t l y s u m m a r i z e d b y N o r m a n d (31) a r e i n g e n e r a l a g r e e m e n t w i t h those i l l u s t r a t e d i n F i g u r e 8. T h e d a t a t a k e n b y M i y a k e a n d K a w a m u r a (30) i n J a p a n w i t h a p h o t o t u b e a n d n i t e r s a r e also s i m i l a r t o those o b t a i n e d b y S t a i r (47) a n d b y F r i t z (15) a t W a s h i n g t o n a n d b y L e j a y (26) i n F r a n c e . T h i s i s t o be e x p e c t e d , as t h e l a t i t u d e s of t h e t h r e e s t a t i o n s differ b u t l i t t l e . Vertical

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I t was pointed out earlier that the n a t u r a l e q u i l i b r i u m condition for the p r o d u c t i o n a n d d i s s o c i a t i o n of ozone r e s u l t e d i n a n ozone m a x i m u m a t some i n t e r m e d i a t e l e v e l i n t h e u p p e r a t m o s p h e r e . F r o m m e a s u r e m e n t s at different t i m e s a n d b y different o b s e r v e r s (6, 12, 16, 32, 33, 37, 39, 41, 42, 50) t h i s m a x i m u m has been f o u n d to v a r y i n h e i g h t , t h i c k n e s s , a n d i n g e n e r a l s t r u c t u r e , w i t h s h o r t a n d l o n g p e r i o d fluctuations p o s s i b l y a s s o c i a t e d w i t h w e a t h e r c o n d i t i o n s a n d seasonal v a r i a t i o n s . O n o c c a s i o n d o u b l e m a x i m a m a y a p p e a r (41)· L i t t l e is y e t k n o w n , h o w e v e r , a b o u t specific r e l a t i o n s h i p s b e t w e e n t h e ozone changes a n d m e t e o r o l o g i c a l c o n d i t i o n s e x c e p t i n t h e i n s t a n c e of seasonal v a r i a t i o n s (6, 8, 10, 15, 26, 30, 4?) a n d c e r t a i n l a r g e a i r m a s s m o v e m e n t s (9, 56, 59). T h e r e is a great need f o r c o n t i n u o u s o b s e r v a t i o n s of b o t h t h e t o t a l a n d t h e v e r t i c a l d i s t r i b u t i o n of ozone i n o r d e r t o e s t a b l i s h possible r e l a t i o n s h i p s b e t w e e n ozone d i s t r i b u t i o n a n d c o n c e n t r a t i o n a n d solar v a r i a t i o n , a t m o s p h e r i c circulation, a n d weather. T h e v e r t i c a l d i s t r i b u t i o n of ozone m a y be m e a s u r e d b y o p t i c a l m e a n s w i t h i n s t r u ments situated p e r m a n e n t l y at the earth's surface, although better b y their transport t h r o u g h t h e ozone l a y e r . S o m e of t h e e a r l y m e a s u r e m e n t s b y G o e t z et al. (16, 17) were b y t h e f o r m e r m e t h o d — k n o w n as t h e U m k e h r effect. T h i s m e t h o d p e r m i t s t h e d e t e r m i n a t i o n of t h e ozone d i s t r i b u t i o n i n t e r m s of t h e change i n t h e r e l a t i v e s c a t t e r i n g of t h e u l t r a v i o l e t s o l a r r a d i a t i o n ( a t t w o selected w a v e l e n g t h s ) of t h e z e n i t h s k y as a f u n c t i o n of t h e z e n i t h angle of t h e s u n . T h i s m e t h o d c o n t i n u e s t o b e e m p l o y e d , a l t h o u g h i t does n o t p e r m i t as h i g h a c c u r a c y as t h e t r a n s p o r t m e t h o d s . E a r l y w o r k a t t h e N a t i o n a l B u r e a u of S t a n d a r d s (50) a n d a t a n u m b e r of o t h e r

279


l a b o r a t o r i e s (16, 37, J$) i n t h e s t u d y of t h e v e r t i c a l d i s t r i b u t i o n of ozone m a d e use of balloons, u s u a l l y u n m a n n e d , for c a r r y i n g radiometric or photographic equipment t h r o u g h a p a r t o r m o s t of t h e ozone l a y e r . T h e N B S i n s t r u m e n t c o n s i s t e d of a p h o t o t u b e a n d f i l t e r - t y p e u l t r a v i o l e t m e t e r (Ifi, 50, 51) w h i c h w a s c o m b i n e d w i t h a n audiofrequency generator a n d radio t r a n s m i t t e r . T h e radiofrequency wave was m o d u l a t e d b y t h e p h o t o t u b e response t o s u n l i g h t as t h e i n s t r u m e n t w a s c a r r i e d u p w a r d through the atmosphere. T h e m a g n i t u d e of t h e response w a s p r o p o r t i o n a l t o t h e s o l a r i n t e n s i t y t h r o u g h t h e s e v e r a l f i l t e r s . T h e a m o u n t of ozone a b o v e t h e i n s t r u m e n t a n d hence, i t s v e r t i c a l d i s t r i b u t i o n , w a s o b t a i n e d b y c a l c u l a t i o n f r o m t h e

OZONE PER Figure 9.

KM.

Vertical distribution of ozone in stratosphere

NBS curve is mean of June and July measurements obtained in 18 unmanned balloon flights using phototube and filter ozone meters. NGS measurements obtained with spectrograph in manned balloon. Regener measurements obtained with spectrograph in unmanned balloons

o b s e r v e d changes i n t h e a p p a r e n t f i l t e r t r a n s m i t t a n c e s . T h e m e a n of d a t a o b t a i n e d i n 18 b a l l o o n f l i g h t s a t t h e N a t i o n a l B u r e a u of S t a n d a r d s e m p l o y i n g t h i s m e t h o d (47, 50) i s g i v e n i n F i g u r e 9 a l o n g w i t h o t h e r d a t a b y o t h e r e a r l y o b s e r v e r s (32). T h e R e g e n e r s (37, 39, 1$) e m p l o y e d s p e c t r o g r a p h s i n s i m i l a r u n m a n n e d b a l l o o n flights a n d e v a l u a t e d t h e ozone v e r t i c a l d i s t r i b u t i o n i n t e r m s of s p e c t r a l c h a n g e of t h e u l t r a v i o l e t s o l a r r a d i a t i o n a t specific w a v e l e n g t h s . R e c e n t d a t a o b t a i n e d b y V . H . R e g e n e r (38) i n u n m a n n e d b a l l o o n flights o v e r N e w M e x i c o i n 1950 a r e i l l u s t r a t e d i n F i g u r e 10. I n t h e R e g e n e r f l i g h t s (38, 41) t h e e n t r a n t s p e c t r o g r a p h t u b e w a s d i r e c t e d d o w n w a r d onto a white m a g n e s i u m oxide surface w h i c h was directly i l l u m i n a t e d b y t h e

280

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Figure 10. Vertical distribution of ozone (in dicating double maxima) over New Mexico in 1950 using spectrographs in unmanned balloons

s u n . T h e e x p o s u r e t i m e was 8 m i n u t e s , d u r i n g w h i c h t h r e e exposures w e r e o b t a i n e d t h r o u g h a three-step filter c o n s i s t i n g of d i f f e r e n t a b s o r b a n c e s of evaporated p l a t i n u m on a quartz plate. T h u s , definite t r a n s m i t t a n c e r a t i o s b e t w e e n t h e t h r e e sets of s p e c t r a w e r e e m p l o y e d i n t h e e v a l u a t i o n of the v e r t i c a l d i s t r i b u t i o n of t h e ozone. T h e i l l u s t r a t e d d a t a ( F i g u r e 10) s h o w t h e p o s s i b i l i t y of d o u b l e ozone m a x i m a o n t h r e e of t h e f o u r flights. H o w e v e r , a l t h o u g h i r r e g u l a r i t i e s exist i n s o m e of t h e N B S a n d N R L c u r v e s , i n no case has a n y definite evidence of a d o u b l e m a x i m u m a p p e a r e d i n t h e i r d a t a . I t m a y be i n t h i s case t h a t e r r a t i c o b s e r v a t i o n s h a v e n o t been p r o p e r l y t a k e n i n t o a c c o u n t (23) ; h o w e v e r , P a e t z o l d (33) has r e p o r t e d definite d o u b l e o r t r i p l e ozone m a x i m a o n n u m e r o u s occasions. T h e w o r k of t h e N a t i o n a l G e o g r a p h i c S o c i e t y g r o u p (32) differed p r i m a r i l y i n t h a t a m a n n e d b a l l o o n was e m p l o y e d so t h a t m o r e e l a b o r a t e e q u i p m e n t c o u l d be u s e d a n d m o r e c a r e f u l c o n t r o l of t h e p h o t o g r a p h i c processes a n d c a l i b r a t i o n s c o u l d be e s t a b l i s h e d . T h e results of t h i s flight differ f r o m t h a t of a l l t h e u n m a n n e d f l i g h t s i n i n d i c a t i n g a v e r y s h a r p ozone m a x i m u m a t a s o m e w h a t l o w e r a l t i t u d e . R e s u l t s f r o m f l i g h t t o f l i g h t m a y differ because of differences i n i n s t r u m e n t a t i o n a n d t e c h n i q u e o r because of a c t u a l change i n a t m o s p h e r i c c o n d i t i o n s . H e n c e , t h e r e exists a g r e a t need for routine experiments w i t h stable equipment. T h e d e v e l o p m e n t of a u n i q u e m e t h o d (12), o r i g i n a l l y d e s c r i b e d b y S t r o n g (55) i n 1941, w h e r e b y i t is possible t o d e t e r m i n e t h e m e a n h e i g h t of t h e ozone l a y e r t h r o u g h m e a s u r e m e n t s of t h e p r e s s u r e - s e n s i t i v e i n f r a r e d a b s o r p t i o n of t h e ozone m o l e c u l e a n d c o m p a r i s o n w i t h d e t e r m i n a t i o n s of t h e t o t a l a m o u n t of ozone b a s e d u p o n u l t r a v i o l e t s p e c t r o s c o p i c m e a s u r e m e n t s has g i v e n a d d e d i m p e t u s t o r o u t i n e ozone v e r t i c a l d i s t r i b u t i o n d e t e r m i n a t i o n s . T h i s m e t h o d p e r m i t s t h e e v a l u a t i o n of t h e a p p r o x i m a t e v e r t i c a l d i s t r i b u t i o n of t h e ozone solely f r o m t h e use of s u r f a c e observations. P r e l i m i n a r y results (12) i n d i c a t e t h a t t h e r e is n o s i m p l e r e l a t i o n b e t w e e n t h e v e r t i c a l d i s t r i b u t i o n of ozone a n d t h e t o t a l a m o u n t p r e s e n t i n t h e a t m o s p h e r e a t any given time. W h e n r o u t i n e m e a s u r e m e n t s are e s t a b l i s h e d o v e r a n e t w o r k of

281


s t a t i o n s b y t h i s ( o r a n y o t h e r s a t i s f a c t o r y ) m e t h o d , i t m a y b e possible t o e s t a b l i s h some definite r e l a t i o n s h i p b e t w e e n ozone d i s t r i b u t i o n a n d m o v e m e n t a n d o t h e r f a c t o r s s u c h as w e a t h e r c o n d i t i o n s , a t m o s p h e r i c c i r c u l a t i o n , a n d s o l a r v a r i a t i o n s . T h e use of r o c k e t s offers a u n i q u e o p p o r t u n i t y f o r t h e p e n e t r a t i o n of t h e e n t i r e ozone l a y e r i n c o n t r a s t t o t h e p a r t i a l p e n e t r a t i o n a f f o r d e d b y b a l l o o n s . E x c e p t f o r t h e m e a g e r d a t a o b t a i n e d b y t h e U m k e h r effect (17, 56) a n d f r o m c a l c u l a t i o n s b a s e d o n t h e p r e s s u r e - s e n s i t i v e i n f r a r e d a b s o r p t i o n of ozone (12, 55), n o a b s o l u t e d a t a o n t h e v e r t i c a l d i s t r i b u t i o n of ozone h a v e b e e n o b t a i n a b l e (before t h e use of r o c k e t s ) t o a l t i t u d e s a b o v e a b o u t 32 k m . L i t t l e credence c o u l d b e g i v e n t o d a t a o b t a i n e d b y e i t h e r t h e U m k e h r effect o r t o t h e i n f r a r e d m e t h o d f o r a l t i t u d e s a b o v e a b o u t 4 0 t o 50 k m . , as b o t h m e t h o d s a r e i n d i r e c t a n d r e q u i r e a p p r e c i a b l e o p t i c a l a b s o r p t i o n t o permit practical application. R o c k e t e x p e r i m e n t s b y p e r s o n n e l of t h e N a v a l R e s e a r c h L a b o r a t o r y (22-24) have e x t e n d e d ozone o b s e r v a t i o n s t o a l t i t u d e s u p t o 70 k m . I n t h i s w o r k V - 2 o r A e r o b e e

r o c k e t s w e r e e m p l o y e d a n d a g r a t i n g s p e c t r o g r a p h was c a r r i e d i n t h e h e a d of t h e v e h i c l e . S i d e w i n d o w s p r o v i d e d w i t h a b e a d of l i t h i u m f l u o r i d e s e r v e d t o i l l u m i n a t e t h e s p e c t r o g r a p h s as t h e r o c k e t s r o t a t e d i n f l i g h t . O n t h e f i r s t f l i g h t e x p o s u r e t i m e s of 3.6, 0.66, a n d 0.12 seconds were e m p l o y e d . O n l a t e r flights a single e x p o s u r e t i m e of a b o u t 1 second w a s e m p l o y e d . C a l i b r a t i o n of t h e r e s u l t i n g s p e c t r a p e r m i t t e d t h e e v a l u a t i o n of t h e r e l a t i v e s p e c t r a l e n e r g y d i s t r i b u t i o n of t h e s u n l i g h t a n d hence, of t h e a m o u n t of ozone i n t h e l i g h t p a t h a b o v e t h e i n s t r u m e n t i n a m a n n e r s i m i l a r t o t h a t e m p l o y e d i n some of t h e b a l l o o n f l i g h t s . #

D a t a o n t h r e e successful flights b y N a v a l R e s e a r c h L a b o r a t o r y p e r s o n n e l a r e r e p r o d u c e d i n F i g u r e 11 (24). W h i l e data obtained b y a rocket spectrograph i n this m a n n e r cannot be considered h i g h l y accurate, t h e large variations i n t h e v e r t i c a l d i s t r i b u t i o n of ozone a t h i g h a l t i t u d e s a r e i n d i c a t i v e of a c e r t a i n a m o u n t o f a i r m o v e m e n t a t these l e v e l s . I t a p p e a r s t h a t a i r m o t i o n u p s e t s t h e ozone e q u i l i b r i u m c o n d i t i o n s b y t h e e q u i v a l e n t of a l t i t u d e changes a t t i m e s of as m u c h as 5 o r 6 k m . F u r t h e r m o r e , i t has been e s t a b l i s h e d b y these e x p e r i m e n t s t h a t ozone extends u p t o a n d s o m e t i m e s a b o v e 70 k m . T h e m a x i m u m r e l a t i v e c o n c e n t r a t i o n of ozone h a s been

A D V A N C E S IN

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f o u n d t o v a r y b e t w e e n a b o u t 25 a n d 30 k m . T h i s m a y be c o n s i d e r e d as t h e ozone equilibrium level. T h i s m a x i m u m r e l a t i v e c o n c e n t r a t i o n m u s t n o t be c o n f u s e d , h o w e v e r , w i t h t h e m a x i m u m of t o t a l ozone w h i c h u s u a l l y exists a t a m u c h l o w e r l e v e l — t h a t i s , b e t w e e n a b o u t 20 a n d 25 k m .

Surface

Ozone

T h e c o n c e n t r a t i o n of ozone i n t h e s u r f a c e a t m o s p h e r e has b e e n f o u n d t o v a r y w i d e l y b o t h i n t i m e a n d l o c a t i o n . I t s presence o r d i n a r i l y h a s l i t t l e o r n o significance e x c e p t i n c e r t a i n l o c a l i t i e s , as, f o r e x a m p l e , w h e r e r u b b e r goods m a y be s t o r e d o r e m p l o y e d or w h e n associated w i t h intense s m o g c o n d i t i o n s as exist i n t h e L o s A n g e l e s b a s i n . I t s presence r e s u l t s f r o m a n u m b e r of causes, m o s t l y n o t y e t w e l l u n d e r s t o o d . S o m e is p r o d u c e d b y discharges f r o m h i g h t e n s i o n w i r e s a n d o t h e r e l e c t r i c a l m a c h i n e r y , some b y l i g h t n i n g , a n d some diffuses o r is c a r r i e d d o w n w a r d b y a i r c u r r e n t s . M o s t of t h e h i g h c o n c e n t r a t i o n s a r e f o u n d i n c o n n e c t i o n w i t h c o m b u s t i o n of h y d r o c a r b o n s a n d a r e a p p a r e n t l y p r o d u c e d t h r o u g h c h e m i c a l a n d p h o t o c h e m i c a l processes n o t y e t f u l l y e s t a b l i s h e d (28). A s n o t e d p r e v i o u s l y , ozone is u n i q u e i n h a v i n g a h i g h o p t i c a l o p a c i t y w i t h i n t h e H a r t l e y b a n d c e n t e r e d a r o u n d 2500 t o 2600 A . T h i s f u r n i s h e s a n i d e a l basis f o r t h e p h y s i c a l m e a s u r e m e n t of t h e s u r f a c e c o n c e n t r a t i o n of t h e ozone i n t e r m s of i t s r e l a t i v e s p e c t r a l a b s o r p t i o n a t these w a v e l e n g t h s i n r e l a t i o n t o t h a t a t l o n g e r w a v e l e n g t h s . S t a i r et al. (49) d e v i s e d a f i l t e r r a d i o m e t e r f o r t h i s p u r p o s e , w h e r e i n t h e a m o u n t of ozone i n p a r t s p e r h u n d r e d m i l l i o n w a s d e t e r m i n e d as a f u n c t i o n of t h e r a t i o s of f i l t e r t r a n s m i t t a n c e s of t h e r a d i a n t e n e r g y f r o m a m e r c u r y a r c l a m p s i t u a t e d a t a d i s t a n c e of 1450 feet f r o m t h e d e t e c t i n g d e v i c e . T h e d e t e c t i n g s y s t e m c o n s i s t e d of a p h o t o m u l t i p l i e r tube a n d associated equipment. I n this arrangement the light b e a m is m o d u l a t e d so t h a t a l t e r n a t i n g c u r r e n t e q u i p m e n t m a y be e m p l o y e d t h r o u g h out. Some results obtained w i t h this equipment on a n evening at W a s h i n g t o n , D . C , w h e n a s m a l l a m o u n t of ozone w a s i n t e r m i t t e n t l y p r e s e n t , are i l l u s t r a t e d i n F i g u r e 12 (49). A l t h o u g h t h e p a r t i c u l a r e q u i p m e n t s e t u p was a d a p t e d o n l y t o n i g h t o p e r a t i o n , b y t h e use of u l t r a v i o l e t filters a n d of m e r c u r y arcs r i c h i n t h e e m i s s i o n lines of 2804 a n d 3655 Α . , d a y o p e r a t i o n s h o u l d be feasible. R e g e n e r (40) e m p l o y e d a s p e c i a l s p e c t r o g r a p h i n t h e m e a s u r e m e n t of ozone i n t h e presence of s m o g a t L o s A n g e l e s , u s i n g a n i n t e n s e h y d r o g e n a r c as t h e l i g h t source. T h e l i g h t p a t h w a s 1000 feet. T h r e e w a v e l e n g t h s w i t h i n t h e s p e c t r a l range of 2665 t o 2807 A . w e r e e m p l o y e d i n t h e ozone e v a l u a t i o n . B y u s i n g a n e c c e n t r i c r o t a t i n g d i s k as a s h u t t e r , a g r a d u a l l y v a r y i n g e x p o s u r e a l o n g t h e slit l e n g t h was o b t a i n e d . F r o m t h e r e s u l t i n g s p e c t r a a n u m b e r of ozone d e t e r m i n a t i o n s were m a d e f r o m e a c h p h o t o g r a p h . R e s u l t s i n a g r e e m e n t t o w i t h i n 1 0 % of those o b t a i n e d w i t h t h e p o t a s s i u m i o d i d e m e t h o d w e r e r e p o r t e d . I n R e g e n e r ' s w o r k i t was a s s u m e d t h a t t h e p r i n c i p a l selective a b s o r p t i o n w i t h i n t h i s p a r t of t h e u l t r a v i o l e t s p e c t r u m w a s due t o o z o n e . S u c h a n a s s u m p t i o n a p p e a r s r e a s o n a b l e i n t h e l i g h t of p r e s e n t i n f o r m a t i o n o n t h e r e l a t i v e s p e c t r a l t r a n s m i t t a n c e s of k n o w n substances i n t h e a t m o s p h e r e . R e c e n t l y t h e A i r P o l l u t i o n F o u n d a t i o n (43) set u p a s p e c t r o r a d i o m e t r i c ozone m e t e r (designed b y R . S. E s t e y ) w h i c h i n c o r p o r a t e s m a n y of t h e features a n d p r i n c i ples e m p l o y e d i n t h e i n s t r u m e n t s d e v e l o p e d b y t h e N a t i o n a l B u r e a u of S t a n d a r d s (49) a n d b y R e g e n e r (40). T h i s meter employs a high intensity m e r c u r y arc (type C H 3 ) t o g e t h e r w i t h a C o r n i n g 9863 f i l t e r w h i c h confines t h e r a d i a n t e n e r g y p r i n c i p a l l y t o t h e u l t r a v i o l e t s p e c t r u m . A s i m p l e s p e c t r o r a d i o m e t e r e m p l o y s f o u r 30° q u a r t z p r i s m s w i t h a 1 P - 2 8 p h o t o m u l t i p l i e r as a d e t e c t o r . T h e d e s i g n of t h e e l e c t r o n i c s a n d s t r i p r e c o r d e r f o l l o w c l o s e l y t o t h a t of t h e N B S i n s t r u m e n t e x c e p t t h a t a f r e q u e n c y of 120 i n s t e a d of 510 cycles p e r s e c o n d is e m p l o y e d (the n a t u r a l m o d u l a t i o n of t h e l a m p as o p e r a t e d o n 60 cycles p e r s e c o n d a l t e r n a t i n g c u r r e n t ) . I n t h i s i n s t r u m e n t

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8:00

8:30

9:00 EST.

Figure 12.

9:30

PM, W A S H I N G T O N ,

10:00

10:30

D.C.

Variation in surface ozone on two evenings at Washington, D. C.

Measurements made with mercury arc source and photomultiplier and filter ozone meter through 1450-foot distance (49)

t h e a i r p a t h l e n g t h i s 3 0 0 feet a n d t h e ozone c o n t e n t i s c a l c u l a t e d i n t e r m s o f r e l a t i v e a b s o r p t i o n a t 2650, 2804, a n d 3 1 3 2 A . T h e s p e c t r u m m a y b e s w e p t as d e s i r e d , b u t i n t h e o r i g i n a l i n s t r u m e n t t w o sweeps, 1 m i n u t e a p a r t , w e r e m a d e e a c h 15 m i n u t e s . T h e N B S i n s t r u m e n t s i m i l a r l y s w e p t t h r o u g h t h e different filter c o m b i n a t i o n s once each m i n u t e . A c o m p a c t p h o t o e l e c t r i c i n s t r u m e n t (28) r e c e n t l y d e v e l o p e d b y H . K r u g e r A s sociates of S a n G a b r i e l , C a l i f . , t a k e s a d v a n t a g e of t h e h i g h o p a c i t y of ozone a t 2537 A . t h r o u g h t h e use of a b e a m of r a d i a n t e n e r g y of t h a t w a v e l e n g t h . A l i g h t p a t h of o n l y 10 i n c h e s i s e m p l o y e d . T h i s i n s t r u m e n t is essentially a double-beam u l t r a v i o l e t filter p h o t o m e t e r a n d h a s a n e x t r e m e l y h i g h s e n s i t i v i t y , f u l l - s c a l e s e n s i t i v i t y b e i n g s o m e t h i n g less t h a n 100 p . p . h . m . of ozone. T h e e l e c t r o n i c s e n s i t i v i t y of t h i s i n s t r u m e n t is p r o b a b l y t o o h i g h f o r p r a c t i c a l p u r p o s e s . I m p r o v e d operation could possibly be h a d t h r o u g h i n c r e a s i n g t h e p a t h l e n g t h , t h e r e b y p e r m i t t i n g t h e use o f less s e n s i t i v e e l e c t r o n i c c i r c u i t s as w e l l as d e c r e a s i n g t h e effect of t h e s m a l l a m o u n t o f ozone p r o d u c e d n e a r t h e l a m p source. O p t i c a l m e t h o d s of ozone d e t e r m i n a t i o n m a y b e c o m b i n e d w i t h t h e c h e m i c a l t y p e s i n one w a y o r a n o t h e r . F o r e x a m p l e , L i t t m a n a n d M a r y n o w s k i (28) i m p r o v e d a n d m o d i f i e d a m e t h o d e a r l i e r e m p l o y e d b y P a n e t h a n d E d g a r (34) u s i n g a s i l i c a gel t o a b s o r b t h e ozone gas f r o m t h e a t m o s p h e r e . A f t e r w a r d t h e ozone w a s flushed i n t o a n o p t i c a l c e l l , w h e r e i t s c o n c e n t r a t i o n w a s m e a s u r e d i n t e r m s of i t s u l t r a v i o l e t a b s o r p t i o n as c o m p a r e d w i t h t h a t r e s u l t i n g f r o m k n o w n c o n c e n t r a t i o n s o f s y n t h e t i c ozone-oxygen mixtures. I n conclusion, i t n o w appears t h a t t h e spectroradiometric methods of ozone e v a l u a t i o n h a v e , c o n t r a r y t o a n e a r l i e r a p p r a i s a l (34), b e c o m e b o t h p r a c t i c a l a n d p r e f e r r e d f o r use i n r o u t i n e ozone m e a s u r e m e n t s . A s t h i s i s a s u m m a r y p a p e r c o v e r i n g s o m e of t h e w o r k of m a n y i n v e s t i g a t o r s , i t has n o t b e e n possible t o g i v e a l l t h e d e t a i l s i n a n y case. I n s o m e cases i m p o r t a n t

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work m a y have escaped notice. However, the list of references at the end of the paper should provide the reader with an opportunity to pursue further along any particular line of investigation.

Literature C i t e d (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45)

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